G. BRENT DALRYMPLE ALLAN COX U. S. Geological Survey, Menlo Par\, Calif. RICHARD R. DOELL

Potassium-Argon Age and Paleomagnetism of the Bishop Tuff, California

Abstract: Duplicate potassium-argon age determi- years. The samples used for previously published nations on each of three samples from widely age determinations of about 1 million years were separated localities indicate that the age of the probably contaminated with older material. Paleo- Bishop Tuff, California, is about 0.7 million years. magnetic results from five widely separated localities Two of the samples are from the basal ash fall that indicate that the welded part of the Bishop Tuff preceded the ash flow eruptions; one of these two became magnetized when the geomagnetic field samples was collected within 1 m of the contact of was normal and that it may have cooled in several the Bishop Tuff with the underlying Sherwin Till. centuries or less. The Brunhes-Matuyama polarity The third sample is from near the present exposed epoch boundary is now uncertain in the range of 0.7 surface of the Bishop Tuff. The minimum age of to 1.0 million years. the Sherwin Till (Kansan?) is thus 0.7 million

CONTENTS Introduction 665 the Bishop Tuff, California, and potassium- Acknowledgments 667 argon and paleomagnetic sampling sites. 667 Potassium-argon investigation 667 2. Directions of magnetization of the Bishop Tuff, Paleomagnetic investigation 668 California 669 Discussion of results 670 Conclusions 671 Table References cited 671 1. Published potassium-argon age determinations Appendix: Sample locations and descriptions . . 673 on the Bishop Tuff, California 666 2. New analytical data for potassium-argon age Figure determinations on the Bishop Tuff, Cali- 1. Index map showing approximate exposure of fornia 668

argon age of the Bishop Tuff using only primary INTRODUCTION pumice fragments free of xenolithic inclusions. The radiometric age of the Bishop Tuff has We find the Bishop Tuff to be only about 0.7 been important in establishing a minimum age million years old. for the earliest North American glaciation that The Bishop Tuff is a rhyolite ash flow that is can be closely related to a deposit with a radio- exposed along the east side of the Sierra Nevada metric age; it has also been important in the between Bishop and Mono Lake, California, development of a time scale for geomagnetic over approximately 350 square miles (Fig. 1). field reversals. Evernden and others (1957; This unit was named and first described in 1959; 1964; in press) have reported radiometric detail by Gilbert (1938) and has been dis- ages of 0.87-1.2 million years for the Bishop cussed and mapped in part by Bateman (1956), Tuff. The Bishop Tuff contains xenoliths from Rinehart and Ross (1956; 1957), and Putnam crystalline country rock (Gilbert, 1938), most (1960). It has an average thickness of about of which is Cretaceous or older. Because of the 500 feet and varies from an unconsolidated ash importance of the age of the Bishop Tuff, and to a densely welded vitrophyre. The Bishop because contamination by xenoliths always Tuff comprises a number of members that poses a problem in radiometric dating of ash resulted from different eruptions all closely flows, we have reinvestigated the potassium- associated in time (Gilbert, 1938). In the

Geological Society of America Bulletin, v. 76, p. 665-674, 2 figs., June 1965 665

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literature, however, the Bishop Tuff has been assigned to the Matuyama reversed-polarity treated as a single formation representing one epoch (Cox and others, 1964). volcanic event. The 0.98-million year date reported by In Owens Gorge, near sample locality OC071, Evernden and others (1964) was important to and in a roadcut along U. S. Highway 395, our placing of the boundary between the near sample locality 4G002, the Tuff overlies Brunhes and the Matuyama polarity epochs at Sherwin Till (Gilbert, 1938; Rinehart and 1.0 million years. The dense distribution of 20 Ross, 1957; Putnam, 1960), which, as Black- reversely magnetized rocks with ages between welder (1931) and Sharp and Birman (1963) 1.0 and 1.6 million years clearly establishes that have suggested, may be correlative with the the boundary can be no older than 1.0 million Kansan Glaciation. years. The lower limit of this boundary, how-

TABLE 1. PUBLISHED POTASSIUM-ARGON AGE DETERMINATIONS ON THE BISHOP TUFF, CALIFORNIA

Reported K-Ar Identification age (106 years) number References* Remarks

The higher value was obtained by cor- 0.681; 0.783 \ „ 87 KA210R1- (1) P- 2 87 KA210Rlt recting the lower value for 15 per cent 0.830; 0.955 / °' (1) p. 2 loss of radiogenic argon during pre- heating

0.9 KA277 (2) 1.2 KA278 (2) KA 305-KA 328 represent determina- 0.96; 0.98 KA305 (2); (3) p. 175 tions on different size fractions of the 0.96; 0.98 KA320 (2); (3) p. 175 same sample. Different values for the 0.91; 0.93; 0.91 KA321 (2); (3) p. 161; p. 175 same determination have been reported 0.91; 0.92 KA328 (2); (3) p. 175 in different publications.

* References: (1) Evernden and others (1957); (2) Evernden and Curtis (in press); (3) Evernden and others (1964). 10 1 t These ages were calculated using a different decay constant for electron capture (\€ = 0.557 X 10~ yr" ) than 10 -1 has been used since 1958. The effect of the more recent decay constant (\f = 0.585 X 10~ yr ) would be to de- crease the calculated age by about 4J^ per cent.

Our interest in the age of the Bishop Tuff ever, is much less firmly fixed. Between 0.5 stems primarily from the application of po- and 1.0 million years there are only four tassium-argon dating techniques to the problem normally magnetized units. Moreover, the of developing a time scale for reversals of the analytical precision of some of these dates is geomagnetic field. The earth's field is now so low that, were it not for the 1-million year known to have two stable configurations, a date on the Bishop Tuff, the Brunhes-Matu- normal one directed toward the north, as at yama boundary might easily be as young as present, and a reversed one directed toward the 0.7 million years. Thus, the accuracy with south. A time scale showing at least seven which the age of this boundary may presently worldwide switches in polarity during the past be defined depends strongly on the accuracy of 4 million years has recently been presented the age of the Bishop Tuff. (Cox and others, 1964). Of 30 volcanic rock Previously published age determinations on units with radiometric ages within the range sanidine from the Bishop Tuff are presented of 1.6 million years to the present for which in Table 1. Evernden and others have dis- magnetic polarities are known, 10, including carded ages as young as 0.78 million years for the Bishop Tuff, are magnetically normal. All an age of 0.96-0.98 million years because they 10 have radiometric ages between 0 and 1.0 believe that the original samples had been million years and all are assigned to the Brunhes inadequately heated (Evernden and others, normal-polarity epoch. The remaining 20 re- 1964; Evernden and Curtis, in press). In the versely magnetized units all have ages within present investigation the possibility is explored the range of 1.0 to 1.6 million years and are that the dates within the range of 0.9 to 1.2

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million years are older than the date of em- by about 55 km (Fig. 1). The two samples from placement of the Tuff because of contamination the more southern part of the formation are by older rocks. from basal ash falls which preceded the glowing avalanches that formed the bulk of the Bishop ACKNOWLEDGMENTS Tuff. The third sample, to the north, is from We thank Professor Charles M. Gilbert, of near the top of the present surface of the the Department of Geology and Geophysics, formation.

EXPLANATION Exposed Bishop Tuff (approximate)

K-Ar Sampling Site

Paleomagnetic Sampling Site

5 10 MILES

Figure 1. Index map showing approximate exposure of the Bishop Tuff, California (after Gilbert, 1938; Bateman, 1956; Rinehart and Ross, 1957) and potassium-argon and paleomagnetic sampling sites University of California, Berkeley, and Richard To minimize the possible effect of contami- J. Janda, of the U. S. Geological Survey, Menlo nation by older granitic and metamorphic Park, for collecting two of the samples. Po- material, we worked only with fragments of tassium analyses were made by H. Collins primary pumice separated from the ash or the Whitehead, of the U. S. Geological Survey. E. flow. The surface of each fragment was cleaned, Roth assisted with the magnetic measurements. and the fragment was broken open and in- spected for accidental inclusions. The samples POTASSIUM-ARGON INVESTIGATION were then crushed and sieved, and sanidine The samples for our potassium-argon analyses was separated from the —20 to -|-60 mesh were collected from three localities, separated fraction by magnetic and heavy liquid methods.

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Four splits, two for argon and two for po- Two splits of an intralaboratory standard sani- tassium analyses, were taken from each of the dine were run concurrently with the Bishop three sanidine concentrates with a Jones-type Tuff samples and were within 0.6 per cent of the microsplitter. mean of previous analyses. Two recent determi- Argon analyses were done by isotope dilu- nations on B3203 are within 2 per cent of the tion. The argon extractions were done on two mean of all previous analyses as reported by 40 extraction lines that have an Ar radiogemc blank Hurley (1962), and our value for G-1, the of about 1 X 10~12 moles. The samples were U. S. Geological Survey standard granite, is subjected to preheating of 300° C for 13 hours within 0.2 per cent of the arithmetic mean of to reduce the atmospheric argon. They were preferred values reported by Stevens and Niles then fused to a clear glass by induction heating. (I960).

TABLE 2. NEW ANALYTICAL DATA FOR POTASSIUM-ARGON AGE DETERMINATIONS ON THE BISHOP TUFF, CALIFORNIA

Per cent K2O Argon analyses Calculated

Sample Material ALr«rad „ lfl a e (1) (2) Average /•„ \ (moles)t iUn 6§ * no. analyzed Ar40 total (10 years)

4G001 Sanidine 10.63 10.56 10.60 6.734 7.758 X IQ"11 61.6 0.736 ±0.07 7.837 9.250 X JO"11 45.8 0.754 ±0.08 4G002 Sanidine 10.69 10.72 10.70 5.671 6.544 X IQ-" 56.0 0.730 ±0.07 6.799 7.441 X 10-11 78.6 0.692 ±0.06 4G003 Sanidine 10.97 10.94 10.96 7.533 7.799 X 10-" 42.2 0.639 ±0.07 11 8.535 9.904 X IQ- 60.9 0.717 ±0.07

10 1 10 1 40 4 * Using Xe = 0.585 X lO" yr" , \g = 4.72 x lO" yr" , and atomic abundance K /K = 1.19 X lO" . Each age represents a calculation using the argon analysis with the average K.2O value for that sample. The ± figure is our estimate of the precision of the determination at the 95 per cent confidence interval. 12 40 t Corrected for the extraction line blank of 1.12 X 10~ moles Ar rad.

CuO and Ti foil were used for gas cleanup, and Our experimental results for the age of the a molecular sieve (Linde Type 5A) was used Bishop Tuff are given in Table 2. The ± values for water removal. A bulb tracer with an ap- for each sample are an estimate of the repro- proximate tracer size of 3 X 10~10 moles Ar38 ducibility of the age determinations at the per release was used. The tracer was calibrated 95-per cent confidence interval (2 a) and are with a volume of Linde purified air argon based on the results from interlaboratory measured with a McLeod gauge and with an standards and the internal consistency of other interlaboratory standard mineral (63 Alel); replication experiments. None of the ages for calibration was checked before and after the any sample is significantly different from the Bishop Tuff analyses. The gas was analyzed on average value of 0.71 million years calculated a Reynolds-type mass spectrometer. from all six determinations. Four recent determinations on B3203, the M.I.T. interlaboratory standard biotite, made PALEOMAGNETIC INVESTIGATION by using the techniques and equipment de- A paleomagnetic study of 103 oriented scribed, gave an average value of 1.733 X 10~9 samples from the Bishop Tuff was made to 40 moles Ar rad per gram. The total spread in the determine whether the formation consists of four determinations is 0.75 per cent and the two or more flow units which cooled at much mean of our results is within 0.21 per cent of the different times. Because the age of the Bishop mean of all previous determinations reported Tuff may be close to the boundary between by Hurley (1962). the Brunhes normal- and Matuyama reversed- The potassium analyses made by Mr. White- polarity epochs, there was a possibility that part head were by flame photometry using a Baird of the Bishop Tuff would be normal and part KY-2 flame photometer and natural gas-air reversed if the formation included several ash fuel. The chemistry was done by the method flows of different ages. The paleomagnetic described by Shapiro and Brannock (1962). samples were collected from five outcrops span-

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ning much of the lateral extent of the Bishop field was reversed at the time the unit cooled. Tuff (Fig. 1). At two outcrops along the Owens Fortunately for stratigraphic applications, such River (OC051 and OC071), closely spaced as the present one, self-reversals are rare in samples were obtained throughout the entire young rocks and occur only in certain minerals thickness of the formation which is exposed which have had rather specific cooling histories (for a brief review of this subject, see Cox and others, 1964). The observation that samples which cooled at different depths in the Bishop Tuff all have the same polarity reduces the possibility that self-reversal has occurred. The abundance of xenoliths in the Bishop Tuff NORTHERN offered yet another opportunity to test for OUTCROPS self-reversal because inclusions of different IC62I + origin usually contain different ferromagnetic IC6I5 * minerals. Ten foreign inclusions from the Bishop Tuff, among which are granites of 0° 30° 60° 90° 60° 30° 0" varying composition, one metamorphic rock, DEMAGNETIZED AT 400 OERSTEDS and several mafic volcanic rocks, all have the same normal polarity as the Tuff body. This essentially eliminates the possibility of miner- alogically controlled self-reversal. Additional insight into the cooling history SOUTHERN of this formation may be obtained from the OUTCROPS paleomagnetic results because of the close OC003 • grouping of directions of magnetization of OC05I • samples from all outcrops. Geomagnetic secular OC07I - variation typically produces changes of several DEMAGNETIZED AT 400 OERSTEDS tens of degrees in the direction of the earth's field at any locality during intervals of hun- T N dreds to thousands of years. Because the magnetic field returns repeatedly to the same direction, coincidence between the directions of magnetization of two outcrops cannot be XENOLITHIC interpreted as conclusive evidence that they INCLUSIONS cooled at the same time. However, if their directions of magnetization are not the same, they cannot have cooled at the same time. Of the 103 samples from the Bishop Tuff, 23 are from poorly consolidated or friable material UNDEMAGNETIZEO in the upper strata or at the margin of the flow. These 23 have somewhat scattered di- LOWER rections of magnetization that may reflect EQUAL AREA PROJECTION HEMISPHERE partial alteration by surficial weathering. All Figure 2. Directions of magnetization of the results from the other more consolidated and Bishop Tuff, California. Each direction is that of welded sections are shown in Figure 2. The an individual oriented sample after partial scatter in directions from outcrop to outcrop demagnetization in an alternating field of 400 is sufficiently small to be accounted for by oersteds to remove the effects of lightning. Inclu- faulting and tectonic tilting (Rinehart and sions, some of which are weakly magnetized, Ross, 1956; Putnam, 1960). However, initial were not demagnetized. dip of this formation cannot be distinguished positively enough from tectonic tilting to there. All 103 samples are normally magnetized. determine whether the tilting is responsible for The possibility must be considered that the all the observed differences. Thus, although ferromagnetic minerals in the Bishop Tuff are the nearly coincident directions of magnetiza- self-reversing; the normal magnetization of the tion at all outcrops do not conclusively prove Tuff would then indicate that the geomagnetic contemporaneity of cooling, the paleomagnetic

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results are exactly those expected if all the overlies Sherwin glacial deposits near the type outcrops of the Bishop Turf were emplaced locality; hence its mean age of 0.71 million within several centuries or less and subsequently years is the appropriate one for purposes of subjected to tectonic tilting of 10 degrees or assigning a minimum age to the Sherwin Till. less, as appears to have occurred in this area. The discrepancy between our results and those previously published clearly lies outside DISCUSSION OF RESULTS the range of calibration errors. A possible ex- The primary question to be considered in planation is that different cooling units were discussing the age of the Bishop Tuff is whether sampled, and in the absence of information as a single igneous event or multiple events are to the locality and stratigraphic position of the involved. Answers to this question will depend samples used for the previous determinations somewhat on the resolving power of the tools this possibility cannot be excluded. This appears available to each investigator. Geologic map- unlikely, however, in view of the geologic ping, which probably has the highest resolving studies by Gilbert (1938), Bateman (1956), power of all methods now available, indicates Rinehart and Ross (1956; 1957), and Putnam that on a small scale multiple events are clearly (1960), none of whom report evidence for two involved. The basal ash fall is easily distinguish- depositions of Bishop Tuff separated by a able from the main part of the flow, and Gil- hiatus of 0.25 million years or more. bert (1938) distinguishes several additional Alternatively, if the samples analyzed in the cooling units. There remains, however, the previous and the present investigations are from question of the time that elapsed between the the same cooling unit or from cooling units emplacement of these geologically distinguish- closely related in time, as is likely, then there able units. are two possible explanations for the different The paleomagnetic results are relevant only dates: (1) either all our samples were subjected to the welded part of the formation and are to subsequent heating resulting in argon loss, or consistent with the interpretation that this (2) the samples of the previous study were con- was emplaced within several centuries. The taminated by older accidental inclusions. The radiometric ages from the basal ash appear former explanation appears impossible because slightly older than those from the welded part no later igneous rocks overlie or intrude the of the flow. However, the differences between Bishop Tuff at our sample localities. Moreover, the ages are so close to the estimated precision it is improbable that the same argon loss would of the individual determinations that it is un- have occurred at the three widely separated certain from the present data whether or not localities we sampled but not at the localities of the differences are real. Generally, therefore, the previous study. our main conclusion is that the Bishop Tuff is Contamination of the previously studied the result of a single igneous event which samples by accidental inclusions appears to us occurred about 0.7 million years ago. The the most probable explanation of the difference radiometric dates establish a maximum limit of in the calculated ages. The Bishop Tuff con- about 70,000 years for the time required for the tains numerous fragments of older granitic and emplacement of the Tuff and its associated ash metamorphic rocks (Gilbert, 1938; Putnam, falls. The paleomagnetic results are consistent 1960) as is typical of this type of deposit (Ross with a much shorter time for the emplacement and Smith, 1961, p. 35). Unless extreme care is of the welded part of the formation, but here exercised in selecting samples, xenocrystic the evidence is only permissive. orthoclase or microcline, which are difficult to For purposes of paleomagnetic and glacial detect, may be concentrated with sanidine dur- stratigraphy, we prefer, on principle, not to use ing mineral separation. Because the tempera- the average age of the formation but rather the ture of emplacement of the Bishop Tuff was radiometric ages of the stratigraphically rele- less than 1000° C (Ross and Smith, 1961) and vant outcrops, even though the differences in may have been as low as 500-750° C (Gilbert, the ages between outcrops are small. The set of 1938, p. 1856), xenocrysts probably would not paleomagnetic samples 1C620 and the radio- have been completely degassed of their pre- metric sample 4G003 are from the same out- viously accumulated radiogenic argon, leading crop, so that the mean age of 0.68 million years to a spuriously high radiometric age (Curtis and is the appropriate one to be assigned to a time others, 1961, p. 344; Evernden and others, when the geomagnetic field was normal. 1964, p. 154; Evernden and James, 1964, p. Similarly, radiometric sample 4G002 directly 947; Evernden and Curtis, in press).

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The possibility of contamination may be shown that the age of the Bishop Tuff would be even more serious than was recognized in the raised from 0.7 to 0.95 million years provided cited investigations. Evernden and others the sanidine concentrate contained only 3 per (1964, p. 154) state, "The source of crystals in cent feldspar from the Sierra Nevada granitic extrusive flows is of little concern for the tem- rocks (approximately 100 million years old) peratures of these deposits after emplacement and provided this feldspar had retained only 8 were such that any pre-eruption content of per cent of the previously accumulated radio- argon was eliminated (Evernden and others, genic argon. Contamination further provides a 1960; Dalrymple, 1963)." Our experience in simple explanation for the scatter of the ages dating young extrusive flows has lead us to pre- found in the previous studies because the dif- cisely the opposite conclusion (Dalrymple, ferent samples would not be expected to have 1963; 1964). The most direct demonstration the same amount of contamination. that xenocrysts may retain significant amounts of pre-eruption argon may be found in the CONCLUSIONS study by Dalrymple (1964) of a xenolith of (1) The Bishop Tuff probably was erupted Sierra Nevada granite, 10 cm in diameter, col- during a single igneous event about 0.7 million lected 3 m below the top of a lava flow. The lava years ago. flow has a potassium-argon age of about 0.060 (2) The minimum age for the Sherwin Till + 0.050 million years, which is consistent with (Kansan?) is about 0.71 million years. Although the local glacial stratigraphy. The calculated the present investigation reduces the age by age of the inclusion is 2.0 +0.1 million years. 0.25 million years, this Till still represents the At least 2 per cent and possibly as much as 5 oldest American glaciation that is closely re- per cent of the radiogenic argon from the potas- lated to a radiometric age determination. sium feldspar in the granite was retained, even (3) The boundary between the Brunhes nor- though the temperature of emplacement of the mal- and Matuyama reversed-polarity epochs flow was probably more than 1000° C. Dal- is now uncertain within the limits of 0.68-1.0 rymple (1964) concludes that it is the diffusion million years. dimension that controls argon retention rather (4) These results emphasize the importance than the size of the inclusion, so that micro- of careful selection of samples for age deter- scopic inclusions may significantly alter whole- minations on ash flows. The most satisfactory rock dates. material for potassium-argon dating of these Applying the equation developed by Dal- deposits is a mineral concentrated from pumice rymple (1964) to estimate the effect of inclu- fragments free from xenolithic inclusions. sions on potassium-argon ages, it may be

REFERENCES CITED

Bateman, P. C., 1956, Economic geology of the Bishop tungsten district, California: Calif. Div. Mines Rept. 47, 87 p. Blackwelder, Eliot, 1931, Pleistocene glaciation in the Sierra Nevada and Basin Ranges: Geol. Soc. America Bull., v. 42, p. 865-922 Cox, Allan, Doell, R. R., and Dalrymple, G. B., 1964, Reversals of the earth's magnetic field: Science, v. 144, p. 1537-1543 Curtis, G. H., Savage, D. E., and Evernden, J. F., 1961, Critical points in the Cenozoic: Annals New York Acad. Sci., v. 91, Art. 2, p. 342-351 Dalrymple, G. B., 1963, Potassium-argon dates of some Cenozoic volcanic rocks of the Sierra Nevada, California: Geol. Soc. America Bull., v. 74, p. 379-390 1964, Argon retention in a granitic xenolith from a Pleistocene basalt, Sierra Nevada, California: Nature, v. 201, p. 282 Evernden, J. F., 1959, Dating of Tertiary and Pleistocene rocks by the potassium/argon method: Geol. Soc. London Proc., no. 1565, p. 17-19 Evernden, J. F., and Curtis, G. H., in press, The present status of potassium-argon dating of Tertiary and Quaternary rocks: Warsaw Internal. Assoc. Quaternary Research Cong. Proc., 1961 Evernden, J. F., and James, G. T., 1964, Potassium-argon dates and the Tertiary floras of North America: Am. lour. Sci., v. 262, p. 945-974

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Evernden, J. F., Curtis, G. H., and Kistler, R., 1957, Potassium-argon dating of Pleistocene volcanics: Quaternaria, IV, p. 13-17 Evernden, J. F., Curtis, G. H., Kistler, R. W., and Obradovich, J., 1960, Argon diffusion in glauconite, microcline, sanidine, leucite and phlogopite: Am. Jour. Sci., v. 258, p. 583-604 Evernden, J. F., Savage, D. E., Curtis, G. H., and James, G. T., 1964, Potassium-argon dates and the Cenozoic mammalian chronology of North America: Am. Jour. Sci., v. 262, p. 145-198 Gilbert, C. M., 1938, Welded tuff in eastern California: Geol. Soc. America Bull., v. 49, p. 1829-1862 Hurley, P. M., Editor, 1962, Variations in isotopic abundances of strontium, calcium, and argon and re- lated topics: Mass. Inst. Technology Dept. Geology and Geophysics Tenth Ann. Prog. Rept., 153 p. Putnam, W. C., 1960, Origin of Rock Creek and Owens River gorges, Mono County, California: Univ. Calif. Pub. in Geol. Sci., v. 34, p. 221-280 Rinehart, C. D., and Ross, D. C., 1956, Economic geology of the Casa Diablo Mountain quadrangle, California: Calif. Div. Mines Special Rept. 48, 17 p. • 1957, Geology of the Casa Diablo Mountain quadrangle, California: U. S. Geol. Survey Geol. Quad. Map GQ-99, scale 1:62,500 Ross, C. S., and Smith, R. L., 1961, Ash-flow tuffs: Their origin, geologic relations and identification: U. S. Geol. Survey Prof. Paper 366, 81 p. Shapiro, Leonard, and Brannock, W. W., 1962, Rapid analysis of silicate, carbonate and phosphate rocks: U. S. Geol. Survey Bull. 1144-A, p. A1-A56 Sharp, R. P., and Birman, J. H., 1963, Additions to classical sequence of Pleistocene glaciations, Sierra Nevada, California: Geol. Soc. America Bull., v. 74, p. 1079-1086 Stevens, R. E., and Niles, W. W., 1960, Chemical analyses of the granite and the diabase: U. S. Geol. Survey Bull., 1113, p. 3-43

MANUSCRIPT RECEIVED BY THE SOCIETY JANUARY 18, 1965 PUBLICATION AUTHORIZED BY THE DIRECTOR, U. S. GEOLOGICAL SURVEY

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APPENDIX: SAMPLE LOCATIONS AND DESCRIPTIONS 4G001. Bishop 1:62,500 quadrangle (U. S. Survey, 1949 ed.) and Casa Diablo Mtn. 1:62,500 Geol. Survey, 1949 ed.), 500 m S. 27° E. from (U. S. Geol. Survey, 1953 ed.), 0-1000 m south of NW.cor.sec. 4, T. 6 S., R. 33 E., in pumice quarry. NW.cor. sec. 22, T. 5 S., R. 31 E. Thirteen oriented Collected from Bishop Tuff approximately 2 m cores collected along the access road to the power above the base of a 4-m-thick ash-fall deposit. The house span the 100-m thickness of the welded part ash fall is underlain by gravels and coarse-bedded of the Bishop Tuff, the base of which is not exposed sands and is overlain by a glowing avalanche deposit at this locality. Two of the xenolithic inclusions are 7-10 m thick. The sanidine used for the analyses from this section. was concentrated from 30-40 pumice fragments the OC071. Casa Diablo Mtn. 1:62,500 (U. S. Geol. diameters of which ranged from 2 cm to about 7 cm. Survey, 1953 ed.), 1000 m southwest of NE.cor.sec. 4G002. Casa Diablo Mtn. 1:62,500 quadrangle 9, T. 5 S., R. 31 E. Fourteen oriented cores were (U. S. Geol. Survey, 1953 ed.), sec. 34, T. 4 S., collected along the access road to the power house. R. 30 E., northeast of roadcut on U. S. Highway The base of the Bishop Tuff is exposed at this 395 near its junction with Rock Creek and old locality, and the entire welded part, which is > 100 Sherwin Grade Road. Collected from the basal ash m thick, was sampled. fall of the Bishop Tuff less than 1 m above the con- OC003. Casa Diablo Mtn. 1:62,500 (U. S. Geol. tact with the Sherwin Till. The sanidine used for Survey, 1953 ed.), 800 m southwest of NE.cor.sec. the analyses was concentrated from 50-60 pumice 19, T. 4 S., R. 30 E. Twenty six oriented cores fragments the diameters of which ranged from collected from road cuts and quarries immediately about 2 cm to about 10 cm. Collected by Richard east of Long Valley Dam span the exposed welded J. Janda, 1964. part of the Bishop Tuff, the base of which is not 4G003. Cowtrack Mtn. SE., 1:24,000 quad- locally exposed. Eight xenolithic inclusions are rangle (U. S. Geol. Survey 1962 Adv. Sheet), from this locality. SE.cor.sec. 28, T. 1 S., R. 29 E., collected from 1C615. Mono Craters 1:62,500 (U. S. Geol. outcrop on southeast side of road at 8420-foot Survey, 1953 ed.), 500 m southwest of NE.cor.sec. elevation. The sample is from the exposed upper 26, T. 1 S., R. 26 E. Six oriented cores collected surface of the Bishop Tuff, within 100 m of the from vertical outcrop about 50 m east of U. S. depositional contact with the rhyolite and obsidian Highway 395. of Glass Mtn. The sanidine used for the analyses 1C621. Approximately 300-500 m north of the was separated from one pumice block 40 X 20 cm. locality of 4G003 and approximately 20 m lower in Collected by C. M. Gilbert, 1964. the same cooling unit. OC051. Mount Tom 1:62,500 (U. S. Geol.

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