BULLETIN OF THE GEOLOGICAL SOCIETY OF AMERICA VOL. 71, PP. 645-768, 35 FIGS. JUNE 1960
REVIEW OF PALEOMAGNETISM BY ALLAN Cox AND RICHARD R. DOELL
ABSTRACT This review is an attempt to bring together and discuss relevant information con- cerning the magnetization of rocks, especially that having paleomagnetic significance. All paleomagnetic measurements available to the authors are here compiled and eval- uated, with a key to the summary table and illustrations in English and Russian. The principles upon which the evaluation of paleomagnetic measurements is based are sum- marized, with special emphasis on statistical methods and on the evidence and tests for magnetic stability and paleomagnetic applicability. Evaluation of the data summarized leads to the following general conclusions: (1) The earth's average magnetic field, throughout Oligocene to Recent time, has very closely approximated that due to a dipole at the center of the earth orientetl parallel to the present axis of rotation. (2) Paleomagnetic results for the Mesozoic and early Tertiary might be explained more plausibly by a relatively rapidly changing magnetic field, with or without wander- ing of the rotational pole, than by large-scale continental drift. (3) The Carboniferous and especially the Permian magnetic fields were relatively very "steady" and were vastly different from the present configuration of the field. (4) The Precambrian magnetic field was different from the present field configuration and, considering the time spanned, was remarkably consistent for all continents.
RESUME Ce compte-rendu s'efforce de rassembler et de discuter les donnees utiles sur la mag- netisation des roches, et en particulier celles qui ont un interet paleomagnetique. Toutes les mesures paleomagnetiques connues des auteurs sont rassemblees et eValuees (avec un indexe du tableau-resume et des illustrations en anglais et en russe). Les principes sur les quels se base 1'evaluation des mesures paleomagnetiques sont resumes, en appu- yant sur les methodes statistiques et sur les donnees et les tests qui rcvelent la sta- bilite magne'tique et 1'utilisation possible en paleomagnetisme. L'etude des donnees mene aux conclusions gdnerales qui suivent: 1. Le champ magnetique moyen de la terre, de POligocene a la periode actuelle suit de tres pres ce qui resulterait de la presence d'un dipole au centre de la terre, oriente parallelement a 1'axe de rotation actuel. 2. Les donn&s paleomagnetiques du Mesozoique et Tertiaire inferieur pourraient etre explique'es d'une maniere plus plausible en supposant un champ magnetique variant rapidement, plutdt qu'une derive continentale a grande echelle, avec ou sans deplacement du p61e de rotation. 3. Les champs magnetiques du Carbonifere et particulierement du Permien etaient tres "stables" en comparaison de ceux des periodes anterieure et posterieure, et avaient une configuration profonde"ment differente de celle du champ actuel. 4. Le champ magnetique precambrien avail une configuration tres differente de celle du champ actuel, mais, si Ton tient compte de la rturee representee, etait remarquable- ment constant pour tous les continents.
ZUSAMMENFASSTJNG Diese Zusammenstellung ist ein Versuch, wichtige Informationen zusammen zu bringen und zu diskutieren, die den Magnetismus der Gesteine behandeln, vor allem solcher, die die palaomagnetische Bedeutung haben. Alle den Autoren zur Verfugung stehenden palaomagnetischen Messungen sind hier zusammengestellt und ausgewertet worden, mit einem Schliissel in Englisch und Russisch fur die zusammenfassende Tabelle und die Illustrationen. Die Prinzipien, auf welchen die Auswertung der palaomagnetischen Messungen basiert, sind zusammengefasst, wobei besonderes Gewicht auf statistische Methoden und auf die Ergebnisse und Versuche betreffend magnetischer Stabilitat und palaomagnetischer Anwendbarkeit gelegt wird. 645 646 COX AND DOELL—PALEOMAGNETISM
Die Auswertung der zusammengestellten Ergebnisse fiihrt zu den folgenden ail- gemeinen Schlussfolgerungen: 1. Das durchschnittliche magnetische Feld der Erde vom Beginn des Oligozans bis zur Gegenwart ist fast gleich geblieben, vermutlich infolge eines Dipols im Mittel- punkt der Erde, wobei der Dipol parallel zu der augenblicklichen Rotationsaxe orien- tiert ist. 2. Die palaomagnetischen Ergebnisse aus dem Mesozoikum und dem Unteren Tertiar konnen besser durch ein verhaltnismassig schnell wechselndes magnetisches Feld als durch eine weitraumige Kontinentalverschiebung, mit oder ohne Wanderung des Rota- tionspols, erklart werden. 3. Verglichen mit denen friiherer oder spaterer Zeiten, waren die magnetischen Felder des Carbons und besonders des Perms, sehr "bestandig". Sie waren vollkommen verschieden von der augenblicklichen Gestaltung des Feldes. 4. Das magnetische Feld des Prakambriums war verschieden von der augenblick- lichen Konfiguration des Feldes und, wenn man die Zeitspanne in Betracht zieht, auf- fallend einheitlich fur alle Kontinente.
OB3OP flBJIEHHfl HAJIEOMArHETHSMA AnjiaH KOKC H PHiap« P. Ronn PeaioMe HacTOHirniH o63op aBJiaeTca nonHTKofi co6paTb Boe^HHo H o6cyflHTi> CBe^e- HHH o HaMarHHiHBaHHH nopo«, oco6eHHo naHHbie, HMeioinHe sHaieirae. Bee «o- CTyrmbie aBTopaM flaHHue o najieoMarneTHaMe 6bijin TrnaTCJibHO npoanajiH3H- i. ycjioBHbie o6oaHatieHHH Ha cBOflnoft Ta6JIH^e H Ha pacyHKax cna6- noHCHeHHHMH Ha aHrjiHficKOM H pyccKOM H3HKe. Cp;eJiaHO o6o6m;eHHe npHHi(HnoB, Ha KOTOPBIX ocHOBana o^eHKa najieoMarHHTHHx HawepeHHit. Oco6o noniepKHBaeTCH snaieHHe cTaTHCTHiecKHX MeiojtOB H jjaHHHx 06 HSMepCHHH MarHHTHOH CTaSnjlbHOCTH, B CB6Te HX npHMeH6HHH K H3yieHHK) najieoMarHeTHSMa. AnajiHB co6paHHHx flauntix HPHBOHHT K cjienyiomeMy o6meMy saKJiioieHino: 1. B TeieHHH ojiHroi;eHa, BIUIOTB jj,o nosflHeieTBepTH^Horo BpeMeHH, cpe^nee Mar-HHTHoe none SCMJIH npaBjiHacaeTCfi BecbMa SJIHSKO K nojiio, oSpasosaHHoMy MarHHTHbiM flHnoneM B i;eHTpe seiajiH, opueHTapoBaHHOMy napajuiejibno K COBpeMeHHOfl OCH BpameHHH. 2. PeayjibTaTbi HaMepeHnit najieoMarHHTHbix csoitcTB MeaoaoftcKHx H TPCTHIHHX nopoH cBHfleTejibCTByioT 06 oTHocHTejibHo 6ncTpo HaMenaromeMca MarnnTHoM none, HeatejiH o KpynHbix nepeMemeHunx MaTepHKOB, co cMenjeHHeiw HJIH 6es cMemeHHH noJiKcoB BpameHHa. 3. MarHHTHbie HOJIS KaMeHHoyrojibHoro H, oco6eHHo, nepMCKoro nepaofla 6bijiH oieHb "nocTOHHHbi", no cpaBHeHHK c HOJIHMH 6oJiee pannero HJIH noafl- nero BpeMeHH, H Becbina oTjin^aJiHCb OT cospeMeHHoro oiepTaHHH nojia. 4. floK6M6pHHCKoe MarHHTHoe nojie oTJiHianocb OT coBpemeHHoro oiepTaHHH MarHHTHoro HOJIH H, npHHHMan BO BHHMaHHe npoMeatyTOK BpeMeHH, oT«ejiHio- m;HH Hac OT flOKeMSpHH, 6bijio B BHCineH cTeneHH nocTOHHHbiM Ha Bcex Ma- TepHKax.
CONTENTS TEXT Page Self-reversed magnetization 652 Page Other processes affecting remanent Introduction 64:7 magnetization 654 Acknowledgments 649 Tests for paleomagnetic applicability 655 The basis of paleomagnetism 649 General statement 655 Magnetization of rocks 649 Field tests 655 General statement 649 Laboratory tests 658 Isothermal remanent magnetization 650 The earth's magnetic field 661 Viscous magnetization 650 Description of field 661 Thermo-remanent magnetization 650 Origin of the field 665 Depositional magnetization 652 Statistical analysis of paleomagnetic data. . 668 Crystallization or chemical magnetization 652 General statement 668 INTRODUCTION 647
Page Figure _ Page Statistical analysis of sets of vectors or for various types of remanent magneti- lines 668 zation 659 Ovals of confidence about virtual geo- 9. Directions of magnetization before and magnetic poles 671 after alternating-field partial demagneti- Sources of error 672 zation 660 Design of experiments 673 10. Theoretical magnetic fields of a geocentric Other statistical methods 673 axial dipole and a geocentric inclined Paleomagnetic data 673 dipole, with observed field directions. . . 662 General statement 673 11. Changes in directions of earth's magnetic Key to Table 1 675 field with time, at four magnetic ob- Discussion of results 733 servatories 664 General statement 733 12. Relationships between location of observa- Post Eocene 733 tory or sampling site, field direction, and Late Pleistocene and Recent 733 virtual geomagnetic pole 665 The reversal problem 735 13. Virtual geomagnetic poles calculated from Oligocene through early Pleistocene 736 observations at various latitudes in Precambrian 739 1945 666 Early Paleozoic 741 14. Changes in virtual geomagnetic poles with Carboniferous 743 time 667 Permian 746 15. Probability density functions (? and equiva- Triassic 749 lent point-percentage contours 669 Jurassic 751 16. Dependence of circle of confidence on Cretaceous 753 number of points or vectors 670 Eocene 754 17. Late Pleistocene and Recent virtual geo- Polar wandering and continental drift 756 magnetic poles 734 General statement 756 18. Correlation diagram for late Pleistocene Axial dipole theory 756 and Recent poles 736 Late Tertiary polar wandering and conti- 19. Magnetic directions in baked zones 736 nental drift 756 20. Oligocene through early Pleistocene virtual Paleomagnetic polar wandering and conti- geomagnetic poles 737 nental drift 757 21. Correlation diagram for Oligocene through Evaluation and interpretation of pre- early Pleistocene poles 738 Oligocene paleomagnetic data 760 22. Precambrian virtual geomagnetic poles. .. 739 Directions of paleomagnetic research 763 23. Early Paleozoic virtual geomagnetic poles. 742 References cited 764 24. Carboniferous virtual geomagnetic poles.. 744 25. Permian virtual geomagnetic poles 746 26. Poles and confidence intervals for the ILLUSTRATIONS Supai formation 747 27. Test for axial, nondipole field 749 Figure Page 28. Triassic virtual geomagnetic poles 750 1. Acquisition of thermo-remanent magneti- 29. Jurassic virtual geomagnetic poles 752 zation in weak magnetic fields 651 30. Cretaceous virtual geomagnetic poles 753 2. Reversal in pyrrhotite caused by magneto- 31. Eocene and Eocene? virtual geomagnetic static interaction between two different poles 755 constituents 653 32. Postulated Tertiary polar-wandering 3. Self-reversal by Ja—T differences in two paths 757 antiparallel sublattices 654 33. Postulated paleomagnetic polar-wandering 4. Cpnsistency-of-reversals test for stability.. 656 curves for Europe and North America . 758 5. Field relations indicating stable magneti- 34. Postulated paleomagnetic polar-wandering zations by Graham's "fold" and "con- curves for Europe, North America, glomerate" tests 656 Australia, India, and Japan 759 6. Reduction in scatter of directions of 35. Permian and Carboniferous virtual geo- magnetization by applying correction for magnetic poles from Europe, North tilt of beds. 657 America, and Australia 761 7. Application of simple "tilt correction" to Table Page plunging fold 658 1. Paleomagnetic data 678 8. Alternating field-demagnetization curves 2. Key to Figures 17, 20, 22-31, 35 733
INTRODUCTION been greatly stimulated by interpretations of the magnetic data as evidence relevant to two Studies of the magnetic properties of rocks persistent geologic hypotheses, continental have accelerated so rapidly during the past drift and polar wandering. Moreover, further 2 decades that the accumulated information interest in paleomagnetism has been aroused and special techniques of rock magnetism may by observations suggesting that the earth's now well be regarded as a separate geologic magnetic field has undergone periodic reversals. discipline. Current interest in the subject has While continental drift and polar wandering 648 COX AND DOELL—PALEOMAGNETISM hypotheses have been proposed for many (1951), working in France, found that the decades, and remanent magnetization has been presence or absence of reversals in otherwise recognized for centuries, a brief description of indistinguishable lavas depended on the strati- the recent rise of interest in paleomagnetism graphic position of the flows. Their data through the union of these rather diverse ele- strongly pointed to field reversals, with the ments will form an appropriate introduction to most recent one occurring early in the Pleisto- this review. cene. Research on the reversal problem is con- The classic early work in paleomagnetism is tinuing, and Uyeda's review (1958) of the self- that of Chevallier (1925), who demonstrated reversal mechanism of the Haruna dacite is that the remanent magnetizations of several one of the outstanding contributions in recent lava flows on Mt. Etna were parallel to the years. earth's magnetic field measured at nearby Methods of using field relationships for observatories at the time the flows erupted. demonstrating stability of remanent mag- (For earlier studies of rock magnetism see the netization have been developed by Graham references cited in Chevallier, 1925, and (1949) and are now standard techniques in Matuyama, 1929.) Mercanton (1926, p. 860) paleomagnetic investigations. appears to have been the first to foresee clearly With the development of a very sensitive the possibility of using rock magnetism as a spinner magnetometer by Johnson and McNish tool for testing the theories of polar wandering (1938), and an astatic magnetometer by and continental drift, and he also anticipated Blackett (1952) with a sensitivity close to the the field-reversal hypothesis. theoretical limit, measurements of weakly Since these early studies, great progress has magnetized sediments became possible. A large been made in understanding the processes by number of measurements by Graham (1949; which rocks become magnetized. For the past 1955), by Clegg and others (1954a), and by 20 years, Thellier and his colleagues have been Creer and others (1954) soon confirmed the concerned with several of these processes, fragmentary evidence from previous studies including magnetization acquired by rocks over indicating that pre-Tertiary rocks do not long periods of time in weak fields and magneti- usually have magnetizations parallel to the zation by heating and cooling in weak fields. present field. Graham (1949) pointed out pos- They have applied the results of these studies sible applications to an evaluation of the con- principally to determinations of the intensity of tinental-drift and polar-wandering hypotheses. the earth's field in the past. (See Thellier and Concurrently advances were made by Thellier, 1959, for an important review of this Elsasser and Bullard in explaining the origin of work.) Important contributions to an under- the earth's field by the dynamo theory (re- standing of magnetic minerals and magnetizing viewed in Elsasser, 1955; 1956). These studies processes have also been made by other workers, suggested that the earth's axis of rotation and including Rimbert, Haigh, Gorter, Nicholls, the average axis of the earth's magnetic field and especially the Japanese workers Nagata, should coincide. Encouraged by theoretical Uyeda, Kobayashi, and Akimoto. developments, Creer and others (1954) pro- The periodic field-reversal hypothesis in posed a paleomagnetically determined polar its modern form, with the last reversal ending wandering path from Precambrian to present in early Quaternary time, was first proposed by times. Matuyama (1929, p. 205). Recent interest in When more measurements from other con- reversals stems from Graham's observations tinents became available, it soon appeared to (1949, p. 156) of opposing directions of mag- many workers that polar wandering alone would netization in sediments, which lead Neel (1951) not adequately explain all the data. At the to propose four mechanisms by which a mag- present time relative displacements of nearly netization might be acquired in a direction all continents, with respect to Europe, have opposite to that of the magnetic field acting. been suggested, for North America by Runcorn Neel's theoretical prediction was brilliantly (1956b, p. 83) and by Irving (1956a, p. 40), confirmed when Nagata and Uyeda discovered for India by Clegg and others (1956, p. 430), that the Haruna dacite reproducibly acquired for Australia by Irving and Green (1958, a remanent magnetization opposing the applied p. 71), for South America by Creer (1958, field (Nagata and others, 1951; Nagata, 1953b; p. 389), for Africa by Creer and others (1958, Uyeda, 1958), suggesting that reversed mag- p. 500), and for Japan by Nagata and others netizations were not due to reversals of the (1959, p. 382). earth's field. However, at about the same time One purpose of this review is to bring together Hospers (1951), working in Iceland, and Roche the relevant paleomagnetic data. In order INTRODUCTION 649
properly to evaluate these data, the reader The first two components of magnetization should be acquainted with the principles of to be distinguished are remanent magnetization paleomagnetism, and a review of these prin- and induced magnetization. Whereas induced ciples precedes the table of data. After a dis- magnetization requires the presence of an cussion of these data, we conclude with a short applied field, remanent magnetization does not. subjective evaluation of some of the paleo- In fields as weak as the earth's, induced mag- magnetic interpretations and suggest a few netization is proportional to the field; the con- topics for future study. stant of proportionality is called the suscepti- bility. Magnetic-anomaly maps of the earth's ACKNOWLEDGMENTS field are usually interpreted by assuming that the magnetization in the rocks producing the This review was begun in 1958, when Doell anomalies is induced magnetization and paral- was a member of the faculty at the Massachu- lel to the earth's field. However, remanent mag- setts Institute of Technology, aided by a grant netizations are often not parallel to the earth's from the National Science Foundation, and field and may be stronger than the induced when Cox was studying at the University of magnetization. (See Nagata, 19S3a, p. 128-129, California, Berkeley, aided by grants from the for some typical values.) Therefore, the assump- American Petroleum Institute and the Insti- tion of a predominance of induced magnetiza- tute of Geophysics of the University of Cali- tion should be tested by sampling wherever fornia. It was completed after both authors possible. joined the U. S. Geological Survey. Our col- Each grain of magnetic material in a rock leagues at these and other institutions aided consists of one or more magnetic domains, and greatly in the preparation of the review, and although the directions of magnetization are we wish especially to acknowledge the value of different in different domains, the intensity of discussions and correspondence with J. R. magnetization per unit volume, termed the Balsley, M. R. Banks, J. C. Belshe, A. F. spontaneous magnetization J$, is the same in Buddington, J. A. Clegg, R. W. Girdler, L. G. all domains of the same mineral. Quantity Jg Howell, E. Irving, S. K. Runcorn, S. Uyeda, decreases with increase in temperature and and J. Verhoogen. Paleomagnetic data were vanishes at the Curie temperature. Sufficiently generously furnished in advance of publication small grains consist of single domains, and in by D. W. Collinson and S. K. Runcorn, by the absence of an external magnetic field the R. W. Girdler, and by D. Bowker. Special direction of magnetization in each domain will thanks are due J. R. Balsley and J. Verhoogen lie along one of several preferred axes. The for their critical reviews of the manuscript. Mrs. directions of these preferred axes and the E. Harper prepared the illustrations. heights of the magnetic energy barriers that separate them are determined by the shape of THE BASIS or PALEOMAGNETISM the grain, the crystalline anisotropy of the mineral, or both. In an applied magnetic field Magnetization of Rocks of increasing intensity the direction of mag- netization in the grain is pulled away from the General statement.—The magnetization ob- preferred axis toward the field direction. If the served in a rock is determined by two factors, energy supplied by the applied field is not the magnetic field applied to the rock up to the greater than the magnetic energy barriers, the time of observation of the magnetization, and direction of magnetization will return to its the occurrence of one or more of the several former position when the field is removed. This processes by which materials become magne- magnetization, which is reversible and depends tized. Different magnetizing processes may on the applied field, is by definition an induced operate on the rock at different times during magnetization.1 its history; the earth's magnetic field may When the applied field is increased above a change from time to time, and the magnetiza- critical value termed the coercive force, the tion acquired may not in all cases be parallel direction of magnetization in a single-domain to the applied field. It is not surprising, there- grain crosses over a magnetic energy barrier, fore, that magnetization is one of the most com- and when the field is removed the magnetic plex properties that the geologist can study in vector comes to rest along a new direction. In rocks. Moreover, geologic interpretations of 1 magnetic measurements are critically depend- The term induced magnetization is used, as defined here, in geophysical prospecting applications ent on the availability of techniques for dis- and should not be confused with induction, B, of tinguishing the various magnetic components. the usual hysteresis curve. 650 COX AND DOELL—PALEOMAGNETISM this case the irreversible change in magnetiza- nent magnetization, provided the field is larger tion is a remanent or permanent magnetization. than the lowest coercive force of the magnetic Larger grains contain many domains, and, in an minerals in the rock. The process is simply one increasing magnetic field, domains with mag- in which domains having magnetic energy netizations nearly parallel to the applied field barriers with corresponding coercive forces less grow at the expense of others. Magnetic energy than the applied field align their magnetic barriers again prevent an unlimited reversible moments with the field. When the field is growth of domains, and the coercive forces of removed, the energy barriers prevent these multidomain grains correspond to the magnetic domains from returning to their former posi- fields necessary to overcome these energy tions, and a net magnetization results. Since barriers. minerals in rocks usually have coercive forces A single rock sample may contain several of the order of 100 oersted or more, the earth's magnetic minerals with a wide range of grain field of about 0.5 oersted is, in general, not sizes and a wide coercive force spectrum strong enough to produce isothermal remanent (Graham, 1953, p. 249). The intensity of natural magnetization (IRM). On the other hand, the remanent magnetizations is commonly several large magnetic fields associated with lightning orders of magnitude less than the maximum bolts may impart a substantial IRM to rocks intensity that could be developed if the rock (Cox, 1959). The IRM may easily be removed, were placed in a magnetic field much larger or changed in direction, by any field as large as than any of the coercive forces of the different that which produced it. magnetic constituents. This indicates that only Viscous magnetization.—If a rock remains in a small fraction of the domains in a naturally a field too weak to cause IRM for a sufficiently magnetized rock have a preferred direction of long period of time, it is often possible to magnetization causing the observed remanent measure a new component of remanent mag- magnetization; the majority have random netization in the direction of the field (e.g., directions. Whether the domains with preferred Rimbert, 1956b, p. 2536; Brynj61fsson, 1957, orientations occur in constituents with high or p. 250-251). Such a magnetization, requiring a low coercive forces depends on the process by relatively long time to form, is termed viscous which the remanent magnetization was ac- magnetization. In rocks, as in other materials, quired. The natural remanent magnetization it is due to the Boltzman distribution of thermal acquired by some processes is very "hard" and energy which, when converted to magnetic similar to the remanent magnetization of a energy, allows the magnetic domains to cross good permanent magnet, whereas that acquired energy barriers that they otherwise could not by other processes is "soft," corresponding cross in the weak field of the earth. Although quite closely with the magnetization of "soft" the thermal-energy distribution has a random iron. Because rocks are not homogeneous mate- nature, the weak field of the earth provides a rials and because many of them have been sub- slight bias sufficient to cause a net change of jected to several magnetizing processes, both magnetization in the direction of the field. The types may be found in the same rock. (See theory of viscous magnetization is similar to Graham, 1953, p. 249-252; Clegg and others that of thermo-remanent magnetization. 1954a, p. 593.) During the past several decades Thermo-remanent magnetization.—Of much considerable progress has been made in under- more importance for paleomagnetic studies is standing some of the processes by which natural the process of thermo-remanent magnetization. remanent magnetization is acquired by rocks, As a rock cools in the earth's magnetic field it and in developing techniques for analyzing the begins to develop spontaneous magnetization observed magnetizations into components corre- at the Curie temperature Tc and a preferential sponding to the various processes. alignment of domains parallel to the field. The In the following paragraphs consideration resulting magnetization is thermo-remanent will be given to the principal processes causing magnetization (TRM). It is important to note natural remanent magnetization in rocks. Proc- that not all of the TRM is acquired at the esses leading to remanent magnetizations Curie temperature, but rather over a tempera- parallel to the applied field will be discussed ture interval extending some tens of degrees first. Then factors leading to remanent mag- below TC- If, during a cooling experiment, a netizations which are not parallel to the applied weak magnetic field is applied only in the tem- field will be considered. perature interval Ti to T2 (Tt < Tc), with Isothermal remanent magnetization.—A rock zero magnetic field at all other temperatures, placed in a magnetic field at room temperature a magnetization known as the partial thermo- and subsequently removed will acquire a rema- remanent magnetization (PTRM) is developed. THE BASIS OF PALEOMAGNETISM 651
An example of the PTRM acquired in equal field between the Curie temperature and room temperature intervals on cooling from the Curie temperature. (Compare Figs, la and b.) temperature is shown in Figure la, where the The theory of TRM for single-domain values are plotted as a function of the mean particles (N6el, 1955, p. 209-212) explains many of these characteristics. In Neel's model (which will be followed here) each grain has 3 AJ/Hx I0 two directions in which the magnetic vector can lie with minimum magnetic energy in the absence of a magnetic field; these directions 10 are 180° apart and are separated by a magnetic barrier of energy
E = vHcJa/2 (D 200 400 600°C (a) Partial TRM where 11 is the volume of the grain, Hc the coercive force, and Js the spontaneous mag- netization of the mineral. When E is greater 4o J/Hx than the thermal energy (kT), where k is Boltz- r mann's constant and T the temperature, the thermal fluctuations are not able to move the direction of magnetization across the energy 30 barrier. However, for sufficiently small values of v or sufficiently high values of T, the thermal fluctuations can cause the magnetic moment to 20 move across the barrier. Thus, a total remanent magnetization of initial amount Jo due to the preferential alignment of a large number of 10 identical single domain grains will, after time t, have decayed to the value JR given by JB = / exp (-I/To) (2) T O 200 400 600' where TO is termed the relaxation time. As in (b) Total TRM other decay processes, one may speak of the FIGURE 1.—ACQUISITION OF THERMO-REMANENT "half-life" of thermo-remanent magnetization MAGNETIZATION IN WEAK MAGNETIC FIELDS which has the value 0.693 TO. Quantity TO is (a) Partial thermo-remanent magnetization given by the equation (PTRM) acquired in field H over equal temperature intervals as a function of mean temperature of inter- w l/r0 = A (n/T) exp (-vB cJs/2kT) (3} val; (b) thermo-remanent magnetization (TRM) = A(v/T)™ exp (-yv/T) V' acquired on cooling from Curie temperature to any temperature T in field H and from T to ambient temperature in zero field (for weak field the quantity Quantities A and 7 depend on the elastic and J/H is approximately constant). magnetic properties of the minerals, and the other quantities are as defined for equation (1). temperature of the interval. Experimentally An important feature of this model for TRM it is found that for lavas and baked sediments is that a small change in the quantity (v/T) can the PTRM acquired in a weak field over any cause a very large change in TO. For example, temperature interval T\ to Tz is independent of the physical constants necessary to evaluate A the magnetization acquired in adjacent tem- and 7 are known for iron (Neel, 1955, p. 211); and values for the quantity (v/T) of 3.2 X 10~21, perature intervals (Thellier, 1951, p. 213; 21 21 Nagata, 1953a, p. 142-153); Thellier reports 7.0 X 10~ , and 9.6 X 10~ correspond respec- 1 9 that the rock preserves an exact memory of the tively to values of 1CT seconds, 10 seconds 2 temperature and field which produced the (3.4 X 10 years), and 3.4 X 10" years for rc. PTRM (quoted in Neel, 1955, p. 212). If the At room temperature the grain diameters rock is heated to temperatures up to T\ no corresponding to these values of (v/T) are effect on the PTRM acquired between T\ and roughly 120 A, 160 A, and 180 A. Thus, the T2 is observed, whereas it is completely de- direction of magnetization in a grain with a stroyed at temperatures above TV The total diameter less than 120 A is easily and quickly TRM acquired in a given field is very close to changed by the thermal fluctuations, and the the sum of the PTRM's acquired in the same application of a weak field h to a number of 652 COX AND DOELL—PALEOMAGNETISM such grains causes a net magnetization in the previously magnetized magnetic particles direction of the field. This "equilibrium" mag- attain a preferential alignment during deposi- netization is given (Neel, 1955, p. 211) by the tion and maintain this alignment after con- equation solidation, giving the sediment a remanent magnetization (Nagata and others, 1943, NvJ tanh (vhJ /kT) a B (4) p. 277-279; Johnson and others, 1948, p. 357- where N is the number of grains with volume v. 360; King, 1955, p. 120). The stability of such Because of the strong dependence of TO on a magnetization depends upon the process by (i!/T) in equation (3), there is a critical blocking which the grains originally acquired their mag- diameter for a given mineral, dependent only netization. (Processes that cause depositional on the temperature; grains with smaller diam- magnetization to have a direction other than eters come to equilibrium very quickly with the that of the applied field will be discussed later.) magnetization indicated in equation (4), while Crystallization or chemical magnetization.-— those with substantially larger diameters main- Although the magnetization of some sediments tain their original magnetizations over long is undoubtedly acquired by the depositional intervals of time, regardless of the external field. process, studies by Martinez and Howell Similarly, there is a critical blocking temperature (1956, p. 205) and by Doell (1956, p. 166) for all grains of the same diameter. indicate that the magnetization of sediments The acquisition of TRM by single-domain may also be associated with chemical changes grains is very simple in terms of this model. As taking place after consolidation. Moreover, a rock cools from its Curie temperature, a given Haigh (1958, p. 284-285) and Kobayashi grain assumes the equilibrium magnetization, (1959, p. 115-116) have shown in the laboratory JE, until the temperature passes through the that a remanent magnetization is acquired by critical blocking temperature of the grain. As magnetic materials undergoing a chemical the temperature goes below this critical value, change (e.g., reduction of hematite to magne- TO for the grain increases rapidly, and the mag- tite) at constant temperature in a weak mag- netization becomes "frozen" at the equilibrium netic field. These authors also show that the level. The independence of partial thermo-rema- stability of this magnetization, under the effects nent magnetizations acquired in different tem- of higher temperature and demagnetizing fields, perature ranges is thus explained as due to the is very similar to that for TRM, although the magnetization residing in grains of different intensity is not so great. diameter. This simple theory explains many of Haigh (1958, p. 278-281) points out the the characteristics of TRM such as its great theoretical similarity of the processes causing stability to disturbing fields and its remarkably chemical magnetization and TRM of small slow decay. grains. As the grains of magnetic material grow The acquisition of TRM by most rocks is chemically, the value of the critical quantity certainly more complex than indicated here, (v/T) in the equations for TRM increases since many rocks contain magnetic minerals because of an increase in v rather than a de- differing in physical properties as well as in crease in T. As the grain grows through the grain size. Moreover, rocks containing multi- critical blocking diameter appropriate to the domain grains, and even massive ferromagnetic temperature at which the chemical reaction mineral specimens, also acquire TRM which, occurs, the equilibrium magnetization JE commonly, has the characteristics described (equation 4) is, as in the case for TRM, effec- above. Verhoogen (1959) suggests that the tively frozen in. Theoretically, the stability TRM of these materials may reside in small, properties of crystallization magnetization and highly stressed regions within the ferromagnetic TRM should be similar, and laboratory experi- crystals. ments indicate that this is true. The Curie temperatures of magnetic mate- Self-reversed magnetization.—The most strik- rials in igneous rocks lie below 700° C and, in ing example of a magnetization acquired in a many rocks, below 600° C. The major portion direction other than that of the field acting of the natural remanent magnetization meas- during the acquisition of the magnetization is ured in many igneous rocks appears to be TRM. that of self-reversal. In many paleomagnetic (For more complete discussions of TRM see studies directions of magnetization fall into Nagata, 1953a, p. 123-192; Neel, 1955, p. 208- two distinct groups nearly or exactly opposed 218, 225-241; Verhoogen, 1959.) to each other. Two interpretations of this phe- Depositional magnetization.—As demon- nomenon have been proposed: that the earth's strated in artificially deposited sediments, magnetic field may periodically reverse itself, THE BASIS OF PALEOMAGNETISM 653 or, alternatively, that some rocks may become netization of the Haruna dacite (Uyeda, magnetized in a direction opposite to that of 1958, p. 120), which is one of the two or three the field acting on them by a process called rocks reported to be reproducibly self-reversing. self-reversed magnetization. The spontaneous magnetization of some Several mechanisms may theoretically give rise to self-reversed magnetization. The first to APPLIED be considered requires two magnetic constitu- ents A and B in the rock. Constituent A FIELD has a higher Curie temperature than B and acquires a TRM parallel to the applied field. As the rock cools through the Curie tempera- ture of constituent B, the TRM of constituent A acts by one of several interaction mechanisms to order the magnetization of B in a direction exactly opposite to that of A and, hence, reversed with respect to the original applied field. A self-reversal occurs if, after cooling, mmw/////,. the total magnetization of B exceeds that of A (Neel, 1951, p. 92), or if constituent A is later selectively removed chemically (Graham, 1953, p. 252-255). The simplest type of interaction is magneto- static (Neel, 1951, p. 100; Uyeda, 1958, p. 50- 56), in which the field in the region of constitu- ent B at the time the temperature passes | | MAGNETITE (Tc = 578°C) through its Curie point is controlled by the magnetization of A, and is reversed with respect ^/Z\ PYRRHOTITE (Tc = 3IO°C) to the applied field. The relationship is shown schematically in Figure 2. For this type of >- NORMAL TRM interaction to lead to a self-reversal, very stringent requirements are placed on the geo- -<-- REVERSED TRM metrical arrangement of the two constituents FIGURE 2.—REVERSAL IN PYRRHOTITE CAUSED BY and on the ratio of the applied field to the spon- MAGNETOSTATIC INTERACTION BETWEEN Two DIFFERENT CONSTITUENTS taneous magnetization of constituent A when On cooling, magnetite with higher Curie tempera- B becomes magnetized. In rock-forming min- ture becomes magnetized first. Further cooling erals this mechanism could occur only in very results in magnetization of pyrrhotite in "reversed" weak applied fields; it is possible but rather field between magnetite layers. Net TRM is improbable in fields as strong as the earth's, 'normal". (After experiment by Uyeda, 1958) and no example has been found in nature (Uyeda, 1958, p. 52). minerals, for example magnetite, is actually A second type of interaction between the two made up of two superimposed opposing spon- constituents is an exchange interaction across taneous magnetizations, each associated with a their common boundary. If good registry exists separate sublattice in the magnetic mineral. between the crystal lattices of the two constitu- If these two spontaneous magnetizations have ents, the spontaneous magnetizations on one different temperature coefficients, the total net side of the boundary will tend to become spontaneous magnetization may change sign aligned either parallel or antiparallel to the with temperature, as shown in Figure 3. This spontaneous magnetization on the other side. type of self-reversal mechanism has been dem- The Weiss-Heisenberg exchange interaction onstrated by Gorter and Schulkes (1953, p. 488) between spinning electrons, which is also in certain synthetic materials but has not been responsible for spontaneous magnetization, found in rocks. provides the coupling, which may be very A mineral may also undergo self-reversal strong. Uyeda (1958, p. 104) finds that members when cations migrate from disordered to of the ilmenite-hematite series xFeTiO3- (1 — *) ordered distributions on cooling (Neel, 1955, Fe2O3, with .45 < x < .6, become self-re- p. 204; Verhoogen, 1956, p. 208). Moreover, versed, even when the applied field is as high when cooled quickly, cations may be frozen in as 17,000 oersted. This type of interaction a disordered state corresponding to a high- appears to be responsible for the reversed mag- temperature equilibrium. Over very long 654 COX AND DOELL—PALEOMAGNETISM periods of time the cations will then slowly that the inclination measured in the samples migrate to the equilibrium-ordered positions, ranged some 20 to 30 degrees less than the and the process may be accompanied by a self- inclination of the field acting, although the reversal of the TRM. Verhoogen (1956, p. 208) declination was faithfully reproduced. This shows that this mechanism is possible for natu- "inclination error" decreased as the field in- clination approached the vertical or horizontal. Like most sedimentary minerals, magnetic mineral grains are rarely uniformly equidimen- sional; moreover, the common magnetic min- erals tend to have directions of magnetization parallel to their longest dimension. The "in- clination error" arises during the depositional .Temp. process since the grains will tend to lie with VJ their longest dimension, and hence magnetic ^^^ / direction, parallel to the horizontal bedding plane and not exactly along the applied field tii ^^^ • 6^X'j 1TC direction. King has also demonstrated that an Jr\ I x C; error in the direction of magnetization can occur ^ .0 due to rolling of grains as they settle on the 1 bottom, caused either by deposition on sloping ^^ I surfaces or by currents. *«^^. The magnetic properties of minerals resemble ^ (> other physical properties in that they are not, FIGURE 3.—SELF-REVERSAL BY J s— T DIFFERENCES IN Two ANTIPARALLEL in general, completely isotropic; in particular, SUBLATTICES individual mineral grains usually cannot be On cooling, sublattice A is initially dominant and magnetized with equal ease in all directions. is aligned with applied field. Sublattice B is locked In all minerals there exist easy and hard direc- antiparallel to A and is dominant at low tempera- tions of magnetization systematically oriented tures. with respect to the crystal lattice, a property called magneto-crystalline anisotropy. A single ral magnetites containing impurities; he esti- crystal of magnetite, for example, is magne- mates that the ordering process would require tized more easily along the [111] axes than at least 106 to 106 years. Such a self-reversal along the [100] axes, and a crystal of hematite mechanism would therefore not be reproducible much more easily in the c plane than along the in the laboratory. c axis. A second factor causing anisotropy is the This brief and incomplete review of a rather shape of the individual grain. An aggregate of large field of research serves to emphasize randomly oriented magnetite crystals should several important points about reversely mag- have no crystalline anisotropy, but a single netized rocks. The reversed magnetization grain of the aggregate will be more easily mag- of some rocks is now known to be due to a netized parallel to its longest dimension. In self-reversal mechanism. Moreover, many theo- any of the magnetization processes considered retical self-reversal mechanisms have been pro- above, except depositional magnetization, in posed, and additional mechanisms will doubt- which the grains are already magnetized, the less be suggested in the future. However, in magnetization direction of a single crystal or of order definitely to reject the field-reversal an elongated grain will lie between a direction hypothesis it is necessary to show that all of easy magnetization and the direction of the reversely magnetized rocks are due to self- applied field. However, when preferred-crystal reversal. This would be a very difficult task directions or longest-grain dimensions are ran- since some of the self-reversal mechanisms are domly oriented within a rock sample, the net difficult to detect and are not reproducible in magnetization direction will be that of the the laboratory. A further discussion of this applied field. problem will be postponed until some of the Deformation of rocks with a remanent mag- relevant paleomagnetic data have been con- netization may also cause a change in the mag- sidered. netization due to a mechanical rotation of the Other processes affecting remanent magnetiza- magnetic particles. A vertical compaction in tion.—King (1955, p. 120), in his experiments sediments might, for example, be expected to on artificially deposited varved silts, found reduce the inclination of the magnetization THE BASIS OF PALEOMAGNETISM 655
vector (Clegg and others, 1954a, p. 596). from a given geologic formation is undertaken, Graham (1949, p. 156-158) has considered the the paleomagnetist is usually less interested in effects of plastic deformation on remanent the magnetism itself than in the direction of magnetization in the limbs of a fold. However, the magnetic field that produced it. A paleo- this phenomenon has rarely been cited as a magnetic study of rocks should therefore yield cause of scattered directions of magnetization, two pieces of information: the average direction probably because highly deformed beds are of the magnetic field at the locality where the usually not chosen for paleomagnetic investiga- rocks were collected, and the time or geologic tions. age when the field had that direction. It is Magnetostriction—the effect of stress on usually assumed for paleomagnetic purposes magnetization-—is another phenomenon which that the magnetization measured is in the may be important in the magnetization of direction of the earth's magnetic field existing rocks. In the investigation by Graham and at the time rocks were magnetized, and that others (1957, p. 471-472) axial compressive the magnetization was acquired during the stresses of slightly more than 2500 lbs/sq, in. formation of the rocks or soon after. We have changed the magnetization in the rocks studied noted in the preceding sections, however, that (mostly gneisses and iron ores) by as much as rocks may receive a magnetization in several 25 per cent; moreover, the magnetization of different ways, some of which do not satisfy some of the samples did not return to the origi- the assumptions just outlined. For example, nal state after the stress was removed. Many depositional magnetization or TRM acquired rocks are subjected to large stresses during by rocks with a crystalline or shape anisotropy their histories—the stresses developed during may not be parallel to the field acting during the cooling of basalt, for example, are sufficient the magnetization process; and viscous or to fracture the rock—, and the research de- chemical magnetization may be acquired long scribed above strongly suggested that magneto- after the formation of the rocks. strictive effects might, in general, cause the Fortunately, the magnetizations acquired recorded remanent magnetizations of rocks to by the different processes commonly have very be in directions that are not those of the fields different properties which in many cases can be acting when remanent magnetization was investigated in the laboratory. Many of the originally acquired. Stott and Stacey (1959, magnetic anisotropic properties of rocks can p. 385) investigated this possibility for TRM also be measured. Finally, certain geological by cooling several types of igneous rocks (in- field tests give very definite limits to the time cluding basalts, dolerites, andesites, and rhyo- at which the magnetization took place. The im- lites) from above their Curie temperatures in portance of these tests in paleomagnetic studies the earth's field while under compressive cannot be overemphasized. Because the critical stresses of 5000 lbs/sq, in. Identical samples reader must know whether or not the mag- were similarly cooled without an applied netization was acquired at the time of forma- stress, and in all cases the resulting TRM, tion of the rocks, and also whether or not it was measured at room temperature after the stress acquired parallel to the field acting, it is im- had been removed, was parallel to the applied portant to consider the field and laboratory field. tests in some detail. Since some magnetostrictive processes may Field tests.—Consistency among the direc- be time-dependent (Graham and others, 1959), tions of magnetization of many samples is field tests are also of interest in evaluating the sometimes used as a criterion for stability. role of magnetostriction in paleomagnetism. Although this test is far from conclusive, direc- Different magnetic minerals respond in differ- tions of magnetization that are tightly grouped ent ways to the same stresses; thus the consist- away from the present field direction have more ency of results from rocks of the same period significance than is often realized. Such a con- that have different mineral assemblages, or sistency demonstrates immediately the absence were magnetized by different processes, or have of a dominant component of magnetization had different stress histories would indicate parallel to the present field, such as might be that, for such rocks, magnetostrictive effects caused by viscous magnetization or chemical have not been important. magnetization associated with surface weather- ing. Moreover, gross petrologic differences of Tests for Paleomagnetic Applicability rocks within a formation are usually recognized, General statement.—When a study of the and similar differences exist in the magnetic remanent magnetism of a suite of samples minerals. These differences are commonly indi- 656 COX AND DOELL—PALEOMAGNETISM cated by large differences in the intensity of reversals. This test applies to reversals due magnetization from sample to sample. Con- either to field or self-reversal, since in both sistency of directions of magnetization in such cases the mean directions of magnetization are 180° apart. If, subsequent to the original mag- NORMAL SET REVERSED SET netization, both groups acquire an additional component of magnetization as shown in Figure 4, the two resultant groups will no longer be 180° apart. This test is very powerful, since • Original Magnetizations it is also valid for completely homogeneous • Secondary Magnetization groups of samples and does not depend on the -< Resultant Magnetizations relative intensity or direction of the secondary magnetization. FIGURE 4. •CONSISTENCY-OF-REVERSALS TEST TOR STABILITY In the above field tests the tacit assumption Two sets of magnetization with initial direction has been made that the rocks have not been 180° apart are no longer exactly reversed if secon- tilted or folded. Rocks are, of course, sub- dary component has been added. jected to folding, and Graham's classic fold
FIGURE 5.—FIELD RELATIONSHIPS INDICATING STABLE MAGNETIZATIONS BY GRAHAM'S "FOLD" AND "CONGLOMERATE" TESTS a case strongly suggests that the magnetization test (Graham, 1949, p. 158) uses folding to was acquired in an unchanging magnetic field. establish stability of magnetization. The test If it were acquired by one or more processes is very simple and has great significance. acting when the field had different directions, Suppose the directions of magnetization of the inhomogeneities would probably result in samples collected from one limb of a fold have magnetization directions spread out between a mean direction significantly different from the two field directions rather than tightly the mean direction of samples collected from grouped at some fixed angle between them. the other limb (see Fig. 5). If on conceptually Magnetic directions of samples from the same "unfolding" the beds and rotating the directions formation are frequently distributed along a of magnetization along with them, the mean plane passing through the present direction of directions from the two limbs coincide, then the earth's field (Runcorn, 1956a, p. 305; the following conclusion is valid: the beds Creer, 19S7a, p. 132-136; Howell and Martinez, received a magnetization of uniform direction 1957, p. 390). The consistency test is not satis- at some time prior to the folding, and the fied in this case, and two components of mag- magnetization has not subsequently changed netization in varying amounts are present, direction. The application of this test to some one of which is parallel to the present field. The Precambrian sedimentary rocks is shown in significance of a consistency test depends Figure 6. This "tilt correction" is usually largely on the extent of the sampling and the made by rotating the beds into the horizontal range in size and composition of the magnetic about the strike direction, a procedure which minerals represented. tacitly assumes that the axes of the folds are Parallelism between tightly grouped mean horizontal. If the fold is plunging and the directions of magnetization in two groups of magnetic inclinations are other than vertical, samples which are reversely magnetized with this method of correction can lead to serious respect to each other is a much stronger test errors. An extreme example showing how a than simple consistency of directions without serious error may be introduced is shown in THE BASIS OF PALEOMAGNETISM 657
Figure 7. The field direction erroneously re- don in stratigraphically higher conglomerates constructed by the simple "tilt correction" and measuring the directions of magnetization differs in azimuth by 90° from the correct in these fragments. Since aligning forces
O Before Correction Lower Hemisphere • After Correction Equal Area Projection FIGURE 6.—REDUCTION IN SCATTER OF DIRECTIONS OF MAGNETIZATION BY APPLYING CORRECTION FOR TILT OF BEDS Graham's fold test applied to directions of magnetization in folded Keweenawan sediment. (After Du Bois, 1957) direction, and, moreover, a false "reversal" associated with the magnetic moment of these has been generated. Although errors this large fragments are very much smaller than large occur only for steeply plunging folds and other forces acting during deposition, the small inclinations, one should, before applying earth's field will not be effective in aligning the simple tilt correction, be sure that the them. Therefore, a completely random set of fold axes are horizontal. A proper correction directions from the fragments is to be expected for plunging folds can, of course, be made with if the fragments are stably magnetized. Sta- an additional operation. bility of magnetization of the parent formation The conglomerate test of Graham (1949, is then usually inferred from random directions p. 158) may also be used to establish magnetic of magnetization in the fragments, as depicted stability. The stability of a formation is tested in Figure 5. Care must be taken in establishing by locating cobbles or pebbles from the forma- that the random magnetization of the fragments 658 COX AND DOELL—PALEOMAGNETISM has not been caused by other than the depo- ponents of natural magnetization found in sitional process, and the test gains in significance rocks. when different samples from the same fragment An important laboratory experiment is that have parallel magnetizations, while samples of examining the magnetic properties of a
(a)
(d) FIGURE 7.—APPLICATION OF SIMPLE "TILT CORRECTION" TO PLUNGING FOLDS (a) Original uniform directions of magnetization; (b) direction in fold with horizontal axis—"tilt correc- tion" restores to condition (a); (c) directions in fold with steeply plunging fold axis; (d) result of apply- ing simple "tilt correction" to fold (c)—false "reversal" has been generated. from adjacent pebbles with the same lithology rock under the effects of demagnetizing proc- have different directions of magnetization. esses. Magnetic minerals in rocks consist of Laboratory tests.—Field tests yield important many domains with a wide spectrum of coercive but not particularly detailed information; at forces, and, as noted previously, natural best they tell us that the magnetization has remanent magnetization is due to a preferential been stable since the occurrence of some event alignment of only a few per cent of these such as folding. Laboratory tests, on the other domains. Different magnetizing processes tend hand, give more specific and detailed informa- selectively to align domains concentrated in tion useful in unraveling the often complex different parts of the coercive force spectrum, nature of the magnetization found in rocks. and by means of demagnetization techniques Laboratory techniques are also useful for it is possible to learn whether a given natural "washing" out unstable components of mag- remanent magnetization resides in domains netization as well as the effects of other ran- with low coercive forces ("soft" magnetization), domizing processes. Much is now known about high coercive forces ("hard" magnetization), the properties of some types of magnetization or perhaps is distributed throughout the due, in large part, to the extensive and careful coercive force spectrum. In a demagnetization experiments of Nagata and his group, and to analysis, the "soft" magnetization in the the works of Thellier, Rimbert, and Haigh. rock is destroyed first by giving low coercive Thus, it is now often possible, by laboratory force domains a random orientation; the re- analysis, to distinguish the principal corn- maining remanent magnetization is then meas- THE BASIS OF PALEOMAGNETISM 659 ured, and the process is repeated with pro- 1959, p. 104). The IRM acquired in a 100- gressively stronger demagnetizations. Two oersted field is effectively destroyed in an demagnetization processes may be used: heating alternating field with a peak value of 100 the rock to a given temperature followed by oersteds; however, the TRM acquired in a cooling in zero magnetic field, or placing it in field of 0.5 oersted has decreased only slightly an alternating magnetic field, whose amplitude slowly decreases to zero. Although Figure lb shows the acquisition of TRM TRM as a sample is cooled from above the Curie point, it may also be used to show the CRM amount of TRM remaining after heating to any temperature. As discussed in more detail in the section on TRM for single-domain grains, heating to a given temperature in zero field causes a random orientation in all domains with magnetic barriers having energies less than or equal to the thermal energy. An upper temperature limit beyond which heat de- magnetization is not useful is frequently set by chemical changes or phase transitions which o.o may occur at temperatures as low as a few 250 500oe. hundred degrees Centigrade. If a rock is placed in an AC magnetic field A. C. Field- with peak value fi, all domains with coercive FIGURE 8.—ALTERNATING FIELD-DEMAGNETIZATION CURVES FOR VARIOUS TYPES OF REMANENT forces less than H will follow the field as it MAGNETIZATION alternates. As the AC field is then slowly de- Normalized isothermal, chemical, and thermo- creased to zero, domains with progressively remanent magnetizations remaining after demag- lower coercive forces become fixed in different netization in A. C. fields, shown as a function of the orientations, and hence all domains with peak value H of the demagnetizing field. TRM and CRM were acquired in 0.5 oersted field, IRM in 30 coercive forces less than H will have random oersted field. (After Kobayshi, 1959) orientations. If a constant magnetic field is superimposed on the alternating field, or if in the 100-oersted alternating field, and a the variation of the magnetic field with time measurable part still remains above 500 oersted. is not symmetrical, an anhysteretic magnetiza- Rimbert (1956a), p. 892 in other experiments tion will develop (Thellier and Rimbert, 1954, noted an appreciable TRM remaining above 900 p. 1400) which may mask the remaining rema- oersted and only a small change between 500 nent magnetization. The development of this and 900 oersted. Chemical magnetization has magnetization may be prevented by performing a stability comparable with that of TRM, as the AC demagnetization in the absence of a was suggested by the similarity of the TRM and constant field with the even harmonics filtered CRM theories for single domains. Thus, with out from the current supplying the AC field these and similar experiments (see aspecially coil (As and Zijderveld, 1958, p. 310), or by Thellier and Rimbert, 1955, p. 1406), it is spinning the sample as the alternating field relatively simple to distinguish IRM in rocks decreases (Brynj61fsson, 1957, p. 248; Cox, from CRM or TRM, but not to distinguish 1959, p. 122). CRM from TRM. Many of the processes causing remanent Viscous magnetization differs from IRM in magnetization can be reproduced in the labora- requiring, for its destruction, an alternating tory, and demagnetization experiments on such field larger than the field in which it was magnetization of known origin are important produced. Rimbert (1956b, p. 2538) found that in interpreting similar experiments on natural the magnitude of the AC field needed to destroy remanent magnetism. Figure 8 shows the viscous magnetizations acquired by volcanic results of alternating field demagnetization rocks over periods of time up to 2 months experiments on thermoremanent and chemical varies linearly with the logarithm of the time. magnetizations produced in weak fields and For example, the viscous magnetization ac- on isothermal remanent magnetization pro- quired in a 5-oersted field during 5 minutes duced in a relatively strong field (Kobayashi, required a 37-oersted alternating field for its 660 COX AND DOELL—PALEOMAGNETISM
O Original Measurement Lower Hemisphere • After 300 oe. A.C. Equal Area Projection Treatment FIGURE 9.—DIRECTIONS OF MAGNETIZATION BEFORE AND AFTER ALTERNATING-FIELD PARTIAL DEMAGNETIZATION All samples are from the same lava flow. (Data from Cox, 1959) destruction, and that acquired in 2 months in magnetization due to lightning are probably the same field required a field of 180 oersted. common sources of scatter in paleomagnetic Although it is dangerous to extrapolate these measurements, these experiments suggest an results to geologic times, they suggest that obvious way of "washing" away these unstable viscous magnetization acquired during a secondary components. Partial thermal and million years in a field of 1 oersted would prob- alternating field demagnetization have been ably be destroyed in alternating fields of the used by a number of workers for this purpose order of a few hundred oersted. A rough verifica- (Doell, 1956, p. 165; Cox, 1959, p. 122; tion of the extrapolation may be found in Brynjolfsson, 1957, p. 253; Hood, 1958; Creer, demagnetization studies by Brynjolfsson (1957, 1958, p. 379; As and Zijderveld, 1958, p. 318). p. 251) and Cox (1959, p. 129) in which a Figure 9 shows an example of the effects of viscous magnetization in volcanic rocks about alternating field demagnetization on volcanic half a million years old was destroyed in alter- rocks; most of the initial scatter in these nating fields of 50 to 100 oersted. measurements has been shown to be due to Since viscous magnetization acquired in lightning (Cox, 1959, p. 135). the earth's field and isothermal remanent Demagnetization experiments are important THE BASIS OF PALEOMAGNETISM 661
in paleomagnetic studies not only for decreasing direction that is not parallel to the field acting scatter in the data but also for shedding light are associated with magnetic anisotropy in on the origin of the remanent magnetization. the rocks. Moreover, natural magnetizations remaining However, magnetic anisotropy of a rock after demagnetization in fields of the order of may be as complex as its remanent magnetiza- 400 oersted are very stable "hard" magnetiza- tion, and no single measurement can completely tions and will certainly not have been disturbed describe it. Anisotropy of the induced magneti- by the effects of sampling, transporting, coring, zation is, to our knowledge, the only magnetic or measuring operations, or by any process anisotropy property that has been measured in capable of magnetizing only low coercive force paleomagnetic studies (Howell and others, domains. 1958, p. 286). If the susceptibility is plotted as A special series of laboratory tests has been a function of the orientation of the magnetic devised by Nagata and hi group (Nagatas and field with respect to the sample, a triaxial others, 1954, p. 184-185; Nagata and others, ellipsoid is described. For example, the sus- 1957, p. 32) for determining whether reversely ceptibility ellipsoid of a rock containing only magnetized rocks represent field or self- hematite crystals with parallel c axes is a very reversals. The tests are primarily concerned flat oblate spheroid with its short axis, the with the detection of self-reversal properties axis of minimum susceptibility, parallel to the in the rocks, and for details the reader is c axes of the hematite crystals. In using sus- referred to the works cited as well as to that ceptibility anisotropy as a test for paleomag- of Uyeda (1958). netic applicability, care must be taken that The field and laboratory tests discussed above the magnetic anisotropy measured corresponds are primarily concerned with establishing the to the remanent magnetization of interest. stability of natural remanent magnetizations For example, the remanent magnetization in and removing the scattering effects of "soft" a rock might be due to hematite with strong magnetizations. However, the very important susceptibility anisotropy, but this would not question of whether the magnetization was be detected if a small proportion of isotropic acquired parallel to the magnetic field that magnetite were also present. produced it remains unanswered. In order to devise tests to answer this question one must The Earth's Magnetic Field first consider processes whereby magnetiza- tions are acquired in directions that are not Description of field.—The present shape of the parallel to the applied field direction. earth's magnetic field and its changes during Nonparallel magnetization will be acquired the last several hundred years are of primary if the magnetic grains in a rock have a shape importance in paleomagnetism, since these data or crystal anisotropy; rocks with thin layers of furnish an estimate of the irregularities and magnetite crystals or hematite crystals with variations likely to be encountered in studies of parallel axes would possess, respectively, these past magnetic fields. This might be called the two types of magnetic anisotropy. Inclination expected "signal to noise ratio" for paleomag- errors associated with depositional magnetiza- netic studies. The present field at the surface of tion also cause nonparallel magnetization and the earth may be described in terms of three probably cause anisotropy as well, since flat or components: a relatively small component due elongated grains tend to lie with their longest to processes occurring above the earth's surface; axes in the bedding plane. Nonparallelism a dipole component equivalent to the field of a in depositional magnetization may also arise magnetic dipole located at the center of the when grains are rolled down inclined deposi- earth and inclined 11K° from the axis of ro- tional planes or moved by bottom currents; tation; and a nondipole component, which Granar (1959, p. 32) has shown that anisotropy would remain if the externally produced field will probably be associated with bottom and dipole field were removed. currents, since elongated grains tend to roll If the earth's magnetic field is represented by with their long axis normal to the current means of spherical harmonics, one may easily direction. recognize and separate these three components. Magnetostriction might also cause a non- The first such analysis was made by Gauss in parallel magnetization, but tests for its oc- 1839 and has been repeated at various intervals currence cannot be devised until the process is since (Chapman and Bartels, 1940, p. 639). better understood. It appears therefore that The results of the analysis are expressed as a most processes known to cause a magnetization series of terms, each a simple algebraic combi- 662 COX AND DOELL—PALEOMAGNETISM
ROTATION AXIS
ORIENTATION OF ORIENTATION OF AXIAL DIPOLE " "INCLINED DIPOLE
0.6 GAUSS > Theoretical field due to geocentric axial dipole > Theoretical field due to geocentric inclined dipole > Earth's field, 1945, projected onto meridional plane 290° east FIGURE 10.—THEORETICAL MAGNETIC FIELDS OF A GEOCENTRIC AXIAL DIPOLE AND A GEOCENTRIC INCLINED DIPOLE, WITH OBSERVED FIELD DIRECTIONS Plane of projection passes through geomagnetic poles, and observed field is projected onto this plane. Observation points are at 30-degree intervals from geomagnetic pole. nation of sin m
produced field, those for which n = 1 are col- random but shows a systematic westward lectively termed the first-order harmonic and drift, estimated by several methods at one- those for which n = 2, 3, • • • are known as fifth degree of longitude per year. The move- the higher-order harmonics. The nondipole ment is independent of the latitude of the component of the earth's field is represented by feature (Bullard and others, 1950, p. 83). the higher-order harmonics, and the dipole The geomagnetic pole does not coincide component is completely described by the with the magnetic dip pole, which is defined as first-order harmonic. Therefore, if the earth's the place where the horizontal component of magnetic field were due solely to a dipole at the earth's field vanishes, because a horizontal the center of the earth, only the first-order component due to the nondipole field is present harmonic would appear in the analysis, and at the geomagnetic pole. At the magnetic conversely the first-order harmonic completely dip pole, the nondipole horizontal component specifies the orientation and intensity of a exactly cancels the horizontal component of geocentric dipole. Of all the dipoles that might, the dipole field. Whereas the geomagnetic pole by various criteria, be chosen best to approxi- has not changed since adequate measurements mate the irregular field of the earth, the one were available, the position of the dip pole has that is inclined 11 J£° from the axis of rotation changed relatively rapidly with changes in gives the best average fit, in the sense of least the nondipole component. squares, over the entire surface of the earth. The description of the earth's field in terms The point on the surface of the earth toward of spherical harmonics is a purely mathematical which this dipole points is, by definition, the procedure and carries no implication that each geomagnetic pole, and its present co-ordinates term (or group of terms) is physically significant are 78 K° North Latitude and 69° West Longi- in the sense that it corresponds to a separate tude (Finch and Leaton, 1957, p. 316). The physical event. However, other evidence shows closeness of the fit is shown graphically in that the external field (corresponding to certain Figure 10. The present earth's field has been of the harmonic terms) is physically different. projected into the plane which passes through With respect to the terms of internal origin, the geomagnetic pole and the earth's axis of the fact that the first term of the spherical rotation. The directions are shown at intervals harmonic analysis is predominant does not in of 30° from the geomagnetic pole. For com- itself imply that the dipole term has special or parison, the field directions caused by an axial separate physical significance. At greater depths dipole are also shown at these points. within the earth the higher-order terms become Although theoretical considerations and larger relative to the first, and at the core- paleomagnetic results suggest that the geo- mantle boundary the two are approximately magnetic pole has not always been at its present equal (Elsasser, 1956, p. 87). The strongest location, it is important to note that there is evidence that the first term may correspond to a no direct evidence that it has moved. It was process different from the higher-order terms not until the latter part of the nineteenth lies in the observation that their rates of move- century that data adequate for an accurate ment relative to the surface during the past 75 determination of the geomagnetic pole became years have been very different. available, and since then it has remained within With the exception of the important work of about half a degree of its present location Thellier and Thellier (1959) on the intensity (Bullard and others, 1950, p. 86). of the earth's past magnetic field, most paleo- Superimposed on this stable dipole field is magnetic data give the field direction only. the comparatively irregular, rapidly changing Therefore, of the various methods of comparing nondipole field represented by the higher-order the dipole and nondipole field components, the harmonics. The nondipole field is made up of angular departure of the observed field from the irregularly distributed regions of high and low field of a dipole is of most direct interest in field intensity which range in diameter from paleomagnetism. Examples from four observa- about 25° to 100° (Bullard and others, 1950, tories of the angular departure of the observed p. 70). Moreover, these regions wax and wane field from that due to the inclined dipole field much as the centers of cyclonic activity in the (and the axial dipole field) are shown in Figure atmosphere do, and present rates of change 11 on an equal-area projection. The axial suggest an average life for an individual cell dipole field is that due to a magnetic dipole of the order of 100 years (Elsasser, 1956, p. 87). aligned along the axis of rotation of the earth The movement of these nondipole features and is of more interest in paleomagnetism than over the surface of the earth is not entirely is the inclined field. The average departure 664 COX AND DOELL—PALEOMAGNETISM between the observed field and the axial dipole to the geomagnetic pole, is given by the field at the present time is 8.5° in the northern 'dipole" formula: hemisphere and 17.3° in the southern (Cox, cot p = ^2 tan 7 1959, p. 11). The largest departure disclosed (7) by most of the available observatory records is Quantities 6' and $' are the latitude and 29°. longitude of the geomagnetic pole; 6 and
-(- cj= AXIAL DIPOLE FIELD
X X INCLINED DIPOLE FIELD
OPEN SYMBOLS: UPPER HEMISPHERE EQUAL AREA SOLID SYMBOLS: LOWER HEMISPHERE PROJECTION FIGURE 11.—CHANGES IN DIRECTIONS OF EARTH'S MAGNETIC FIELD WITH TIME, AT FOUR MAGNETIC OBSERVATORIES Successive observations of the field direction are spaced 40-50 years apart. The axial and inclined dipole field directions shown for the four locations are computed from the present positions of the geographic and geomagnetic poles using equations (5) to (7).
It is often convenient in paleomagnetic the latitude and longitude of the observatory; studies to represent the data not in terms of and D and 7 are the declination and inclination the field direction measured, but rather in of the field at the observatory. The position of terms of the geocentric dipole that would the geomagnetic pole consistent with a given produce the measured field direction. This is field direction may also be found graphically usually done by specifying the geographic by means of a Schmidt or Wulff projection. co-ordinates of the geomagnetic pole that The relationships are shown graphically in corresponds to the orientation of this inferred Figure 12. dipole. Given the declination and inclination Equations (5), (6), and (7) establish a one- of the field at an observatory (or as determined to-one mapping relation between all possible paleomagnetically), the position of the geo- field directions at an observatory and their magnetic pole consistent with the observed equivalent pole locations distributed over direction may be found by the following re- the earth's surface—given one quantity, the lations : other is uniquely determined. "Poles" may thus be formally computed from any observed sin 9' = sin 6 cos p + cos 9 sin p cos D (5) field direction whether due entirely to a geo- sin (<*>' - 0) = (sin p sin D)/ cos 0' (6) centric dipole or not. Such poles will here be termed virtual geomagnetic poles. When non- where p, the angular distance along the great dipole components are present, the virtual circle from the observatory (or sampling site) geomagnetic poles calculated at different THE BASIS OF PALEOMAGNETISM 665
P- Geographic Pole tan 1 = 2 cot p S~ Observatory Location V.G.P.-Virtual Geomagnetic Pole FIGURE 12.—RELATIONSHIPS BETWEEN LOCATION OF OBSERVATORY OR SAMPLING SITE, FIELD DIRECTION, AND VIRTUAL GEOMAGNETIC POLE Calculation of virtual geomagnetic pole from field-direction data. 6 is the latitude and
GEOMAGNETIC POLE
90°E 9Q°W
900-60°N,60-90°S DATA FROM 60°-300N,30-60°S OBSERVATORIES 30°N-30°S FIGURE 13.—VIRTUAL GEOMAGNETIC POLES CALCULATED FROM OBSERVATIONS AT VARIOUS LATITUDES IN 1945 Data from Vestine and others, 1947 objections. The high rate of change and west- at depths greater than 25 km is probably ward drift of the nondipole field with velocities above 750° C (Jacobs, 1956, p. 219), it Mows up to 20 km per year are difficult to explain as that only this outer region could possess a due to geologic processes in the mantle or crust. remanent magnetization. The required average Such processes would certainly proceed at a intensity would be 6 emu/cc, and in the light much more leisurely pace. A further difficulty of the present knowledge of crustal materials is that the earth probably does not contain fulfillment of this requirement seems virtually enough ferromagnetic materials below their impossible. Magnetite-rich igenous rocks may Curie temperatures to account for the in- acquire remanent magnetizations as large as tensity of the field. The Curie temperature of 0.6-1.8 emu/cc in magnetic fields of the order iron is 780° C, of nickel 350° C, and of mag- of 1000 oe (Nagata, 1953a, p. 107); however, netite 580° C; moreover, there is no evidence their natural remanent magnetizations usually that these values increase significantly with range between 0.001 and 0.05 emu/cc. pressure. Since the temperature in the earth A theory that most nearly explains all the THE BASIS OF PALEOMAGNETISM 667
+ NORTH GEOGRAPHIC POLE X GEOMAGNETIC POLE FROM SPHERICAL HARMONIC ANALYSIS, 1945 FIGURE 14.—CHANGES IN VIRTUAL GEOMAGNETIC POLES WITH TIME Poles were calculated from the field-direction data at four observatories shown in Figure 11. Time be- tween points is 40-50 years. (Symbols in Figures 11 and 14 correspond.)
observations is that the fluid core of the earth field are interpreted as the result of fluid eddies acts as a self-exciting dynamo (Elsasser, 1956, near the core-mantle boundary, and the west- p. 88-90). In addition to the existence of an ward drift as due to a smaller angular velocity electrically conducting fluid the theory requires in the outer layer of the core with respect to a source of energy to keep the fluid in convective the mantle. If we accept this model of the motion. Moreover, it requires that the fluid be earth's field, then, as a consequence of the rotating so that order is established in the relative motion of the core and mantle, the otherwise random convective motions. The nonaxial components of both the dipole field and earth's rotation fulfills the last requirement, nondipole field should cancel when averaged and aside from transient effects the orientation over a sufficiently long time (Runcorn, 1959, of the magnetic field should be symmetrically p. 91). The dynamo theory requires that one related to the axis of rotation (Runcorn, 1954, layer in the core be rotating at the same ve- p. 61). The rapid changes in the nondipole locity as the mantle, however, and this last 668 COX AND DOELL—PALEOMAGNETISM argument would not apply to fields generated a single oriented sample, or of the average in that layer. Although the differential motions directions of magnetization of each oriented of the core and mantle, as well as the basic sample from a single lava flow or outcrop. An rotational requirement of the dynamo theory, analysis could also be made of the mean direc- lead to an average field with axial symmetry, tions of magnetization of lava flows from a it is not necessarily a dipole field—i.e., the single formation, or perhaps of the virtual average declination may be zero, but the geomagnetic poles corresponding to the mean inclination need not show the dipolar variation directions of magnetization of the lava flows. with latitude. Since the method is concerned with an The dynamo theory has had some success in analysis of directions, each datum is given unit explaining features of the magnetic fields of weight by representing it as a vector with unit the sun and of some stars; moreover, as pre- length—there is no weighting in favor of more dicted by the theory, there appears to be a intensely magnetized specimens. An equivalent correlation between the sun's magnetic field representation is to regard each datum, or and its axis of rotation (Elsasser, 1956, p. 101). vector, as a point on a sphere of unit radius. In order rigorously to justify the use of Fisher's Statistical Analysis of Paleomagnetic Data statistics, the population from which the sample is drawn must satisfy two conditions: (1) the General statement.—The basic data obtained vectors in the population must be distributed in paleomagnetic studies consist generally of with axial symmetry about their mean direc- many directions of magnetization measured in tion; (2) the density of the vectors in the popu- oriented rock samples. Although the samples lation must decrease with increasing angular are collected from areas never greater than a displacement ^ from the mean direction ac- very small fraction of the earth's total surface, cording to the probability density function their stratigraphic distribution is often such that they represent directions of the earth's exp (K cos i (8) field over long periods of time. Paleomagnetic 4ir sinh data might thus be compared with a long record from a single tide gauge in a harbor, whereas Quantity K is a constant called the precision the data used in a contemporary spherical parameter and describes the tightness of the harmonic analysis are analogous to a topo- group of vectors in the population about their graphic map of the water surface in the harbor mean direction. High values of K indicate at some particular instant. Different sorts of tight groups, and K = 0 corresponds to a popu- information can be obtained from the two ap- lation uniformly distributed over the entire proaches, and different mathematical tech- surface of the unit sphere. The probability niques are necessary for their analyses. density function (P has the following meaning: Statistical analysis of sets of vectors or lines.— given a small area of size 8a on the unit sphere, An analytical tool that has proved very useful at an angular distance \[t from the mean direc- for paleomagnetic interpretations is the sta- tion, the proportion of the total points ex- tistical method developed by Fisher (1953). pected in Sa is K = 10 FIGURE 15.—PROBABILITY DENSITY FUNCTION P AND EQUIVALENT POINT-PERCENTAGE CONTOURS The population of points shown has a symmetrical distribution with mean direction at the pole of the projection (i.e., vertical). (P is the probability density function, J) is a function showing the distribution of point percentage contours for 1 per cent test areas. Cross sections of these functions are shown along the horizontal line. shown in the same figure. As may be seen in the cross sections, the two representations are A' = 2 cos /; cos Di (North component) (10) similar except for a slight leveling of the peak in the density-contour diagram owing to the K finite size of the test circle. Y = S cos It sin Di (East component) (11) Provided the conditions of the statistical model are satisfied, Fisher (1953, p. 296) shows R = (X2 + Y2 + Z2)"2 (12) that the direction of the vector sum of the N unit vectors of the sample is the best estimate sin IK = Z/R (13) of the true mean direction of the population. Ff the «th unit vector has declination or azimuth tan DI{ = Y/X (14) 670 COX AND DOELL—PALEOMAGNETISM where Z, X, and F are the components of the In paleomagnetic analyses, P is usually taken resultant vector, R is its length, and DR and as 0.05, which means that there is 1 chance in IK its declination and inclination, respectively. 20 that the true mean direction of the popula- K = IO EQUAL AREA PROJECTION FIGURE 16.—DEPENDENCE OF CIRCLE OF CONFIDENCE ON NUMBER OF POINTS OR VECTORS Population is same as in Figure 15 The best estimate, k, of the precision parameter tion lies outside the "cone of confidence" K is given (Fisher, 19S3, p. 303), for k > 3, by specified by 0195 and the direction of R. Some approximate relationships, valid for small N - 1 values of a, are k = (IS) N - R 67.5° (P = 0.5) (17) At a probability level of (1 - F), the true mean direction of the population lies within a 140° circular cone about the resultant vector .R with (P = .05). (18) a semivertical angle a(i_p), given (Fisher, 1953, p. 303) for/c > 3, by For a discussion of these and other useful relations, reference is made to Watson (1956) N - R cos ct(l_P) 1 — (16) and Watson and Irving (1957, p. 289-293). As the number of vectors included in the THE BASIS OF PALEOMAGNETISM 671 analysis increases without limit, k, the best if the vector sum is to be the true mean direc- estimate of the precision parameter, approaches tion of the lines, it is necessary that it be normal the true value of K, the precision parameter; to the dividing plane. In practice the proper on the other hand a becomes infinitely small as plane may be found as follows: (1) a provisional N becomes infinitely large. The relationship is mean direction is estimated and the sphere is shown graphically in Figure 16 for K = 10. divided by the plane normal to this direction, Thus, even for small values of K, it is possible (2) the vector sum of the points in one hemi- to determine the mean direction of a population sphere is computed yielding a more accurate with any desired degree of accuracy provided a mean direction, (3) the sphere may then be sufficient number of independent measurements redivided by the plane normal to this new are made. direction, and a second vector sum may be To determine whether the paleomagnetically computed, (4) finally, the above steps may be determined mean direction differs significantly reiterated until no change in the direction of from some known direction such as the present the vector sum takes place. Quantity a has earth's field at the sampling site, a may be the same significance as for vectors, except used directly. The two directions are signifi- that the cone defining the confidence interval cantly different at the probability level used if is reflected in the plane perpendicular to the the angle between them is greater than a. mean direction. Fisher's statistical method may also be used to Ovals of confidence about virtual geomagnetic calculate a, k, and the mean position of virtual poles.—Equations (5), (6), and (7) establish a geomagnetic poles corresponding, for example, 1-to-l mapping relationship between any mean to the mean directions of lava flows. As is the field direction at a given locality and a cor- case for directions, the mean position of the responding mean virtual geomagnetic pole. virtual geomagnetic poles differs significantly The equations also map the circle of confidence from some known position, such as the north about the field direction into a closed curve geographic pole, if the distance between them around the virtual geomagnetic pole which, exceeds a. because of the mapping function, is an oval It is often desirable to compare one paleomag- rather than a circle. If a is the semivertical netically determined field direction with another angle of the circle of confidence we may write rather than with a known direction. A criterion sometimes used is that the two mean directions a = d 1 = dD cos I (19) are significantly different if the two cones of where 7 is the inclination of the mean field confidence do not intersect, and conversely direction, and dl and dD are changes in in- that they are not different if the cones do inter- clination and declination, respectively. From sect. This criterion is not rigorously correct, and equations (5), (6), and (7) we may then find the more exact significance tests are now available semiaxes dp and 8m of the oval of confidence (Watson, 1956, p. 157; Watson and Irving, about the mean virtual geomagnetic pole 1957, p. 293). from: Sets of lines or axes rather than vectors are 2 encountered in many geologic applications, Sp = M(l+3 cos p) dl = cia (20) and in paleomagnetic studies the problem dm = sin p dD = sin p/ cos / = cxx (21) arises when normal and reversed magnetiza- tions are analyzed together. In this case the where p is the distance from the sampling site field is known to be parallel to a certain line, to the virtual geomagnetic pole (Irving, 1956a, but the sense along that line is not specified. p. 26). The semiaxis Sp lies along the great If a set of lines or axes has axial symmetry circle passing through the point of observation about its mean direction and approximates the and the virtual geomagnetic pole, and the semi- density distribution of equation (8), it may be axis 8m is perpendicular to 5p. analyzed using Fisher's statistics. However, it is The constants c\ and c?, which determine first necessary to convert the lines to vectors— the "ellipticity" of the oval of confidence, that is, to give an arbitrary sense to each line. depend only on the inclination of the field and This is most easily done by dividing the unit increase with increasing inclination. Quantity sphere into two hemispheres and regarding each Cj increases from a value of J^ for 0 inclination line in one of the hemispheres as a positive unit to 2 for vertical inclination, and c2 increases vector. The vector sum is then calculated in from 1 to 2 over the same range. Thus, for the usual manner. The choice of the plane vertical inclinations a circle of confidence dividing the sphere is not arbitrary, however; about the mean field direction with radius a 672 COX AND DOELL—PALEOMAGNETISM maps into another circle about the pole with It can be shown that the "precision parameter" radius 2a, whereas for flat inclinations the for the 300 vectors is about 25, given the above semiaxes of the oval of confidence for the same values for KSP, KSA, and KF. We may use this circle are Y^a. and a respectively. value in the approximate formula (18), with N If a is calculated for a probability of 95 equal to 300, to estimate the "circle of con- per cent, there is a 95 per cent probability that fidence" which would be found in this incorrect the virtual geomagnetic pole lies within the analysis; "an" = 140°/V25 X 300 = iy2°, oval determined from equations (20) and (21). which is too small by a factor of 7. Sources of error.—Fisher's statistical methods, The same problem arises in a paleomagnetic like others, can be incorrectly applied, and a study of sediments, where the different values review of the published data suggests to us that of K might describe the precision at different underestimates of 0:95 are not uncommon. Too hierarchal levels in the sampling scheme such small a value for a^ often results when the N as the stratigraphic units in a formation, the vectors constituting the sample have not been sampling sites within a stratigraphic unit, or randomly drawn from the same population. the oriented samples at a site. This error is analogous to that which would When it can be established that a group of arise if, in finding the mean chemical composi- data at some level in the sampling scheme tion of the Sierra Nevada batholith, five sam- corresponds essentially to one point in time, as, ples were collected at random, 20 separate for example, the group of measurements from chemical analyses performed on each sample, one lava flow, then the variations with time and the resultant data treated statistically as if of the earth's field set a lower limit to the value 100 samples had been collected and a single of Q!95 that can be obtained from N groups of chemical analysis made on each sample. data representing N points in time, no matter As a numerical example of the error that may how many data are in each group. This lower arise in this way, suppose that the true mean limit for 0:95 may be estimated by the approxi- directions of magnetization of five lava flows mate formula (18): are scattered with a precision parameter KF. Let 15 oriented samples be collected from each 095 = UOVVkW (22) flow with a between-sample precision parameter where k' is the precision parameter correspond- KSA, and let four specimens be measured from ing to variations of the earth's field with time. each sample with a between-specimen precision A rough estimate of k' may be made by as- parameter KSP. The scatter in directions of suming that variations with time of the field magnetization within samples and within lava at a locality are similar to the present variation flows is usually much smaller than that between of the field around the circle of latitude passing flows in the same formation; fairly typical through the locality. Creer (1955) estimates values are KF = 30, KSA = 200, and KSP = 1000 that k' at the equator is about 16 and that it (Cox, 1959, p. 76). increases poleward, reaching a value of about If each mean sample direction is first found 70 in high latitudes. Cox (1959, p. 38), using a by taking the mean of its four specimens, and different method of analysis, finds a similar if each mean flow direction is then found by a range of values with large irregularities in the Fisher analysis of its 15 mean sample directions, latitude dependence. For the latitudes where the resulting estimated mean flow directions most paleomagnetic sampling has been done will deviate slightly from the true flow direc- 30 is a good average value for k' to be used tions. Therefore, the estimated precision pa- in equation (22). Using this value, a lower limit rameter kp will be slightly smaller than that for the value of 0:95 obtainable from four lava for the true mean flow directions, KF. However, flows is 12°. If the present irregularities in the for the values of KSP, KSA, and KF indicated, earth's field existed in the past, some of the the difference is small, and it can be shown that reported values of age based on a large number kp is equal to 29.7. The circle of confidence of samples collected from a few lava flows are using this value is: 0:95 2 1400/ 29.7 X 5 = probably too small. , by the approximate equation (18). This One final note should be made concerning is a very close approximation to the smallest the use of Fisher's statistics. The vector popu- value of 0:95 that can correctly be calculated lation which is represented by the magnetic from these data. measurements should satisfy the density distri- An incorrect procedure is to analyze the 300 bution given by equation (8) and have sym- specimen measurements as if each had been metry about the mean direction. Watson and chosen randomly from the same population. Irving (1957, p. 293) have tested these re- THE BASIS OF PALEOMAGNETISM 673 quirements on two sets of stably magnetized 5i of the unit vectors from the mean direction. rocks and one set of unstable rocks. To the Wilson (1959, p. 755) shows that extent allowed by the number of measurements available, the two stable magnetizations satis- N}112 = [2(N - R)/N}M (24) fied Fisher's requirements, whereas the unstable one did not. Design of experiments.—The variation in where TV is the total number of vectors, and directions of magnetization encountered in a R is the length of the vector sum. If the distri- paleomagnetic investigation may arise from bution of the vector population is that of many sources, and values of the corresponding Fisher's model, then 63 per cent of the vectors precision parameters may differ by several will be within a cone with radius 8r.m.s. (Creer orders of magnitude. Statistical methods are and others, 1959, p. 316). useful in designing sampling schemes and experiments so that the greatest accuracy can PALEOMAGNETIC DATA be obtained from the smallest possible number of measurements. General Statement Watson and Irving (1957, p. 296) have considered the case of the two-level sampling Paleomagnetic results are of interest in scheme in some detail. If a total of N oriented several fields of study, and an attempt has been samples are collected at B sites or from B lava made in Table 1 to assemble all the available flows, «95 is given by the approximate formula basic data in as compressed and accessible a form of reference as feasible. All information available to us that could possibly be used for «95 140° J_ , JLV' (23) uN PBJ paleomagnetic purposes is here tabulated. No data have been excluded, even where there is no where co is the within-site precision parameter evidence for stability or where there are other at all sites, and 0 is the between-site precision reasons for rejection. It is our belief that suffi- parameter. Watson and Irving conclude that, cient basic data should be available to enable whatever the values of w and /3, the smallest the critical reader to decide for himself whether number of samples needed to achieve a given a individual determinations are sufficiently reli- is made up of a single observation at each of able for his purposes. Description of the number the B sites, but that in practice two samples are of samples collected, their lithology, the areal desirable to test for gross experimental error and stratigraphic extent of the sampling, pos- and magnetic stability. If, as is often the case, sible stratigraphic uncertainties, and tests for the number of sampling sites is limited, a paleomagnetic applicability are fully as im- preliminary estimate of co and j3 may be used iu portant as a list of pole positions. equation (23) to estimate the number of The numerical data presented include the co- oriented samples which should be collected at ordinates of the sampling locality, the direction each site. In the example cited by Watson of magnetization after correcting for tilt of the and Irving (1957), the use of this method gave strata, and the co-ordinates of the corresponding an 6*95 8 per cent lower than that found using geomagnetic pole. Confidence limits about the magnetic direction and pole position are also mean-site directions. included. In many paleomagnetic studies not Frequently a small a is desired not only for all these quantities are given, frequently be- the entire formation but also at each site for pos- cause the appropriate statistical techniques sible use in stratigraphic correlation or other ge- were not available when the study was made, ologic applications. From a preliminary estimate and in other cases because the original author of the appropriate precision parameters an esti- did not intend that "poles" be calculated from mate of the number of samples required at each the data. Because of this, separate entries are site can be made using equation (18). used for each source of data in Table 1. Exten- Other statistical methods.—A measure of the sive use has been made of previous reviews, dispersion of a set of vectors that makes no notably those of Hospers (1955), Creer and assumption about their density distribution is others (1954), Runcorn (1955a), Irving (1956a), the angular standard or root mean square Creer and others (1957), and Irving (1959). In deviation dr.m.s. defined (Wilson, 1959, p. 755) general, a review is cited only if it lists data as the root mean square of the angular distances which did not appear in the original source or if 674 COX AND DOELL—PALEOMAGNETISM there is a difference in the numerical values reference is intended as a verification rather cited. than a correction of the earlier result. The quantities listed in entries designated by In a few cases it has not been possible to an asterisk were found by us in the following reconcile sampling-area co-ordinates, field di- ways. The co-ordinates of sampling sites, when rections, and pole positions listed in original not listed in original sources, were located in sources with our computational methods. We standard atlases and are reported to the nearest have attempted to reach the authors concerned, half degree. When the original source lists di- stating the methods we have used, so that the rections of magnetization with no mean di- reasons for the differences might be determined. rection or pole position we have made a Fisher Where this has not been possible we have used statistical analysis of the data, and the results the methods discussed in this paper with the are listed to the nearest half degree. Frequently, available data so that all results would be as directions of magnetization are shown only on nearly consistent as possible. stereographic or equal-area projections, and we The proper application of statistical methods have scaled the magnetization directions from is especially important when paleomagnetic these diagrams. Since some of the diagrams are results are interpreted in terms of continental small, errors may arise during the scaling drift and polar wandering, and errors arising process; however, other workers have used this from the treatment of each specimen measure- procedure in earlier reviews (e.g., Irving, 19S6a, ment as an independent datum were discussed Tables 1, 2, 3; Irving, 1959, Table 1), and it is in the section on statistics. Where adequate encouraging to note that the different determi- data have been given in original sources, and nations usually agree to within 1-2 degrees. where, in our view, confidence intervals are too Several examples showing our values and those low, new analyses have been made for Table 1. of previous reviewers are listed in the tables. In all these cases both the original values and Virtual geomagnetic pole positions can be the procedures used by us are described. Even in found from locality co-ordinates and mean di- the absence of complete data, it occasionally rections of magnetization by using equations has been possible to make realistic estimates of (5) through (7) or, alternately, by using a stereo- confidence intervals. For example, when speci- graphic or equal-area projection. The pole men measurements have been made on samples positions in Table 1 attributed to the present from a few lava flows, the secular variation of authors were calculated on a Schmidt equal- the earth's field and the number of flows sets a area projection 20 cm in diameter. These pole lower limit to «%. (See equation (22).) In positions were read to the nearest half degree, Table 1 numerous confidence intervals have but because of a slight distortion in the pro- been recalculated using this equation. For sedi- jection used they are probably accurate only to ments it is usually difficult to judge what level about 1 degree. in the sampling scheme represents an inde- Occasionally pole positions but not mean di- pendent point in time, and, therefore, few such rections of magnetization are listed in original changes have been made in the statistical data sources. Although it would be possible to re- for sediments. However, when the statistical calculate the original direction data from the analysis has been applied to many individual pole position and sampling-locality co-ordinates specimen measurements made from only a few using equations (5) to (7), we have preferred to oriented samples, it is possible that the resulting scale the original individual directions of mag- circle of confidence is too small. netization from diagrams and compute mean Values of k, the precision parameter describ- field directions from them. The virtual geo- ing variations in the observed directions of magnetic poles calculated from these mean field magnetization, are listed, where possible, for directions usually differ by only 1-2 degrees each entry in Table 1 because of the importance from the pole position listed in the original of this quantity in evaluating paleomagnetic source, again indicating that analyses based on data. As discussed previously, the present ir- data scaled from small diagrams can be quite regularities in the earth's magnetic field in accurate. Where this procedure has been em- moderate latitudes may be described by a value ployed we have listed the virtual geomagnetic for k of approximately 30. If this value is taken pole corresponding to the mean field direction as a measure of the variations of the field at one calculated by us so that the two will be con- locality over an interval of time in the past, sistent. A pole position calculated by us and then a set of paleomagnetic data in which each differing by 1-2 degrees from that in the original datum corresponds to an individual point in PALEOMAGNETIC DATA 675 time would be expected to have a similar value netization during metamorphism or other of k. Values of k considerably lower than 30 alteration? (6) To what geological interval of indicate a variation larger than that presently time and what geographical region do these observed in the earth's field and may be due to results apply? (Rather extensive sampling is a greater amount of variation in the past field, necessary before a pole position can be regarded to experimental error in measurement, or pos- as representing, say, "the Carboniferous of sibly to the presence of anomalous components Asia"). of magnetization in the rocks. On the other hand, higher values of k may arise in two differ- ent ways. If each specimen measured has effec- Key to Table 1 tively averaged the earth's field direction over a long interval of time, then each sample will have The data are arranged in three sections: the a direction close to the mean field direction, and numerical data, a list of references from which k will be large; alternatively, if many of the these data were obtained, and relevant re- samples measured were magnetized at the same marks. Each entry is designated by a serial time, the mean direction of the group will not be number and contains data from one source only; parallel to the average direction of the field, but there is no significance in the ordering of the the scatter will be small, and k will again be entries. The entries in boldface type are those large. from which the text figures in the next section The stratigraphic subdivisions of Table 1 are were made and include values in all columns if somewhat arbitrary and simply reflect the order these were available or could be calculated from in which the data will be discussed. Since many data in the original reference. rocks giving consistent paleomagnetic results Rocks sampled.—The formation name or are poorly dated geologically, the assignment of other descriptive designation from the original a particular paleomagnetic investigation to one reference is given here, together with an indica- of the subdivisions in Table 1 has sometimes tion of what petrologic type is represented. been difficult. In several instances stratigraphic Within each section the results from a given assignments different from those in previous continent are grouped together. reviews have been made. In these cases we have No.—This column contains a number for each noted the original stratigraphic assignment and entry. In each section numbers begin with 1, have, where possible, given some indication of and the complete serial designation of an entry the stratigraphic uncertainty. We feel ourselves is understood to be the letter describing the unqualified to pursue this problem further, and, table subsection followed by the entry number— for additional information concerning ages, for example A 3 and D 4. reference is made to the original sources. No Locality.—The two columns under this head- stratigraphic ordering is implied by the order of ing give the latitude and longitude of the place listing in each section of the table. where the samples were collected. The latitude In evaluating the reliability of individual (Lat) is given in degrees north (N) or south studies listed in Table 1, the following questions (S) of the equator, and the longitude (Long) in should be carefully considered: (1) Do field and degrees east (E) or west (W) of Greenwich. laboratory tests indicate that the magnetization Magnetic direction.—The columns under this is stable and parallel to the field in which it de- heading give the magnetic direction and statisti- veloped? (2) Do the samples represent enough cal data. The declination of the average mag- time to insure that rapid variations in the netic direction (Decl) is given in degrees east of earth's field have been averaged out? (3) Have geographic north, and the inclination of the appropriate statistical methods been applied, average magnetic direction (Incl) is given in and is the value of 0:95 realistic considering degrees below the horizontal or above the probable variations in the earth's field? (4) Has horizontal (the latter is indicated by a minus a sufficiently large geographical area been sign). Correction has been made for tilt of the sampled to insure that the direction of mag- strata. The statistical data listed are the circle netization has not been influenced by local of confidence in degrees at a probability level of effects such as magnetic field anomalies or small, 95 per cent (a96), the precision parameter (k), undetected tectonic movements? (5) Are the and the number of vectors (N) used in obtaining geological structure and history sufficiently well these values. Values of k and N apply to mag- known to allow proper bedding corrections to be netic field directions for all those entries which made and to eliminate the possibility of remag- list values for the magnetic direction. In other 676 COX AND DOELL—PALEOMAGNETISM entries k and N apply to statistical analyses of the sampling as well as the manner in which sets of pole positions. statistics were applied to the data. Pole position.—These columns give the lo- (3) Stability — Any field or laboratory tests cation of the pole of the theoretical geocentric that indicate stability or instability are noted dipole (the virtual geomagnetic pole) consistent here. If no entry is present, no specific test for with the co-ordinates of the sampling locality stability has been reported. Even if a stable and the mean direction of magnetization. The component of magnetization is indicated, it location is given by the values in columns (Lat) does not necessarily follow that the results have and (Long), with the same conventions as for paleomagnetic applicability. the sampling locality. Each magnetic dipole has (4) Reversals — For those studies that show a north and a south pole, and the pole listed is mixed polarities, remarks concerning the num- the one that falls in the northern hemisphere. ber of samples in each group, their stratigraphic In column (P) which designates the polarity of relationship, and other relevant observations the pole, the letter S is used if the direction of are noted. magnetization corresponds to a magnetic south (5) Other — Remarks that do not come under pole in the northern hemisphere (the present the above headings are listed here. magnetic field is of this type), and the letter N is used if it corresponds to a magnetic north IIOflCHEHHE K TAEJIHIJAM pole in the northern hemisphere. Poles calcu- lated from sets of samples having approximately Ta6jiHn,bi COCTOHT Ha Tpex opposing or reversed polarities are designated incnoBbix Hamix, cnncKa JIHTC- (M). (5m) and (dp) are the values in degrees paTypbi H3 KETopoft 3TH n.aHHbi noiepKHyTbi, of the semimajor and semiminor axes, respec- H cooTBeTCTByiomHx npHMeiaHHH. KaJKRoe tively, of the 95 per cent confidence oval about HccJieflOBaHbie noMe^eno cepHHHbiM HOMepoM the mean pole position, corresponding to the H cojjepJKHT flaHHbie no o6pacny TOJIKO HS oflHoro ncTOHHHKa; nopHHOK nocneHOBa- value of 0:95 about the mean magnetic direction HoMepoB anaieHHH He HMeeT. in each entry. Quantity dp is measured along naHHbie npe«cTaBjiHK>T the great circle passing through the sampling KaToporo 6bijia sanrbi tmcnoBbie site and the mean pole position, and 8m along i, noMemeHHbie B TeKCTe cjieny- the great circle at right angles to the first lacra; OHH AaioT iHCJioBbie circle. Kaatjuaro cTOJi6n,a, Kar^a BTH The last column in the numerical data section, bi HJIH MoryT 6biTb (S), indicates the publication source from which no jjaHHbiM HCTOiHHKa. HHJKC the data in that entry were obtained. Lower- AeTaJibHoe noHCHemie *rro Haxomrrca IIOA aarojioBKOM B KaatAOM cTOJi6n.e. case letters refer to the references listed in the Rocks sampled. — HaaBaHnne AaHHbia. SHaieirae CHTeJibHO cpe«nero MarHHTHOro HanpafljieHHa cpeAHero HanpaBjieHHa no Kaac«OMy onpe«ejieHHio. Sp HSMepaeTca (Ded.) AaeTca B rpa^ycax Ha BOCTOK OA no OKpymnocTH 6ojiacoro Kpyra, npoxoflamero reorpa^nreecKoro Mepn^HaHa, a SHa^enne lepes MBCTO oT6opKH o6pacna H cpeftnee HaKJioHeHHfl cpeAHero HanpaBjieHHH Hama- nojioacenne nojiroca, a dm Bflojib OKpyatHocTH rHHienocTH (Incl.) B rpaflycax OT ropn- SoJibmoro Kpyra nepneHAHKyjiapnoro K nep- soHTajra, co 3HaKOM MHHyc HJIH Hanpa- BOMy Kpyry. B nocJieAHen KOJIOHKB (S) BJieHHa BBepx OT nee. BBenena nonpaBKa sa iHCJiOBbin AaHHbix yKasan HCTOHHHK, H3 HaKJioHenne njiacTa. IIpHBeneHHbie CTaTnc- KaToporo 6bijiH BsaTbi «!HcjioBbJe jianHLie. THiecKHe AaHHbia yuasHBaiOT "Kpyr cxo- 6yKBbi OTHOCHTCa K HCTOIHHKaM AHMOCTH" KaTopnH coflepsKHT B ce6a B cjieAyromea KOJIOHKB, a CHMBOJI nojioaceHHe cpenneH TO^KH c BepoaTHOcm (*) oSosHaiaeT ITO nenaTopbie HS sejiHHHH 95% (ass), Mepa TOIHOCTH (k) H HHCJIO B npeflHAynP 5 SECTION A POST EOCENE EUROPE 2 — — — — MT. ETNA LAVAS 1 37MN 15 E 4H 56 11 s — a Chevallier Age: 394 B.C. to A.D. 1911 2 37MN 15 E tte 56 6)4 50 11 86 N 126 E s 9H 7 * a Hospers, 1955 Sampling: 3 to 9 oriented samples were taken from 1 1 3 — — — — — — 11 86HN 125J4E s 10 7H b b Irving, 1959 historic lava flows. The last 2 directions agree with c Chevallier, 1925 observatory data. o _ _ _ _ _ O CARTHAGE FISED 4 37 N 10 E 359 54« 88 N 155 W s * Thdlier and Age: 2 dates, 146 B.C. and 300 A.D. CLAYS Thellier Sampling: 9 oriented samples from 2 kilns (146 B.C.) a Thellier and and 9 oriented samples from one kiln (300 A.D.) Thellier, 1951 were measured. Values in entry (4) are based on the o average declination and inclination given on p. 1478, o Ref. a. H Other: N = 2 is not sufficient for calculating Fisher f statistics. _ — PRASTMOX 5 63 N 17HE 357H 73 H 3H 46 — s a Bancroft Age: 0 to A.D. 1000 VAHVES 6 63N 17HE 8STM 73H 3H 42 46 86 N 150 W s 6 5J4 * a Hospers, 1955 Sampling: 46 "groups" of specimens were measured. b Bancroft, 1951 BRITISH FIRED 7 St N 0 E 0 661-j 2W 242 14 87 N 180 E s 4 3H * Cook and Belshe Age: First to 15th centuries A.D. CLAYS a Cook and Belshe', Sampling: 10 archaeological fired clays were measured 1958 from the 1st through 4th centuries and 4 from the 12th, 13th, and 15th centuries. Values for entry (7) are based on data scaled from Fig. 3, Ref. a, which a gives declination and inclination values corrected to Cambridge, England. Other: Statistical values have no rigorous significance because individual measurements do not represent independent points in time. ANGESMAN 8 63 N 17'AE 2 74H 4W 34 29 88 N 150 E s 8 7H * Griffiths Age: 1100 B.C. to A.D. 750, based on Liden's varve RIVER VARVES 9 — — — — — — 88 HN 160 E s 13 12 b a Griffiths, 1955 chronology — b Irving, 1959 Sampling: About 150 samples from two localities a few kilometers apart were measured. These were averaged into 29 groups, each representing about 100 years. Data for the calculations of entry (8) were scaled from Fig. 3(b), Ref. a. _ _ ICELAND LAVAS 10 - 6 31 21 89 HN 54 E S 6 6 * Brynjdlfsson Age: Postglacial (3400 B.C. to 1950 A.D.) a Brynj61fsson, Sampling: 21 flows were sampled covering about 5000 1957 years. Entry (10) is based on pole-position data for I each flow scaled from Fig. 3, Ref. a. Stability: The samples were partially demagnetized in 140 oersted A.C. fields before measurement. _ __ _ ICELAND LAVAS 11 1 74 8 S — S a Hospers Age: Postglacial 12 64 N 19 W 1 74 8 36 8 86HN 153 E S 15 13 H * a Hospers, 1955 Sampling: 8 flows were measured covering a period of at least 4000 years. The value of k for entry (12) was calculated by the approximate formula. _ _ _ SWEDISH VARVES 13 12H 16 10 86 N 98 W S 12H 12H * Granar Age: Glacial and postglacial a Granar, 1959 Sampling: 10 varve sections were sampled over a lateral distance of 800 km. Fisher statistics were applied to the pole positions calculated from mag- netic direction and locality data on p. 27, Ref. a. ^ _ _ — Roche Age: late Pleistocene W CHAINE DES 14 — 353 62 — 10 84 N 50 W S a 3 E 353 62 10 84HN 106 W S -~ * a Roche, 1958 Sampling: 10 flows were sampled. j^. PUYS LAVAS 15 45WN — — — -_ — ICELAND LAVAS 16 64 HN 22 W 181 -75 7 51 — N a Hospers Age: early Quaternary Q 17 64HN 22 W 181 -75 7 9 51 87HN 150 E N 13 11 H * a Hospers, 1955 Sampling: The samples were collected over a lateral ^ extent of 125 km. ^ _ J PLATEAUX 18 206 -63 H _ 8 72 N 100 E N a Roche Age: early Quaternary BASALTS 19 45HN 3 E 206 -63 H 8 71HN 84 E N * a Roche, 1958 Sampling: 8 flows were sampled. O — — — — H FRENCH LAVAS 20 45 N 3 HE 197 — 62H 13 — 6 — — N — b Roche Age: Pliocene and Pleistocene, The oldest flows are >• 21 45 N SHE 197 -62J.4 13 28 6 78 HN 93 E N 20 15 ^ * a Roche, 1951 Villafranchien. b Hospers, 1955 Sampling: 6 flows were sampled. ICELAND LAVAS 22 _ 19 33 77HN 74 E S a Sigurgeirsson Age: Pliocene and Pleistocene * Sampling: 33 lava flows from three normally magne- 23 _ _5H 19 33 77HN 74 E S 5H 5J^ a Sigurgeirsson, _ — — — tized groups and 26 flows from three reversely mag- 24 _ — — IS 26 88 N 149 E N — a 1957 25 7 15 26 88 N 149 E N 7 7 * netized groups were sampled. Fisher statistics were — — applied to the pole positions calculated from each flow. The values of «95 for entries (23) and (25) were calculated by the approximate formula. Stability: A.C. demagnetization of 110 oersted reduced the scatter in the determinations, especially in the reversed groups. TABLE 1.—Continued Locality Magnetic Direction Pole Position Rocks Sampled No. References Remarks Lat. Long. Decl. Incl. aog * A- Lat. Long. P ft. sp 5 SECTION A— Continued _ _ _ CHELEKAN 26 12 37 — — — S — — a Khramov Age: Pliocene and Pleistocene — SEDIMENTS 27 39 N 53 E 12 37 69 N 161 W s * a Khramov, 1957 Sampling: 650 oriented samples were measured from — — — — — — localities as far as 170 km from each other. 28 — — 196 -30 — — N — a 29 S9N 53 E 196 -30 6SN 163 W N * Other: In 4 bore holes 2-4 km apart, transition from — — — — — north-seeking to south-seeking magnetizations oc- curs at the same level. Samples from several "nor- mal" and "reversed" zones are included in the o average directions cited. Additional details appear o in Khramov, 1958. FRENCH LAVAS 30 45HN 3E -51 14 5 — — N — — b Roche Age: Miocene and Pliocene. The oldest flows are 176H — 31 «HN 3E 176H -51 14 27 5 76 N 164 W N 19 12H * a Roche, 1951 Pontian. — — 32 — — 5 73 N 167HW N 19 13H c b Hospers, 1955 Sampling: 5 flows were sampled. — — c Irving, 1959 ICELAND LAVAS 33 65 N 20 W 1H 78 5H — 102 — — M — — a Hospers Age: Miocene 34 65N 20 W 1H 78 5H 7 102 88 N 10W M 10H 10 * a Hospers, 1955 Sampling: An average of 25 flows from each of 4 f normal and reversed zones were measured. f VOGELSBERG Angenheister Age: Cited as Miocene BASALTS a Angenheister, Sampling: More than 200 oriented samples were col- (35 8 57 8 29 S a 1956 lected from 42 flows. The values of k for entries Normal flows \36 SOHN 9HE 8 57 8 11 29 76 N 163H S 11H 8 * (36), (38), and (40) were calculated by the approxi- /37 188M -60 15 13 N a mate formula. Reversed flows \38 SOHN 9HE 188H -60 15 7 13 79 N 155 E N 22 17 * /39 8H 57H 6 42 M — a I All flows — — * \40 SOHN 9HE 8H 57H 6 13 42 76HN 160 E M 8H 6H ENGLISH NORTH 41 SSHN 3 W 179H -73 15H 16 7 87 N 8 W N 28 25 * Bruckshaw and Age: Cited as Oligocene or Miocene in Refs. a and b — — WEST DYKES 42 55H_ N 3 W 174_H -73H 15_H 7 N b Robertson Sampling: The samples were collected from 4 dikes — — — 43 — 7 86 N 76 W N 25 21 c a Bruckshaw and at 7 sites. The sites covered an area of about 50 — Robertson, 1949 by 140 miles. Values for entry (41) were based on b Hospers, 1955 data scaled from Fig. 7B, Ref. a. c Irving, 1959 „ . Roche LIMAGNE BASALT 44 _ 180 -73 — N — a Age: Cited as Aquitanian (lower Miocene) 45 — — — — — 77 N 3E N b a Roche, 1950b Sampling: One locality was sampled. — — — 46 46N 3E ISO -73 77N 3E N * b Irving, 1959 — — — — — _ _ FRENCH INTRU- 47 46N 3E 201 -57 11 9 N — a Roche Age: Cited as Oligocene in Ref. a SIVE ROCKS 48 «N 3E 201 -57 11 24 9 72N 114 E N 15H 11H * a Hospers, 1955 49 — — — — — 73 N 119 E N 16 12 c b Roche, 1950a — — c Irving, 1959 NORTH AMERICA NEW ENGLAND 50 43 N 72HW 355 H 51 H 10 — 6 — — S — — a Johnson, Murphy, Age: 13,000 to 7000 B.C. VARVES $1 43 AT 72 H W S55M 51 H 10 50 6 79 N 127 E S 13 Vt 9 « and Torreson Sampling: The measurements were averaged into ti — — 52 — — — — — 80 H 132 E s 13H 9H b a Hospers, 1955 groups, each representing 1000 years, and the cal- b Irving, 1959 culations were based on the average directions for c Johnson, these 6 groups. Murphy, and Torreson, 1948 CANADIAN 53 348H 75 4 28 46 M a Du Bois Age: Cited as late Tertiary in Ref. a BASALTS 54 61 N 134J.4W 348' -2 75 4 28 46 85N USE M 6H 7J-5 * a Du Bois, 1959a Sampling: 29 samples were collected at 60K N, 135 W; 9 at 60 N, 130H W; 6 at 63 N, 138 W; and 4 at 59M N, 134 W; 2 widely divergent samples from the second group were discarded. There is no indication of the number of flows sampled. Location data for entry (54) are averages of the above 4 localities. JJ Reversals: All the samples from the first and third t~* groups were reversed with respect to the present g field direction. g 29 — — Age: Magnetization is parallel before correcting for £> NEBOLY FOR- 55 — — 7 58 3 — S — — a Doell MATION 56 S7HN 122 W 7 58 3 89 29 85N 42 W S 4 3 » a Doell, 1955a post early Pleistocene folding with dips of 50°; O (sediments) b Doell, 1956 therefore the magnetization was acquired post ^ 1 early Pleistocene, although the rocks were deposited ^ in late Miocene tune. X 1 Sampling: 29 oriented samples were collected from j 3 areas about 50 miles apart. The value of * for 5 entry (56) was calculated using the approximate i— J formula. '•** j Stability: Partial heat demagnetization to 100°C. decreased the scatter in directions. PAYETTE FOR- 57 43 N 115HW VA 62H VA 258 13 I8HN 60 W s 4 3 . Torreson et al. Age: Cited as Pliocene in Ref. a although now re- MATION a Torreson, garded by U. S. Geological Survey as Miocene and (sediments) Murphy, and Pliocene(?) Graham, 1949 Sampling: 13 oriented samples were collected at one site. Entry (57) was based on data given in Table 4, Ref. a. ON OO NJ TABLE 1.—Continued Locality Magnetic Direction Pole Position Rocks Sampled No. References Remarks Lat._^ Long. Decl. Incl. SECTION A— 'Continued COLUMBIA Campbell and Age: Miocene RIVER Runcorn Sampling: 73 separate flows are included in the cal- BASALTS a Campbell and culations out of a total of 114 examined in the area. (58 — 11*4 73*4 7*4 5 44 — S — — a Runcorn, 1956 7 localities were sampled over an area of 300 by N — \59 46*4N 120 W 11*4 73*4 7*4 5 44 75*4N 9—7 W s 13*4 12*4 * b Irving, 1959 200 miles. Values for entry (62) were obtained by (60 177 -66 10*4 4 29 N a averaging the values for declination, inclination, Reversed flows \61 46*4N 120 W 177 -66 10*4 4 29 87 N 170 W N 17*4 14*4 * and k given in entries (58) and (60). The value for QT96 was then calculated by the approximate (62 46*4N 120 W 3*4 69*4 8 4 73 83N 105 W M 13*4 11*4 * All flows formula. \M 87 N 40 E M 4*4 b ELLENSBURG 64 46 ^N 120*4W 4 66 *4 10*4 11 19 86*4N 73 W S 17*4 14 * Torreson et al. Age: Cited as Miocene in Ref. a. Now regarded as FORMATION 65 A/Rl£fJ 120*4W *4 68*4 9 12 23 85N 115 W S 15*4 13 * a Torreson, late Miocene and early Pliocene by U. S. Geological (sediments) Murphy, and Survey. Graham, 1949 Sampling: 23 oriented samples were collected at one b Graham, 1949 site. The calculations for entry (64) were based on data scaled from Fig. 11, Ref. b, and those for entry (65) were based on data given in Table 4, Ref. a. Stability: Stability is indicated by the application of Graham's conglomerate test. Reversals: One sample showed reversed polarity. ARIKAREE 66 44 N 103 W 66 69 25*4 3 21 47 N 49 W S 43*4 37 * Torreson et al. Age: Cited as Miocene. FORMATION a Torreson, Sampling: 21 oriented samples were collected at one (sediments) Murphy, and site. Entry (66) was based on data given in Table 4, Graham, 1949 Ref. a. Other: The scatter in directions is extreme. DUCHESNE 67 40*4 N now 2 65 5 14 85 83 N WW s 8 6 a Collinson and Age: Tertiary. RIVER FOR- Runcorn Sampling: 85 specimen measurements were made on MATION a Collinson and 24 oriented samples. (sediments) Runcorn, 1960 i [ SOUTH I AMERICA NEUQUEN LAVAS Crcer Age: Cited as Quaternary. -68 350 -46 8 31 12 S a a Creer, 1958 Sampling,: In all these entries, ^V is the number of Normal flows < 69 38 S 70 W 350 -46 8 31 12 T6XH 112 W S 10 6 b Irving, 1959 specimen measurements which were made on 20 -70 188 64 5 17 46 ~m N a oriented samples collected from 10 flows (both Reversed flows-* ~6H 71 38_ S 70 W 188 64 5 17 46 76 E N 1« normal and reversed). Lateral sampling extent was — about 200 by 400 km. Entry (74) was based on data 72 — 1 -61 5 15 58 — — M — a 73 83 N 126 E M 7 6 b scaled from plots in Ref. a. Entry (75) is based on All flows < 74 38 S vow 3 -69 3H 15 65 75 N 102 E M 6 5 the ''secular variation precision" k' =* 30 and the 75 38 S 70 W 3 -69 8 k' 10 75 N 102 E M 13K U, * number of flows sampled, and a« was calculated by the approximate formula. Stability: The scatter in directions was very greatly reduced by partial A.C. demagnetization at 250 oersted. AUSTRALIA 143HE 8 8 35 13 — — S — a Irving and Green Age: Pliocene to Recent NEWEK VOL- 76 38 S -S9H — CANICS OF 77 38 S 143}$E 177 60 6H 31 16 — — N — a a Irving and Sampling: At least 2 oriented samples were collected — VICTORIA 78 38 S 14SKE 3M -60 5 37 32 86MN 78 W M 7 5^ a Green, 1957 at each of 32 sites covering an area of 50 by 150 miles. The values cited here for k are between-site precision parameters (see Watson and Irving, 1957, p. 296-297). NEW ZEALAND 79 _ 79 N 35 E S b Hatherton Age: Cited as Pliocene — * a Hatherton, 1954 Sampling: 52 oriented samples were measured as well IGNIHBSITES 80 37J4S 175 E 351 -65 — — 79N 26 E S — b Irving, 1959 as 65 samples cut from vertical drill cores. Values for entry (80) were based on the inclination and declina- tion data given in Table 3, Ref. a. Lateral sampling extent was 10 miles. ASIA JAPANESE 81 35MN 140 E 358 52 3 58 45 86H 5 W S 4 3 * Watanabe Age: From 5600 to 4400 B.C. and 300 to 1800 A.D, FIBED CLAVS a Watanabe, 1958 Sampling: Data from 43 archaeological fired clays and 2 dated lava flows were reported, 22 in the period 300 to 1800 A.D. and 23 in the period 5600 to 4400 B.C. Values for entry (81) were based on data scaled from Figs. 1 and 2, and cited in Table 1, Ref. a. PS TABLE 1.—Continued Locality Magnetic Direction Pole Position Rocks Sampled No. References Remarks Lat. Long. Decl. Incl. a H k N Lat. Long. P ft. tp S SECTION A— Continued NOKIH IZTJ AND 82 — — — — 7 11 42 TON 45HE M 7 7 * Nagata et al. Age: Quaternary n HAKONE VOL- 83 — — 76HN 37ME M 7 b a Nagata, Akimoto, Sampling: 42 flows were measured more or less uni- — — — — — 10H CANIC ROCKS Uyeda, Shimizu, formly throughout the Quaternary. The calculations Ozima, Kobay- for entry (82) are in the form of a Fisher analysis on ashi, and Kuno, the pole positions for each group of flows cited in z; 1957 Table 2, Ref. a. Since values of JAPAN VOLCANIC 84 36 N 138 E 11 61H 11 5 39 79HN 17SW M 17 13 . Maluyama Age: The rocks sampled are Tertiary and younger. ROCKS a Matuyama, 1929 Sampling: 39 oriented samples were collected at 35 localities in Honsyu, Kyusyu, Tydsen, and Man- churia, the majority being collected in Honsyu. The data used in the calculations for entry (84) were § scaled from a figure on p. 204, Ref. a. The latitude & longitude of the sampling area are an average for en Honsyu ; thus there may be somewhat more dispersion in the values given than there would have been at a single sampling area. Reversals: About half the samples measured showed reversed polarity. JAPAN VOLCANIC 85 S6N USE 359 47^ 11 IS 11 82 HN 35 W S 14H 9H * Kumagai et al. Age: Pleistocene to Recent ROCKS a Kumagai, Kawai, Sampling: 11 lava flows were sampled. Data for the and Nagata, 1950 calculation of entry (85) were taken from Tables I and II, Ref. a (nos. J, 2, 3, 5, 6, 7, 17, 18, 19, 20, 21). The Narita bed, no. 4, is stated by Kawai (1954, p. 209) to be unstable and was not included here. Co- ordinates for the sampling area are averages. j JAPAN VOLCANIC 86 36 N 138 E 8»M 48 12H 37 7 79N 13 E M 16 10W * Kutnagai et al. Age: Late Tertiary ROCKS a Kumagai, Kawai, Sampling: 7 lava flows were sampled. Data for the cal- and Nagata, 1950 culation of entry (86) were taken from Table I, Ref. a. Nos. 9 through 15 were used. No. 8 was dis- carded because of instability (Kawai, 1954, p. 209). Co-ordinates for the sampling area are averages. Reversals: One of these flows showed reversed polarity. _ _ KAWAJIRI 87 _, — M SON 49 E N a A sami Age: early Pleistocene BASALT a Irving, 1959 Sampling: One flow was sampled. b Asami, 1954a Other: Normal and intermediate directions of magne- c Asami, 1954b tization were found in closely associated lavas (Ref. c, p. 151). JAPAN VOLCANIC Nagata et at. Age: late and early Pliocene ROCKS a Nagata, Aki- Sampling: 2 basalt lavas were sampled from the upper 2 — 12 79 N' 57 W S 3 2 a moto, Shimizu, Pliocene and 2 andesite lavas from the lower Plio- upper Pliocene 88 — — 3 42 _ — 14 52 4 — 10 77 N 109 W S 6 4 a Kobayashi, and cene. Values for calculation of entry (90) are based lower Pliocene 89 — + combined 90 10 82 4 79HN 88HW S 10 10 Kuno, 1959 on direction data given in Table II, Ref. a, and the — § — — localities cited in Table 1, Ref. a. Fisher statistics > ] were applied to the 4 pole positions calculated from o these data. JAPAN VOLCANIC Nagata et al. Age: late, middle, and early Miocene nd ROCKS _ a Nagata, Aki- Sampling: A dolerite sheet, andesite sheet, and 2 27 59 3 32 68 N 152 W M 5 4 a moto, Shimizu, andesite lavas were sampled from the upper and o upper and mid- 91 — — dle Miocene _ _ Kobayashi, and middle Miocene and a dolerite sheet and 2 andesite lower Miocene 92 32 4_0 11 — 20 59 N 113 W M 13 8 a Kuno, 1959 lavas from the lower Miocene. Values for calculation 22 g 7 73N 144 W M 22 22 * of entry (93) are based on direction data given in combined 93 i — — Table II, Ref. a, and the localities cited in Table I, Ref. a. Fisher statistics were applied to the 7 pole positions calculated from these data. i | Reversals: 2 units from the upper and middle Miocene i and one from the lower Miocene had reversed polarity. i i TABLE 1.—Continued Locality Magnetic Direction Pole Position Rocks Sampled No. References Remarks Lat. Long. Decl. Incl. cm k N Lat. Long. P 5m IP 5 SECTION B PRECAMBRIAN EUROPE TORRIDONIAN Irving and Age: late Precambrian, overlying Lewis gneiss and SANDSTONE Runcorn overlain by Lower Cambrian rocks. Entries (1) 295 -34 — — — — — a a Creer, Irving, through (12) are Aultbea, Applecross, and top Dia- (1 — — — — — 58 N 6W 295 -34 — — 3N 53 E N — — * and Runcorn, baig formations. Entries (13) through (16) are lower 1 2 — 3 58 N 6W 294 -28 9 9 28 — — — — — e 1954 Diabaig. Aultbea overlies Applecross which overlies n 4 58 N 6 W 294 -28 9 9 28 IN 56 E N 10 5H * b Runcorn, 1955b Diabaig. 5 127 52 — — — — — — a c Runcorn, 1955a Sampling: Lateral extent, 60 miles; vertical extent, — — — — 6 58 N 6W 127 52 IIJ^N 37 E S — * d Day and Run- 10,500 feet in upper Torridonian and 1,900 feet in Upper — — — — 7 58 N 6W 129 51 5 15 53 — — — — — e corn, 1955 lower Torridonian. N refers to the number of sam- 8 58 N 6W 129 51 5 15 53 ION 36 E S 6H 4H * e Irving and Run- pling sites; the mean direction at each site was used 9 304 -38 — — — — — — a corn, 1957 in the statistical analysis. 11 sites in the upper o — — — — * o 10 58 N 6W 304 -38 — — — 2N 45 E M — — f Creer, Irving, Torridonian were magnetized in directions oblique 11 — — — — — 5 N 49 E 8 5 b and Runcorn, to the mean direction of the remaining 81 sites; these, — — — P 12 123 44 S 12 81 6N 43E M 6 4 f 1957 as well as 6 sites with directions parallel to the — — present field, were not used in the statistical analysis. 13 37 N 109 W A g Irving, 1957a A> 14 . 307 34 7 40 ^_ . c "Normal" and "reversed" groups of sites differ Lower ]15 307 34 7 _40 _13 35 N 112 W S 8 5 f significantly in their mean directions but were com- — lie 58 N 6W 307 34 7 35 N 118 W S 8 5 6 bined in entry (12) for mean axial direction of mag- § netization. Other: The change in magnetic direction between upper and lower Torridonian occurs stratigraphically in less than 300 feet with no evidence of unconformity. W Stability: Torridonian pebbles in conglomerates of H later age have randomly oriented directions of magnetization. Directions of magnetization are scattered in beds showing penecontemporaneous deformation (Ref. e, p. 93). Directions of magnetiza- tion of samples collected from a fold of Caledonian age are parallel only after correcting for dip, indicat- ing stability at least since the Caledonian (Ref. e, p. 92-93). Reversals: About 16 zones in the upper Torridonian have magnetizations which alternately lie in approxi- mately opposite directions. In two cases obliquely magnetized zones occur stratigraphically between two reversed zones. No reversals occur in the lower Torridonian. _ LONGMVNDIAN 17 — 111 19 _ _ a Creer — — — — Age: late Precambrian, possibly equivalent to Tor- (Sediments) 18 53 N 3 W 111 19 — — 4MN 115WW M — — * a Creer, Irving, ridonian — 19 — — — — — — 2N 118 W — 13 7 b and Runcorn, — Sampling: Lateral extent, 20 miles. Vertical extent, 20 — — 114 29 12 5 40 2 N 120 W M 13 7 c 1954 probably several thousand feet. 40 samples were 21 53 N 3 W 114 29 12 5 40 IVsN 121 W M 13 7 * b Runcorn, 1955b collected at 12 sites. Statistical analysis apparently c Creer, Irving, is of directions of individual samples, not of mean and Runcorn, site directions (Ref. d). 1957 Stability: Magnetization unchanged in 300 oersted d Creer, 1957b alternating field and also unchanged after 1 year's random orientation in the laboratory. Reversals: Present, but number of alternating zones not known. NORTH AMERICA — — — — CHEQUAMEGON 22 — 30 74 6 — 15 69 N 47 W a Du Bois Age: late Keweenawan f^ SANDSTONE 23 47 N 88).4W 30 74 6 36 15 68 N 47 W S 11 10 * a Du Bois, 1957 Sampling: N is listed as number of "specimens." The W value of k for entry (23) was calculated by the ap- ^ proximate formula. ^ O JACOBSVILLE 24 — — 250 -11 13 — 15 14 N 10 E N — a Du Bois Age: late Keweenawan, older than Chequamegon -^ — SANDSTONE 25 47 N 88HW 250 -11 13 8 15 17HN 13 E N 13 6H * a Du Bois, 1957 Sampling: N is listed as number of "specimens," The S value of k for entry (25) was calculated by the £* approximate formula. _ _ _ FREDA SAND- 26 — — _ 20 N 165 E a Du Bois Age: late Keweenawan, older than Jacobsville; con- ^ — — — — STONE AND 27 - — 285 -1 3 — 68 9N 169 E — — — b a Du Bois, 1955 formable on Copper Harbor conglomerate £> NONESUCH 28 46J^N 88HW 285 -1 3 32 68 ION 170 E S 3 VA * b Du Bois, 1957 Sampling: Lateral extent, probably about 100 miles. SHALE N is number of "specimens." Value of k for entry (28) was calculated by the approximate formula. Stability: Fold tests indicate stability. _ _ _ _ — COPPER HARBOR 29 294 32 7 25 SON 176 E — a Du Bois Age: late Keweenawan (Sediments and 30 47 N 88HW 294 32 7 16 25 29N 176 E S 8 4H * a Du Bois, 1957 Sampling: 13 samples are from lava flows, and 12 are Lava flows) b Du Bois, 1955 from sediments. The value of k for entry (30) was calculated by the approximate formula. Stability: Copper Harbor (?) basalt and andesite- pebbles in Copper Harbor conglomerates are ran- domly magnetized. Thermal demagnetization curves (of basalts?) are similar to those for TRM. Other: Sediments have 10° smaller inclination than lavas above and below them (Ref. b, p. 507). —I TABLE 1.—Continued Locality Magnetic Direction Pole Position Rocks Sampled No. References Remarks Lat. Long. Decl. Incl. a« k N Lat. Long. P h, it 5 SECTION B— Continued POBTAGE LAKE 31 282 41 4 — 31 25 N 170 W — — a Du Bois Age: middle Keweenawan; conformably overlain by — — — LAVA SERIES 32 47 N 88MW 2S2 41 4 40 31 25 N MOW S 5 3 * a Du Bois, 1957 Copper Harbor Sampling: N is number of specimens. The value of k for entry (32) was calculated by the approximate formula. Stability: Thermal decay curves are similar to those for TRM. MICHIGAN DIA- 33 46HN SSHW 82 -86 1 82 36 45N 99W N 2 2 a Graham Age: late Precambrian. Overlain in adjacent localities BASE DIKES a Creer, Irving, by Jacobsville and by flat-lying Cambrian sediments. and Runcorn, Probable age about 1100 million years (James, 1957 1958, p. 40). b Graham, 1953 Sampling: Statistical analysis is based on 36 samples from 2 dikes 8 miles apart. 20 samples from 1.8-foot thick dike span a distance of 3H feet. 16 samples from 40-foot thick dike span a distance of 15 feet. Samples are from both chilled borders and coarse interiors. Several samples from a third dike agree with these directions; samples from two additional dikes are widely scattered in direction. Stability: One sample retained direction and 70% of intensity of magnetization in alternating magnetic field of 493 oersted. Reversals: Graham (1953, p. 252-254) believed dikes may have undergone self-reversal in field essentially parallel to present field, but laboratory evidence for self-reversal is not conclusive. i ADIRONDACK 34 1 _ _ — 44 N 74 W N _ a Balsley and Age: Du Bois (Ref. a) believes remanent magnetization METAMOHPHIC Buddington was acquired during metamorphism of Grenville age ROCKS | a Du Bois, 1958 (ca. 1000 million years). i | i b Du Bois, 1957 Other: Pole entry (34) was calculated on the basis of ! c Balsley and the report by Balsley and Buddington (1954) that Buddington, 1954 rocks containing titano-hematite as the principal magnetic constituent tend to be vertically and reversely polarized. Co-ordinates were scaled from i | Fig. 1, Ref. a. Balsley and Buddington (1958, p. 790-792) question the interpretation that the rem- anent magnetization of these rocks is simple TRM parallel to the field in which the rocks cooled. GABBRO INTRU- Hood Age: Precambrian (Grenville province). Radio-isotope SIVE ROCKS, a Hood. 1958 ages of igneous rocks in this part of the Grenville BANCROFT province generally range between 1000 and 1200 AREA, ONTARIO million years (Shillibeer and Cumming, 1956, p. 56-57; Wilson, 1958, p. 762). Boulter Intru- 35 45 N 77«W 297 h 55 5 19 43 42HN 157 W S 7 5 a Sampling: Statistical analysis is based on directions of sive I 43 specimens from 8 oriented samples. Stability: Alternating field decreased scatter in direc- tions of magnetization. Other: Gabbro is locally gneissic, approaching meta- gabbro. Laboratory observations suggest suscepti- _ _ bility anisotropy. Umfraville (36 45 N 78 W 115 42^ 7 8 58 — a Sampling: Statistical analysis is based on 58 specimens — — * Instrusive \37 45 N 78 W 115 42H 7 8 58 1 N 22 W S 9 5W from 12 oriented samples; anomalously magnetized o specimens were not included in the analysis. j Other: Umfraville intrusive is less metamorphosed | _ _ than Boulter. Thanet Intru- /38 45 N 77HW 1 92 H 62 H 14M 5 28 a Sampling: Statistics are based on 28 specimens from 6 — — — sive \39 45 N 77HW 92H 62H 14^ 5 28 28 N 23HW S 22 18 * oriented samples. Locality is 5 miles from Umfraville site. | Stability: Alternating magnetic field demagnetization ] decreased scatter in directions of magnetization. Thanet and (40 45 N 77HW 110 49 7 6 86 34 N 22 W S 9 6 a Sampling: Combination of two previous groups. 86 8HN S 9 6 « Umfraville \« 45N 77HW 110 49 7 6 2_2 W _ — Tudor Intru- (42 44HN 77HW 327H 11H 9 5 68 — a Sampling: Statistics are based on 68 specimens from — * 11 oriented samples. Tudor locality is 8 miles south sive l« 44HN 77HW 327H llM 9 5 68 41HN 149 E S 9 4H l of Thanet. TABLE 1.—Continued Locality Magnetic Direction Pole Position Rocks Sampled No. References Remarks Lat. Long. Decl. Incl. «95 4 N Lat. Long. P ft. *P 5 SECTION B— Continued SUDBURY Hand Age: Precambrian. Radio-isotope age of between 1200 INTRUSIVE a Hood, 1958 and 1800 million years is generally assigned (Russell and others, 1954, p. 307-308; Wetherill and others. 1957, p. 412). Four localities (Azilda, Blezard, Gar- o son, Creighton) spanning 16 miles were sampled o on the south side of the Sudbury Basin. One lo- X cality (Levack) was sampled on the north side. Levack 44 46MN 81WW 320 H 70 5 20 41 64 N 140HW S 9 8 a Sampling: Statistics based on 41 specimens from 14 oriented samples; some anomalous specimens were not included. o Azilda 45 46HN 81 W 67 — Sampling: Statistical analysis is based on 45 speci- o 194^ 66H VA 45 — — — a — mens from 13 oriented samples. Lateral sampling extent is 1.6 miles. B Blezard 46 46HN 81 W 184 86 — Sampling: Lateral sampling extent is 1.4 miles. Sta- 71}£ m 78 — — — a — tistical analysis based on 78 specimens from 12 oriented samples. Stability: Partial demagnetization in 124 oersted A.C. field decreased scatter. Garson 47 46HN 81 W 155H 59 10 44 6 — a Sampling: Statistical analysis is based on 6 specimens — — — — from 2 oriented samples. § Creighton 48 46HN 81 W 171H 57 4 47 26 — a Sampling: Statistics are based on 26 specimens from — — — — 6 oriented samples. a Stability: 125 oersted alternating field had little effect on intensity or direction of magnetization. South Range (49 46HN 81 W 183 68 1H 49 155 7HN 94HW S 3 2 a Sampling: Entry (49) is based on all 155 specimens of Sudbury, 50 46HN 81 W 183 68 m 49 1S5 7HN 82HW s 3 2 * from 33 samples collected at all 4 sites. Entry (51) combined (51 46MN 81 W 174 64 UM 65 4 * is based on the mean direction of magnetization — — — — — _ at each of 4 sites. Both sides of 46HN 81 W 245H Other: The magnetic direction in entry (52) was chosen /S2 82 H — — 196 38HN 99MW s — a Sudbury \53 «HN 81 W 300 78 11 53 N 115 W s 21 20 * midway between the direction for Levack on the Basin, com- — — north rim of the basin and the average direction bined entry (49) for the sites on the south rim. Entry (53) is based on the following geologic considerations. Thomson (1956, p. 44-45) concludes that the gently dipping north limb and steeply dipping south limb of the Sudbury syncline were largely developed I | before intrusion; most of the post-intrusion folding was in the south limb. The magnetic evidence sup- ports this view; accordingly the direction of the ! | | 1 north range (Levack) is unfolded 10° about the i axis of the syncline, and the mean direction of the south range is unfolded 35° about the same axis, which makes them coincide. The circle of confidence is estimated as that of entry (51). _ _ _ _ SON 148 W , 10 9 a Runcorn Age: Precambrian HAKATAI SHALE, f54 _ _ 34 148 W S b a Runcorn, 1955b Sampling: Statistical analysis for entry (55) is based GRAND CANYON [55 _ 268 73 5H 22 30 HN 9H 8V4 [56 — 268 73 5 22 31 N 150 W S 10 9 c b Runcorn, 1956a on 34 specimen measurements from 15 oriented —34 * c Creer, Irving, samples collected at one locality. l« 36 N 112 W 268 73 5 22 29HN 149 W S 9M 8H and Runcorn, Other: Doell (I955b, p. 1167) incorrectly states that 1957 these values are not corrected for geologic dip. _ _ 144 W S — — a Doell Age: Precambrian. Bass limestone underlies the ^ HAKATAI SHALE fS8 _ _ 246 72 17W 18 N AND BASS 159 215 76 18 — 21 N 130 W S — — b a Doell, 1955b Hakatai shale; the contact is gradational. f-i — LIMESTONE, 160 36 N 112 W 215 76 18 13 N 127 W S — — * b Creer, Irving, Sampling: 10 oriented samples from 1 locality span W — — GRAND CANYON (61 36N 112 W 20S 65 21 6 10 4N 52 E N 33H 27H * and Runcorn, a stratigraphic interval of 450 feet. Locality is ^ 1957 same as that of entries (54-57). ^ c Doell, 1955a Stability: an of directions of magnetization before O correcting for individual geologic dips of beds is j2j smaller than after dip correction (19° vs. 21°) in- 2 dicating some instability. ^ Other: Entry (58) was not corrected for geologic dip. Entry (59) corrects for regional dip. Entry (61) J*-jJ was recalculated from data in Ref. c, correcting ^ direction of magnetization of each sample for local J> attitude of bed. _ HAKATAI SHALE, (62 36 N 112 W 291 58 41 36 N 177 W S — — a Collinson and Age: Precambrian GRAND CANYON \tt 36 N 112 W 245 31 9N 6E N a Runcorn Sampling: Entry (62) is based on 41 specimen meas- — — — — — a Collinson and urements from 14 oriented samples from a locality Runcorn, 1960 several miles from the site of the samples in entries (54-61). Directions of magnetization show streaking toward direction of present field; in entry (63) a mean direction was estimated excluding directions tending toward the present field direction. _ BASS LIME- /64 36 N 112 W 232 52 5 23 43 6 N 26 E N 6 3 a Collinson and Age: Precambrian; conformably overlain by Hakatai STONE, GBAND (es 36N 112 W 229 34 20 N 21 E N a Runcorn shale CANYON — — — — a Collinson and Sampling: Statistical analysis is based on 43 speci- Runcorn, 1960 mens from 13 oriented samples. 1 ON 1 1 >O TABLE 1.—Continued Locality Magnetic Direction Pole Position Rocks Sampled No. References Remarks Lat. Long. Decl. Incl. SECTION B- Continued SHINUMO QUART- f« 36 N 112 W 288 65 61 37 N 166 W S a Colliscn and Age: Precambrian; overlies Hakatai shale ZITE, GRAND \67 36N 112 W 246 33 7N IE N a Runcorn Sampling: Entry (66) is based on 61 specimens from — CANYON — — — — a Collinson and 14 oriented samples. Directions of magnetization Runcorn, 1960 show streaking toward direction of present field; in entry (67) a mean direction was estimated ex- cluding directions tending toward the present field direction. _ _ _ HAZEL FOR- (68 49 N 175 W S a Howell, Martinez, Age: Precambrian — — — — — — * MATION 69 SIN 105 W 316 56H 6M _35 _15 53 N 173 W S 9H 7 and Statham Sampling: Entries (68, 69) are based on 15 samples (sediments) 70 60 N 151 E S ~- a a Howell, from flat-lying beds exposed at 5 localities scattered — — — — — — In SIN 105 W 328 S7H 17 3 37 59HN 154 E S 20 12 * Martinez, and over an area of 2 square miles. Entries (70, 71) Statham, 1958 based on 37 oriented samples from dipping beds from 9 localities scattered over 20 miles, correction for dip having been made. Entries (69) and (71) are based on data scaled from Fig. 3 and Fig. 4, respectively, in Ref. a. Stability: Directions of magnetization of some samples changed 2°-16° in several months. Other: Rocks may be slightly metamorphosed. BELT SERIES McNamara 72 47 N 114 W 26 -43 4 30 53 14 N 42E S 5 3 a Collinson and Age: Precambrian Formation Runcorn Sampling: Statistics are based on 53 specimens from (sediments) a Collinson and 20 oriented samples. Runcorn, 1960 Miller Peak 73 47 N 114 W 2S4 SO 7 20 23 UN 14 E N 9 4 a Collinson and Age: Precambrian Formation Runcorn Sampling: Statistics are based on 23 specimens from (sediments) a Collinson and 14 oriented samples. Runcorn, I960 [75 47 N 112 W 232 55 4 IS 71 5N 152 W S 6 4 a Collinson and Age: Precambrian; younger than Grinell formation J Spokane Shale Runcorn Sampling: Statistics are based on 71 specimens from a Collinson and 39 oriented samples. Runcorn, 1960 76 49 N 114 W 206 39 8 10 19 16 N 41 E N 10 6 a Collinson and Age: Precambrian; younger than Grinell formation Runcorn Sampling: Statistics are based on 19 specimens from a Collinson and 5 oriented samples. Runcorn, 1960 Appekunny 77 49 N 113)5 W 223 i 29 6 15 38 15 N 24 E N 7 4 a Collinson and Age: Precarabrian; younger than Appekunny argillite Argillite 1 Runcorn Sampling: Statistics are based on 38 specimens from a Collinson and 15 oriented samples. 1 Runcorn, 1960 Grinell For- 78 49 N 11SH W 225 48 6 15 42 3N 28E N 8 5 a Collinson and Age: Precambrian mation i Runcorn Sampling: Statistics are based on 44 specimens from (sediments) a Collinson and 16 oriented samples. Runcorn, 1960 Bonito Canyon 79 36 N 109 W 31 -25 4 19 74 33 N 34 E S 4 2 a Collinson and Age: Precambrian Quartzite Runcorn Sampling: Statistics are based on 74 specimens from a Collinson and 16 oriented samples. Runcorn, I960 BLACKHEAD 80 47 N 53 W 232 51 10 25 10 5 N 84 E 13H 9 a Nairn, Frost, and Age: Precambrian * SANDSTONE, 81 47 N 53 W 232 51 10 25 10 2N 95 W S 13H 9 Light Sampling: Statistical analysis is based on mean direc- NEWFOUND- a Nairn, Frost, tion of each of 10 oriented samples from one lo- LAND and Light, 1959 cality. 4 specimen measurements were made on each ^ oriented sample. r4 Other: CCW rotation of Newfoundland of 20° is sug- S gested in Ref. a in order to bring these results into g closer agreement with results from Hakatai shale; ;> "corrected" pole is then at 10° N 110° W. O _ SIGNAL HILL 82 47 N 53 W 283 20 11M 21 9 16 N 142 W 12 6 a Nairn, Frost, and Age: Precambrian, possibly 8500 feet stratigraphically >-j # SANDSTONE OF 83 47 N 53 W 283 20 11M 21 9 16J.4N 145 W S 12 6 Light below Blackhead sandstone H NEWFOUND- a Nairn, Frost, Sampling: Statistics are based on 9 oriented samples t. LAND and Light, 1959 from one locality. 4 specimen measurements were ^ made on each sample. t-j Stability: Beds at the Signal Hill site dip steeply **" eastward, those at the Blackhead site dip less steeply; agreement between the two sites is presumably better after correction for dip. Other: Correction for hypothetical rotation of New- foundland was made as in the case of the Black- head sandstone. "Corrected" pole is at 29° N, 163° W. 53 W 262 39 13H 8 19 11 N 122 W 16 9H a Nairn, Frost, and Sampling: Analysis was made on entire 19 oriented BLACKHEAD AND 84 47 N — SIGNAL HILL Light samples of last entries. FORMATIONS, a Nairn, Frost, UNDIFFEK- i and Light, 1959 ENTIATED 1 1 1 o\ TABLE 1.—Continued Locality Magnetic Direction Pole Position Rocks Sampled No. References Remarks Lat. Long. Decl. Incl. OEM k N Lat. Long. P Sm sp S SECTION B— Continued AUSTRALIA BDIXIIVA 85 US 132 E 243 as 12 SON 121 W N 14 8 a Irving and Green Age: latest Precambrian; contains fossils of primitive QUASTZITE — — a Irving and life Green, 1958 _ NULLAGINE 86 21 S 120 E 143 64 8 mmm 51 N 18 W N 13 10 a Irving and Green Age: late Precambrian, part of Catherine River group; LAVAS a Irving and probably younger than the Edith River volcanic Green, 1958 rocks _ __ EDITH RIVER 87 US 132 E 90 48 18 6N 14 E N 24 IS a Irving and Green Age: earliest late Precambrian VOLCANIC 88 13 S 132 E 90 48 IS — 6N 14 W N 24 15 b a Irving and ROCKS — Green, 1958 b Irving, 1959 AFRICA _ _ _ _ PlLANSBERG 89 26 S 28 E 24 7HN 42HE S a Gough Age: Generally regarded as pre-Karroo, post-Water- 69H — DYKES 90 26 S 28 E 24 69H 6 124 5 7HN 42 HE S 10 9 * a Gough, 1956 berg. Possibility of their being late Precambrian is mentioned in Ref. a. Recently determined radio- isotope age is 1290 m.y. (Schreiner, 1958, p. 1330). Sampling: Sampling sites span 54 miles. Vertical sampling extent varied, but usually was at least several hundred feet. Between 8 and 58 samples were collected from each dike studied, giving circles of confidence of 3.3° to 7°. Statistical analysis of entry (90) is based on the mean direction of mag- netization of each of 5 dikes. A comparatively small number of randomly magnetized samples is not included in this analysis. Stability: Most specimens were stable in alternating fields of the order of 100 oersted. BXJSHVELD 91 25 HS 28 E 5 23 N 36 E S a Gough and Age: Concordant radio-isotope results indicate an — GABBEO 92 25HS 28 E .12 40 5 23 N 36E S 12 12 * van Niekerk age of 2.0 X 10* years. — — a Gough and van Sampling: Between 12 and 29 oriented samples were Niekerk, 1959 collected at 5 sites in the Main Gabbro zone of the complex. Lateral sampling extent is 150 miles. Circles of confidence at the 99% level for the direc- tions at each site range between 2° and 9.6°; all data were included in these calculations with the excep- tion of 4 anomalous samples at one site. Scatter in mean site directions is reduced on correcting for pseudo-stratification in the gabbro. Statistics for entry (92) were applied to magnetic poles correspond- ing to corrected magnetic field directions at each of 5 sites. Stability: Reduction of scatter in directions of magne- tization upon correcting for pseudo-stratification of gabbro suggests deformation after cooling of the intrusive and stability since that deformation. 13 > f M O § > O 3 d n o JO TABLE 1.—Continued Locality Magnetic Direction Pole Position Rocks Sampled No. References Remarks Lat. Long. Decl. Incl. CMS k N Lat. Long. P &m Sp ! 5 1 n SECTION C EARLY PALEOZOIC o X DEVONIAN EUROPE 52 N 2HW 233 -22 Age: Devonian; Lower part of Old Red sandstone, OLD RED SAND- 39 a Clegg, Almond, 52 N »MW 233 -22 4 39 31 N Ill E N 12 * and Slubbs Brownstone series from Mitcheldean in Gloucester- STONE {; 11H 6H — — — — 25 N 102 E N — b a Clegg, Almond, shire U — — and Stubbs, Sampling: Measurements were made of 39 specimens 1954a from 3 oriented samples collected alt one locality. b Irving, 1959 an (entry 2) was recalculated from vaue of ago given in Ref. a; this is probably based on If = 39. The value of k was calculated by the approximate formula. Stability: Magnetizations were stable in D.C. fields of several oersted and in alternating fields of several hundred oersted. _ 4 199 -2 N _ a Creer Age: Devonian; the Old Red sandstone of the Angleo 5 52 N 3 W 199 2 — — 37 N 153 E N — * a Creer, Irving, Welsh cuvette is divided into an upper and lowr- — — 6 — 34 -2 — S — a and Runcorn, unit; the lower unit consists of the Down Ionian and — — — — — — 7 52 N 3 W 34 -2 — 31HN 136 E S * 1954 the overlying Dittonian. Entries (6) and (7) refer to — — — — OLD RED SAND- 8 — — — — 34 N 156 E M — a b Runcorn, 1955a the Downtonian, entries (4) and (5) to the two over- — — — — STONE 9 — 198 -2 — 45 N 155 E N — b c Runcorn, 1955b lying units, and all the other entries refer, presum- — — — — 10 — — — 34 N 156 E N 10 5 c d Creer, 1957b ably, to the entire section of Old Red sandstone. — — — — 11 52 N 3 W 196 -4 5 19 35 d e Creer, Irving, Sampling: Samples were collected at 17 localities with — — — — — 12 196 -5 5 19 35 SON 159 E N 5 3 and Runcorn, different amounts of folding. Only the samples from — — e IS 52 N 3 W 196 4 5 19 35 38HN 156 E N 5 VA " 1957 6 localities with flat-lying strata gave consistent results. The statistical analysis of entries (11), (12), and (13) is based on 35 oriented samples from these 6 localities. The stratigraphic thicknesses sampled at these localities are 30, 80, 250, 0.5, 34, 0.5, and 1200 feet, respectively. Stability: The lack of agreement between strata which have been folded by different amounts indicates that at least some of the magnetization is unstable. i Reversals: Ref. d states that reversals are absent from the Old Red sandstone. However, in Ref. a, reversals were noted at two localities in the Downtonian. NORTH AMERICA ONONDAGA 14 42«N 74 W 177 79 4 19 65 21 N 73 W S 7H 7 • Graham Age: Top of lower or bottom of Early or Middle Devo- LIMESTONE a Graham, 1956 nian (late Ulsterian) Sampling: 65 oriented samples were collected at 2 localities. Analysis of entry (14) is based on data scaled from Fig. 3 of Ref. a. Stability: Directions of magnetization are approxi- mately parallel throughout both stratiform beds and beds showing pen econ tempo ran eo us deformation, indicating that remanent magnetization is post- ^ depositional. £> r~* AUSTRALIA o _ _ AINSLIE VOL- 15 35S 149 E 17 -30 12 66 N 168 W S 13 7 a Irving and Green Age: There is some uncertainty as to whether the £? CANIC ROCKS a Irving and volcanic rocks are Devonian or Silurian (Ref. b). ^ Green, 1958 3 b Irving, 1959 H SILURIAN n o EUROPE 1-3 /1 6 205 -16 - 40 N 140 E N a Creer Age: The Ludlow series at Pembrokeshire is Late LUDLOT W SERIES \17 52 N 5 W 205 -16 41 N 141 E N * a Creer, Irving, Silurian. — — — — and Runcorn, 1954 _ _ _ URAL 18 67 N 66 E 98 38 16 N 140 E S a Komarov Age: Silurian PERIDOTITES a Komarov, 1959 Sampling: 6 samples were collected at one quarry. NORTH AMERICA ROSE HILL FOR- [19 40 N 78 W 322 -39 5 — 19 N USE S 6 4 b Graham Age: early Middle Silurian (lower Niagara series) 322 -39 S 27 — 19 N 138 E 6 4 c a Graham, 1949 MATION or 20 — — s Sampling: 35 oriented samples were collected at 6 SWARTZ, 1923 21 39 HN 79 W 321 -40 6 TABLE 1.—Continued Locality Magnetic Direction Pole Position Rocks Sampled No. References Remarks Lat. Long. Decl. Incl. a» k N Lat. Long. P 8m tp 5 SECTION C— Continued folds at 2 localities and on steep limbs of large fold- at the remaining localities. Directions of magnes n tization are widely scattered before correction for dip. All but 3 samples have nearly parallel directions after dip correction. Deformation was near the end of the Paleozoic, indicating magnetic stability for at least 200 million years. o Reversals: Of the 3 aberrant samples, 2 that are strati- o graphically adjacent have directions of magnetiza- tion approximately opposed to the mean direction for the entire group. _ CLINTON IRON f22 .— 35 N 138 E N a Howell et al. Age: Middle Siluvian (Niagaran series) — — — — — — — ORE \23 33^N 86HW 143 19H HH 107 16 34 N 139 E N 12 6H * a Howell, Mar- Sampling: 16 specimens were measured from 7 oriented S tinez, and samples. Lateral sampling extent: 300 feet. Analysis Statham, 1958 (entry 23) is based on data scaled from Fig. 1 of Ref. a. Since specimen rather than sample directions are used, cm may be too small. Q 2 Other: Planes of maximum susceptibility tend to lie in bedding plane. AUSTRALIA (24 35 S 149 E -30 22 S 14 Irving and Green Age: Late Silurian (Ref. a) MCCA PORPHYRY 26 - - 60 N 157 W 24 a \25 35S USE 26 -30 22 60 N 153 W S 24 14 * a Irving and Green, 1958 ASIA RED SILTS-TONES /26 . . 293M 55H 8H 17 49 N 12 E S a Chang Wen- You and Age: Middle Silurian FROM YUMEN \27 40 N 97 E 293H 55M V& 16 17 381/2N 25HE S 12 8H * Nairn Sampling: Analysis of Ref. a is based on measurements a Chang Wen-you of 17 specimens from 3 oriented samples. Collecting and Nairn, 1959 site is described as southern part of Yumen, Kansu province. ORDOVICIAN | l EUROPE 1 [28 51 N 26 E 140 75 28 NT 46 E S a Komarov Age: Shown as Ordovician(?) in Table 2 of Ref. a, but UKRAINIAN J29 51 N 26 E 140 74 H 10^4 40 6 27 ^N 46E S 19VS 17J4 * a Komarov, 1959 Komarov (1959) believes European polar-wandering BASALTS 30 51 N 26 E 255 58 — — — 21 N 27 W s — — a curve suggests Cambrian age for entries (28, 29) and [31 51 N 26 E 255 56 J-2 11W 23 8 20 N 29 W s 16)^ 12 * Ordovician age for entries (30, 31). Sampling: 6 samples collected in a quarry were reported for entries (28) and (29), and 8 samples from 2 quarries for entries (30) and (31). Data for calculating entries (29) and (31) were taken from Table 1, Ref. a. RED SANDS AND 32 60 N 30 E 211 -35 - - N a Khratnoy \ Age: Early Ordovician — — — — — BROWN CLAYS 33 60 N 30 E 27 58 — S — a a Khramov, 1958 Sampling: 29 oriented samples were collected over a NEAR LENIN- 34 60 N 30 E 38 41 — — — — — s — — a stratigraphic thickness of 10 meters. Entry (34) is — — — — GRAD 35 60 N 30 E — 18'/2 42 N 169 E M — a i an estimate that excludes an unstable component in ^ [36 60N 30 E 18M 44N 162 E M 22 13 • i entry (33). Entry (35) is from Table 25, Ref. a, and > — — — — entry (36) is the mean of the poles in entries (32) £] s 1 and (34). Q | ! S NORTH i AMERICA 1 W TRENTON GROUP I H i (sediments) | i ! o Sprakers, New j 37 42 hN 75 W 177H 71 5Vi 23 28 S'/sN 74 W S 9V5 8 * i Graham \ Age: Middle Ordovician y York ! a Graham, 1956 Sampling: 28 oriented samples were collected, pre- >• 1 sumably at 1 locality; samples are from different H 1 ! cobbles in a limestone conglomerate. Data for entry (37) are based on values scaled from Fig. 2 of Ref. a. Stability: Agreement in directions of magnetization in different cobbles indicates that the remanent mag- j netization is not that of the parent limestone body but was acquired after the deposition of the Trenton j group. Trenton Falls, 38 43 hN 75 W 179 82 5 23 45 27HN 75 W S 10 9J.2 ' b Graham, 1954 Sampling: Data for entry (38) were scaled from Fig. 5 New York of Ref. b showing 35 measurements from flat-lying beds and 10 from a local deformed bed after correct- ing for dip. i Stability: Flat-lying beds have well-grouped directions i of magnetization. A distorted zone enclosed by fiat- lying beds has scattered directions, which tend to | move toward the other group on correcting for dip; ! significant stability since deposition is thus indicated QS | j (Graham, 1954, p. 219). Jg TABLE 1.—Continued Locality Magnetic Direction Pole Position Rocks Sampled No. References Remarks Lat. Long. Decl. Incl. a »6 * N Lat. Long. P ft. sp 5 SECTION C— Continued JUNIATA FOR- Collinson and Age: Late Ordovician MATION (sedi- 40 40 N 78MW 131 26 8 6 56 20 N 153 E N 9 5 a Runcorn Sampling: Statistical analysis is based on 56 specimens ments) a Collinson and from 12 oriented samples. Runcorn, 1960 CAMBRIAN n EUROPE o _ _ 187 39 _ 15 N 173 E N „ a Creer Age: Cambrian. Caerbwdy sandstone is in the lower f41 — CAERBWDY 42 — — 191 41 — — — 15 N 170 E N — — b a Creer, Irving, part of the Cambrian Caerfai series. SANDSTONE 43 52 N 5 W 187 39 8 — — 15 N 173 E — 10 7 c and Runcorn, Sampling: 12 samples span a stratigraphic interval of J44 187 39 8 32 12 15 N 173 E N 10 8 d 1954 350 feet. o — — o 1*5 52 N 5 W 187 39 8 32 12 15HN 168HE N 9H 5H * b Runcorn, 1955a Other: Values cited in entry (41) also appear in Ref. b. H c Irving, 1956a Details appear in Ref. e, p. 123-124. d Creer, Irving, and Runcorn, 1957 e Creer, 1957b NORTH AMERICA o _ TAPEATS SAND- 46 — 22 N 27 E „ a Runcorn Age: Early Cambrian — — — — STONE, GRAND a Day and Run- Other : No other data about this research are available. CANYON corn, 1955 According to Ref. b, the pole of Ref. a is based on b Creer, Irving, inadequate data and should be disregarded. and Runcorn, 1957 _ _ __ WILBERNS FOR- 47 SOHN 99 W 98 24H 0 158 E N a Howell and Mar- Age: Late Cambrian; Point Peak shale member of MATION (sedi- — tinez Wilberns formation, Llano uplift area ments) a Howell and Sampling: 185 samples were collected at 10 localities Martinez, 1957 spanning a distance of 55 miles. Statistical analysis was not made of this data because directions of mag- netization show "streaking" toward direction of present field. Values for declination and inclination indicated are for the group of measurements farthest from the present field direction; they were scaled from Fig. 5, Ref. a, and do not exactly correspond to the pole position cited. i ! Stability: "Streaking" indicates partial instability. A single locality with steeply dipping beds shows wide scatter in directions of magnetization after correcting for dip. Reversals: No systematic reversals occur, but some widely scattered points are on the upper hemisphere. SAWATCH 48 — — — — 49 N 125 E N b Howell and Mar- Age: Late Cambrian; magnetization may be post "QUARTZITE" 49 39 N J106!.2W 148 -15 4 44 31 47 N 125 E N 4 2 * tinez depositional SANDY DOLO- a Howell and Sampling: 36 samples were collected at 2 localities. MITE Martinez, 1957 Analysis for entry (49) was based on data scaled i b Howell, Mar- from Fig. 7 of Ref. a, omitting 5 samples with direc- tinez, and tions parallel to present field. i i t ; i Statham, 1958 Stability: 5 of the 36 samples have directions of mag- 1 netization parallel to the present field; 31 have tight 1 grouping approximately perpendicular to the present field. > Other: Remanent magnetization may have been ac- quired at tune of dolomitization, possibly in the late i Paleozoic, Ref. a, p. 391. _ _ _ _ LODORE FOR- fso 41 N 109H VV 59 4 8 14 26 — a Collinson and Age: Cambrian o MATION 51 41 N 109H W 234 13 13 25 7 — — — — __ a Runcorn Sampling: Based on 33 specimens from 11 oriented 2 (sediments) [a 41 N 109K W 23 N 6 E M 7 4 a a Collinson and samples. — — — — Runcorn, 1960 Reversals: Entry (50) is reversed with respect to (51). o3 Stratigraphic distribution of samples in these two groups is not known. a DEADWOOD 53 42 N 107H W 151 -14 7 15 34 47 N 117 E N 7 4 a Collinson and Age: Cambrian. Ref. a cites a possible Mississippian FORMATION j Runcorn age. However, it is definitely regarded as Cam- (sediments) i a Collinson and brian by U. S. Geological Survey. Runcorn, 1960 Sampling: 34 specimen measurements were made on 7 oriented samples. AUSTRALIA ELDER 54 16 S 126 E 231 -15 10 —_ — 34 N 172 W N 10 5 a Irving and Green Age: Middle Cambrian (Ref. a) MOUNTAIN 55 16 S 126 E 231 -15 10 34N 165 W N 10 5 * a Irving and SANDSTONE — Green, 1958 I ANTRIM 56 16 S 126 E 53 -2 12 36 N 154 W M 12 6 a Irving and Green j Age: Early Cambrian, possibly latest Precambrian PLATEAU — — a Irving and (Ref. b) BASALTS I Green, 1958 Reversals: Reversals are indicated in Ref. b, but no b Irving, 1959 details are given. O TABLE 1.—Continued o to Locality Magnetic Direction Pole Position Rocks Sampled No. References Remarks Lat. Long. Decl. Incl. OC95 k 2V Lat. Long. P &m sp 5 SECTION D CARBONIFEROUS EUROPE GLOUCHESTER 1 33 35 14 — — S a Clegs et al. Age: late Carboniferous (late Coal Measures) PENNANT 2 51HN 2WW 33 35 10H 13 14 48HN 126! 2 E S 12 6H * a Clegg, Almond, Sampling: 14 specimen measurements were made on SANDSTONE 3 — — — — — — 43 N 114 E S — — b and Stubbs, 1 oriented sample. Calculations for ass and k in — 1954a entry (2) were based on aeo = 5, cited in Ref. a. o b Irving, 1959 Stability: The magnetization of the specimens was unchanged after application of fields of several > hundred oersted A.C. 200 IS CLEE HILL SEDI- 4 — — 45 — — N — — a Clegg et al. Age: Probably late Carboniferous o MENTS AND 5 52HN 2 W 200 15 3H 36 45 «HN 156 E N 3M 2 * a Clegg, Deutsch, Sampling: Samples were collected at 6 sites over a IGNEOUS ROCKS 6 — — — — — — — 27 N 155 E N — — b Everitt, and lateral extent of 30 miles. It is not known whether Stubbs, 1957 N is the number of oriented samples or specimen b Irving, 1959 measurements. Calculations for entry (5) were based on data scaled from Fig. 1, Ref. a. Igneous and sedimentary samples could not be distinguished in this plot. Stability: Stability is suggested by the tight grouping of the baked sediments compared with scattered directions and lower intensity of nearby unbaked Q sediments. ___ TlDESWELLDALE 7 218 36 5 a Clegg et al. Age: These sediments were baked by an intrusive * C/3 BAKED SEDI- 8 53 WN 2 W 219 41 8 61 5 6MN 142 Vj E N 9H 6 a Clegg, Deutsch, of late Carboniferous age. g MENTS 9 — — 5 N 143 E N — b Everitt, and Sampling: It is not known whether N is the number — — — — — — Stubbs, 1957 of oriented samples or specimen measurements. b Irving, 1959 Calculations for entry (8) were based on data scaled from Fig. 1, Ref. a. LANCASHIRE 10 54N 3 W WA 23^ 4 64 19 44N 142HE S 4 2 * Seishi Age: Carboniferous PENDLE a Belshe, 1957 Sampling: 19 oriented samples were collected at 3 MONOCLINE sites from the lower Coal Measures, Dandy Rock and Old Lawrence Rock. Calculations for entry (10) were based on data scaled from Fig. 3, Ref. a. Stability: Application of Graham's fold test indicates that the magnetization is stable and was imparted to the rocks before late Carboniferous folding. I LANCASHIRE 11 54 N 3 W 27 24 6 29 IB 43 N 139 E S 6W SH * Belshe Age: late(?) Carboniferous ROCKS a Belshe, 1957 Sampling: Calculations for entry (11) were based on data scaled from Fig. 2, Ref. a. This figure shows the directions of magnetization for 40 (out of a reported 49) samples. These 18 samples form 1 group and the 13 used for the following entry (12) form another group. 9 scattered directions were excluded from these calculations. LANCASHIRE 12 54N 3 W 188H 9H 3 176 13 SOWN 167 E N 3 1H . Belshe Age: early late Carboniferous MILLSTONE a Belsht, 1957 Sampling: Calculations for entry (12) were based on GRIT data scaled from Fig. 2, Ref. a. Also see remarks for entry (11) above. DERBYSHIRE 13 26 37 S , a Behhl Age: early and late Carboniferous — — — — — SANDSTONE 14 — — — 36 N 137 E S — b a Runcorn, 1955a Sampling: 103 oriented samples were collected from AND SlLTSTONE _ 15 — — 27 36 — — 103 — — S — _— c b Runcorn, 1955b 14 sites. 16 53N 1HW 27 36 103 50HN 136 E S * c Belsh£, 1957 Stability: Stability is suggested by remeasurement — — 17 53 N 1HW 26 37 — — 51 N 143 E S — d d Irving, 1956a after 6 months with no change in the magnetization. — — Other: A later study cited in Ref. c of 142 samples from 34 localities showed much greater scatter. DERBYSHIRE 18 53 N 1HW 26 43 _ 55 N 148 E S a Belshf Age: The lavas are interbedded with sediments of TOADSTONES 48 early Carboniferous age and are probably pre- 19 — — 47 13 9 — — — — b a Irving, 1956a 20 53 N 1HW 48 47 13 13 9 4—7 N 105 E S 17 11 * b Belshe, 1957 Millstone Grit (Evans, 1918, p. 172). Sampling: 9 oriented samples were obtained from 3 interbedded units. Calculations in entry (20) were based on data scaled from Fig. 1, Ref. b. KISGHORN LAVAS Clegs el al. Age: early Carboniferous (pre-Millstone Grit) f21 56 N 3^W 20 15 — — — — — — — — a a Clegg, Deutsch, Sampling: The sequence of flows is as indicated, with Flows 64-65 •22 56 N 3HW 20 15 — — 39 N 150HE S — — * Everitt, and 65 uppermost. Calculations for entries (27) and — — — — — — — b 23 — — 42 N 150 E S — Stubbs, 1957 (29) (flows 50-54 and 41-46) were based on data — — — — — — 24 56 N 3HW 200 38 — — a b Irving, 1959 scaled from Fig. 2, Ref. a. Data for flows 64-65 Flows 48-54 •25 56 N 3HW 200 38 — — — 10WN 157WE N — — * and 48^49 were not given. Entry (31) is a Fisher — — — — — — 26 — — 12 N 157 WE N — b statistical treatment of the pole positions given Flows 50-54 27 56 N 3WW 202 H 34 5H 23 27 13 N 154WE N 6W 3W * in entries (22), (25), and (27). — — — [28 56 N 3HW 26 -42 — — 18 — — a Flows 41-46 29 56 N 3WW 26 -42 3 156 18 6HN 153 E S 4 2W * 30 — 8N 153 E S b 1 — — — — — — — — Kinghorn ; 31 28 21 3 18HN 154 E M 28 28 * Average — — — — TABLE 1.—Continued Locality Magnetic Direction Pole Position Rocks Sampled No. References Remarks Lat. Long. Decl. Incl. ans k N Lat. Long. P Sm sp 5 SECTION D— Continued — — — — SHATTERFORD 32 — — — 32 N 137 E M — — b Clegg et al. Age: Cited as Carboniferous (undifferentiated) in INTRUSION 33 51WN 2 W MM 3 4 63 21 32HN 1S9ME M 4 2 * a Clegg, Deutsch, Ref. b Everitt, and Sampling: The samples were collected at 2 sites 200 Stubbs, 1957 yards apart; it is not known whether N is the num- b Irving, 1959 ber of oriented samples or specimen measurements. 8 Calculations in entry (33) were based on data scaled from Fig. 3, Ref. a. Reversals: 11 of the 21 samples were oppositely polarized from the other 10. a _ 34 175 -9 27 5 a Blundell Age: Cited as Permian and Carboniferous in Ref. a LUNDV — — — — — — — 35 — — — 43 N 180 E N — b a Blundell, 1957 and as late Carboniferous in Ref. b. GRANITES — — — — — I 36 BIN 4MW 175 -9 27 5 5 43 N 177 W N 27 ISM * b Irving, 1959 Sampling: 10 specimen measurements were made on 5 oriented samples. The value of k for entry (36) i was calculated by the approximate formula. NORTH AMERICA o NACO FOK- Runcorn Age: Pennsylvanian 25 MATIOK f37 36 N 113 W 149M -3M 4 40 31 48MN 120 E N 4 4 a a Runcorn, 1956a Sampling: Entries (37)-(39) are based on measure- Carizzo Creek 3* 36N 113 W 149H -3^ 4 40 31 4SHN 114 E N 4 2 * b Irving, 1959 ments of 31 specimens from 8 oriented samples col- (sediments) (39 41 N 120 E N 8 4 b lected at 1 site with a stratigraphic extent of 200 Fossil Creek 40 S4HN 111H W 125 16 7 22 20 23 N 130 E N 7 4 a Collinson and feet. Entry (40) is from a later collection of 9 oriented (limestone) Runcorn samples made at Fossil Creek. a Collinson and Runcorn, 1960 _ _ _ — BARNETT 41 31 N 99 W — — — 39 N 124 E N a Bowell and Age: Mississippian FORMATION 42 31 N 99 W — — — — — 41 N 128 E N b Martinez Sampling: 60 oriented samples from 8 sites (N poles) — — * (sediments) 43 SIN 99W 148H 19 6M 11 60 39N 122HE N 6M S a Martinez and and 8 oriented samples from 1 site (S poles) were 44 31 N 99 W — — — — 42 N 142 E S a Howell, 1956 collected over a lateral extent of 73 miles. Calcu- — — — 45 SIN 99 W 319 8 3M 200 8 42HN 144 E s SH 2 * b Howell and lations for entries (43) and (45) were based on data Martinez, 1957 scaled from Fig. 3, Ref. b. CODKOV GROCP 46 48 N 59 VV 166 8 | 856 38 9 43 N 139 E N 855 4H a Nairn et al. Age: Cited as Mississippian in Ref. a (sediments) 47 4»N 59 W 166 8 | 85£ 38 9 36HN 139 E N 8H 4H * a Nairn, Frost, Sampling: 36 specimen measurements were made on | and Light, 1959 9 oriented samples. BONAVENTURE, 48 48 N 66 W 163 Hi 195-a S 46 a Du Bois \ Age: Carboniferous; Bonaventure is Late Mississip- KENNEBECASIS 49 48 N 66 W 163H I9H 5 17 46 30 N 133 E N 5 2H * a Du Bois, 1959b pian or Early Pennsylvanian, Kennebecasis is and BATHURST Pennsylvanian (Ref. a) FORMATIONS Sampling: 22, 14, and 10 oriented samples were col- (sediments) lected from these 3 formations from groups of sites spanning 250 miles. The mean direction of 2 speci- men measurements from each sample was reduced to the common mean locality cited, and a statistical analysis was made on the resulting directions. a»s may be too low because mean site or formation directions were not used in the analysis. The value of k for entry (49) was calculated by the approximate formula. AUSTRALIA _ KATTUNG VAE- SO 33 S 151 E 90 84 6 75 32 N 15 W M 12 12 a Irving Age: Age cited as late Carboniferous in Ref, b VOID SEDI- a Irving, 195 7b Sampling: 75 oriented samples were collected at 4 O MENTS b Irving and localities. Green, 1958 Stability: Application of Graham's fold test indicates stability. n Reversals: The direction of magnetization of samples I from one locality was reversed to that at the other 3 localities. KATTUNG LAVAS 51 33 S 151 E 5 -85 8 43 N 30 W S 16 16 a Irving and Green Age: Cited as late Carboniferous in Ref. a — — a Irving and Green, 1958 AFRICA DWYKA VABVED 52 US 29E 360 -81 5H 84 10 36 N 151 W S 10 10 a Nairn Age: Cited as late Carboniferous in Ref. a CLAYS 53 18 S 29 E 333 76 7 57 9 7N 17 E 13 12 a a Creer, Irving, Sampling: The 19 specimen measurements (including — 54 US 29 E 333 76 7 57 9 5MN 1754E S j 13 12 * Nairn, and "normal" and "reversed" groups) were made on ! Runcorn, 1958 j 4 oriented samples. o Cn TABLE 1.—Continued Locality Magnetic Direction Pole Position Rocks Sampled No. References Remarks Lat. Long. Decl. Incl. 0:96 k N Lat. Long. P 5m Sp S SECTION E PERMIAN EUROPE _ _ f 1 — — 189 -9 — 47 N 147 E N — — a Creer Age: Cited as Permian — . EXETER VOL- I 2 — — — — 48 N 168 E N — — c a Creer, Irving, Sampling: 34 oriented samples were collected from O —_ — — CANIC SERIES 1 3 — 189 -9 20 15 5 43 N 164 E N 20 10 f and Runcorn, 5 flows. Pole position cited from Ref. a was scaled ^ I* 51 N 4 W 189 -9 20 5 43N 164 E N 20 10 d 1954 from Fig. 1. This figure also appears in Ref. b. Values > — b Runcorn, 195Sa cited for Ref. c were also scaled from a map. Data ^ c Runcorn, 1956a for the individual flow directions are given in Ref. ^ d Irving, 1956a e. The dispersion within flows is much less than G e Creer, 1957b that between flows. O f Creer, Irving, p and Runcorn, f* 1957 4 MAUCHLINE (5 187 -6 12 26 37 N 163 E N a Du Bois Age: Cited as Permian t"J * SEDIMENTS \6 55HN 4HW 187 -6 12 5 26 37 N 166 UE N 12 6 a Du Bois, 1957 Sampling: 26 is the number of specimen measuree Q ments; number of samples is not known. The valu3 S? of k for entry (6) was calculated by the approximat- > formula. 2 MAUCHLINE /7 180 -4 8 34 36 N 175 E N a Du Bois Age: Cited as Permian t-3 — — * LAVAS 1 " 55 W 4HW 180 -4 8 9 34 36 N 175 E N 8 4 a Du Bois, 1957 Sampling: 34 is the number of specimen measure- C/3 ments. The number of flows sampled is not given. S The value of k for entry (8) was calculated by the approximate formula. ESTEREL VOL- Age: Cited as Permian CANIC ROCKS _ 9 210 -16 — 5 a Roche Sampling: The samples were collected "several dozen ( — — — — — — — * Pyromeride R4 10 43HN 7E 210 -16 11H _k' 5 46N 142 E N 12 6H a Roche, 1957 meters apart and from different levels." The value of 11 46 N 141 E N b b Irving 1959 NlEDECK 23 — — 193 _7 4H 22 49 43 N 168 E N — _ a Nairn Age: Saxonian(P), middle Permian PORPHYSE 24 193 -7 4H 22 49 43 N 168 E N VA VA * a Nairn, 1957b Sampling: 49 specimen measurements were made on 14 — — j oriented samples. Other: Locality is in the Vosges of France; exact locality j could not be determined. _ MONTCENIS 25 — — 197 6 4 93 | 14 38 N 162 E N — a j Nairn Age: Cited as Saxonian, middle Permian SEDIMENTS 26 46J.2N 4HE 197 6 4 93 | 14 38 N 162 E N 4 2 * j a Nairn 1957b Sampling: 14 specimen measurements were made on 3 1 | oriented samples. Some specimens were not included i in the statistical analysis because they were believed to be unstable. TABLE 1.—Continued Locality Magnetic Direction Pole Position § Rocks Sampled No. References Remarks Lat. Long. Decl. Incl. OT95 * N Lat. Long. P Sm sp 5 SECTION Ei— Continued SAINT-WENDEL 27 — — 181 -9 3H 27 27 45 N 175 W N — — a Nairn Age: Cited as Autunian, early Permian SEDIMENTS 28 49 HN 7E 181 -9 SH 27 27 45N 175 W N 3H 2 * a Nairn, 1957b Sampling: 27 specimen measurements were made on 5 oriented samples. UrlMSKIJ AND 29 58 N 56 E 221 -40 15M — — 45 N 178 E N — — a Khramov Age: late Permian KAZANSKIJ 30 58 N S6E 221 -40 15H 45N ITS E N 18H 11H * a Khramov, 1958 Sampling: 16 oriented samples were collected from SEDIMENTS — — Ufimskij over a lateral extent of 100 km and a thick- Q ness of 70 m, and 24 were collected from Kazanskij s^ over a lateral extent of 100 km and a thickness of 90 m. Data for entry (29) are an estimate which excludes ^ "partially" stable samples. Q Other: A large amount of scatter is present in the ^ plotted measurements. Q W TARTARSKIJ 31 59 N 50 K 10 52 N 176 E 17 a Khramov Age: late Permian M SEDIMENTS — — — — — — a Khramov, 1958 Sampling: 74 oriented samples were collected over a 1 lateral extent of 100 km and a thickness of 485 m. ^ Khramov states that two groups are present, center- 2^ ing at 38 E, plus 57 and 254 E, plus 42. He states that & the latter group has a stable component at 211 E, Q minus 38. ^ Other: A plot of some of these data shows very extreme Q scatter. 2 NORTH Ij AMERICA § CUTLER FORMA- Graham Age: Permian TION (sedi- a Graham, 1955 ments) Glenwood (32 39HN 107 HW — — — — 2 36 N 122 E N — — a Sampling: 2 samples were collected 50 feet apart later- Springs, Colo. \33 39HN 107HW 140 6 — — 2 33HN 123 E N — — * ally and 1 foot apart stratigraphically. Entry (32) values were scaled from a map on p. 343, Ref. a. Entry (33) is based on data scaled from Fig. 7, Ref. a. (N = 1 is not sufficient to calculate out or k.) Monument 34 37 N now 161 32^ 9H 96 4 33 N 92 E N 11 6 * Sampling: 12 samples were collected over a strati- Valley, Utah graphic thickness of 200 feet and a lateral extent of yi mile. Entry (34) is based on data scaled from Fig. 7, Ref. a, for 4 samples, which form a group away from the present field direction. _ _ i _ ! _ CUTLER, AVER- 35 13 2 34 N 107 E N 13 13 * Sampling: This pole is midway between the poles of AGE entries (33) and (34); 13° is the distance to either pole and has no statistical significance. ST'PAI FORMATION Age: Permian and Pennsylvanian Upper Supai 37 35 N 112 W 161 10 8 13 32 46 N 96 E N 8 4 a Collinson and Sampling: Statistical analysis is based on 32 specimen a ree , Runcorn measurements on 14 oriented samples. rizona a Collinson and Runcorn, 1960 Oak Creek, (38 35 N 111HW — — — — 43 N 122 E N — a Graham Sampling: 52 specimen measurements were made on — — Arizona \39 35 N 111HW 143W 9>4 7H 17 24 37 HN 117 E N 7 H 4 * a Graham, 1955 17 oriented samples spanning 1.4 miles laterally and b Irving, 1956a 100 feet stratigraphically. Specimens from con- c Irving, 1959 glomerates had rather scattered directions. 11 speci- mens from flat-lying sediments show streaking to- ward the present field. The analysis of entry (39) is based on the remaining 24 specimens from flat-lying sediments. Entry (38) was scaled from Fig. 6 of Ref. a. Carizzo Creek, (40 34 N 110HW — SON 104 E N — — a Sampling: Statistical analysis of entry (41) is based on — — — — Arizona \41 34 N 110HW 159 2 3 i 33 59 50 N 103 E N 3 1H * 59 specimen measurements made on 30 oriented samples collected over a lateral distance of 10 miles and a stratigraphic thickness of 1000 feet. Entry (40) was scaled from Fig. 6 of Ref. a. Q Supai Com- (42 35 N 104 W 150 3 5 — 37 N 107 E 5 3 b Sampling: Described in Ref. b as Supai Beds, Arizona — — bined \43 — — — — — — 41 N 117 E N — — c and New Mexico. I — n (44 23 N 119 E N 8 6 a Runcorn Sampling: 31 samples were collected over a lateral dis- Grand Canyon o J45 36 N 112 W 132H 23 7H 16 34 26 N 119 E N 8 b a Runcorn, 1955b tance of 75 miles and a stratigraphic thickness of 46 36 N 112 W 23 16 34 24MN 120 E N 8 4 * b Runcorn, 1956a 500 feet. 21 specimen measurements from 6 samples a u 132H 7H (47 36 N 113 W 133 23 8 26 N 121 E N 9 5 c c Irving, 1956a were smeared toward the present pole. The calcula- — — tions were based on the remaining 34 specimen meas- urements from 25 oriented samples. | •< Grand Canyon, (48 — 146 8 7 — 39 N 115 E N — — b Doell Sampling: 42 oriented samples were collected over a — — Arizona \49 36 N 112 W 146 8 7 33 12 39 N 114 E N 7 3H * a Doell, 1955a lateral extent of 2 miles and a stratigraphic thickness b Doell, 1955b of 500 feet. 24 of the samples showed agreement between 2 or more specimen measurements. 12 of these were smeared toward the present pole, and the calculations were based on the remaining 12 (Ref. a). Hunter's Point 51 35J4 N 109 W 164 5 5 32 24 SON 96 E N 5 3 a Collinson and Sampling: 24 specimen measurements were made on 7 Runcorn oriented samples. Lower Supai, 52 35 N 112 W 141 18 4 25 55 I 33 N 116 E N 4 2 a a Collinson and Sampling: 55 specimen measurements were made from Oak Creek, ! Runcorn, 1960 16 oriented samples. Arizona SUPAI AVERAGE 53 _ i — — 9 45 7 40HN 110 E N 9 9 * Sampling: This mean pole position was found by apply- — ing Fisher statistics to the pole positions cited for entries (37), (39), (41), (46), (49), (51), and (52) above. TABLE 1.—Continued Locality Magnetic Direction Pole Position Rocks Sampled No. References Remarks Lat. Long. Decl. Incl. ass k N Lat. Long. P Sm sp * SECTION E<— Continued _ _ _ YESO FORMA- 54 35HN 105MW 22 41 N 127 E N a Graham Age: Permian (Leonard) — — — TION (sedi- 55 35 MN 105MW 143 -1 3 99 22 41 N 127 E N 3 1*4 * a Graham, 1955 Sampling: 26 oriented samples were collected over a ments) lateral extent of 200 feet and a stratigraphic thick- ness of 36 feet. The pole position for entry (54) was scaled from Fig. 6 of Ref. a. Entry (55) was based on data scaled from Fig. 7, Ref. a; 4 widely scattered samples were excluded from the calculations. ABO FORMATION Graham Age: early Permian (sediments) o /56 34*4N 8 44 N 120 E N a a Graham, 1955 Sampling: 11 oriented samples were collected over a G Abo Canyon \57 34HN 106HW 149 8 17*4 5 17 42 N 117 E N 17*4 9 * lateral distance of 400 feet and a stratigraphic thick- ness of 50 feet. 8 of the 20 specimen measurements made from these samples form a group and were used to give the pole position of entry (56) which was scaled from Fig. 6 of Ref. a. Entry (57) is based on data scaled from Fig. 7 of Ref. a, excluding 3 of the 20 specimen measurements. * Zuni Mts. 58 3SHN 108 H W 160M 55 12 7 25 17 N 87ME N 17 12 Sampling: 13 oriented samples were collected over a lateral extent of 150 feet and a stratigraphic thickness of 25 feet. Entry (58) is based on directions of all _ specimens as scaled from Fig. 7, Ref. a. i ABO AVERAGE 59 35 N 107HW SON 100 E N 18 18 * Sampling: Average of pole positions of entries (57) and — — — — (58) is listed. 18° is the distance from average posi- tion to either pole and has no further significance. * SANGRE DE 60 S5HN 105MW 175^ SOM 11 9 19 38 N SOME N 11 6M Graham Age: early Permian and Middle or Late Pennsylvanian CRISTO FOR- a Graham, 1955 Sampling: 19 oriented samples were collected over a MATION (sedi- lateral extent of 100 feet and a stratigraphic thickness ments) of 10 feet. Entry (60) was based on data scaled from Fig. 7, Ref. a. AUSTRALIA _ UPPER MARINE 61 35S 151 E 67 SI 11 27 N 11 W N 21 21 a Irving and Green Age: Cited as Permian in Ref. a VOLCANIC a Irving and Sampling: Ref. b states that 3 flows were sampled. SERIES Green, 1958 b Irving, 1959 _ LOWER MARINE 62 33 S 151 E 110 80 _ — | 38 N 6 W N i — a Irving and Green Age: Cited as Permian in Ref. a VOLCANIC i a Irving and Sampling: Ref. b states that 1 flow was sampled. SERIES Green, 1958 b Irving, 1959 AFRICA MAJI VA CHUMVI 63 3S 39 E 267 38 11 M 9 5 4N 150 E N 13H 8 a Nairn Age: Cited as late Permian FORMATION a Creer, Irving, Sampling: 21 specimen measurements were made on 5 (sediments) Nairn, and oriented samples. Runcorn, 1958 TARU GRIT 64 3S 39 E 87 61 ISM 23 8 ON 87 E S 29 «H a Nairn Age: Cited as early Permian a Creer, Irving, Sampling: 32 specimen measurements were made on Nairn, and 8 oriented samples. Runcorn, 1958 Other: Values cited for A', 095, and k are inconsistent in entries (63) and (64), and it is impossible to deduce ^ whether specimen measurements or mean sample C~* measurements were used in computing the statistics. 59 M d o d H TABLE 1.—Continued Locality Magnetic Direction Pole Position Rocks Sampled No. References Remarks Lat. Long. Decl. Incl. an k If Lat. Long. P fan sp S SECTION F TRIASSIC EUROPE KEUPER MARLES Clegg et al. Age: Late Triassic; sandstones sampled are close to base a Clegg, Almond, of Keuper Marie series 1 — 29 34 10 58 — — — S — — b and Sutbbs, Sampling: 12, 2, 6, 3, and 2 samples, respectively, were — — — — — Normal sites 2 — — 26 28 — — s — b 1954a collected at each of 5 sites over a lateral distance of * Q u S3N 2 W 26H 34H 13H 34 5 SOHN 137HE S 15H 9 b Runcorn, 1955a 140 miles. Values from Ref. b (citing an analysis of c Runcorn, 1955b Creer, 1955) are from p. 272 and p. 284, respectively. d Runcorn, 1956a Calculations for entry (3) were based on mean site 55 e Irving, 1956a directions cited in Table 1, Ref. a. o f Day and Run- Stability: The magnetization was not changed after corn, 1955 random orientation in the earth's field for several D weeks, nor by the application of a 300-oersted A.C. field. Graham's fold test on dips of 15° and less also indicate stability. 4 — 13 — — — — — 219 -16 27 N — b Sampling: 6, 2, 2, and 8 samples, respectively, were Reversed sites (5 214 -28 — — N b collected at each of 4 sites over a lateral distance of I — — — — — — — f u 53 N 2 W 218 -16 27 13 4 35HN 129HE N 28H 15 * 180 miles. For basis of entry (6) see remark above for entry (3). § _ Stability: See previous remark about stability. g 7 48 N 155 E M — b Sampling: Values for entry (9) were scaled from Fig. 9 — — — — — — — f 8 47 N 122 E 13 7 c in Ref. d. Entry (12) was based on average directions Average of — — — — — 1 9 46 N 131 E 13 7 d at normal and reversed sites (Table 1, Ref. a). All w J 53 N 2 W 33 27 12 — — 43 N 131 E — 12 7 e entries are presumably based on these same data. everse Uo en — — — — — — 47 N 133 E — — — f Other: A detailed study of unstable Keuper Maries from 111 — § (a 53 N 2 W 32 26 13 16 9 43 N 133 E M U 8 * another locality is given by Creer (1957a). VOSGES SAND- 13 48N 6E 218 10 12 2 61 27HN 1*2 E N 12 6 . Clegg et al. Age: Cited as Early Triassic in Ref. a — — — — — STONE 14 — — 28 N 143 E N 12 6 b a Clegg, Deutsch, Sampling: 61 specimens or samples (?) were collected Everitt, and at 7 sites. Entry (13) was based on data scaled from Stubbs, 1957 Fig. 5, Ref. a. b Irving, 1959 VILLA vicio SA IS 43MJN 5HW « 56 2 SO 87 82 N ISO E S VA 2 * Clegg et al. Age: Cited as Triassic in Ref. a SANDSTONE a Clegg, Deutsch, Sampling: It is not known whether 87 is the number Everitt, and of oriented samples or specimen measurements. Stubbs, 1957 Entry (15) was based on data scaled from Fig. 6, Ref. a. Stability: Stability is suggested by the tightness of the group which in situ (i.e., before correction for geo- logic dip) has a direction 30" from the present field direction. Of 7 sites sampled in this region, only Villaviciosa, Alcolea, and Aguilar had consistency of directions of magnetization. * ALCOLEA AND 16 41 N 2HW 349^ 51'/2 SH 50 16 78 N 135 W S 7H 5H Clegg et al. Age: Cited as Triassic in Ref. a AGUILAR a Clegg. Deutsch, Sampling: 16 samples or specimens (?) were collected SANDSTONE Everitt, and at 2 sites. Entry (16) was based on data scaled from Stubbs, 1957 Fig. 7, Ref. a. VETLt'jsKij 17 59 N 50 E 235 21 — 9 — — — a Khramov Age: Early Triassic — * SEDIMENTS 18 59 N 50 E 235 21 9 7HN 176 E N — a Khramov, 1958 Sampling: 9 oriented samples were collected over a 19 59 N 50 E 222 -19 — ~_ — — — — — — a lateral extent of 100 km and a thickness of 40 m. 20 59 N 50 E 222 -19 31 N 179HE N * Entries (19) and (20) refer to the direction of the — — — "stable" (Ref. a) component. Much scatter is present in the plotted data. NORTH AMERICA SPRINGDALE 21 37HN 113 W 350 39 8H 17 18 S a Runcorn Age: Late Triassic (?) — § SANDSTONE 22 37HN 113 W 360 39 »M 17 18 72H ME S 10 6 * a Runcorn, 1956a Sampling: 8 oriented samples were collected at 1 M MEMBER OF 23 — — — — — — — 60 N 110 E S — — b b Runcorn, 1955b locality. The statistical analysis of entry (21) is tl MOENAVE — — — — 24 338 16 — 55 N 107 E s — — a based on 18 specimens from 7 of these samples. The n FORMATION pole position cited in entry (24) is suggested (Ref. a) as more probable owing to "smearing" toward the d present pole. * REDONDA FORMA- 25 35 N 104 W 16H 55H 4H 57 20 77 N 23 W S 6H 4Ji Graham Age: Late Triassic, equivalent to the upper part of the TION (sedi- a Graham, 1955 Chinle ments) Sampling: 20 specimen measurements were made on 17 oriented samples collected over a lateral extent of H mile and a stratigraphic thickness of 150 feet. Calcu- lations for entry (25) were based on data scaled from Fig. 7, Ref. a. CHINLE FORMA- Age: Late Triassic. The Chinle includes the Redonda TION (sedi- formation (upper portion) and the Shinarump mem- ments) ber of the Chinle formation (lower portion). Romeroville, 26 35 WN 105 W 15H 9 9 14 16 56 N 47 E M 9 4H * Graham Sampling: 16 oriented samples collected at 1 site over New Mexico a Graham, 1955 a horizontal distance of 100 feet and a vertical thickness of 15 feet. Entry (26) was based on data scaled from Fig. 7, Ref. a. Reversals: \ specimen is reversely magnetized. TABLE 1.—Continued Locality Magnetic Direction Pole Position Rocks Sampled No. References Remarks Long. Decl. Incl. ags N Lat. Long. P 6m Lat. * Sf 5 SECTION F— Continued _ _ — Las Vegas, fa 35M N 105 W 48 32 — — — — — b Graham Sampling: 8 specimen measurements were made on 6 New Mexico \28 35 H 105 W 33 47 H 16H 12 8 61 N iow S 21H 14 * a Graham, 1955 oriented samples. Lateral sampling extent is 100 feet, b Kintzinger, 1957 vertical sampling extent is 5 feet. Entry (28) is based on data scaled from Fig. 7 of Ref. a. — Site 1, Moab, /29 38J$N 109H W 160 44 — — 39 23 N 90 E N — a Collinson and Sampling: 39 specimen measurements were made on 14 -8 — 109 E N Utah j\30 38>*N 109H W 156 — 39 49 N — — a Runcorn oriented samples. a Collinson and Stability: Because of "smearing" these samples are Runcorn, 1960 considered to be only partially stable; entry (30) is an estimate (Ref. a) of the stable component of o magnetization. o 73 — S — Site 2, Moab, f31 38HN 109K W 59 — 60 48 N 66 W — a Collinson and Sampling: 60 specimen measurements were made on 10 Utah \32 38HN 109H W 160 10 — — 60 SON 114 E N — — a Runcorn oriented samples. a Collinson and Stability: Because of "smearing" these samples are Runcorn, 1960 considered to be only partially stable; entry (32) is an estimate {Ref. a) of the stable component of magnetization. — — — Site 1, Colo. fa 39 N 108J3 W 356 66 5 25 29 — — a Collinson and Sampling: 29 specimen measurements were made on 6 National 134 39 N 109 W 356 66 5 25 29 SOHN 125 W S 8H 7 * Runcorn oriented samples. Monument a Collinson and Stability: Cited as unstable (Ref. a) o Runcorn, 1960 — — — Site 2, Colo. [35 39 N 108H W 34 60 7 14 31 — — a Collinson and Sampling: 31 specimen measurements were made on 7 National \36 39 N 108H W 34 60 7 14 31 64N 35 W S IOH 8 * Runcorn oriented samples. Monument a Collinson and Stability: Cited as unstable (Ref. a) § Runcorn, 1960 SHINARUMP 37 36 N 111MW 355 43 6H 27 17 78HN 90 E M 8 5 * Graham Age: Late Triassic — — MEHBER OF 38 36 N 111HW 356 33 — — 17 — — — b a Graham, 1955 Sampling: 20 specimen measurements were made on CHINLE FOR- b Kintzinger, 1957 17 oriented samples taken over a lateral extent MATION of 20 feet and a stratigraphic thickness of 5 feet. (sediments) Entry (37) was based on data scaled from Fig. 7, Ref. a. 3 widely scattered specimen measurements were excluded. Reversals: 4 specimens are reversely magnetized. _ _ _ CHINLE 39 27H 5 8 78 N 16 E M 27H 27H * \ Sampling: These values were calculated by applying AVERAGE — Fisher statistics to the pole positions cited in entries (25), (26), (28), (30), (32), (34), (36), and (37). These include the poles for the equivalent Redonda for- mation and the Shinarump member of the Chinle formation. Other: Owing to "smearing" and instability at some sites (see above) the mean pole position cited here is probably too far north. The confidence intervals for the data used in this computation vary widely, and the value of ags listed is therefore of doubtful significance. ^ CHUGWATER Collinson and Age: Cited as Triassic in Ref. a; generally taken as FORMATION Runcorn Triassic and Permian (sediments) | Troublesome 41 42 N 106H W 134 -12 7 10 43 36 N 135 E N 7 4 a a Collinson and Sampling: 43 specimen measurements were made on Creek, Runcorn, 1960 10 oriented samples. Wyoming § Alcova i 42 42HN 1°7 W 152 -18 8 10 48 49 N 118 E N 8 4 a Sampling: 48 specimen measurements were made on Reservoir, 10 oriented samples. Wyoming Sheep Mtn., 43 44HN 108 W 148 -6 3 58 35 40 N 115 E N 3 2 a Sampling: 35 specimen measurements were made on Wyoming 9 oriented samples. 44 44HN 107 H W 155 -23 7 16 16 — — N — a Sampling: 5 specimen measurements for the normal 3 — * o 45 44H N 107H W 155 -23 7 16 j 16 SOHN 113 E N 8 4 group were made on 1 oriented sample. 16 specimen 5 _ — — measurements for the reversed group were made a Shell, Wyoming •46 44HN 107H W 340 36 — — S — a > — 60 N — * on 4 oriented samples. Entry (48) is an average 47 44HN 107 H W 340 36 — 5 110HE S — H — — 4 for the normal and reversed groups. 48 44H N 107H W — — — 56 N 113 E M 8 a 49 43 N 109 W 146 6 5 34 28 N a Sampling: 25 specimen measurements for the normal 50 43 N 109 W 146 6 5 34 28 34HN 114 E N 2H group were made on 6 oriented samples. 28 speci- Fort Washakie, 51 43 N 109 W 346 14 6 22 25 S a men measurements for the reversed group were Wyoming ~1 52 43 N 109 W 346 14 6 22 25 51HN 94HE S 3H * made on 5 oriented samples. Entry (53) is an aver- age for the normal and reversed groups. 53 43 N 109 W — — — — 44 N 105 E M 5 3 a — — — — 'S4 43HN 109H W 154 -20 7 17 22 N — a Sampling: The 22 specimen measurements for the Dinwoody 55 43HN 109H W 154 -20 7 17 22 SON 112 E N 8 4 * reversed group were made on 6 oriented samples. Lake, 56 43H N 109H W 339 25 6 30 20 — S — — a The 20 specimen measurements for the normal Wyoming 57 43HN 109H W 339 25 6 30 20 55 N 107 HE S 6H 3 H * group were made on 4 oriented samples. Entry — — — (58) is an average for the normal and reversed groups. 58 43HN 109H W — 52 N 110 E M 5 3 a — — — 59 42H N 108H W 157 15 9 8 37 N — a Sampling: The 37 specimen measurements for the 60 42HN 108H W 157 15 9 8 37 35 N 99 E N 9M * reversed group were made on 8 oriented samples. Lander, 61 42MN 108H W 344 27 4 55 12 S a The 12 specimen measurements for the normal Wyoming * 62 42HN 108H W 344 27 4 55 12 58 N 101 E S 5 lH group were made on 2 oriented samples. Entry (63) 63 42HN 108H Wl — — — — — 47 N 100 E M 8 4 a is an average for the normal and reversed groups. TABLE 1.—Continued Locality Magnetic Direction Pole Position Rocks Sampled No. References Remarks Lat. Long. Decl. Incl. cm k N Lat. Long. P Sm dp S SECTION F— Continued Q 64 43*4 N 108 W 170 -19 11 13 15 N — a Sampling: 18 specimen measurements for the normal rN — — — 65 43*4 N 108 W 170 —19 11 13 15 54HN 90 E N 6 * group were made on 3 oriented samples. 15 speci- £> Thermopolis, UH 66 108 W 332 16 5 56 18 S a men measurements for the reversal group were -^ Wyoming 67 43*4 N 108 W 332 16 5 56 18 46HN 114 E S 5H 3 * made on 3 oriented samples. Entry (68) is an average ^ 68 43*4N 108 W — — — 52 N 103 E M 7 4 a for the normal and reversed groups. O — 69 42 N 149 — 107*4 W -19 6 25 21 — — N — — a Sampling: 21 specimen measurements for the reversed Q 70 42 N 107*4 W 149 -19 6 25 21 47*4N 122 E N 7 3H * group were made on 6 oriented samples. 12 speci- ^H '. 71 42 N 107*4 W 328 12 4 43 12 S a men measurements for the normal group were made t""1 Wyoming — — — — 72 42 N 107*4 W 328 12 4 43 12 44 N 120*4E S 3H 2 * on 3 oriented samples. Entry (73) is an average J. 73 42 N 107*4 W 46 N 121 E M 5 3 a for the normal and reversed groups. j> — — — — 74 41 N 106 W 151 -—6 6 12 52 — N a Sampling: 52 specimen measurements for the reversed t"1 — — — 75 41 N 106 W 151 -6 6 12 52 44N 116 E N 6 3 * group were made on 13 oriented samples. 16 speci- Q R d Mt _ 76 41 N 106 W 335 15 6 30 16 — S — a men measurements for the normal group were made •<* Wyoming«7 " ' — 77 41 N 106 W 335 15 6 30 16 SON 114 E S 6^ 3 * on 3 oriented samples. 2 samples were not used J> 78 41 N 106 W — — 47 N 116 E M 5 3 a in the calculations. Entry (78) is an average for 2 — — — the normal and reversed groups. M _ _ _ CHUGWATER 79 5 57 17 48 N 112 *4E M 5 5 * Sampling: These calculations were made by applying c/5 AVERAGE — Fisher statistics to the pole positions given for S each site; reversed groups are considered as separate sites. The 17 entries used are (41), (42), (43), (45), (47), (50), (52), (55), (57), (60), (62), (65), (67), (70), (72), (75), and (77). MOENKOPI Age: Early to Middle (?) Triassic FORMATION (Sediments) _ Sampling: 18 oriented samples were collected over [80 36 N 111*4W 0 27*4 — — — — — — — a Runcorn a lateral extent of 250 miles. Entry (81) was based Zion Nat. Park * \81 36 N 111*4W 1*4 28 13 9 16 69 N 64 E S HH 8 a Runcorn, 1956a on mean directions of 16 samples (Table 6, Ref. a); two samples with widely divergent directions were not included. (82 37 N 111WW 325 35 11 a Kintzinger Sampling: 11 oriented samples were collected over a Marble Canyon 83 37 N 111HW 340 28 — — 11 _ — j _ — — a a Kiutzinger, 1957 lateral extent of 1 mile and a stratigraphic thick- [84 37 N 111HW 345 29 Yi 15H 11 11 65 N 105 E JS 17 9H * ness of 230 feet. The values cited for entries (82) and (83) appear in a table and were scaled from Fig. 1, respectively, in Ref. a. Entry (84) was based on data scaled from Fig. 1 in Ref. a. Echo Cliffs, 86 37 N 111J.4 W 349 28 6 17 42 66 N 95 E S 6 4 a Collinson and Sampling: The statistical analysis was based on 42 Arizona Runcorn specimen measurements made on 18 oriented sam- a Collinson and ples; 6 specimen measurements on 4 samples were Runcorn, 1960 discarded. Poverty Tank, 87 36 N HIM W 337 36 7 22 27 64 N 127 E S 8 5 a Collinson and Sampling: The statistical analysis was based on 27 Arizona Runcorn specimen measurements made on 10 oriented sam- a Collinson and ples; 3 specimen measurements from 1 sample Runcorn, 1960 discarded. Vernal, Utah 88 40HN 109H W 158 -4 6 18 38 46 N 103 E N 6 3 a Collinson and Sampling: The statistical analysis was based on 38 Runcorn specimen measurements made on 8 oriented sam- a Collinson and ples. 3 specimen measurements from 1 sample were Runcorn, 1960 discarded. ^ Split Mtn., 89 40HN 109 W 156 -4 6 28 23 46 N 106 E N 6 3 a Collinson and Sampling: 23 specimen measurements were made on t"* Colorado Runcorn 5 oriented samples. S a Collinson and g Runcorn, 1960 Sand Canyon, 90 40HN 109 W 148 — 7 9 10 27 43 N 118 E N 9 4 a Collinson and Sampling: The statistical analysis was based on 27 O Colorado Runcorn specimen measurements made on 7 oriented sam- 2 a Collinson and pies. 3 specimen measurements from 1 sample were h-j Runcorn, 1960 discarded. ^ _ _ _ _ MOENKOPI 91 12M 20 8 62 N 103 E M 12H 12 H » Sampling: These calculations were made by applying 2 AVERAGE Fisher statistics to the pole positions of entries H (81), (84), (86), (87), (88), (89), and (90). Data > on partially unstable Capitol Reef outcrop (Ref. a) not included. _ CONNECTICUT 92 42 N 73 W 10 14 11 8 54 N 90 E S 8 6 a Du Bois et al. Age: Cited as Triassic VALLEY 93 42N 72HW 10 14 15 k' 3 54 N 90 E S 15 7H * a Du Bois, Irving, Sampling: 8 oriented samples were collected from 3 MASSACHU- Opdyke, Run- flows. Entry (93) is a minimum value of a»s based SETTS LAVAS corn, and on 3 points in time (3 flows) and a "secular varia- Banks, 1957 tion precision" k' = 30. _ CONNECTICUT 94 42 N 73 W 12 14 15 12 55 N 88 E 15 8 a Du Bois et al. Age: Cited as Triassic in Ref. a — 12 14 3 — 32 41 N — LAVAS AND 95 — 91 E M — b a Du Bois, Irving Sampling: 12 oriented samples were collected (Ref. a); SEDIMENTS 96 42N 73 W 12 14 15 7 12 53 N S6E M 15 7H * Opbyke, Run- 32 specimens were measured (Ref. b). The value corn, and of k for entry (96) was calculated by the approxi- Banks, 1957 mate formula. d Du Bois, 1957 Reversals: A plot in Ref. b shows that about half the specimens are reversed. H- TABLE 1.—Continued Locality Magnetic Direction Pole Position Rocks Sampled No. References Remarks Lat. Long. Decl. Incl. an k N Lat. Long. P 6m ip i SECTION F— Continued BRTJNSWICKIAN 97 41 N 75 W 6 28 3 — 71 63 N 93 E S 6 3 a Du Bois et al. Age: Late Triassic, top of the Newark group; referred FORMATION* 98 40HN 75 W 2 22 3 86 70 61 N 102 E s 3 2 b a Du Bois, Irving, to as "Newark formation" in Ref. b. (sediments) Opdyke, Run- Sampling: 71 specimen measurements were made on corn, and 21 oriented samples (Ref. a). n Banks, 1957 b Collinson and Runcorn, 1960 NEW OXFORD 99 39HN 77HW — — — 63 N 158 E S a Graham Age: Late (?) Triassic, bottom of the Newark group FORMATION 100 39HN 77HW 334 48 6M 36 13 66N 174 E s 8H 5H * a Graham, 1955 Sampling: 13 oriented samples were collected over a o (sediments) lateral distance of 300 feet and a stratigraphic thickness of 20 feet. Values for entry (100) were based on data scaled from Fig. 7, Ref. a. One sample I is not included. The pole position for entry (99) was scaled from Fig. 6, Ref. a. _ _ NOVA SCOTIA 101 _ 5H 74 77HN 72 E 5H 5H a Bowker Age: Cited as Triassic in Ref. a LAVAS 102 11H 14 12 77N 75HE 11M 11H * a Bowker, 1959 Sampling: 217 specimen measurements were made on — — — — — (letter of Nov. 16, 74 oriented samples collected at 12 sites. The values 1959 to Doell) for entry (102) were based on averages for each § site. In both entries, the statistical calculations were made on pole positions rather than magnetic directions. Stability: A.C. partial demagnetization decreased the scatter. DINOSAUR CAN- 103 37 N 111HW 21 30 — — — — — a Kintzinger Age: Late Triassic (?); younger than the Chinle for- YON SANDSTONE 104 37 N niHW 13!^ 27 — — — — — — — a a Kintzinger, 1957 mation MEMBER OF 105 37 N 111HW 17 31 11 1—2 17 65 N 28 E s 12 6H * Sampling: Sampling extent was over 1 mile laterally THE MOENAVE and 80 feet stratigraphically. Values for entry (103) FORMATION are from the table in Ref. a, and entry (104) was scaled from the plot in the same Ref. Entry (105) is based on individual sample data scaled from Fig. 1 in Ref. a. f Editor's Note: Data for No. 98 withdrawn from Ref. b in final version of manuscript. i . AUSTRALIA _ _ TASMANIAN VOL- 106 42S 147 E -81H 17 42N 33 W M >16 >16 * Almond et al. Age: The tuffs underlie the Tasmanian dolerite and CANIC TUFFS a Almond, Clegg, are Triassic (?) ! and Jaeger, Sampling: 20 feet of azimuthally unoriented core was i 1956 sampled. The inclination indicated is an average i of data in Table 4, Ref. a. Other: Since / = 81H° corresponds to latitude 74°, the pole should be roughly 16° away from the sam- pling area. Reversals: 2 of the 17 samples have reversed polarity. _ _ _ BRISBANE TUFF 107 27HS 153HE 35 -83 39 N 37 W M a Irving and Green Age: Cited as probably Early Triassic — — a Irving and Reversals: Reversals are noted, but no details are 1 Green, 1958 given. •TJ AFRICA 1 J-H BECHUAXALAND 108 — — 326 -16 9K 6 44 54 N 79 W S — — a Nairn Age: Late Triassic, immediately underlying the Karroo S CAVE SAND- !l09 23 S 27 E 326 -16 9H 6 44 54 N 44 W S 10 5 b a Nairn, 1957a basalts (Du Toit, 1953, p. 301) g STONE b Creer, Irving, Sampling: 44 specimen measurements were made on j> Nairn and 8 oriented samples. O Runcorn, 1958 Other : Con temporaneous sedimen ts from ad jacen t ±j !1 regions have scattered directions. ^ o H TABLE 1.—Continued 8 Locality Magnetic Direction Pole Position Rocks Sampled No. References Remarks Lat. Long. Decl. Incl. an k N Lat. Long. P &m IP S SECTION G JURASSIC EUROPE NORTH YORK- 1 . 3H 67 88 36 . a Nairn Age: Early Jurassic (middle Lias-lower Corallian) 2H — SHIRE SEDI- 2 54 N 1 W 3H 67 4H 88 36 S5N 150 E S 7^ 6 * a Nairn, 1956 Sampling: 36 specimen measurements were made on MENTS b Nairn, 1957c 6 oriented samples. These were later rejected as unstable (Ref. b), because MIDFORD SANDS 6 51 HN 2MW 103H 70 7H 10 42 S3N 41 E S 13 11 a Girdler Age: Early Jurassic (upper Lias) 2 a Girdler, 1960 Sampling: 42 specimen measurements were made on ^ 20 oriented samples collected at 3 sites separated g by more than 1 mile. Stability: Scatter was reduced by application of Graham's fold test. Specimens were remeasured after 6 months with no change in magnetization. Other: Samples at nearby sites were unstable. COTSWOLD 7 S1MN 2HW 262VJ -64 10 7 38 38 N 5»HE N ISH 11H a Girdler Age: Early Jurassic (upper Lias); older than the SEDIMENTS a Girdler, 1960 Midford sands Sampling: 38 specimen measurements were made on 17 oriented samples collected at 2 sites. Stability: Specimens were remeasured after 6 months with no change in magnetization. PYRENEES VOL- 8 43 N i 1HE ss 60 6 IT I 38 49 N 76HEJ S 8H 6J4 a Girdler Age: Early Jurassic (lower Lias), on the basis of inter- CANIC ROCKS | a Girdler, 1960 cola ted limestone beds 1 Sampling: 38 specimen measurements were made on i 18 oriented samples collected from 4 sites separated by more than i mile. Stability: Correction for rather uniform dips up to 40° did not significantly reduce the scatter. Other: The value of ALPINE RADIO- 9 47MN 12HE 36H 48 5)4 100 21 58)4N 128 E S a Hargraves and Age: Cited as Middle Jurassic * LARITE 10 47HN 12HE 36H 48 5)4 100 21 56N 122HE S 7 4^ Fischer Sampling: 21 specimen measurements were made on a Hargraves and 15 oriented samples taken at one site over a strati- Fischer, 1959 graphic thickness of 7 meters. ALPINE LIME- H 47HN 12)4E 48 50)2 6)4 71 30 53 N 112 E S — a Hargraves and Age: Cited as Early Jurassic STONE 12 47MN 12HE 48 50)4 6)4 71 30 SON 109ME S 9 6 * Fischer Sampling: 30 specimen measurements were made on a Hargraves and 16 oriented samples taken at 1 site over a strati- Fischer, 1959 graphic thickness of 4 m. § g NORTH AMERICA I KAYENTA Age: The Kayenta is Early Jurassic (?) (Glen Canyon g FORMATION _ group) . n — — 353 9H 50 6 — S _. a Runcorn Central (13 43)4 — Arizona \14 35 N Ill W 353 43)4 9)4 50 6 79 N 102 E S 12 7H * a Nairn, 1956 Kayenta 15 36)4 N 110H W 20 50 6 14 43 72 N 6 W S 8 6 a Collinson and Sampling: 43 specimen measurements were made on Arizona Runcorn 8 oriented samples. 52 a Collinson and Runcorn, 1960 Echo Cliffs, 16 37 N 111)4 W 14 43 6 14 43 73 N 19 E S 7 5 a Collinson and Sampling: 43 specimen measurements were made on Arizona Runcorn 14 oriented samples. a Collinson and Runcorn, 1960 Navajo /17 36)4 N 111 W 335 50 11 7 27 — S — a Collinson and Sampling: 27 specimen measurements were made on — — National \18 36)4 N 111 \V 335 50 11 7 27 68)4 N 151 E S 15 10 » Runcorn 7 oriented samples. Monument a Collinson and Runcorn, I960 Kanab, Utah 19 37 N 112)4 W 4 53 9 12 33 85 N 23 E S 13 9 a Collinson and Sampling: The statistical analysis was based on 33 Runcorn specimen measurements on 8 oriented samples. 6 a Collinson and specimen measurements from 2 oriented samples Runcorn, 1960 were discarded. The circle of confidence includes the present dipole direction. TABLE 1.—Continued Locality Magnetic Direction Pole Position Rocks Sampled No. References Remarks Lat. Long. Decl. Incl. OC95 k N Lat. Long. p 6m 5p 5 on _ _ _ _ SECTION G— Continued !*! KAYENTA 20 21 20 4 83 N 63E S 21 21 * Sampling: These values were computed by applying !> AVERAGE Fisher statistics to the pole positions for entries 2 (14), (15). (16), (18), and (19). ° O CARMEL FORMA- 21 38WN 109H W 319 63 9 10 31 SON 160 W S 14 11 a Collinson and Age: Cited as Jurassic in Ref. a ^ TION (sedi- Runcorn Sampling: 31 specimen measurements were made on f ments) a Collinson and 9 oriented samples. V Runcorn, 1960 AFRICA 4 >r* W KARROO Graham and Hales Age: The dolerites were emplaced during and some- Q DOLERITES a Graham and what after extrusion of the Karroo basalts (Du Toil, gj Hales, 1957 1953, p. 369) . Age is Late Triassic (Walker and Polder- > vaart, 1949, p, 598-599) or Early Jurassic (Du Toit, 2 1953, p. 370). M Winkelhaak 22 26HS 29 E 173 58 S 17 77 N 127 W N 7 6 a Sampling: 17 oriented samples were taken in mine H — Upper sill shafts. A 75-foot thickness of the sill was sampled at gj 4 sites a maximum of 1 mile apart. § Winkelhaak 23 26HS 29 E 307 -63 12 — 8 44 N 98 W S 18 14 a Sampling: 8 oriented samples were taken in mine Lower sill shafts. A 50-foot section of the sill was sampled at 4 sites a maximum of 1 mile apart. Estcourt re- /24 29 S 30 E 167 51 6 — 15 77 N 90 W N 8 6 a Sampling: 15 oriented samples were collected in a versed dol- \25 29 S 30 E 157 51 6 15 79 N 79 W N 8 6 * tunnel about 1 mile long. erite — Estcourt nor- /26 29 S 30 E 331 -64 g 9 67 N 86 W S 12 10 a Sampling: 9 oriented samples were collected in a tunnel — mal dolerite \27 29S 30 E 331 -64 8 — 9 62 N 105 W S 13 10 * about 1 mile long. _ _ _ _ DOLERITE 28 20 23 4 66N 101HW M 20 20 * Sampling: These values were computed by applying AVERAGE Fisher statistics to the pole positions of entries (22), (23), (25), and (27). ESTCOURT BAKED Graham and Hales Sampling: These samples were baked by the reversed SEDIMENTS a Graham and and normal dolerites at Estcourt, respectively, and — — — — Reversed 29 29 S 30 E 180 55 11 — 8 — a Hales, 1957 were collected in the same tunnel. They are in good Normal 30 29 S 30 E 324 -62 7 — 7 — — — — — a agreement with the dolerite samples in both cases. _ KARROO SURFACE 31 30HS 28BE 352 -62 12 33 76 N 128 W M 19 14 a Graham and Hales Sampling: 33 oriented samples were collected over an SAMPLES a Graham and area 250 by 400 miles. Hales, 1957 Reversals: 8 of the 33 samples had reversed polarity. KARROO BASALTS 32 330 -40H 4H 19 44 61 N 76 W S a Nairn Age: The Karroo basalts are the top of the Stonnberg — -40 44 — — — b 33 — 332 4H 19 — — a Nairn, 1956 series immediately overlying the Cave sandstone. 34 18 S 26 E 332 -40 5 19 44 63 N 78 W S 6 2H c b Nairn, 1957a The age is Late Triassic (Walker and Poldervaart, 35 18 S 26 E 328 -40 4H 19 44 60 N 77 W S SB 3 d c Creer, 1958 1949, p. 598-599) or Early Jurassic (Du Toit, 1953, 36 US 26 E 332 -40 9H k' 7 63B N 79 W S 11 H 7 * d Creer, Irving, p. 370). 1 Nairn, and Sampling: 44 specimen measurements were made on 11 Runcorn, 1958 oriented samples collected from 6 or 7 flows. Values 1 for entry (36) are based on the "secular variation precision" k' = 30 and N = 7, the number of flows. Other: The pole position at latitude 2 N, longitude 8 W S mentioned in Ref. b apparently is a misprint. SOUTH AMERICA I Q PARANA BASIN Creer Age: Exact age within the Mesozoic is not known BASALTS _ a Creer, 1958 f37 29 S 57 W 348 —47 2 233 22 — — a Sampling: The 48 specimen measurements (normal and Normal group — — * 38 29 S 57 W 348 -47 2 233 22 SON 146 W S 2B Hi reversed) were made on 12 oriented samples collected 39 29 S 57 W 174 39 7 18 26 — — — — — a at 4 sites. Reversed group 40 29 S 57 W 174 39 7 18 26 81 N 96 W N 8Vi 5 * [ Stability: The scatter in directions was considerably 41 29 S 57 W 351 -42 2 82 48 — — — — a decreased by partial demagnetization in A.C. fields Basalt average — * 42 29 S 57 W 351 -42 2 82 48 81 N 118 W M 2B 1H of 250 oersted. TACUAREMBO Creer Age: These sandstones were baked by the Parana Basin BAKED SAND- a Creer, 1958 basalts. Thus the age of their magnetization within STONE _ the Mesozoic is not known. 43 29 S 57 W 357 -42 3 25 71 — — — a Sampling: The 81 specimen measurements (normal and Normal group — 44 29 S 57 W 357 -42 3 25 71 84 N 96 W S 4 2 * reversed) were made on 12 oriented samples collected /4S 29 S 57 W 176 43 14H 12 10 — — — a at 4 sites. Reversed group — — * 146 29 S 57 W 176 43 14H 12 10 7_7 N 132 W N 18 11 Sandstone 47 29 S 57 W 356 -43 3M 22 81 — — — — a 48 29 S 57 W 356 -43 3B 22 I 81 102 W M 4 * Average 1 84H N 2B TABLE 1.—Continued Locality Magnetic Direction Pole Position Rocks Sampled No. References Remarks Lat. Long. Dec!. Incl. an k ff Lat. Long. p Sm sp 5 SECTION G— Continued PARANA BASALT 49 29 S 57 W 354 -43 2H 24 129 83 N 126 W M 3 1H a Creer Sampling: For entry (49) Fisher statistics were applied AND SANDSTONE 50 29 S 57 W 354 -43 5 k' 24 83HN 112 W M 6H 4 * a Creer, 1958 to the 129 specimen measurements (basalts and 51 29 S 57 W 354 -43 7H k' 12 83HN 112 W M 9 5H * sandstones) made on the 24 oriented samples (Ref. 8 52 29S 57 W 354 -43 12H k' 4 MMN mw M 16 9^4 * a). Entries (50), (51), and (52) are based on the "secular variation precision" k' = 30 and the 3 fol- lowing assumptions about the number of different points in time that were sampled. Entry (50) : the 12 sandstone samples were baked by 12 different flows, and in addition 12 other lava flows were also sampled. H Entry (51): the sandstone samples were baked by 12 £ different flows, and in all cases both a now and its baked sediment were sampled. Entry (52): only 1 lava flow and its associated baked sediment were sampled at each of the 4 sites. Ref. a does not specify the number of flows sampled. t § > SECTION H CRETACEOUS O EUROPE _ WEALDEK 1 345 63 a Wilson Age: Early Cretaceous — — — — — — — — SEDIMENTS 2 SOHN 1HW 345 63 2 260 19 79N 117 W S 3 —2 * a Wilson, 1959 Sampling: 97 specimen measurements on 19 oriented samples collected over a lateral extent of 4 miles were made. Mean sample directions were used in the statistical analysis, and the data for entry (2) were calculated from other statistical parameters reported in Ref. a. Stability: Correction for small-amplitude folding re- duced scatter. Partial A.C. demagnetization at 180 oersted reduced scatter. Sands and clays of different lithology and different intensities had closely parallel directions. Other: 27 additional cretaceous samples were rejected as unstable. NORTH i AMERICA I DAKOTA SAND- 3 34 N no w 164 -62 10 76H 127 E N 11 8H a Runcorn Age: Lies above the Trinity group; Early(P) and Late STONE 4 34N now 169 | -57 15 17 5 SON 176 W M 21 H 16 * i a Runcorn, 1956a Cretaceous l Sampling: 6 samples were collected over a lateral distance of about 1 mile. The analysis of entry (3) is based on 10 specimen measurements made on 3 of the 6 oriented samples collected; all 10 specimens have reversed polarity. Values for entry (4) are based on average sample-direction data obtained from Table 8, Ref. a; one divergent sample was discarded. AUSTRALIA TASMANIAN Age: The dolerite sills cut Late Triassic (possibly Early DOLERITES Jurassic) rocks and are involved in early Tertiary ^ faulting; they therefore are Jurassic or Cretaceous. t""1 Assignment to the Jurassic is favored in Ref. b. 9 Surface samples & 42 S 147 E 325 -85 4 48 30 SON 23 W S 8 8 a Irving Sampling: 2 samples were collected at each of 30 sites S* a. Irving, 1956b covering an area of more than 9000 square miles. The J> b Irving, 1959 mean site directions were used in the analysis. Q Stability: Stability is indicated by the application of S Graham's conglomerate test to Tertiary breccia 1-3 containing fragments of the dolerite. Q 6 42 S 147 E -85H 57 M a Almond et al. Sampling: 3 cores were sampled over the depths 1-947 Core samples ! — * 42S 147 E -85H 57 42N 33 W M >9 >9 a Almond, Clegg. feet, 8-152 feet, and 500-636 feet, respectively. The Ejj 1* — — — and Jaeger, 1956 cores were vertical, but azimuthally unoriented. H} Other: Since / = 85H° is appropriate to latitude 81°, >* the pole should be roughtly 9° from the sampling area. Reversals: About one-fifth of the samples near the bottom of one core had reversed polarity. TABLE 1.—Continued Locality Magnetic Direction Pole Position Rocks Sampled No. References Remarks Long. Lat. Long. 5m Lat. Decl. Incl. a« * N P Sf s SECTION H—Continued INDIA o UPPEK RAJMAHAL 8 25 N 88 E 328 -64 , 33 13 N 70 W S a Clegg et al. Age: The lower Rajmahal traps are interbedded with TEAPS 9 25 N 88 E 327 -64 4 36 33 13 N 69 W S 6H 5 * a Clegg, Rada- sediments of Early to Middle Jurassic age (Krish- 10 25 N 88 E 327 -64 10H *' 6 13 N 69 W s 17 13 * krishnamurty, nan, 1956, p. 273). The upper part of the traps are and Sahas- undated but petrologically similar to post-lower rabudhe, 1958 Cretaceous Deccan traps (Hobson, quoted in Pascoe, 1929, p. 146). Assignment to the Jurassic is favored in Ref. a Sampling: The samples came from the upper 250 feet of the traps. 33 oriented samples were collected from 3 quarries about 20 miles apart. Samples from 2 additional badly weathered outcrops had scattered directions of magnetization. Entry (9) is based on data scaled from Fig. 1, Ref. a. Entry (10) is based on the "secular-variation precision" k' = 30 and an estimate of the number of flows. AFRICA g _ » MADAGASCAX 11 — — 13 H 32 5 58HN 162HW s 13)^ 13H Roche et al. Age: Cited as Turonian stage of the Cretaceous § LAVAS AND 12 — — — — 68 N 168 W s b a Roche, Cattala, Sampling: Entry (11) was obtained by a Fisher statis- — — — — — DYKES IS 9 31 10 66HN 163HW s 9 9 * and Boulanger, tical analysis of 5 pole positions calculated from — — — — 1958 mean directions of magnetization of 5 sites (p. 2923, b Roche and Ref. a). The 5 sites span a distance of 500 miles. Cattala, 1959 Entry (13) was based on pole positions corresponding to the mean directions at 10 sites (p. 1050, Ref. b) ; these include (?) the data given in Ref. a. Entry (12) was found by grouping the results according to sampling area and averaging the results. Stability: Stability was checked by repeated measure- ments (Thellier's test) and by partial heat demagnet- ization. SECTION H—Continued JAPAN INKSTONE RED Nagata et al. Age: Cited as Middle to Early Cretaceous SHALES a Nagata, Ald- Sampling: 24,15, and 23 oriented samples were collected 81 24 I — | — [ S — | — moto, Shimizu, at Okoti, Imoziya, and Okuhu, respectively. Okoti 34HN 131HE 81 54 24 ! 25HNJ 164V4WI S | 5u! 4 Kobayashi, and Other: The between-site dispersion is much greater 50 46 S Kuno, 1959 than within sites. The value of «»s cited in entry Imoziya 34HN SO 46 82 47 N 143HW S (20) is therefore probably much too small. The value 43 46 of k for entries (15), (17), and (19) was calculated by Ohuku 34HN 43 46 52MN the approximate formula. Combined 20 58 50 42N 153 W SECTION I EOCENE EUROPE •-d LUNDV^DVKES 197 Blundell Age: Not known but assigned to the early Tertiary >1 64 75 N i 129 H E N 10 H a Blundell, 1957 (Ref. a) because the magnetic direction is similar to tr 51 N 4HW 194 W that for other lower Tertiary rocks from the British O Isles g Sampling: At least 2 samples were taken from each of > 13 dikes and 2 dike contact rocks. 5 dikes and the 3 O contact rocks were stable. Entry (2) was based on data from Table 1, Ref. a. Stability: Directions of magnetization were remeas- O ured after 1 month and had not changed. ARRAN DYKES — I 10 '• 79 N 154 E M ! 15 Leng Age: Cited as early Tertiary in Ref. a a Irving, 1959 MULL INTRUSIVE 56HN 6 W 58 ! 14 20 7 72 N 173 E I M | 21 15H Vincenz Age: Early Tertiary, post Mull lavas and pre-North- ROCKS — — 72 N 139 E I M i 14 a Vincenz, 1954 west Dikes b Irving, 1959 Sampling: An average of about 9 samples was taken from each of 12 intrusive bodies. Only 7 of these bodies could be shown to possess a stable magnet- ization. The maximum separation of the sites was about 6 miles. Data for entry (4) were taken from Table I, Ref. a. Reversals: 2 of the 7 stable bodies showed reversed polarity. to TABLE 1.—Continued 00 Locality Magnetic Direction Pole Position Rocks Sampled No. References Remarks Lat. Long. Decl. Incl. 096 k N Lat. Long. P Sm ip 5 SECTION I— Continued MULL LAVAS 6 56&N 6 W 137 -78 23H 16 — — — b Bruckshaw and Age: Early Tertiary, probably Eocene (Ref. a) but may — — — 7 56HN 6W 154H -WS 17H 11 S 76HN 74 W N SlH 28« * Vincenz be Oligocene or even Miocene (Ref. b). 8 — — — — 8 82 N 71 W N 31 28 c Sampling: An average of about 7 samples was taken — — a Bruckshaw and Vincenz, 1954 from each of 16 flows at 3 sites. 8 of the 16 flows could o b Hospers, 1955 be shown to have stable magnetizations, and the c Irving, 1959 data for entry (7) were for these stable flows, cited in Table I, Ref. a. Entry (6) is based on the data for both stable and unstable flows. _ „ ANTRIM BASALTS 9 55 N 6HW 194 -60 5H 24 — N a Hospers and Age: The interbasaltic zone covering these basalts is 10 55N 6HW 1M -60 5H 31 24 7JN 135 E N 8H 6H * Charlesworth "probably Eocene or Oligocene but may be later" 11 — 194 -60 — — 75 N 118 E N — b a Hospers, 1955 (Charlesworth, as reported in Ref. a). Simpson — — — 12 55 N 6W 194 -60 5 — 74 N 133 E — 8 6 c b Creer, Irving, (reported in Ref. a) considers this zone to be late — and Runcorn, Miocene or early Pliocene. 1954 Sampling: 6 flows at each of 3 sites and 5 at a fourth c Irving, 1956a site were sampled; 2 to 4 samples were taken from d Hospers and each flow. The sites were about 30 miles apart Charlesworth, (Ref. d). 1954 ANTRIM BASALTS _ _ Wilson Age: Generally regarded as Eocene, but comparison of Lower olivine /13 — 173 — — — — a a Wilson, 1959 Scottish and Irish pollens suggests (Ref. a) an age as — -64H — — basalts \U 55 N 6W 173 -64H 9H 13 19 SON 162 W N 15 12 * young as Miocene Middle tholei- (is — — 206 -62 — — — — — — — a Sampling: Ninety samples were collected from 57 — itic basalts 55 N 6W 206 -62 16 18 6 69HN 108 E N 25 19 • igneous bodies at 47 sites spanning about 75 miles. Intrusive r — — 184 -63H — — — — — — a The remanent magnetizations of one core from each in — — bodies 55 N 6 W 184 -63H 8H 20 16 79HN 157 E N 13 10H * sample and of the entire sample were both measured, r — 183 -64 — — • • — — — — -mL- J !l9 a and the mean of these two directions used in the sta- Cora Dined — — — \M 55N 6W 1S3 -64 6 16 41 MHN 162 E N 9H TH » tistical analysis. Three groups of basalt were studied: older "lower" olivine basalts (19 flows, entries 13 and 14); younger "middle" tholeiitic basalts (6 flows, entries 15 and 16); and intrusive rocks of un- known relative age (16 igneous bodies, entries 17 and 18). Entries (19) and (20) combine all these data. Statistical data were calculated from other statistical parameters reported in Ref. a. 1 1 NORTH AMERICA 1 _ SILETZ RIVER 21 — 70 55 7 — 8 37 N SOW M a Cox \ Age: Early middle to early Eocene age established by VOLCANIC 22 45 N 123H W 70 55 7 50 8 37 N 49 W M 10 7 * a Cox, 1957 interbedded fossiliferous sediments ROCKS Sampling: 57 samples were taken from 8 flows over a lateral distance of 38 miles; the value of k was es- timated by the approximate formula. Stability: Heat demagnetization and application of Graham's fold test decrease scatter. Reversals: 5 flows indicate normal (S) poles, and 3 in- dicate reversed (N) poles. LANEY SHALE |23 86 N 164 W S 10 8 b Torreson et al. Age: Cited as Eocene in Ref. a — — MEMBER OF \24 41HN 109HW 353H 62H 6 30 19 85 N 170 W S 9H 1% * a Torreson, Sampling: 21 oriented samples were collected. 2 of these GREEN RIVER Murphy, and were too weakly magnetized to measure. Data for FORMATION Graham, 1949 entry (24) were taken from Table 4, Ref. a. b Irving, 1959 Stability: The circle of confidence includes the presen pole, thus there is no indication of stability. i GREEN RIVER 25 39MN 108 W S45H 65 4H 168 7 77V4 N 158 W s 7 6 * Torreson et al. Age: Cited as Eocene in Ref. a FORMATION a Torreson, Sampling: 7 out of 9 oriented samples collected were o (sediments) Murphy, and strong enough to be measured. Data for entry (25) 2; Graham, 1949 were taken from Table 4, Ref. a. H 2 O WASATCH FORMA- 26 44HN 109 W 351H 63W 17 30 4 84H N 180 E s 26H 21 * Torreson et al. Age: Cited as Eocene in Ref. a TION (sedi- a Torreson, Sampling: 4 oriented samples were collected. Entry a ments) Murphy, and (26) is based on data from Table 4, Ref. a. Graham, 1949 Stability: No stability is indicated since circle of con- fidence includes present pole. Other: 5 additional samples from the Wasatch at Gard- ner, Colorado, showed random directions. AUSTRALIA OLDER VOLCANIC 27 38 S 14SHE 17 -73 7 35 15 67 N 57 W M 12 11 a Irving and Green Age: Cited as early Tertiary and probably Eocene in ROCKS or a Irving and Ref. a VICTORIA Green, 1957 Sampling: 3 oriented samples at each of 15 sites were collected. The sites cover an area of approximately ! 5000 square miles. Reversals: 9 sites are normal (S poles), 4 sites are re- versed (N poles), and 2 sites are mixed. TABLE 1.—Continued Locality Magnetic Direction Pole Position Rocks Sampled No. References Remarks Lat. Long. Decl. Incl. _ _ SECTION I— Continued TASMANIAN 28 42S 147 E -83 8 42N 33 W S >13H >13H « Almond et al. Age: Cited as early Miocene or Oligocene on basis BASALTS — a Almond, Clegg, of sediments associated with basalts (Ref. a) ; how- and Jaeger, 1956 ever, Banks (1958, personal communication) believes age may be Eocene. Sampling: 8 cores not oriented with respect to azimuth from 2 borings span a stratigraphic interval of 339 O and 53 feet, respectively and penetrate a number ^ of flows. The cores have an average inclination of 83°. • i$ Other: Since I = 83° corresponds to latitude 76H°, Q the pole should be roughly 13H° from the sampling locality. Q Reversals: One sample is reversely magnetized, but H this core may have been inverted (Ref. a). ^H INDIA > DECCAN TRAPS _ Age: Generally considered Cretaceous to Eocene H Undiffer- /29 18 N 74 E 149 56 10 7 28 N 78 W M 15 10 a Ining Sampling: 7 samples were collected at different levels O entiated \30 1SN 74 E 149 56 13H 21 7 28N 78 W M 19H 14 * a Irving, 1956a at sites spanning some 200 miles. The data for the « calculations of entry (30) were taken from the Q appendix of Ref. a. !^ Reversals: 5 of the 7 samples showed reversed polarity M (N poles) from the other 2. HH /31 22 N 79 E 164 Sampling: The 195 oriented samples or specimen g Linga Area 48 195 a Clegg et al. \32 22 N 79 E 164 48 2 25 195 36}i N 83 W N 2H 2 a Clegg, Deutsch, measurements (?) were taken from 4 flows which and Griffiths, were sampled over an area of some 50 square miles. 1956 The lowest of these flows is considered to be the bottom of the Deccan trap sequence. Values for OJE and * for entry (32) were calculated by the ap- proximate formulas from ato given in Ref. a. /33 18J4N 73HE 147 58 233 — — a Sampling: 233 specimen measurements were made on Khand 1 Ar — — — — — * \34 18HN 73HE 147 58 3 9 233 25 N 78 W N 4H 3H 139 oriented samples from "numerous flows" col- lected at 20 sites. About 40 anomalous specimen measurements were not included. The flows are con- sidered to be younger than those at Linga. Values of aat and k for entry (34) were obtained in the manner outlined for entry (32) using data in Ref. a. _ _ _ Linga and 35 155 53 28 N 85 W N — a Sampling: Average of Linga and Khandala results. Khandala combined _ Kambatki (36 17HN 74 E 176 60 — _ 5 — — — — a Clegg et al. and Sampling: Of the 2 flows exposed at this area only * Area \37 17HN 74 E 176 60 5 150 5 31 N 102 W N 8 6 Deutsch et al. 1 was stable, and the 5 oriented samples were from a Clegg, Deutsch, this flow. This flow is considered (Ref. b) to be Everitt, and near the bottom of the sequence. Values of k and Stubbs, 1957 CMB for entry (37) were obtained in the manner out- lined for entry (32) using data in Ref. a. Since only one point in time was sampled, cm as a measure of the average field direction should be larger. (38 19HN 73HE 161 51 — — — b b Deutsch, Rada- Sampling: 4 samples were taken from each of 4 flows Igatpuri Area — - \39 73 HE 161 51 35 N 86HW N krishnamurty (Ref. b). One flow was unstable. and Sahas- rabudhe, 1959 (40 16HN 74 HE 170 57 44 a Sampling: One flow was sampled at two sites (Ref. b). Nipani Area, 16HN 74HE 168 60 44 b 4 anomalous specimens from 1 sample were ex- Lower flows I4' 74HE 168 60 4 44 31HN 95 W N 6 * cluded. Values of otas and k for entry (42) were ob- W tained as outlined for entry (32) using data in Table O 2 of Ref. a. g > J43 16HN 74HE 340 -30 59 a Sampling: At least 3 distinct flows spanning a strati- Nipani Area, 16HN 74HE 338 -32 74 b graphic thickness of 200 feet were sampled at 5 lo- Upper flows § (:: 338 -32 4 14 74 50 N 71HW S Iw 2H calities. These flows probably overlie the lower Nipani flows. Values of 095 and k for entry (45) were obtained as outlined for entry (32) using data in o Ref. b. O J46 17 N 74 E 141 59 54 a Sampling: At least 5 flows were sampled at 8 sites in Amba Area, 47 17 N 74 E 144 60 109 b the Amba area; the results from the 5 lowest sites Lower flows [48 17 N 74 E 144 60 4 10 109 23 N 77 W N 6 IM are combined here. Several samples with widely divergent directions were not used in the statistical analysis. Values of age and k for entry (48) were obtained as outlined for entry (32) using data in Ref. b. 49 17 N 74 E 335 -23 34 a Sampling: The results from the highest 3 of the 8 Amba Area, • 50 17 N 74 E 335 -26 54 b Amba sites are combined here; at least S flows were Upper flows 51 17 N 74 E 335 -26 7 8 54 SOHN 66 W S 7 H 4 * sampled at the 8 Amba sites. Values of an and k for entry (51) were obtained as outlined for entry (32) using data in Ref, b. OJ to TABLE 1.—Continued Locality Magnetic Direction Pole Position Rocks Sampled No. References Remarks Lat. Long. Decl. Incl. CC95 k N Lat. Long. P Sm sp S 8 SECTION I— Continued Pavagadh Area, 52 22HN 71 HE 348 —12 - 41 — — — — - a Sampling: 8 basic flows lying below an elevation of 53 22HN 71 HE 351 -16 I 69 b 2425 feet on Mt. Pavagadh were sampled. Values ow 54 22HN 71HE 351 -16 7 5 69 58 N 91 W S 7H 4 * of cm and k for entry (54) were obtained as outlined o for entry (32) using data in Ref. b. 55 22HN 71HE 357 15 — 17 — — — — — a Sampling: 15 specimen measurements were made on § dh Ar a — 56 22HN 71HE 355 17 — — 15 — — — - b 8 samples of acid tuff collected at an elevation C'lH tuff ' 57 22HN 71HE 355 17 7 26 15 75HN 88 W S 7H 4 * above 2490 feet on Mt. Pavagadh. They are post Deccan traps and may be post Eocene. Values of £1 ass and k for entry (57) were calculated as outlined f for entry (32) using data in Ref. b. Other: Associated rhyolite flows show random direc- tions, and laboratory tests indicate that these flows are unstable. o 58 A131* 742 82 53 N 7514• «72W " * TRAPS Fisher statistics to the upper Amba (51), upper 1en AVERAGE Nipani (45), and lower Pavagadh (54) pole positions. § There is no independent stratigraphic evidence cor- relating all these units, but the occurrence at several localities of normally magnetized flows overlying reversely magnetized flows has been interpreted (Ref. b) as supporting the correlation of all the normal flows and all the reversed flows. _ _ _ _ LOWER DECCAN 59 8H 65 6 SOHN 87 W N 8H 8H * Sampling: These values were computed by applying TRAPS Fisher statistics to the lower Amba (48), lower AVERAGE Nipani (42), Igatpuri (39), Kambatki (37), Khandala (34), and Linga (32) pole positions. See also previous note. DISCUSSION OF RESULTS 733 DISCUSSION or RESULTS done because these results are relatively simple n , „. . . and furnish a norm for comparison with the General Statement olde,, r resultsUT.U The rest* . orf thfi. e stratigraphi*. ±- , •c An attempt is here made to separate a discus- column is then discussed in sequence, beginning sion of the paleomagnetic data from a discus- with the Precambrian. The section concludes TABLE 2.— KEY TO FIGURES 17, 20, 22-31, 35 TABJIHIIA 2.— YCJIOBHtlE OEO3HAHEHLM K PHCYHKAM 17, 20, 22-32 H 35 VIRTUAL GEOMAGNETIC POLES HCTHHHBIE TEOMArHHTHLIE IIOJIJOC&I O EUROPEAN EBPOHEHCKHE D NORTH AMERICAN CEBEPO-AMEPHKAHCKHE A AUSTRALIAN ABCTPAJIHHCKHE V INDIAN a SOUTH AMERICAN J03KHO-AMEPHKAHCKHE O AFRICAN AOPHKAHCKHE D ASIAN A3HATCKHE POLARITY SYMBOLISM (APPLIES TO ALL SHAPES ABOVE AND NOT MERELY TO CIRCLES) CHMBOJIfcl nOJIflPHOCTH /OTHOCflTCfl HE TOJIBKO K KPYJKKAM. HO H KO BCEM BLiniEHPHBEflEHHtlM YCJIOBHHM OEOSHA^EHHHM/ • SOUTH MAGNETIC POLES ("NORMAL" POLARITY). J03KHHE MArHHTHBIE nOJIK)CtI/"HOPMAJIBHAH" HOJIflP- HOCTB/ O NORTH MAGNETIC POLES ("REVERSED" POLARITY). CEBEPHHE MArHHTHBIE IIOJIK>CH/"OEPATHAfl" IIOJIJIP- HOCTB/ (J POLES FROM MEASUREMENTS WITH MIXED POLARITY. nojirocti, nojiojKEHHE KOTOPLIX BHCHHTAHO no HBMEPE- HHHM CMEfflAHHOH HOJIflPHOCTH .* T ". 95 PER CENT CONFIDENCE LIMITS t A y rpAHHn;A 95% TO^IHOCTH sion of the conclusions that have been drawn with a discussion of the applications of paleo- from these studies. Individual paleomagnetic magnetic results to hypotheses such as polar studies are briefly described first, emphasizing wandering and continental drift. tests for stability and paleomagnetic applica- bility. All the virtual geomagnetic poles printed post Eocene in boldface type in Table 1 are shown in the figures that accompany this section, and, since Late Pleistocene and Recent. —Th e virtual geo- the reliability of the determinations varies magnetic pole positions for the data from this widely, frequent reference to the data tables is period are given in Section A of Table 1 and are recommended. The symbols used in the illustra- shown graphically in Figure 17. The total tions are explained in Table 2. sampling area represented is large, since two of The sequence we have used for presenting the determinations are from North America, the data places the post-Eocene first; this was two are from Asia, and the remaining nine 734 COX AND DOELL—PALEOMAGNETISM European studies were made in areas as far 1600, but the earlier Roman measurements apart as Iceland, Sicily, and Northern Sweden. have westerly declinations. The Sicilian measurements (A 2) (Chevallier, Measurements on 150 samples from the 1925, p. 146-158) were made on 11 lava flows Angerman River varves (Griffiths, 1955, p. 108) -(-GEOGRAPHIC POLE X GEOMAGNETIC POLE EQUAL AREA PROJECTIONS FIGURE 17.—LATE PLEISTOCENE AND RECENT VIRTUAL GEOMAGNETIC POLES (TABLE 1, SEC. A) Mean poles and 95 per cent confidence intervals in region north of 70° N. Lat. are shown on the right. Heavy dashed circle is "cW1 calculated from mean pole positions but has no rigorous statistical significance. Numbers refer to entries in section A, Table 1. from Mt. Etna extruded over a period of about collected at two localities a few kilometers apart 2300 years, the last one in 1911. The last two were averaged into 29 groups each representing flows have average directions agreeing with about 100 years; the mean value for each group nearby observatory measurements of the field, was then used in the statistical analysis to com- but there is no evidence of stability, other than pute pole A 8. The period covered is from 1100 the fact that the older flows have directions B.C. to A.D. 750. different from the present field direction. The Brynjolfsson (1957, p. 252) has measured the Prastmon varves (A 6) (Bancroft, 1951) cover direction of magnetization in 21 post-glacial the period from 0 to A.D. 1000 and come from lava flows from Iceland (A 10). Specimens from an area some 25 degrees north of the Sicilian these flows, which cover the period from 3400 lavas. B.C. to A.D. 1950, were partially demagnetized Measurements of the remanent magnetism of in A.C. fields of 140 oersted before measure- kilns from Carthage have been reported by ment, indicating that the reported directions Thellier and Thellier (1951, p. 1477) (A 4). Two were determined from magnetic components of the kilns were last fired in 146 B.C., and the having high coercivities. Post-glacial lava flows third in 300 A.D., so that only two points in from Iceland have also been measured by time are represented, and no statistical calcu- Hospers (1955, p. 63) (A 12). The consistency lations can be made. Measurements on 14 of both Bryj61fsson's and Hosper's measure- archaeological specimens of fired clay (A 7) have ments is good, k = 36 and k = 34, respectively. been reported by Cook and Belsh6 (1958, p. Granar (1959, p. 27) sampled 10 sections of 174). These specimens were all collected from Swedish glacial and post-glacial varves over a Great Britain and are from the Roman period lateral extent of some 800 km (A 13). Measure- (first-fourth centuries) and the Middle Ages ments on 10 upper Pleistocene lava flows from (twelfth, thirteenth, and fifteenth centuries). France (Roche, 1958, p. 3365) give the average The measurements from the Middle Ages show pole numbered A 15. (Data necessary for a easterly declinations, in agreement with direct Fisher analysis were not available.) observatory measurements made in London in Some of the first remanent magnetic measure- DISCUSSION OF RESULTS 735 ments made on rocks from North America were easily explained, however, because each of the those of Johnson and others (1948, p. 366) on 13 paleomagnetically determined poles repre- glacial varves from several localities in New sents an average of several points in time. The England (A 61). These measurements were nondipole components, which cause scatter in grouped into sets representing about 1000 years the present field, apparently have tended to each, covering the period from 13,000 to 7000 cancel each other out. These paleomagnetic B.C. The average virtual geomagnetic pole is results are of great significance, since they displaced from the geographic pole, and inclina- clearly indicate that the basic configuration of tion errors such as those observed in artificially the earth's field was that of a dipole with a deposited varves (King, 1955, p. 121) would fixed axis during the time these groups of rocks cause such a pole displacement in the observed were magnetized. The tight grouping of virtual direction, as Johnson and others (1948, p. 358) geomagnetic poles would not occur if the earth's suggested. The magnetization direction in the field were not dipolar, nor if the average dipolar varves thus probably does not represent the axis had moved between the times the different field direction at the time they were deposited, groups were magnetized. even though a fold test demonstrated the mag- In discussing historic observations of the netic stability of at least some New England earth's field we noted that there is little direct varves (Graham, 1949, p. 137-143). evidence that the earth's inclined dipolar axis The other measurements from North America has moved. If this axis had also been in its are those on the Neroly formation of California present inclined position throughout late (A 56) (Doell, 1956, p. 158). Although the rocks Pleistocene and Recent time, we should expect were deposited much earlier, it was shown that to find the poles in Figure 17 grouped about the the magnetization was acquired after folding of present geomagnetic pole rather than about the post early Pleistocene age because application of earth's geographic pole. Values of a9S should also Graham's fold test causes considerable scatter be considered in evaluating the significance of in the directions (the in situ directions are con- this grouping, and from the correlation diagram sistent). Partial demagnetization by heating to (Fig. 18) it can be seen that two ovals of con- 100°C and cooling in zero field did not alter the fidence encircle both poles, two ovals (from the magnetization, suggesting stability. Neroly formation and the New England varves) Measurements on 11 lava flows of Pleistocene encircle neither pole, no oval includes the geo- to Recent age from Japan are given by Kumagai magnetic but not the geographic pole, and seven and others (1950, p. 62) (A 86). Watanabe include the geographic pole but exclude the geo- (1958, p. 383-384) has reported magnetic di- magnetic pole. There is thus very strong paleo- rections for 45 archaeological fired clays and magnetic evidence that throughout late Pleisto- also two historically dated lava flows from cene and Recent time the earth's field, when Japan (A 81). These measurements cover the averaged over a few thousand years, has been periods from 300 to 1800 A.D. and from about that of a dipole parallel to the present axis of 5600 to 4400 B.C.; thus, some 2700 years has rotation. been sampled. The reversal problem.—In contrast with the Several other groups of measurements which late Pleistocene and Recent results, about half contain rocks from both this and preceding the older post-Eocene rocks have magnetiza- Tertiary time are discussed in the section on tions about 180° removed from the present Oligocene through early Pleistocene measure- direction of the earth's field. Before considering ments. these results in detail, it is necessary, therefore, In summary, each of the 13 average geo- to consider briefly the problem of reversals. As magnetic poles calculated for late Pleistocene noted before there are two interpretations of and Recent time were calculated from sets of this phenomenon, one that the earth's field samples that were probably magnetized over a periodically reverses its polarity or, alterna- period of several thousand years. (One possible tively, that the rocks having reversed polarity exception is the Neroly formation (A 66); the possess a self-reversal mechanism. Although the magnetic history of this rock is somewhat ob- problem is not completely resolved, it appears scure.) These poles are very tightly grouped, to us that some reversals are due to self-reversal and a comparison with Figure 13 indicates, in and others to a reversal of the field. The investi- fact, a much higher precision than for the gation of the Haruna dacite by Uyeda (1958, virtual geomagnetic poles calculated from present p. 29-48) clearly shows that rocks can be self- field directions. This initially surprising result is reversing, and the correlation between inclina- 736 COX AND DOELL—PALEOMAGNETISM tion and mineralogy found by Balsley and Sigurgeirsson, 1955, p. 892;Hospers, 1953-1954, Buddington (1954, p. 180) in metamorphic p. 475), France (Roche, 1953, p. 109; 1956, p. rocks strongly suggests a self-reversal. However, 814), and Russia (Khramov, 1957, p. 851). Al- in thick sequences of alternating normal and though the exact ages of the rocks in most of these studies are subject to some uncertainty, Zone baked by flow N %%& Zone baked by flow R 10 20 30 FIGURE 19.—MAGNETIC DIRECTIONS IN BAKED DISTANCE TO POLE-DEGREES ZONES • With respect to geographic pole O With respect to geomagnetic pole the stratigraphic control is adequate to indicate FIGURE 18.—CORRELATION DIAGRAM FOR LATE that none of the late Pleistocene or Recent rocks PLEISTOCENE AND RECENT POLES which have been studied paleomagnetically are Two points are plotted for each virtual geomag- reversely magnetized. (The Haruna dacite netic pole (VGP); the abscissae of the two points are the lengths of the lines from the VGP to the geo- samples were collected from a tuff for which graphic and geomagnetic poles respectively, and the there are no paleomagnetic data, but a young ordinates are the semiaxes of the oval of confidence flow of this rock would presumably be re- along these two lines. A point in the lower' 'significant versed.) The last appearance of reversely mag- difference" field thus indicates the pole lies outside the confidence interval. Large circles are for mean netized rocks at about the same time in such VGP position shown by heavy dashes in Figure 17. widely separated areas suggests field reversal. Oligocene through early Pleistocene,—The reversed lava flows such as those in Iceland the virtual geomagnetic poles from paleomagnetic reversals are not associated with changes in studies on Oligocene through early Pleistocene lithology (Einarsson, 1957, p. 233), and the rocks are also given in Section A of Table 1. PTRM curves for the reversed flows give no Thirteen of the 28 studies are from Europe, and indication of a self-reversal mechanism (Hos- the remainder from other continents. The most pers, 1953-1954, p. 487). Evidence given by notable feature of these data in comparison with Roche (1953, p. 108) also favors a reversal of the the late Pleistocene and Recent data is the large field; he found that four subjacent days baked number of reversed magnetizations. Nine are of by reversely magnetized lava flows were also mixed polarity, 11 are consistently reversed, reversely magnetized. It seems improbable that and 7 are entirely of normal polarity (Fig. 20). in all four cases the subjacent clays as well as Results from Icelandic lava flows of early the lavas themselves would possess a self-re- Quaternary and Miocene age are given by versal property. Einarsson and Sigurgeirsson Hospers (1955, p. 65, 68) and of Pliocene and (1955, p. 892) have also found numerous similar Pleistocene age by Sigurgeirsson (1957, p. 243). baked zones in Iceland; the baked rocks all have The Quaternary flows studied by Hospers (A 17) the same direction of magnetization as the all show reversed polarity, and although the baking flow (Fig. 19). precision is not high (k = 9) the reversed nature The youngest reversely magnetized rocks are of the magnetization indicates stability. The early Pleistocene or very late Pliocene in all the Miocene flows (102 in number) (A 34) show regions where reversely magnetized rocks have about equal numbers of normal and reversed been found, including Japan (Nagata and polarizations, occurring in four alternately re- others, 1957, p. 32), Iceland (Einarsson and versed and normal zones. The consistency-of- DISCUSSION OF RESULTS 737 reversals test indicates stability, and, because Peninsula. The samples show both normal and of the large number of flows measured, ags is reversed polarity and indicate pole positions small despite the low value of k. Sigurgeirsson's relatively far from the geographic pole. Because studies, in which the samples were "cleaned" in statistical data were not given a confidence A.C. fields of 110 oersted, resulted in much interval could not be calculated. \ RSED POLARITY O MIXED POLARITY + GEOGRAPHIC POLE X GEOMAGNETIC POLE ISO' EQUAL AREA PROJECTIONS FIGURE 20.—OLIGOCENE THROUGH EARLY PLEISTOCENE VIRTUAL GEOMAGNETIC POLES (TABLE 1, SEC. A) Mean poles in region north of 70° N. Lat. are shown on the right. Ovals of confidence are shown for only those poles significantly different from the geographic pole. Heavy dashed circle is "aw" calculated from mean pole positions but has no rigorous significance. Numbers refer to those in section A, Table 1. larger values for k. The mean pole for the re- The circle of confidence calculated from the versely magnetized flows (A 25) is not signifi- directions of magnetization of 42 lava flows cantly different from the geographic pole, but from the Vogelsberg (A 40) (Angenheister, that for the normally magnetized flows (A 23) 1956, p. 190-191) includes neither the present is, an effect which Sigurgeirsson (1957, p. 243) geographic pole nor the geomagnetic pole. attributes to the small number of flows sampled Thirteen of the flows have reversed polarity, in one part of the normal sequence. and, treated separately, the reversed and normal Five determinations of the remanent mag- flows do not have exactly the same average netization of lava flows and intrusives from direction, indicating the presence of a secondary France, ranging in age from Oligocene to early component of magnetization by the consistency- Quaternary, have been reported by Roche of-reversals test. (1958, p. 3365; 1951, p. 1133; 1950b, p. 1604; Bruckshaw and Robertson (1949, p. 316) 1950a, p. 114). Six flows of Pliocene and Pleisto- found that all the samples from the North West cene age (A 21) have an average direction not Dikes of England (A 41) had directions of mag- significantly different from the geographic pole. netization roughly opposed to the present field However, five flows of Miocene and Pliocene age direction. (A 31) and nine determinations on Oliogene In contrast with the European measurements intrusives (A 48) have circles of confidence that which, except for the Russian results, were all do not include the present geographic pole. made on igneous rocks, most of the determina- Statistical data for the Limagne basalt (A 46) tions from North America were made on sedi- and eight flows of early Quaternary age (A 19) mentary rocks. Twenty three oriented samples were not available. All the samples studied in from the Ellensburg formation of late Miocene these five determinations had reversed polarity. and early Pliocene age and 21 from the Arikaree Khramov (1957, p. 850) (A 27 and A 29) formation of Miocene age studied by Torreson studied 650 oriented samples of Pliocene and and others (1949, p. 125) yield poles A 65 and Pleistocene sediments from the Chelekan A 66 respectively. The poles are not significantly 738 COX AND DOELL—PALEOMAGNETISM different from the geomagnetic pole, and there Victoria (A 78) (Irving and Green, 1957, p. is no indication of stability. Torreson and others 351). Data necessary for computing Fisher (1949, p. 125) also measured 13 oriented samples statistics for the Pliocene ignimbrites were not from the Payette formation of Miocene and available. The Newer volcanics of Victoria, sampled at 32 widely separated sites, range in age from Pliocene to Recent; 16 of these sites yielded samples with reversed polarity. Six determinations from Japan are available for this interval—all made on volcanic rocks, and all but one including reversely magnetized samples. The oldest measurements are those of Matuyama (1929, p. 204) on lavas of Tertiary and younger ages collected at 36 localities throughout Japan and Manchuria (A 84). About half the samples have reversed polarity, and the scatter is probably somewhat increased be- cause average co-ordinates for a sampling site had to be used. Forty-two lava flows extruded more or less 0 10 ZO uniformly throughout the Quaternary are re- DISTANCE TO POLE - DEGREES .. ported by Nagata and others (1957, Table 2) • With respect to geographic pole (A 82). Nine of the flows near the bottom of the O With respect to geomagnetic pole sequence showed reversed polarities; the origin FIGURE 21.—CORRELATION DIAGRAM TOR and stability of the magnetization measured in OLIGOCENE THROTJGH EARLY these lavas were very carefully studied by PLEISTOCENE POLES means of several laboratory tests. See caption for Figure 18 Of the seven lava flows of late Tertiary age described by Kumagai and others (1950, p. 62) Pliocene(?) age (A 57). These are all normally (A 86) only one has reversed polarity. One magnetized and have an unusually high pre- reversely magnetized lava flow of early Pleisto- cision parameter (k = 258). Collinson and Run- cene age from Kawajiri was measured by corn (1960) measured 24 samples of the Asami (1954a) giving pole A 87. Duchesne River formation from Utah (A 67). Two dolerite sheets, an andesite sheet, and Like the other Tertiary sediments from North four andesite lavas of Miocene age and two America these are all normally polarized. Du andesite lavas and two basalt lavas of Pliocene Bois (1959a, p. 1618) measured 46 samples col- age have been studied by Nagata and others lected from lava flows in northwestern Canada (1959, p. 380-381). The Pliocene flows (A 90) at four very widely separated localities (A 54). all have normal polarities, but three of the The direction of magnetization at two of the Miocene units (A 93) have reversed polarities. localities was reversed. The Columbia River In addition to the above studies, six lava basalt of Miocene age was extensively sampled flows stratigraphically between normally and re- by Campbell and Runcorn (1956, p. 450) (A 62). versely magnetized flows have intermediate Twenty nine of the 73 flows sampled had re- directions of magnetization (Momose, 1958, p. versed polarity. 18); the data in this preliminary description are Ten Quaternary lava flows from South insufficient for the calculation of confidence America (A 75) have been studied by Creer limits. (1958, p. 381). Some flows are normally, and Except for the occurrence of reversals and some reversely polarized. The samples were all somewhat greater scatter in the virtual geo- partially demagnetized in fields of 250 oersted magnetic poles, the results from Oligocene A.C. before measurement so that the magnetiza- through early Pleistocene time are very similar tions reported are those of the higher coercive to those for the late Pleistocene and Recent. force magnetic constituents. Only those ovals of confidence which do not Two measurements are available from the encircle the present geographic pole are plotted southwest Pacific, one on the New Zealand in Figure 20; Figures 20 and 21 show that the ignimbrites (A 80) (Hatherton, 1954, p. 429), poles farthest from the geographic pole generally and the other on the Newer volcanics of have the largest ovals of confidence. Some of the DISCUSSION OF RESULTS 739 ovals of confidence which do not encircle the poles are shown in Figure 22 with their respec- geographic pole appear, for statistical reasons, live ovals of confidence; each oval is indicated to be too small (Table 1). As may be seen in the by eight dashes around the virtual geomagnetic FIGURE 22.—PRECAMBRIAN VIRTUAL GEOMAGNETIC POLES (TABLE 1, SEC. B) correlation diagram, the poles definitely group pole, a device introduced to avoid giving undue about the geographic pole rather than the geo- weight to data with large confidence intervals. magnetic pole, and it appears that, within some- Two paleomagnetic studies have been made what broader limits than indicated for the late on British rocks of Precambrian age. From a Pleistocene and Recent, the earth's average paleomagnetic point of view the Torridonian magnetic field throughout post-Eocene time sandstone (B 12 and B 16) (Irving and Run- was that of a dipole parallel to the present axis corn, 1957, p. 88) has been studied as carefully of rotation. as any formation. Four hundred oriented samples were collected over a wide area and Precambrian through a thick stratigraphic sequence, and the field tests indicating magnetic stability are very In striking contrast with results from the impressive. In conglomerates containing very upper Tertiary, virtual geomagnetic poles calcu- fine-grained Torridonian pebbles, the directions lated from all reported remanent magnetizations of magnetization within pebbles are uniform, of rocks of Precambrian age are significantly whereas those between pebbles are random; di- different from the present geographic and geo- rections of magnetization in the beds are widely magnetic poles. These virtual geomagnetic scattered before correcting for Caledonian 740 COX AND DOELL—PALEOMAGNETISM folding but are nearly parallel after the cor- of sets of samples collected on the north and rection. Only a relatively small proportion of south rims of this body lie on a small circle the samples show evidence of instability, and having, as its axis, the fold axis of the Sudbury the balance of the evidence certainly favors a basin. The simplest interpretation is that the field significantly different from the present magnetization predates the folding; the ap- field at the time these rocks were magnetized, propriate fold correction has been made in which was before Caledonian time and probably calculating pole B 53. in the Precambrian. Stability of the magnetization of the Michi- In the upper Torridonian (poleB 12) magneti- gan diabase dikes (B 33) (Graham, 1953, zations fall into two groups which are approxi- p. 246) is indicated by their stability in A.C. mately reversed with respect to each other; magnetic fields up to 493 oersted, but other since the axes of the "normal" and "reversed" laboratory tests suggest that the present mag- directions of magnetization differ significantly, netization may have been acquired during the statistical combination of these two may chemical or phase changes after cooling have resulted in too low a value for 0:95. The (Graham, 1953, p. 252-254). The confidence lower Torridonian sequence (B 16) has a di- interval about the pole position is artificially rection significantly different from that of the small because at most only two points in time upper, and in contrast has no reversals. are represented. Stability of the late Precambrian Longmyn- The pole for the Adirondack metamorphic dian formation (B 21) (Creer, 1957b, p. 126) is rocks has not been plotted because the original suggested by A.C. demagnetization experiments workers (Balsley and Buddington, 1958, p. and Thellier's test (that is, there was no change 790-792) have given convincing evidence that in magnetic directions on remeasurement after the directions of magnetization in these meta- storage in the earth's field). The pole lies be- morphic rocks are controlled by the mineralogy tween those for the upper and lower Torridonian and hence are not applicable to paleomagnetic (B 12 and B 16). interpretations. Of the Canadian Keweenawan rocks studied The Hakatai shale has been sampled at two by Du Bois (1957, p. 178) (B 23, B 25, B 28, localities about 1° apart (B 57 and B 63) B 30, and B 32) only the Freda sandstone and (Runcorn, 1956a, 309; Collinson and Rundorn, the Nonesuch shale (B 28) are folded and satisfy 1960). Pole B 65 is based on measurements by Graham's fold test. These data were shown in Collinson and Runcorn (1960) of samples from Figure 6 as an example of a fold test. The the underlying Bass limestone, which has a Copper Harbour formation (B 30) includes both gradational contact with the Hakatai. Com- sediments and lava flows; the sediments have bined results from these two units based on an inclination 10° smaller than the intercalated measurements by Doell (1955a) (B 61) have a lava flows, which is much smaller than the 63° large oval of confidence which intersects the difference between the mean direction of oval for pole B 57 but not that of pole B 65. remanent magnetization and the present direc- The Shinumo quartzite (B 67) (Collinson and tion of the earth's field. Random directions Runcorn, 1960) overlies the Hakatai shale. The of magnetization in pebbles of Copper Har- pole positions for these related formations, bour lava in conglomerates also suggest while scattered, show some measure of con- stability. Two sedimentary formations from sistency. However, the differences emphasize Newfoundland studied by Nairn and others the need for extensive sampling before pole (1959, p. 596) give the pole positions B 81 positions can legitimately be regarded as repre- and B 83. sentative of a given continent and geologic The gabbro intrusives from southeastern period. Canada (B 35, B 41, B 43) (Hood, 1958) show The results for flat-lying beds (B 69) from the some evidence of metamorphism. The gabbro is Hazel formation (Howell and others, 1958, locally gneissic, and laboratory results strongly p. 291) and for dipping beds after correction for suggest susceptibility anisotropy so that these dip (B 71) are not significantly different in mean measurements may not be suitable for paleo- direction, but differ considerably in amounts of magnetic purposes, even though A.C. de- scatter (k = 35 for flat lying, k = 3 for tilted magnetization usually decreased the scatter in beds), suggesting the presence of an unstable directions. A stronger case exists for the sta- component of magnetization. The rocks may be bility of magnetization of the Sudbury slightly metamorphosed and hence unsuited for intrusive. The mean directions of magnetization paleomagnetic applications. DISCUSSION OF RESULTS 741 The Belt series (B 72, B 73, B 76, B 76, B 77, the present field over wide sampling areas, and B 78, and B 79) has been sampled over a wide consistency of reversals. The balance of the region by Collinson and Runcorn (1960). The evidence strongly suggests that the earth's field virtual geomagnetic poles from this series form was not parallel to its present direction through- a remarkably tight group in the vicinity of 10° out Precambrian time. S. and 150° W., and it would be very difficult The distribution and polarity of the virtual to regard this grouping near the present equator geomagnetic poles calculated for Precambrian as random. The mean directions of magnetiza- formations from all continents do not appear tion at two sites (B 73 and B 85) are approxi- to be entirely random. Most of the poles fall mately reversed with respect to the others, in a region covering about one-third of the satisfying the consistency-of-reversals sta- hemisphere, and a concentration appears near bility test. the equator in the vicinity of 160° W. Some of Results are available from three Australian the poles considerably removed from this formations (Irving and Green, 1958, p. 66): region, however, are based on excellent data the Buldiva quartzite (B 85), the Nullagine (for example, poles B 16 and B 28). lavas (B 86), and the Edith River volcanics (B 87) which are probably younger than the Early Paleozoic Nullagine lavas. The Pilansberg dikes (B 90) were sampled at Because of the scarcity of results from the widely spaced localities and show excellent con- Cambrian through Devonian formations, they sistency of magnetizations (Gough, 1956, p. are grouped together, and the virtual geo- 206). Moreover, stability is suggested by the magnetic poles are shown in Figure 23. One small changes of magnetization in alternating Cambrian formation from Europe has been fields of 100 oersted. The Bushveld gabbro studied, the Caerbwdy sandstone from South (B 92) was also sampled over a wide area Wales (C45) (Creer, 1957b, p. 123-124). (Gough and van Niekerk, 1959, p. 131), and The directions of magnetization of the North its stability of magnetization is suggested by a American Wilberns formation, which was sam- considerable reduction in scatter on correcting pled at 10 localities spanning 55 miles, are dis- for geologic dip, as inferred from pseudo- tributed along a plane passing through the stratification in the gabbro. If a preferred present direction of the earth's field (Howell orientation or layering of ferromagnetic grains and Martinez, 1957, p. 390-391). This indicates accompanies the stratification, however, the the presence of varying amounts of a magnetiza- original direction of TRM may not have been tion parallel to the present field, and Howell and parallel to the applied field; Graham's fold test Martinez (1957, p. 391) calculate a pole position does not eliminate this possibility, and meas- (C 47) from the group of measurements farthest urements of susceptibility anisotropy would be removed from the present field direction. The desirable (Girdler, 1959). Recently determined Sawatch "quartzite" (C 49), a sandy dolomite, radio-isotope ages for these two bodies are 1290 may have acquired its magnetization during a m.y. for the Pilansberg (Schreiner, 1958, p. post-depositional dolomitization, and thus the 1330) and 2000 m.y. for the Bushveld (Gough pole may correspond to the earth's field in post- and van Niekerk, 1959, p. 127). In view of the Cambrian time (Howell and Martinez, 1957, large difference in age, the small distance be- p. 391). A reversal occurs in the Lodore tween the poles (18°) is remarkable. formation (C 62) (Collinson and Runcorn, In summary, all the paleomagnetically 1960), satisfying the consistency-of-reversals studied Precambrian formations are magnetized stability test. The Deadwood formation (C 53) in directions significantly different from that is cited as Mississippian by Collinson and of the present field. Although there are grounds Runcorn (1960); however, it contains unequiv- for reasonable doubt about the paleomagnetic ocal late Cambrian fossils at its type locality applicability of some of the formations de- and is therefore listed here. scribed, many of them satisfy the classic tests Two sets of Cambrian data are available from for stability. These tests are Graham's fold Australia, one from the Elder Mountain sand- and conglomerate tests, Thellier's test for stone (C 56) and one from the igneous Antrim change of direction in the laboratory, stability Plateau basalts (C 56) (both from Irving and of magnetization in A.C. fields, concurrent Green, 1958, p. 66). Details of the sampling and results from igneous and sedimentary rocks, evidence for stability have not been published; consistency of directions different from that of concordant results from igneous and sedi- 742 COX AND DOELL—PALEOMAGNETISM mentary rocks like these, however, carry much closed by stratiform beds, have much more more weight than either result would separately. scattered directions. However, most of the Two widely separated poles (C 29 and C 31), directions move toward the other group on FIGURE 23.—EARLY PALEOZOIC VIRTUAL GEOMAGNETIC POLES (TABLE 1, SEC. C) based on measurements of Ukranian basalts correcting for dip, and Graham (1954, p. 219) (Komarov, 1959, p. 1221), are cited as probably concludes that the magnetization has sig- Ordovician in age. However, Komarov (1959, nificant stability. Pole C 37 is based on direc- p. 1223) states that they may not be the same tions of magnetization in limestone cobbles of age, but are both within the lower Paleozoic. Trenton age in a conglomerate (Graham 1956, Early Ordovician sediments near Leningrad p. 738). Since the directions of magnetization have been sampled by Khramov (1958, p. 185). are uniform from cobble to cobble, the mag- Pole C 36 is the mean of a reversed (N) pole and netization must be post-depositional. Agree- a normal (S) pole that has been "corrected" for ment with the previous results suggests, how- an unstable component. Pole position C 38 ever, that not much time elapsed between (Graham, 1954, p. 219) is based on 45 samples deposition and magnetization. Pole position from rocks of the Middle Ordovician Trenton C 40 is based on measurements of red beds in group from New York. Thirty-five of the the North American Juniata formation of Late samples are from undisturbed flat-lying beds Ordovician age (Collinson and Runcorn, 1960). and have well-grouped directions of magnetiza- The two Silurian units from Europe that tion significantly different from the present have been studied are the Ludlow series (C 17) field. Ten samples from a distorted zone, en- (Creer and others, 1954, p. 165) and the Ural DISCUSSION OF RESULTS 743 peridotites (C 18) (Komarov, 1959, p. 1222) studies. Clegg and others (1954a, p. 587-588) for which no confidence intervals or other examined specimens from three oriented details are available. samples collected at a single locality (C 2) and Two Middle Silurian formations from North found that the magnetization was stable in America have been investigated, the Rose Hill weak D.C. and in strong A.C. magnetic fields. formation of Swartz (1923) (C 21) (Graham, Creer (1957b, p. 113-123) sampled much more 1949, p. 148-154) and the Clinton iron ore widely and noted that flat-lying beds gave (C 23) (Howell and others, 1958, p. 287-289). consistent results (C 13), whereas the directions The Rose Hill determination is based on 35 from folded beds after correcting for tilt were oriented samples collected at 6 localities span- widely scattered, indicating the presence of an ning 32 miles. At two localities the samples were unstable component. collected on the limbs of small folds and at the The paleomagnetism of one North American remaining localities on the limbs of large folds. formation of Devonian age, the Onondaga Directions of magnetization are widely scat- limestone, has been studied (C 14) (Graham, tered before correcting for dip and, except for 1956, p. 738). Directions of magnetization are three sample directions, are nearly parallel after parallel throughout both layered beds and beds the dip correction. The deformation occurred showing penecontemporaneous deformation, near the end of the Paleozoic, and Graham indicating post-depositional remanent mag- (1949, p. 151) concludes that the magnetization netization. Although the consistency of the took place before the Permian and very prob- results and their divergence from the present ably at the time of deposition. The Clinton iron field direction point to some stability, the age ore data are from a sampling area regarded by and origin of the magnetization are uncertain. Howell and others (1958, p. 287) as too small The Ainslie volcanic rocks from Australia to give a reliable pole position. The rocks have (C 15) (Irving and Green, 1958, p. 66) are susceptibility anisotropy with the plane of probably Devonian but may be Silurian. maximum susceptibility nearly in the bedding In addition, Graham (1954, p. 216) has plane, and the remanent magnetization as- measured 182 samples from 14 exposures of sociated with such a susceptibility anisotropy Ordovician to Permian age in the northeastern will probably have a smaller inclination than United States, with emphasis on the older rocks. that of the earth's field in which it developed. Most of these have polarizations within about The Clinton formation and the Rose Hill forma- 30° of the present direction of the earth's field, tion of Swartz (1923) are both of Middle and there is evidence that some of them are Silurian age, so that, if the earth's field at this stable and some partially stable. Separate data time was that of a dipole consistent with the for each formation are not given (with the Rose Hill paleomagnetic results, the field at exception of that for pole C 38), so individual the Clinton site would have been D = 320, poles could not be computed. I = -28, or D = 140, I = 28, depending on whether the "normal" or "reversed" sense is Carboniferous taken. The average direction of magnetization of the Clinton rocks is D = 140°, I = \9y2°. Many data are available from lava flows, Since the difference in inclination is in the intrusive rocks, sediments, and baked sediments direction anticipated on the basis of the sus- of Carboniferous age from Great Britain. ceptibility anisotropy, this careful laboratory Virtual geomagnetic poles and associated con- study by Howell and others supports the results obtained from the study of the Rose Hill fidence limits for these and other Carboniferous formation. studies are shown in Figure 24. Igneous rocks, Details of the investigation of the Mugga probably of late Carboniferous age, and sedi- porphyry from Australia (C 25) (Irving and ments baked by them were sampled at six Green, 1958, p. 66) have not been published. sites in England spanning 30 miles (D 5) (Clegg The data for the Silurian Red siltstones from and others, 1957, p. 220). Samples from ad- China (C 27) consist of measurements of speci- jacent unbaked sediments have random direc mens from only three oriented samples and, as tions of magnetization, whereas the baked Chang Wen-You and Nairn (1959, p. 254) sediments have closely parallel directions and state, should be regarded as provisional. intensities of magnetization 100 times larger The British Old Red sandstone of Devonian than those of the unbaked rocks. Pole position age has been the subject of two independent D 8 is from another set of sediments baked bv 744 COX AND DOELL—PALEOMAGNETISM an intrusive of late Carboniferous age (Clegg Wales has widely scattered directions and no and others, 1957, p. 220). correlation with the above results (Belshe, Unbaked sediments from Great Britain that 1957, p. 188). FIGURE 24.—CARBONIFEROUS VIRTUAL GEOMAGNETIC POLES (TABLE 1, SEC. D) have been studied paleomagnetically include the Interbedded with the Derbyshire sediments Gloucester Pennant sandstone (D 2) (Clegg (D 16) are three volcanic units (D 20) with well- and others, 1954a, p. 588), the Pendle Mono- grouped directions of magnetization which are cline sediments of Lancashire (D 10), the not significantly different from the direction of Millstone Grit of Lancashire (D 12), other the sediment (Belsh6, 1957, p. 188). A sequence Lancashire sediments (D 11), and sandstones of 15 lava flows of early Carboniferous age and siltstones from Derbyshire (D 16); the last from Scotland falls into three stratigraphic four are from Belshe's (1957) data. The mag- groups, two with approximately parallel mag- netizations of the 14 specimens from one netizations separated by one with approxi- oriented sample that constituted the data for mately opposed magnetization (Clegg and pole position D 2 were stable in A.C. fields of others, 1957, p. 221). The mean directions of several hundred oersted. Stability for the magnetization of these three groups have been magnetization corresponding to pole position used by the present authors to calculate pole D 10 is supported by a fold test. The data for position D 31. the other pole positions (D 11, D 12, and D 16) Samples from two sites 200 yards apart in show considerable scatter, and an additional the Shatterford intrusion (D 33), and un- set of 50 Carboniferous sediment samples from doubtedly from the same intrusive unit, have DISCUSSION OF RESULTS 745 tightly grouped directions of magnetization (Creer and others, 1948, p. 495) are based on almost exactly 180° apart, satisfying the con- measurements of 19 specimens from four sistency-of-reversals stability test (Clegg and oriented samples which fall into two groups with others, 1957, p. 222). Five samples from the mean directions 157° apart. In the absence of Lundy granite, studied by Blundell (1957, p. stability tests, these poles should be viewed with 191) (D 36), are weakly magnetized; however, some reservation. application of Thellier's test after a month in In summary, the virtual geomagnetic poles the laboratory indicates some magnetic calculated for the sedimentary formations of stability. Carboniferous age from North America show a All the paleomagnetic determinations for the remarkable grouping in the vicinity of 36°N., Carboniferous of North America are on sedi- 115°E. The virtual geomagnetic poles calcu- mentary rocks. The Naco formation of Penn- lated from English rocks also fall in this general sylvanian age was sampled at two localities region but may be subdivided into two groups, yielding poles D 38 (Runcorn, 1956a, p. 309) one of which agrees very closely with the North and D 40, (Collinson and Runcorn, 1960). American results (poles D 20, D 2, D 16, D 11, The angular difference between the mean D 10, and D 33) and one of which does not directions of magnetization of these two locali- (poles D 8, D 31, D 5, D 12, and D 36). Two ties is much less than the difference of either determinations from the latter group (D 31 from the present field direction; however, the and D 36) have large ovals of confidence, and confidence intervals for these two determina- the significance of their deviations from the tions do not intersect, illustrating the other British group is doubtful. The remaining importance of regarding the results from a virtual geomagnetic poles, especially D 5 and single sampling site as applying to that site D 8, based on intrusive rocks and associated only and not necessarily to the entire formation. baked sediments, are undoubtedly significantly The mean directions of magnetization of eight different. Among several possible interpreta- sites spanning 73 miles in the Barnett formation tions of these two groups of poles, Clegg and of Mississippian age (D 43) are approximately others (1957, p. 221-222) considered the follow- reversed with respect to the mean direction of ing: (a) These rocks may have been magnetized a ninth site (D 45) (Howell and Martinez, after the Carboniferous; this appears unlikely 1957, p. 385-388). The magnetization appears in view of the evidence relating the magnetiza- to be chemical in origin, and the fair agreement tion in the baked sediments to the Carbonifer- in direction between normal and reversed sites ous intrusives. (b) The original magnetization gives evidence of magnetic stability. Pole posi- may not have been parallel to the earth's field, tion D 47 (Nairn and others, 1959, p. 596) is (c) The earth's field may not have been that of based on data from the Codroy group of sedi- a dipole which remained relatively fixed with ments from Newfoundland, and pole D 49 on respect to the mantle during the Carboniferous. combined measurements from the Bonaventure, We have tried to fit these data to an axial Kennebacasis, and Bathurst formations of quadrupole field (see Creer and others, 1957, southeastern Canada (Du Bois, 1959b, p. 63). p. 148), for equations of this field) without Two sets of data are available from Australia. success. A considerable interval of time is Irving (1957b) collected 75 oriented samples covered by these determinations, however, and from the Kattung varved sediments at four it is therefore unlikely that the virtual geo- sites (D 50). The mean directions of magnetiza- magnetic poles calculated from the North tion are very widely scattered before correcting American results, with sampling sites as far for Permian folding, but are in striking agree- apart as Arizona and Newfoundland, would be ment after correcting for dip; moreover, samples so well grouped if the earth's field were non- from one site are reversely magnetized with dipolar, or if it were changing rapidly during respect to those at the other three. Thus both this period. The inconsistency between these the fold test and consistency-of-reversals test two groups of European paleomagnetic poles for magnetic stability are satisfied. Pole position remains an important unsolved problem. D 51 for the Kattung lavas (Irving and Green, Viewing the Carboniferous data on a some- 1958, p. 66) does not differ significantly from what larger scale, it would be difficult not to that for the varves, and the agreement between recognize that there is excellent evidence for a the results from igneous rocks and those from magnetic field in Carboniferous times different sediments also points to stability. from the present one. Virtual geomagnetic The only results from Africa (D 52 and D 54) poles calculated from English and American 746 COX AND DOELL—PALEOMAGNETISM paleomagnetic directions form a group which is sedimentary rocks (Du Bois, 1957, p. 178). far from random, and the Australian poles, The Ayrshire kylites (E 22) (Armstrong 1957, calculated from concordant results from igneous p. 1277), which intrude the Coal Measures and ••'IP; »", * . */• 90°E *- * 90° Wi •r -d + + 60 N 30° M t ' °* X 59'oQ^ \ * '3555« FIGURE 25.—PERMIAN VIRTUAL GEOMAGNETIC POLES (TABLE 1, SEC. E) rocks and sediments and with several indica- are cut by Permian volcanic necks, were sam- tions of stability, are certainly significantly pled at five sites. different from the North American and Poles E 16, E 10, E 13, and E 18 are based European group. on studies of several units in the Esterel vol- canic rocks of southeastern France (Rutten and Permian others, 1957, p. 195; Roche, 1957, p. 2953; As and Zijderveld, 1958, p. 317). E 16 is based on Paleomagnetic results are available from 14 samples from a single flow and hence a volcanic, intrusive, and sedimentary rocks of single point in time, and, therefore, the small Permian age from Great Britain and continental value of 0:95 does not have the usual paleomag- Europe; Figure 25 shows the virtual geo- netic significance. Stability is indicated for the magnetic poles computed from these data. Esterel rocks (pole E 18) by the reduction of Pole position E 4 is based on the mean direction scatter in directions on partial demagnetization of five lava flows from the Exeter volcanic at 150°C. and in an A.C. field of 300 oersted, series (Creer, 1957b, p. 112-113). Pole E 8 is and also by a fold test. based on data from the Mauchline lavas, and Two other determinations from European E 6 on data from the associated Mauchline igneous rocks are pole E 24, based on measure- DISCUSSION OF RESULTS 747 ments of samples from the middle Permian only reversals in all reported Permian paleo- Xiedeck porphyry from France (Nairn, 19S7b, magnetic data—the scatter in the measure- p. 722), and pole E 20 based on a number of ments is so extreme as to leave this suggestion separate igneous units sampled in the Oslo unsubstantiated. graben (Rutten and others, 1957b, p. 195). The individual virtual geomagnetic poles 90* W 60* N 30° N 180° FIGURE 26.—POLES AND CONFIDENCE INTERVALS FOR THE SUPAI FORMATION Virtual geomagnetic poles corresponding to mean directions at different sampling sites are indicated by X. Numbers refer to entries in section E, Table 1. Two sedimentary formations from France, corresponding to seven separate studies of the the Montcenis and the Saint-Wendel, give North American Supai formation (Permian and poles E 26 and E 28. Twenty-two additional Pennsylvanian) at sampling sites spanning samples from the Montcenis and other sedi- several hundred miles are plotted in Figure 26, mentary formations (Nairn, 1957b, p. 722) together with the mean pole position E 53. have directions of magnetization which are Poles E 37 (Collinson and Runcorn, 1960) either scattered or roughly parallel to the and E 39 (Graham, 1955, p. 343) correspond to present field; Nairn (1957b, p. 722) regards sets of samples collected at about the same these as unstable. Late Permian sediments locality. Poles E 46 (Runcorn, 1955b, p. 505) have been sampled at two localities in Central and E 49 (Doell, 1955b, p. 1167) are based on Russia (E 30 and E 31) by Khramov (1958, samples collected at a different locality. p. 187). Although reversed magnetizations are The variations in the pole positions com- suggested for the Tartarskij sediments—the puted from the Supai data give some idea of the 748 COX AND DOELL—PALEOMAGNETISM expectable variations in paleomagnetic studies The two widely spaced poles from Africa are of sediments and indicate that different popu- based on sediments of late Permian (E 63) lations of magnetic directions have probably and early Permian (E 64) age (Creer and been sampled at the different sites. Such dif- others, 1958, p. 495). ferences between two undisplaced points within A striking feature of the Permian paleo- a formation are entirely normal and possibly magnetic results is the separation of European are due to their having acquired magnetizations from North American virtual geomagnetic poles. at different times. Again these studies The results from European igneous rocks (poles emphasize the importance of not interpreting E 4, E 8, E 10, E 13, E 18, E 20, E 22, E 24) the results of a statistical analysis of measure- show no systematic difference from those of ments from one sampling site as necessarily the European sediments (poles E 6, E 26, E 28) applying to the entire formation, geologic However, the group of poles calculated from period, or continent in which the sampling site North American sediments is quite probably is situated. significantly different. As one views these results from the Supai A possible explanation of the separation of formation on a somewhat larger scale, the these two sets of data is that the earth's field consistency between the average directions of was not dipolar during the Permian. As noted magnetization is more impressive than the dif- in the section on the earth's field, the relative ferences. Three groups of workers collected motion between core and mantle leads to an oriented samples independently at widely average field symmetrical with respect to the separated sites, transported them in different axis of rotation (Runcorn, 1959, p. 91). To test ways to different laboratories, machined the hypothesis of an axial, nondipole field, cubes, cylinders, or discs from the samples, and three sets of data from three widely spaced measured the remanent moments using several localities would, in general, be required; inter- types of spinner and astatic magnetometers. section at a point P of the three great circles Thus the variations observed between different lying along the directions of the horizontal field sets of data from the Supai include the effects components at the three localities would con- of experimental errors as well as the actual stitute support for the hypothesis. variations in direction of magnetization between In the special case where results from only sampling sites. two sampling areas are available but the areas The mean pole position for the Supai forma- happen to be equidistant from the point of tion (E 53) is the result of a Fisher statistical intersection P, the hypothesis may also be analysis of the seven mean poles. Since these tested, inasmuch as axial symmetry of the field individual data have different values of aK, requires that the sampling areas have the same no rigorous statistical significance should be average inclination. The mean Permian sam- attached to the circle of confidence calculated pling areas in Europe and North America are for pole E 53. both about 62° away from the corresponding The Cutler formation was sampled at two point of intersection P (Fig. 27). The two sets of localities by Graham (1955, Fig. 7) and by inclinations are: Collinson and Runcorn (1960). Pole E 35 is midway between the two mean poles for these Europe: -9°, -6°, -4°, -16°, -13°, sets of data; " X SAMPLING SITE FIGURE 27.—TEST FOR AXIAL, NONDIPOLE FIELD P is the intersection of great circles connecting mean sampling sites and mean virtual geomagnetic poles, as indicated by a heavy square (North America) and a heavy circle (Europe). tion that must be made is that mineral consistent with the excellent Australian Car- assemblages necessary for self-reversal are boniferous data. abundant in Carboniferous and Triassic rocks since both these periods have many reversals, Triassic but are missing in all (or almost all) Permian rocks studied to date. The interpretation of this The Triassic poles are plotted in Figure 28. feature on the basis of the field-reversal hy- The nine sampling sites in the Keuper Marl pothesis is that oscillations in the polarity of sandstones from England (Clegg and others, the earth's field have been intermittent. The 19S4a; 19S4b, p. 195) fall into two groups which latter interpretation appears to us somewhat have directions of magnetization approxi- more plausible. mately reversed with respect to each other. The two results from the Permian of Australia These sediments are stable in weak D.C. fields are consistent with each other but not with the and in strong A.C. fields and satisfy a fold test general grouping of results from North America for rather shallow dips. Pole F 12 is based on 750 COX AND DOELL—PALEOMAGNETISM the nine mean site directions as listed in the the Vetlujskij sediments (F 20) are given by original paper. Creer (1957a, p. 136) showed Khramov (1958, p. 187). that Keuper Marl samples collected at many Several Triassic formations from North FIGURE 28.—TRIASSIC VIRTUAL GEOMAGNETIC POLES (TABLE 1, SEC. F)* other localities have two components of mag- America have been sampled extensively at a netization, one stable, the other unstable; the large number of sites, and in order to present stable one is essentially parallel to that de- all the data on a single plot only one mean pole scribed by Clegg. position is shown for each formation; for intra- Pole F 13 combines the results of measure- formational details the reader is referred to ments on sediments of early Triassic age from Section F of Table 1. Pole F 39 is based on seven sites in the Vosges region of France. work by several workers at eight sampling sites There is considerable scatter in directions of in the Chinle formation or formations of equiva- lent Late Triassic age (Graham, 1955, Fig. 7; magnetization, which has been interpreted by Collinson and Runcorn, 1960). At some of the Clegg and others (1957, p. 225) to be indicative sites an unstable component of magnetization of partial instability. Of the seven localities in is probably present (Collinson and Runcorn, Spain where sedimentary samples were col- 1960), and our use of the actual measured direc- lected, only three localities had consistent tions in the statistical analysis rather than the directions of magnetization (F 15 and F 16) direction of an inferred stable component may (Clegg and others, 1957, p. 225-226). Pole have resulted in a position too far north for (F 16) combines data from two sites. Data for pole F 39. Five of the sites are normally mag- DISCUSSION OF RESULTS 751 netized, one site is probably reversed, and at magnetic pole within 16° of the sampling site. two sites most of the samples are normal, but Deviations of the core hole from the vertical, several are reversed. a common occurrence, may contribute an Pole F 79 is based on a very extensive sam- additional uncertainty. Pole F 107 (Irving and pling of the Chugwater formation by Collinson Green, 1958, p. 66) is based on measurements of and Runcorn (1960). At 7 of the 10 widely oriented surface samples of the Brisbane tuff, spaced sampling sites both normal and reversed and it is of interest to note that the pole in- groups of magnetizations were encountered; ferred from the unoriented vertical core samples in the statistical calculations for pole F 79 each is in agreement with this pole. Pole F 109 is group, whether normal or reversed, was treated based on sediments of Late Triassic age from as a separate measurement. The scatter of poles Africa (Nairn, 1957a, p. 166-167); there is good corresponding to the mean site directions is internal consistency in this study, but con- unusually small (k = 57), and these data show temporaneous sediments from adjacent locali- excellent stability by the consistency-of- ties have widely scattered directions. reversals test. All but one of the poles calculated from The Moenkopi formation of Early and Triassic paleomagnetic measurements are sig- Middle (?) Triassic age (F 91) has been sam- nificantly displaced from the present geographic pled at seven sites by Runcorn (19S6a, p. pole. The North American poles are very 311-312), Kintzinger (1957, p. 931), and roughly grouped at about 60° N. and 105° E., Collinson and Runcorn (1960). The four nor- but many poles for individual formations are mally magnetized and the three reversely displaced considerably from this position. magnetized sites appear to form two separate Reversals in both igneous and sedimentary groups; the poles of the normal group are north formations are common. of the others. Thus the consistency-of-reversals test is not satisfied, indicating the presence of a Jurassic comparatively small unstable component of magnetization. However, as pointed out by Jurassic virtual geomagnetic poles are plotted Creer and others (1957, p. 151), the average in Figure 29. Pole G 2 is based on results from position of the two groups probably corresponds English rocks which may be unstable (Nairn, reasonably well to the stable component of 1957c, p. 311-312). Pole G 5 is based on meas- magnetization. The Springdale sandstone mem- urements of six oriented samples from several ber of the Moenave formation (F 22) (Runcorn, British Lower Jurassic formations; five of the 1956a, p. 312-314) was sampled at one site and six samples had reversed polarity (data are from is probably partially unstable. Nairn, 1957a, p. 311-312). Poles G 6 and G 7 Poles F 93 and F % are based respectively on are based on two groups of sedimentary samples the magnetizations of lava flows and on those from the Early Jurassic (upper Lias) in Britain of sediments and lava flows combined, all from (Girdler, 1960, p. 358-359). One group is very the Connecticut Valley (Du Bois and others, nearly reversed with respect to the other, 1957, p. 1186). Pole F 98 (Du Bois and others, indicating stability by the consistency-of- 1957, p. 1186) is based on data from sediments reversals test. Samples from other nearby sites at the top of the Newark group, and pole were unstable (Girdler, 1960, p. 354-355). F 100 (Graham, 1955, Fig. 7) on sediments Measurements on volcanics from the Pyrenees from the bottom of this same group. Lavas gave pole G 8 (Girdler, 1960, p. 359-361). from Nova Scotia were sampled at 12 sites by The flows are dated as Early Jurassic on the Bowker (letter of November 16, 1959, to basis of intercalated limestone beds. Poles Doell), and pole F 102 is the mean of the virtual G 10 and G 12 are based, respectively, on meas- geomagnetic poles calculated for each of the 12 urements of radiolarite and limestone samples sites. The Dinosaur Canyon sandstone member of Middle Jurassic age from the Alps (Hargraves of the Moenave formation, studied by and Fischer, 1959, p. 38). Kintzinger (1957, p. 931) (F 105), is Late From North America the Kayenta formation Triassic(?). has been sampled at five sites (Collinson and Pole F 106 was calculated from measure- Runcorn, 1960), and pole G 20 is the mean of ments of the average inclination of unoriented the virtual geomagnetic poles for each site. vertical core samples from Tasmanian volcanic The Carmel formation (G 21) (Collinson and tuffs (Almond and others, 1956, p. 775); the Runcorn, 1960) was sampled at one locality. steep inclination of 81}-^° implies a virtual geo- The Karroo dolerites were sampled exten- 752 COX AND DOELL—PALEOMAGNETISM sively both in tunnels and mines (G 28) and have been combined in calculating pole G 52. on the surface (G 31) (Graham and Hales, 1957, The confidence interval of 3° by 1^° (Creer, p. 155). The surface samples have more scat- 1958, p. 385) is probably too low, while the . > '-,<•• ife51? , FIGURE 29.—JURASSIC VIRTUAL GEOMAGNETIC POLES (TABLE 1, SEC. G) Poles H 5, H 7, and H 10 (shown in Fig. 30) may also be Jurassic and possibly should be included here. tered directions of magnetization, but the mean one shown in Figure 29 of 16° by 9%°, estimated directions for both groups agree. Some of the by us, may be too large. The correct value underground sills are magnetized reversely with depends on how many independent points in respect to the others, and associated baked time (i.e., separate lava flows) were sampled. sediments are invariably magnetized in the In addition to the points plotted in Figure 29, same direction as the sill that baked them. Such the Rajmahal traps of India (pole H 10 in Fig. a relationship points strongly to reversal of the 30) and the Tasmanian dolerites (poles H6 and earth's field. Pole G 36 for the Karroo basalts H 7 in Fig. 30) may also represent Jurassic field (Nairn, 1956, p. 936; 1957a, p. 166), which are directions—the ages of these rocks are un- as old as the dolerites or slightly older, does certain. not differ significantly from the pole for the In addition to the Jurassic measurements re- dolerites. ported above, four samples from the Summer- Basalts from the Parana basin of South ville formation and three from the Carmel America, as well as sandstones baked by them, formation were measured by Torreson and have been studied by Creer (1958, p. 377). The others (1949, p. 125) in Utah. Because of the two sets of results agree with each other and extreme scatter in the measured directions, DISCUSSION OF RESULTS 753 no poles could be computed. The Jurassic nearly parallel to the present field. Since no virtual geomagnetic poles calculated for all numerical data were given a pole could not be continents show considerable scatter, and a calculated. FIGURE 30.—CRETACEOUS VIRTUAL GEOMAGNETIC POLES (TABLE 1, SEC. H) Poles H 5, H 7, and H 10 may be Jurassic relatively large number are not significantly The only Cretaceous paleomagnetic data different from the present geographic pole. from North America for which a pole can be calculated consist of measurements made on 6 Cretaceous oriented samples from the Dakota sandstone from the Colorado Plateau (Runcorn, 1956a, Only one Cretaceous pole from Europe is p. 314-315). Three of the samples have mag- available, that from the Wealden sediments of netizations approximately reversed with respect the Isle of Wright (H 2) (Wilson, 1959, p. 753). to the direction of the present field, and 10 Several lines of evidence point to the existence specimen measurements from these samples of a stable component of magnetization about are the basis for the generally cited North 4° from the present field direction. Nairn American Cretaceous pole. For pole H 4 (Fig. (1957c, p. 309) reports that the remanent mag- 30) we have used the mean sample directions netization of a red band 2-6 inches thick at for the three reversed and two normal samples; the base of the Lower Cretaceous Gault Clay one obliquely magnetized sample has been formation from England has both reversed and excluded. normal directions of magnetization which are The Tasmanian dolerite sills cut Upper Trias- 754 COX AND DOELL—PALEOMAGNETISM sic (or possibly Jurassic) rocks and are displaced and therefore some of these virtual geomagnetic by early Tertiary faults. Although they are thus poles are expected to be near the present geo- no more closely dated than Late Triassic to graphic pole, in agreement with the other post- early Eocene, the paleomagnetic results are Eocene results. usually plotted as Jurassic. We have taken the The age of the Lundy dikes (I 2) (Blundell, liberty of placing these virtual geomagnetic 1957, p. 191) is not known, but assignment to poles on the Cretaceous diagram to emphasize the early Tertiary has been suggested because the uncertainty in their ages. The dolerites have the magnetization is similar to that of other been extensively sampled at surface outcrops lower Tertiary rocks from Britain. Thellier's (H 5) (Irving, 1956b, p. 166-167), and measure- stability test, with 1 month between measure- ments have also been made on vertical well ments, and the consistency of this reversed cores unoriented with respect to azimuth (Al- magnetization point to stability. The Arran mond and others, 1956, p. 773). Since the in- dikes (I 3) are described as early Tertiary by clination of the magnetization in the cores is Irving (1959, p. 64). The Mull lavas (I 7) —85} indicating a virtual geomagnetic pole (I 28) In addition to the Eocene measurements re- within 13^° of the sampling site. ported above, five samples from the Wasatch The Deccan traps of India have been studied formation in Colorado and four from Wyoming FIGURE 31.—EOCENE AND EOCENE (?)VIRTUAL GEOMAGNETIC POLES (TABLE 1, SEC. I) Poles for all paleomagnetic studies of rocks described as Eocene, possibly Eocene, or early Tertiary are included. Some are almost certainly post-Eocene. independently by several workers. Pole I 30 is have been measured by Torreson and others based on seven oriented samples collected from (1949, p. 125). The nine measurements were different levels over a wide area (Irving, 1956a, much too scattered to permit the calculation of p. 40). Clegg and others (1957, p. 227-230) and a pole. Deutsch and others (1959) have investigated In summary, the virtual geomagnetic poles the traps in great detail and have collected from calculated from the magnetizations of rocks of several levels at seven sites covering a wide area. Eocene (and probably younger) age fall into Results from horizons which are topographically two groups, one near the present geographic and probably stratigraphically nearer the bot- pole and the other distributed north of lat 30° N. tom of the sequence have been combined for and within about 25° of the 300th meridian. pole I 59, and those from the upper horizons for Three poles in the first group correspond to sedi- pole I 58. Results from a post Deccan trap tuff mentary formations from North America having form the basis for pole I 57.* no evidence for stability, and the remaining poles in this group correspond to volcanic rocks * Pole I 57 was inadvertently not plotted. of somewhat uncertain age from Europe. 756 COX AND DOELL—PALEOMAGNETISM Polar Wandering and Continental Drift constitute strong evidence in support of the dynamo theory for the origin of the earth's mag- General statement.—Paleomagnetic measure- netic field. ments, which have been discussed in terms of It is, of course, possible that the coincidence their equivalent virtual geomagnetic poles, tell of the paleomagnetically determined dipole axis us only the direction of the earth's field at the and the present rotational axis is fortuitous. The time and place of formation of the rocks. If we earth's field may not have been dipolar or axial, are to apply these results to the problems of con- and displacements of the sampling areas or of tinental drift and polar wandering we must in the entire crust may have been of exactly the some way relate the magnetic field to geographic right amount to compensate for such irregulari- configurations. ties in the field. This, however, would be ex- In order to interpret the results from some tremely unlikely. period in terms of continental drift we must first Late Tertiary polar wandering and continental have enough data to know the configuration of drift.—As noted earlier, the paleomagnetic re- the magnetic field during that period: was it, for sults for Oligocene through early Pleistocene example, essentially a dipole field as at present? time are very similar to those for late Pleisto- It is further necessary to assess the expected cene and Recent except for the presence of re- variation between sampling areas that have not versals and somewhat greater scatter in mean been displaced with respect to each other. Only pole positions. However, it is much more diffi- then is it possible to infer that a departure in cult to determine on geological or geophysical direction at some given sampling area indicates grounds what relationship the axis of rotation displacement of that area with respect to others. has had to the present continental configuration Polar-wandering interpretations require further during this same interval. Large displacements that there be some connection between the mag- of the pole have, in fact, been postulated by netic-field configuration and the earth's axis of several workers. Polar-wandering curves of rotation. Continental-drift interpretations do Kreichgauer (1902), Koppen and Wegener not require such a connection, but they do re- (1924), Milankovitch (1938), and Koppen quire that the field configuration be changing at (1940) as given by Gutenberg (1951, p. 202) are a rate which is small in comparison with the shown in Figure 32. (Note that only the paths of degree to which we can establish the contem- Kreichgauer and Milankovitch are in relation to poraneity of the formations compared. the present continental configuration; those of Axial dipole theory.—The paleomagnetic evi- Koppen and Wegener assume continental drift dence indicates, with a high degree of precision, and are with respect to Africa only.) On the that the earth's magnetic field, when averaged other hand, Chaney (1940, p. 486) and Durham over a few thousand years, was dipolar in nature (1950, p. 1260; 1952, p. 339) have cited paleo- and parallel to the present axis of rotation climatological evidence to indicate that early throughout early Pleistocene and Recent time. Tertiary isoclimatic zones were parallel to pres- It follows that the two axes were parallel during ent latitude lines. Further clarification of the this time if we can establish that the earth's axis paleobiogeographic picture during this interval of rotation has also remained fixed. The dis- is greatly to be desired. To the extent that we tribution of the Pleistocene polar ice sheets de- are willing to extrapolate the axial dipole model fines rough limits for possible movements of the back into the past, the paleomagnetic evidence rotational axis during this time, but it is doubt- indicates that no large shift of the axis of rota- ful whether polar wandering of less than 10°-20° tion has occurred during the late Tertiary—a could be detected. Estimates of displacements conclusion previously arrived at by Hospers of the rotational axis during the past several (1955, p. 72-73) from analysis of fewer data. decades, as found from astronomical observa- Some recent theories calling for substantial tions, indicate (Elsasser and Munk, 1958, p. shifts of the pole of rotation in late Tertiary 230) that the geographic pole moved at most 15 time have cited paleomagnetic evidence in sup- feet between 1900 and 1940. If the average rate port of such shifts (Ewing and Donn, 1956, p. of motion during the past half million years had 1065; Hapgood, 1958, p. 308). Although some been twice that amount, the total polar shift Oligocene to early Pleistocene virtual geomag- would have been only 1°. Since there is no other netic poles do, in fact, have ovals of confidence evidence to suggest that the axis of rotation dif- that do not include the present pole, there are fered significantly from the present one, the late several reasons why these probably do not indi- Pleistocene and Recent paleomagnetic results cate polar wandering or displacements of the DISCUSSION OF RESULTS 757 Kreichgauer Koppen and Wegener Koppen Milankovitoh FIGURE 32.—POSTULATED TERTIARY POLAR-WANDERING PATHS C—Carboniferous, P—Permian, T—Triassic, J—Jurassic, K—Cretaceous, E—Eocene, M—Miocene, LQ—lower Quaternary. (After Gutenberg, 1951) sampling areas. For statistical reasons previ- probability level used in these analyses, that 1 ously discussed, some of the confidence intervals out of every 20 ovals of confidence will appear may be too small. Pole A 82, for example, is to be significantly different. Thus, although based on data with widely varying confidence small amounts of polar wandering cannot be ex- intervals, and a circle of confidence cannot be cluded, the paleomagnetic evidence appears to rigorously calculated from such data. Moreover, us to offer no support for theories requiring sub- the virtual geomagnetic poles that are displaced stantial late Tertiary polar wandering; on the from the present geographic pole are distributed contrary, it indicates that the pole has remained throughout the time interval represented and relatively fixed during this time. are not confined to older rocks. This suggests Paleomagnetic polar wandering and continental that the effect may be random rather than drift.—Since the average magnetic dipole and systematic. One also expects, at the 95 per cent rotational axes have been parallel in late Pleisto- 758 COX AND DOELL—PALEOMAGNETISM (1) Europe 1( After Creer, Irving and (2) North America J Runcorn, 1957) (3) Europe 1 (4) North America j (I) Europe 1 (After Creer, Irving and (2) North America] Runcom, 1957) (3) Australia (After Irving and Green, 1958) (4) India (After Clegg, Radakrishnamurty and Sahashrabudhe, 1958) (5) Japan (After Nagata, et al, 1959) FIGURE 34.—POSTULATED PALEOMAGNETIC POLAR-WANDERING CURVES POR EUROPE, NORTH AMERICA, AUSTRALIA, INDIA, AND JAPAN €—Cambrian, S—Silurian, D—Devonian, C—Carboniferous, P—Permian, T—Triassic, J—Jurassic, K—Cretaceous, E—Eocene, M—Miocene, PI—Pliocene results as due to a westward drift of North Amer- Figure 34, and one may readily note that very ica with respect to Europe of 45° (Du Bois, 19S7, large relative drifts and rotations are required p. 179; 1958, p. 512). to bring them into coincidence. The Indian Subsequent paleomagnetic data from India, studies, mostly on the Deccan traps, have been Australia, and Japan have led many authors to interpreted by a number of workers as indi- postulate different polar-wandering curves for cating that India has rotated about 24° counter- each of these regions. The Paleozoic and later clockwise and has drifted 4000-5000 km with re- portions of these curves are reproduced in spect to North America and Europe since the 760 COX AND DOELL—PALEOMAGNETISM Eocene (Clegg and others, 1956, p. 430; Irving boniferous are beginning to emerge as such and Green, 1957, p. 358; Deutsch and others, times (Fig. 35) since there are now enough rela- 1959, p. 53-54). tively consistent results from North America The paleomagnetic results from Australia and Europe to establish an average pole region have also been interpreted as evidence for large and to estimate the expected scatter. Even for displacements of Australia, again with respect these periods, however, the powers of resolution to North America and Europe (Irving and of the paleomagnetic method should not be Green, 1958, p. 71; Irving, 1959, p. 69-72). A overestimated. Displacements or rotations as suggested interpretation for the Japanese results small as 20°, such as have been suggested for is one of polar wandering along the path for Newfoundland (Nairn and others, 1959, p. 596), North America and Europe, on which is super- might well be due to expectable intraperiod vari- imposed the effects of a drift of Japan since the ation rather than to displacements of land Cretaceous and a fairly large rotation since the masses. A subsequent study of several Car- Miocene (Nagata and others, 1959, p. 382-383). boniferous formations from eastern Canada by Paleomagnetic evidence has also been cited in Du Bois (1959b, p. B.A., 63) tends to confirm support of other rotations and displacements of this conclusion. land masses. Nairn and others (1959, p. 596) Viewed on a larger scale, however, the con- have suggested a 20-degree counterclockwise ro- sistency of the Permian and Carboniferous re- tation of Newfoundland with respect to North sults from North America and Europe is most America on the basis of a comparison between impressive and certainly indicates a magnetic- sets of Carboniferous and Precambrian virtual field configuration vastly different from the geomagnetic poles from these two areas. Creer present configuration. If the earth's magnetic (1958, p. 389) suggests a drift of South America field was that of an axial dipole, as it probably with respect to Africa from a comparison of was from Oligocene to early Pleistocene time some Jurassic measurements from these con- and almost certainly was from late Pleistocene tinents, and Creer and others (1958, p. 497-501) to Recent time, then these results constitute a have suggested a displacement of Africa with re- strong case for polar wandering. This interpreta- spect to Europe. The Triassic virtual geomag- tion is also in accord with the axial-symmetry netic poles from Europe have been cited in sup- requirements of the dynamo theory. port of a rotation of Spain with respect to An objection to this axial-dipole interpreta- France and England (Clegg and others, 1957, tion has been voiced by Opik (1955, p. 236), p. 227). A 16-degree counterclockwise rotation however, who suggests that boundary condi- of Japan has been suggested to explain Japanese tions, such as temperature differences at the results from Pleistocene to Holocene (Irving, core-mantle boundary, rather than the earth's 1959, p. 63). Finally, the large displacement of rotation might act to establish order in the con- the Siletz River volcanic series' virtual geomag- vective core motions. The resulting external netic pole from the usual North American polar- field would not be symmetrical with respect to wandering curve has been suggested as possibly the earth's axis of rotation, and, moreover, due to a large clockwise rotation of Oregon with different rates of rotation of the core and mantle respect to the rest of North America (Irving, would not tend to give an average axial sym- 1959, p. 65). metry because the cold and hot spots causing Evaluation and interpretation of pre-Oligocene the convective pattern would move with the paleomagnetic data.—In analyzing the post- mantle. This theory faces some obstacles, how- Eocene, and especially the post-early Pleisto- ever. No analysis has, to our knowledge, been cene, paleomagnetic results, we found a dipolar carried out to determine whether such tempera- field configuration and were able to make an ture differences could exert a substantial in- estimate of the expected scatter in an individual fluence on the convective pattern. Another ob- measurement. With this information we could stacle to the theory is the axial-dipole nature of then discuss, with some confidence, the applica- the earth's field for the past 30 million years. tion of these paleomagnetic data to possible Either the temperature differences did not exist post-Eocene polar wandering and continental during this time, or they were fortuitously sym- drift. In interpreting the pre-Oligocene data, is metrical with respect to the axis of rotation, or there any time for which we may also establish a conceivably they were causally related to the field configuration and an estimate of the rotation axis. Although the principle of uni- scatter? formitarianism favors a polar-wandering inter- The Permian and to some extent the Car- pretation for the Permian and Carboniferous DISCUSSION OF RESULTS 761 paleomagnetic results, Opik's objection should probably not accurate to better than 20 million be kept in mind as a possible alternative ex- years, and this certainly appears to be a reason- planation. able estimate since most of the formations 0- CARBONIFEROUS O PERMIAN A FIGURE 35.—PERMIAN AND CARBONIFERNOUS VIRTUAL GEOMAGNETIC POLES FROM EUROPE, NORTH AMERICA, AND AUSTRALIA North America—squares, Europe—circles, Australia—triangles The Permian data also constitute the strong- studied are lava flows, intrusive rocks, and con- est evidence for a relative displacement between tinental red beds, all subject to more than aver- North America and Europe. An alternative ex- age age uncertainty. Using an estimate for the planation for the displacement of the virtual average rate of movement of the dipole axis geomagnetic poles calculated from the two during the Permian (Creer and others, 1957, p. regions may lie in inaccuracies in the strati- 155), we can compute that an apparent displace- graphic correlation for the Permian on the two ment of 10° would result from a 20 million year sides of the Atlantic Ocean. In discussing and discrepancy in stratigraphic correlation. rejecting this possibility, Creer and others Even assuming that there is no systematic (1957, p. 151) state that the correlations are difference in the age of the rocks from North 762 COX AND DOELL—PALEOMAGNETISM America and Europe listed as Permian, and as- must expect a rather large variation in any given suming also that the field was dipolar, it is im- measurement. Thus, as for the early Paleozoic, a portant to note that the two sets of Permian large uncertainty exists in the basic data avail- data from North America and Europe do not able for polar-wandering and continental-drift uniquely determine the relative positions of the interpretations, and the paths shown in Figure two continents before a hypothetical displace- 33 might best be regarded, at present, as work- ment. The data from each continent must span ing hypotheses. enough time to delineate a segment along a path Paleomagnetic measurements for the Meso- of polar wandering, and a very interesting prob- zoic and early Tertiary present an interesting lem emerges when the Permian and Carbonifer- problem; although the preceding Permian and ous magnetic poles from Europe and North following post-Eocene results are each inter- America are considered jointly. Between Car- nally consistent, the geomagnetic pole positions boniferous and Permian time the average geo- from the Mesozoic and early Tertiary are quite magnetic pole position for North America ap- scattered. Some impressively consistent results pears to have moved roughly N. 85° W., while have been obtained for individual Triassic for- that for Europe moved roughly N. 40°-70° E. mations, and the distribution of results from (Fig. 35). These two path segments cannot be North America and Europe to some extent sug- brought into coincidence by postulating a simple gests relative drift. However, the scatter be- westward movement of North America with tween mean formation directions is quite large respect to Europe since the Paleozoic, but would and weakens the conclusion that a relative dis- require, rather, a large post-Permian movement placement has taken place. The paleomagnetic of North America toward Europe from the results from the Jurassic are at least as scattered southeast. This is not intended as a serious as those for the Triassic, and no conclusive interpretation, but points out difficulties that statements concerning drift or wandering can arise in attempting a unique solution for the be made. (Note also that poles H5, H 7, and data now available. (Geometrical techniques H 10 in Figure 30 are from formations of uncer- useful in experimenting with paleomagnetic re- tain age and may be Jurassic.) constructions are given by Irving 1958, p. 227- The paleomagnetic results from the Creta- 229.) ceous and Eocene may profitably be considered The Australian Carboniferous and Permian together, inasmuch as large amounts of drift paleomagnetic investigations, although few in with respect to North America and Europe have number, include concordant results between been postulated for India, for Japan, and, to sediments and volcanic rocks, with an excellent some extent, for Australia on the basis of geo- fold test for the sediments. However the virtual magnetic data from rocks of these ages. Such geomagnetic poles are quite significantly differ- interpretations are based on the conclusion that ent from the poles for Europe and North Amer- the Eocene and Cretaceous pole positions for ica. If the axial-dipole hypothesis is valid for North America and Europe were within 18° of these periods, the Australian data constitute the present geographic north pole and moved up evidence for a relative displacement of Australia to coincidence with it in the Tertiary, as shown with respect to North America and Europe in Figures 33 and 34. The paucity of paleomag- which cannot be ignored. netic information for these time intervals has In general, there are relatively few measure- already been discussed. In this respect one of ments available from early Paleozoic rocks, and the most reliable virtual geomagnetic poles from those that are available show a considerable the few available North American determina- range in pole positions (Fig. 23). Therefore, tions (the Siletz River volcanic series) falls without sufficient data to determine a field con- much closer to the Indian and Australian paleo- figuration or to assess the expected scatter in magnetic poles than to the Cretaceous and the results, interpretations for these data in- Eocene points on the usual polar-wandering volving continental drift or polar wandering are curves for North America and Europe. Thus, as hazardous. for many of the other periods, the North Ameri- Many more Precambrian data are available, can (and possibly European) field configurations and, as might be expected from older rocks with for Cretaceous and Eocene time are far from large relative age differences, the scatter is large. certain, and, without such a well-defined refer- As mentioned earlier, many of the virtual geo- ence position, the drift interpretations for Aus- magnetic poles tend to lie in the eastern Pacific tralian and Indian Cretaceous and Eocene near the equator. Even if this group of poles results may be questioned. represents an average field configuration, we An alternative explanation of the Cretaceous DISCUSSION OF RESULTS 763 and Eocene measurements, and one that does relevant paleobiogeographic evidence to indi- not require simultaneous rotations of several cate whether these discrepancies could be due to tens of degrees for Oregon and India, would in- continental drift or polar wandering is needed. volve relatively rapid changes in the magnetic- The Permian and Carboniferous periods are field configuration during this period while paleomagnetically very calm, and the measure- maintaining the present continental configura- ments indicate a vastly different field configura- tion. The field may have been nondipolar or, tion from the present one. Thus, Permian and alternatively, may have been that of a dipole Carboniferous formations should also be inter- undergoing somewhat rapid changes in di- esting subjects for paleobiogeographic studies. rection. Such changes may or may not have Recent paleobiogeographic studies of some rele- been accompanied by rapid polar wandering. vance are those of Durham (1950; 1952), Stehli This hypothesis poses many problems. It ap- (1957), and Irving (1956a). pears to us, however, to fit the available data at A further development of the dynamo theory least as well as the drift hypotheses and it for the earth's magnetic field is also highly de- emphasizes the uncertainties connected with sirable. The question of greatest interest con- paleomagnetic interpretations at this time. cerns the axial-dipole requirements of the We have emphasized the tentative nature of theory; the paleomagnetist would like to know some paleomagnetic interpretations. However, if it is at all possible for a nonaxial dipole field we would not like to conclude without empha- to exist for a long period of time. Could stresses, sizing the contributions of paleomagnetism to such as might be provided by a local "hot spot" geology. The post-Eocene results are impressive in the mantle, cause an ordering in core motions and offer very strong evidence for the dynamo that would result in a magnetic field not sym- theory and against substantial Tertiary polar metrical with the axis of rotation? wandering or continental drift. Although the Other applications of rock-magnetism tech- nature of the Mesozoic and early Tertiary mag- niques to special geologic problems might be netic field is obscure, the consistency of the late mentioned. Since the thermo-remanent mag- Paleozoic studies, however, indicates that the netization of some rocks acquired in each tem- paleomagnetic method is quite applicable to perature interval is independent of that ac- older rocks. It seems probable, therefore, that, quired in adjacent temperature intervals, an ap- with additional carefully studied and well-dated plication to unraveling the thermal histories of determinations from Mesozoic and pre-late these rocks may be possible. Magnetic-anisot- Paleozoic time, a clear picture of the field con- ropy properties of rocks may also find im- figurations will evolve, opening the way for portant geologic applications. Techniques that better evaluation of the hypotheses of con- permit a rapid measurement of the preferred tineatal drift and polar wandering. orientation of ferromagnetic grains having either flat or elongate shapes or magnetocrystal- DIRECTIONS OF PALEOMAGNETIC RESEARCH line anisotropy are now available. A field of paleomagnetic research that has In concluding this review, we wish to suggest been merely alluded to in this paper is that de- some lines of investigation in geology and geo- scribed by Thellier and Thellier (1959) in a physics that are of special interest to the worker review of their extensive work on the intensity in rock magnetism, and also to mention some of the earth's magnetic field during historic and important problems that remain for the paleo- Quaternary time. Their report should certainly magnetist. excite more interest in this subject. Although Paleobiogeographic studies probably rank directions of the field are of most use in con- first among those in other disciplines that are of tinental-drift and polar-wandering interpreta- interest in paleomagnetism. The paleomagnetic tions, intensity data should be very useful in data from post-Eocene rocks indicate that the developing an expanded theory for the origin of average geomagnetic pole was close to the the field and, hence, of considerable indirect present geographic pole throughout this time, interest to studies of drift and wandering. and additional independent paleobiogeographic In order to test properly the hypotheses of evidence to show that the geographic pole has large-scale continental drifts since the Creta- also been in its present position throughout this ceous and Eocene, it will be necessary to have period would give added support to the axial- additional paleomagnetic data from well-dated dipole magnetic-field theory. Moreover, large Cretaceous and Eocene rocks from all con- discrepancies appear in the paleomagnetic data tinents, especially from Europe and North of the late Mesozoic and early Tertiary, and America. Rocks of Permian and Carboniferous 764 COX AND DOELL—PALEOMAGNETISM age from localities other than those already negative anomalies with certain minerals: Jour. Geomagnetism and Geoelectricity sampled in North America, Europe, and Aus- [Kyoto], v. 6, p. 176-181 tralia should also be rewarding subjects for 1958, Iron-titanium oxide minerals, rocks, and paleomagnetic studies. aeromagnetic anomalies of the Adirondack In the paleomagnetic work completed, a rela- area, New York: Econ. Geology, v. 53, p. 777-805 tively large number of studies from a given con- Bancroft, A. M., 1951, The magnetic properties of tinent and interval of geologic time have been varved clays: M. S. Thesis, Univ. of Birming- necessary before a clear picture of the field for ham, England that time and place has emerged; the Permian Belshe', J. C., 1957, Palaeomagnetic investigations of Carboniferous rocks in England and Wales: of North America and Europe are examples. Advances in Physics, v. 6, p. 187-191 Even with a large quantity of data, however, Blackett, P. M. S., 1952, A negative experiment the picture may still be cloudy, as for example relating to magnetism and the earth's ro- in the Triassic from North America and Europe. tation: Royal Soc. London Philos. Trans., ser. A, v. 245, p. 303-370 Future investigations may yield more and better Blundell, D. J., 1957, A palaeomagnetic investi- information if they are able to satisfy the fol- gation of the Lundy dyke swarm: Geol. Mag. lowing conditions: v. 94, p. 187-193 (1) The geologic formation to be studied Bruckshaw, J. M., and Robertson, E. I., 1949, The magnetic properties of the tholeiite dykes of should be well dated geologically and accessible North England: Royal Astron. Soc. Monthly at several sites over a relatively large sampling Notices, Geophys. Supp., v. 5, p. 308-320 area. Bruckshaw, J. M., and Vincenz, S. A., 1954, The (2) Careful consideration should be given to permanent magnetism of the Mull lavas: Royal Astron. Soc. Monthly Notices, Geophys. Supp., the sampling scheme so that a minimum of v. 6, p. 579-589 samples will be required, the requirements of Brynj61fsson, A., 1957, Studies of remanent magne- the statistical method to be employed will be tism and viscous magnetism in the basalts of satisfied, and the earth's field over a large area Iceland: Advances in Physics, v. 6, p. 247-254 Bullard, E. C., Freedman, C., Gellman, H., and and at several points in time is represented. Nixon, J., 1950, The westward drift of the (3) Demagnetization and anisotropy studies earth's magnetic field: Royal Soc. London of the samples should always be made in order Philos. Trans., ser. A, v. 243, p. 67-92 to test the stability and paleomagnetic applica- Campbell, C. D., and Runcorn, S. K., 1956, Mag- netization of the Columbia River basalts in bility of the measured magnetizations. 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