BULLETIN OF THE GEOLOGICAL SOCIETY OF AMERICA
VOL. 41, PP. 383-404.4 FIGS. SEPTEMBER 30,1930
AMYGDULES AND PSETJDO-AMYGDULES1
BY FREDERICK K. MORRIS
(Bead before the Geological Society December 26, 1929)
CONTENTS Page Introduction ...... 388 Classes of cavities arid cavity-fillings in igneous rocks...... 384 Comments on certain te rm s...... 385 Amygdaloidal dikes of the Liukiang coal field...... 386 Coal-penetrated dike near C hinw angtao...... 391 Amygdaloidal dikes of the Linsi coal field...... 392 H si Shan am ygdaloid...... 394 The flow near N iangniang M iao...... 396 O ther examples of filling and pseudo-amygdules...... 397 Fillings in the norites of T sin an fu ...... 401 Amygdaloidal dike in Gasp6...... 403 Conclusions ...... 404
I ntroduction
My interest in amygdules and cavity-fillings was stimulated by the study of certain dikes in the coal mines of Linsi, Chihli province, north China, on which a report was written for M. F. F. Mathieu, then geolo gist for the Kailan Mining Administration. Later, cavity-fillings and masses of alteration products were seen in other dike rocks from the Liukiang coal-fields, and from near Chaochiakou. Open cavities in a granite from Mukden and in a remarkable contact rock from Tsinanfu in Shantung were studied, as well as a thick amygdaloid in the Western Hills of Peking. After reviewing the available literature, it seemed to me that a paper which would suggest a genetic classification of cavities and cavity-fillings would be acceptable. Such a classification is offered in this paper and is illustrated with examples which came within my own observation, and with comments upon some published examples, and
1 Manuscript received by the Secretary of the Geological Society, February 1, 1930. (383)
XXV—B u l l . G e o l . S o c . A m ., V o l . 41,1930
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with one case cited by Professor William A. Parks. I am aware that I do not approach full justice to the literature; and in acknowledging what I have found helpful in the writings of others, I wish to express my regret to those whose work I have not seen.
C l a s s e s o f C a v it ie s a n d Ca v it y -F il l i x g s ix ig x e o u s R o c k s
I. Primary cavities: those due to some original process that was completed with the cooling of the magma. 1. Vesicles due to expanding gases in the magma. Common in surface flows, but occasionally found in dikes and sills. These may be classified as follows: (а) Vesicles due to magmatic gases only; (б) Vesicles and irregular cavities due to introduced gases generated from ground-water or pond-water over which lava rode, or to gases distilled from coal or other rocks cut by a dike. 2. The unshaped, very irregular cavities in lava, such as aa lava. 3. “Kneaded” cavities, or spaces due to the movement of crusted lava. Thus one sees in aa lava deep, narrow, rough-sided cavities whose shape is due to the squeezing or folding over of a lava whose scoria- ceous surface is quite or nearly rigid. 4. Flow spaces or caverns formed where the lava ran out from under a rigid surface. They may be quite small. 5. Tensional cracks due to cooling. 6. Miarolites: cavities in granitoid rocks, especially in pegmatites. A subdivision could be made here: (а ) Cavities due to primary magmatic volatiles; (б) Cavities due to water introduced syntectically by the stoping and dissolving of country rocks from which volatiles could be distilled by magmatic heat.
II. Openings due to earth movements. This class includes the following g ro u p s: 1. The large majority of joints—all except those due to cooling or to chemical alteration. 2. The cracks and spaces found in fault planes and fault breccias. 3. The spaces occupied by the large majority of veins and dikes.
III. Secondary cavities: those due to the solution and removal of minerals. They may very closely resemble miarolites. Cavities of this type are common also in other than igneous rocks. 1. Enlarged cavities: primary cavities enlarged beyond their original bounds by the solution of the walls of the primary cavities. 2. Cavities made by the complete removal of a mineral: (а) If the original mineral was automorphic, the cavity will bear its shape. (б) If a xenomorphic crystal or mass of crystal grains has been re moved, the shape will be correspondingly irregular.
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(c) Cavities of either shape may be secondarily enlarged by further removal of the surrounding minerals. This process does not necessarily produce a form of cavity like that of III 2 (6). 3. Cavities made by the alteration of a mineral, a group of crystals or a glass, and the partial removal, in solution, of the alteration products.
IV. Residual cavities due to the incomplete filling of a cavity, made in any of the above ways, by introduced material or by reorganization products.
V. Revived cavities due to the partial or complete removal of a filling from the cavity which it occupied. If the filling is completely removed, it may be impossible to prove that the cavity was ever filled. But even in the zone of weathering, empty cavities will be associated with perfect and partly corroded fillings, suggesting that all or nearly all the cavities were once filled. In a single hand specimen of unweathered vesicular rock one often sees primary fillings that are complete, associated with every stage of revival of the cavity down to vesicles upon whose walls a mere film of introduced mineral remains.
C o m m e n t s o n c e r t a in T e r m s True amygdular fillings are masses of introduced minerals filling pri mary cavities or vesicles in igneous rocks. Any primary cavity may be so filled. The name pseudo-amygdule was applied by Pumpelly2 to a mass of alteration products and introduced minerals formed first by the alteration of a primary mineral to secondary products, such as chlorite and carbonate, and so rearranged as to simulate a cavity-filling. A psuedo-amygdule does not necessarily show that an open cavity ever existed there; so that pseudo-amygdules, considered as a class, can not properly be placed under “fillings.” Irving simplified the spelling to pseud-amygdule, but in this paper the word pseudo-amygdule is used strictly as Pumpelly used it. When an amygdaloid undergoes dynamic metamorphism along with the enclosing rock, as for instance when a Keewatin amygdaloidal basalt is changed to a chlorite schist, the amygdular fillings may be flattened and recrystallized or even destroyed by the redistribution of the filling minerals. Such flattened fillings may be called recrystallized fillings. The term amygdular augen, coined in analogy with the augen in a true metamorphic gneiss, is objectionable, for augen are of at least three genetically diverse kinds. The term metamygdule is not a good one, be cause it would connote an analogy with Lawson’s term metacryst or metaphenocryst, which is not a metamorphosed crystal, but is a new phenocryst developed during metamorphism.
2 Raphael Pumpelly: Lithology of the Keweenawan system, Geology of Wisconsin, vol. 3, 1880, p. 31.
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The term miarolitic is sometimes used for cavities in contact meta- morphic rocks. Thus Barrell,3 in considering the order of crystallization of contact metamorphic lime-alumina-silicate rocks writes:
“Garnet is the mineral which oftenest has the power to assume its own proper form, and does so in the presence of all other minerals. Augite, while molded by garnet, itself molds wollastonite. . . . Calcite is often a secondary mineral, and in such cases fills miarolitic cavities or replaces some previous mineral, and is naturally in such cases highly allotriomorphic.” It would be well to restrict the term miarolitic cavity to cavities formed in igneous rocks under the influence of mineralizers and bounded by crystals which are generally euhedral and arranged centripetally. Filled miarolites properly should have another name than amygdule; they may be called miarolitic fillings. The amygdules considered in this paper are discussed for the purpose of showing the criteria by which some of these types of cavity-filling may be recognized.
A mygdaloidal D ik e s o f t h e L i u k ia n g C o a l F ie l d According to L. F. Yih and C. C. Liu,4 the Liukiang coal-field lies 38 li (13 miles) north of the seaport of Chinwangtao, in the northern part of Chihli province. The succession of rocks is as follows: Upon Archeozoic gneisses lie, in ascending order, Cambrian conglomerate, sandstone and shale, followed by 800 meters (1,760 feet) of Cambro- Ordovician limestone. Upon this rests, with parallel disconformity, 1,000 meters (2,200 feet) of Carboniferous beds; and above these are igneous rocks—andesite, trachyte, and gabbro. The dike rock is a light greenish gray, fine-grained porphyry, containing white rod-shaped phenocrysts of feldspar up to 2 mm. long. The rock shows obscure flow structure. Under the microscope the large feldspars are seen to be composed of basic andesine plagioclase, now replaced by carbonate and less paragonite. In some places a crystal of feldspar is replaced by a single crystal of car bonate ; in others by a coarse mosaic of crystals of carbonate among which lie patches of paragonite and remnants of feldspar. Other pheno crysts are seen whose shape suggests pyroxene, now altered to chlorite, epidote and a dense whitish leucoxene. Probably the original mineral was a titaniferous augite. The groundmass consists of a felt of very minute plagioclase needles, arranged in a distinct flow-structure, and
3 Joseph Barrell: Microscopical petrography of the Elkhorn district. TJ. S. Geol. Survey, 22d Ann. Rept., vol. 2, 1901, p. 548. 4 L. F. Yih and C. C. Liu : The coal-field of Ling-Yung-Hsien, Chihli. Geol. Survey, China, Bull. 1, 1919.
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embedded in a chlorite mass containing numberless minute granules of epidote. Silica, carbonate and a little pyrite are introduced minerals. Along its contacts with the dikes, the coal is changed to graphitic coke which shows prominent columnar structure perpendicular to the contact with the dike. Locally the contact is a sharply defined, irregular boundary with blunt apophyses of dike invading the coal, but with sharper and longer tongues of black, coaly matter invading the dike. Near the contact blocks of coal are included in the dike; but isolated blocks of the dike are also included in the coal. Some of the sharp veins of coaly matter that pierce the dike are 5 mm. thick at the contact, and many extend at least 50 mm. into the dike, forming a network of dark veinlets. The apophyses of dike rock penetrating the coal are extremely irregular and contain many fragments of partly graphitized coal.
F i g u r e 1 .— Contact between Dike and Coal, L iukiang
Slender, sharp veins of coal pierce the dike, and include small xenoliths of porphyry. A few of the thin carbonate veins which pierce the coal are shown as white lines.
The carbonaceous veins cutting the dike may be explained by suppos ing that the magma distilled from the bituminous coal carbonaceous volatiles that entered the magma as veinlets; but how this was done requires consideration. It might be argued that the igneous rock de veloped cooling cracks while still hot enough to allow volatile distillates from the coal to enter the cracks as they developed. To test this, I measured the widths of the black veins along a part of the contact. In the specimens having the best display of veins, the widths aggregated 24.8 mm.' in a contact 155 mm. long; that is, the veins are 16 per cent of
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the contact of this specimen. Assuming that the amounts of feldspar and ferromagnesian minerals were about equal in the original rock (actually there is more feldspar), the mean coefficient of cubical expan sion for this rock would be nearly 0.000021. Assuming further a tem perature of 1,300° C. for the magma at the time of its injection (a figure almost certainly too high), and that the rock cooled to 0° (which it surely failed to reach), then in 155 mm. the total space that could be made by simple contraction would amount to 4.32 mm., as against the actual 24.8 mm. Among the errors of this calculation is the certainly incorrect assumption that the mean coefficient of expansion of the solid rock would remain constant through a range of temperature that rises above the melting point. But most of the errors are such as to exag gerate the cooling contraction; and yet the calculated maximum contrac tion is only about a sixth of that actually observed. Had the coefficients of linear expansion been used in the calculation, the difference between estimated and observed cracks would have been even greater. Volatile distillates from the coal probably exerted considerable pres sure upon the dike. It matters little that the juices could escape out ward through the coal and overlying rocks; if only the rate of escape were slow and the evolution of distillates rapid, the pressure exerted upon the dike might be great enough to produce the phenomena. I have avoided giving a name to the volatiles, for I do not know what volatile would come off from coal when it is distilled under considerable pressure of rock-cover at a high temperature; but I suppose the distillates would contain water, carbon oxides and a considerable series of hydro carbons, together with mineral matter carried in solution by some of the volatiles, notably by the water. The abundance of black carbon in veins, in the groundmass and even in amygdules suggests the breakdown of hydrocarbon molecules and of CO or C02. The amygdules are small and nearly round, ranging in diameter from 0.10 to 0.34 mm. Some are joined, opening into one another; some are connected in chains by slender quartz veins, which enlarge into pinched, lens-shaped vesicles, unlike the round amygdules. Along many of these veins black carbon is distributed, now in the vein itself, now in a smother of black particles in the groundmass along the course of the vein. I tested the possibility that this carbon was merely a smudge on the thin section due to grinding, and found that it is structural. The filling of the vesicles is quartz and chlorite, the quartz in a few large blocky crystals built in from the vesicle-wall, the chlorite filling the irregular central space, though in many vesicles the chlorite reaches the margin at one or more points; and some vesicles, especially the
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spindle-shaped ones near the coal contact, are filled wholly with a mosaic of irregular quartz grains, save for a narrow border of chlorite which grows in about the margin. The simplicity of the structure and coarse ness of the grain of all the fillings are very striking in comparison with the small size of the vesicles. All the quartz shows a pronounced wavy extinction. In many crystals the extinction-band crosses the crystal, as
F i g u r e 2 .—Amygdules in the Liukiang Dike
Thin-section of the margin of the Liukiang dike showing veins and amygdules. Black carbonaceous matter not only forms sharply bounded veins but locally commingles with the igneous rock.
the section is rotated between the nicols, in a fan-shaped course; the fulcrum of the fan is at the margin of the vesicle, and the free end of the moving shadow is toward the center of the vesicle. Therefore I do not attribute the wavy extinction to strain, since the quartz grew into an open cavity, but to a cryptocrystalline internal structure in the quartz, akin to the fibrous structure in some chalcedony, and due to peculiarities in the original growth habit of the crystal. Similar extinction bands have been noted by Buerger and Maury5 in the “colloidal” quartz of a
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cassiterite vein. Some of the amygdular fillings contain tiny crystals of graphite. I suggest an interpretation of the several features just considered. The alteration of the phenocrysts is not like that which takes place in ordinary weathering, but is more like a deep-seated process due to solu tions or volatiles circulating at high temperatures. The smallness and the uniform size of the vesicles suggest that they were formed at depth and in a viscous magma, for in a more fluid magma or under less pres sure the bubbles could have emptied into one another, making bubbles of more diverse sizes. Their generally round and simple shape points to slightness of motion in the magma during their formation. Lastly, I see several reasons for thinking that the gas which made the vesicles was derived from the coal traversed. The presence of vesicles at all in this dike suggests a considerable supply of volatiles, yet the dike is as fine-grained as a surface flow. The carbon invading the dike, the abundant carbon scattered in the groundmass along the veins, the crystal lized carbon in some of the amygdules, all suggest that the gases were introduced. The vesicles would belong to subdivision I I (&) of the classification given above. Another specimen from the same mine, though possibly not from the same dike, contained smaller amygdules, averaging 0.04 mm., entirely filled with chlorite. The rock is similar, but even the smallest feldspars are altered to secondary micas; and the larger ones, which are much smaller than those in the rock just described, are wholly replaced by carbonate. In the same coal-field there is a large sill about 100 meters thick, covering many square kilometers. West of Chutsaoying, about 10 kilo meters north of Tahei Shan, the survey map by L. F. Yih and C. C. Liu shows the basal contact of the sheet passing from Carboniferous to Cambro-Ordovician rocks, so that here, at least, the sheet seems to cut across the bedding. One of my students, Mr. Huang Tien-hua, made a detailed section through the field at Tahei Shan, and his observations confirm those of the Chinese Survey. The rock is an augite-andesite porphyry, of rather fine grain for so great a sheet, but it contains no primary vesicles. There is no doubt in my mind that the sill and the dike came from the same parent magma; they are similar in color and texture as well as in composition, except that the sheet contains more pyroxene, and that the phenocrysts of the dike are somewhat smaller. Mr. Huang reported the dike about 70 feet thick in the mine. The fact
5 M. J. Buerger and Jesse L. Maury: Tin ores of Chocaya, Bolivia. Econ. Geol., vol. 22, 1927. p. 4.
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that vesicles appear in the dike but not in the sill may signify that the dike was richer in volatiles—from whatever source—than the sill. This fact further suggests that the dike, which cuts the coal, obtained its vesicle-making volatiles from the coal, while the sill, remote from the coal, formed no vesicles. It may be objected that large sills do not com-
F ig u r e 3 .—Relations of the “Light” and “Dark” Porphyry, Chinwangtao
Polished section of a core-boring from the coal-field northwest of Chinwangtao. The coal (not shown) is cut by the “dark” porphyry, which in turn invades the “light” porphyry. The tenuous veins of the dark rock suggest the extreme fluidity of the dark magma. The light-colored rock is much shattered ; some of its fragments are rounded and embayed, as though partly dissolved, and some are darkened along their contacts with the dark porphyry. Under the microscope the two porphyries show perfect intermixture along their boundaries.
monly carry amygdules; but true amygdules are found in a larger in trusive in the Hsi Shan, to be mentioned further on.
C o a l - p e n e t r a t e d D i k e n e a r C h i n w a n g t a o
I found a magma penetrated by coaly matter in samples sent to me by M. F. F. Mathieu from a small coal basin northwest of Chinwang tao, Chihli province. The core showed, in descending order, coal, sandy
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shale, black coaly shale and a dark porphyry a foot thick, below which is a light-gray porphyry. The contact between the black shale and the porphyry is similar to that seen in the Liukiang dike. The coal is metamorphosed to a semianthracite having characteristic columnar struc ture. Sharp apophyses of coal invade the dike; twisted vein-like shreds of black carbon are included in the porphyry, which is distinctly darker near the contact and contains finely divided carbon. The dark porphyry invades and brecciates the light porphyry below. Both porphyries are andesitic and both are much altered. The dark rock contained femic crystals, which are wholly altered to a leucoxenic mass; the light rock carried hornblende now changed to penninite. No amygdules were seen, though absorption of carbon by the dark rock seems to be certain. This fact may seem to afford an argument against my judgment that the vesicle-making gases in the Liukiang dike were derived from the coal. But I think the argument is not conclusive, for if the magma of this dike had been more fluid, as its delicately tenuous penetration of the light porphyry seems to prove, the gases generated by distillation of the black shale may have escaped without making vesicles, which require a viscous magma to prevent their collapse. It is quite possible that the “dark magma” was rendered more fluid by the volatiles it had absorbed from the coal and coaly shales, and that this is why it so readily penetrated the “light porphyry.”
A mygdaloidal D ik e s o f t h e L i n s i C o a l F ie l d Some remarkable dike rocks have been found by Messrs. Dupont and Mathieu in the coal mine at Linsi, Chihli. The mine is at the east end of the Kaiping basin, a syncline about 20 miles long, striking about northeast to southwest and enclosed by Ordovician limestone, whose out crops form the hills along the north border of the basin. Igneous rocks have been found only at Linsi. The thickest dike is 1 meter wide. Along the contact the coal is altered to a graphitic coke with columnar struc ture, as at Liukiang. Also as at Liukiang, the dike is pierced by sharp threads of carbonaceous matter which extend 5 cm. or more into the dike. Amygdular structures are very abundant, and these require some dis cussion. Under the microscope the rock is a dark micro-porphyry composed chiefly of ferromagnesian minerals. The most abundant mineral is a pale greenish-gray augite in long slender plates or rods. Other essential min erals are biotite, in hexagon-shaped plates; nephelite and alteration products of nephelite, appearing in the interstices; and feldspar which is
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wholly altered. Eods of apatite and grains of magnetite are plentifully sprinkled between the other minerals, and in countless places the mag netites are arranged in rings about a central mass of green serpentinous aggregate made by the alteration of a primary mineral, probably olivine. Carbonate also appears in some of these altered phenocrysts, commonly in the center; and the serpentine in these phenocrysts commonly forms fringes of parallel fibers about their margins. Other such masses' are seen which have the shape of phenocrysts except on one side, where the secondary minerals, bordered always with green serpentinous aggregate, branch out into the groundmass in irregular growths while the car bonate appears as a large central bolus made of coarse interlocking crystals with well developed cleavage. These appear to be pseudo- amygdules. The euhedral feldspars are represented by areas of carbonate and secondary mica. These areas are pierced by veinlets of green chloritic matter which frequently occupies nearly half the space of the former phenocryst. Some of the pseudo-amygdules partly replace, and were partly derived from altered crystals of feldspar. Altered and partly dis solved crystals of pyroxene and biotite are enclosed in the edges of the green pseudo-amygdular mass, and the ghosts of vanished crystals may be seen where a slight granularity of texture in the green matter is ar ranged in rod-like shapes. It is indeed difficult, where the shape is not diagnostic or the flaky mica is lacking, to tell whether a mass was orig inally a femic or a felsic primary mineral. Round, bubble-like amygdules containing the same green fibrous aggregate and the white carbonate that are found in the pseudo-amyg dules are also seen; a little late chalcedony replaces part of the earlier carbonate. Veins of green chloritic matter connect many of these bodies with one another; and veins of carbonate cut and even fault some of the amygdules and pseudo-amygdules. The green veins swell and pinch irregularly, and their walls are not parallel but are crenulated; the white veins have smoother walls and show a crustified structure of crys tals meeting in a zigzag center line. I infer that alteration of the pri mary phenocrysts and of the groundmass and their reorganization to form pseudo-amygdules, took place at the same time that the primary vesicles were filled, and that the same circulating waters accomplished both processes. Probably, too, the alteration and filling took place immediately after solidification and while the dike was still hot, for only so can we account for the vigorous solution of the silicate minerals and magnetite.
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Hsi Shan Amygdaloid
The chief features of the Hsi Shan amygdaloid are its uniform, fine grained texture, with diabasic structure; and the predominance of feld spar, with much less interstitial pyroxene, now wholly altered. Small granules of magnetite are fairly abundant. The rock may be classified as an amygdaloidal diabasic augite-andesite. According to Dr. W. H. Wong 6 the diabase is a large intrusive sill “ordinarily intercalated be tween the Permo-Triassic sandstone and the lower Jurassic coal series,” rising locally into the coal series. Its average thickness is given as 300 to 500 meters (980 to 1,600 feet), and even its eroded remnants cover large areas, one estimated at 96 square kilometers. These facts make the simplicity of composition and uniform, fine-grained texture the more remarkable. I saw scarcely any evidence of differentiation and no pegmatites, such as the pegmatitic layer near the chilled roof of the Palisade sheet in New Jersey—a sheet of diabase of less thickness than the Hsi Shan. Doctor Wong reports that -“the metamorphic effects of the diabase on the sedimentary rock at its contact are but insensible.” These facts suggest to my mind a magma poor in the solvent-volatiles, or mineralizers, and of a viscosity too high to permit much gravitative differentiation. But if these inferences be granted, the presence of abundant large amygdules, 2 centimeters and more in diameter, becomes a very striking feature. Most of the vesicles are primary, as is shown by their round shapes, their smooth outlines, the fact that several cham bers open into one another, and the fact that the feldspar rods of the rock are crowded back around the bubbles and lie tangent to them. Some amygdules have been enlarged beyond the boundaries of the cavities, replacing the rock with carbonate and epidote, and these should be classed as secondarily enlarged amygdules, under subdivision III, para graph 1 of the classification suggested on page 384. Some of the fillings are composed chiefly of quartz and some of epidote, although both min erals may be present; in some amygdules a layer of carbonate rims part of the margin. The arrangement of the introduced minerals is variable: (1) The amygdule may consist chiefly of carbonate; (2) a carbonate margin may be followed by a narrow rim of green, strongly pleochroic chlorite, which is not continuous but is broken by radiating masses of muscovite, whose ragged star-like periphery pierces the quartz crystals—similar radiating groups are isolated in the quartz and their slender plates project between as well as into the quartz crystals, which form an irregular mosaic showing no comb-strueture or centripetal -ar-
6 L. F. Yih : The geology of the Hsi Shan or Western Hills of Peking. Mem. Geol. Survey, China, ser. A, no. 1, 1920, pp. 38-42.
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rangement; (3) the epidote margin ramifies into the groundmass of the diabase, straggling among the crystals as irregular veinlets, enlarging locally into masses of epidote—“pseudo-amygdules,” in Pumpelly’s sense; within the chamber, quartz and epidote appear in irregular, com plexly interlocking grains; (4) epidote, quartz and chlorite are irregularly distributed within the amygdule, each in restricted areas; (5) small irregular bodies of red hematite, varying from deep brownish red almost to scarlet, are found, some as amygdular fillings, some as veinlets, some rimming the green and white amygdules along one side; most of them ramifying into the surrounding rock, and even including shreds of it. The original amygdules have been enlarged by the alteration and solu tion of the surrounding rock. Doctor Wong estimates the original thickness of the rock-cover over this intrusive at “a few hundreds of meters at most.” He says “the diabase is found generally below the Men Tou Kou series (Jurassic), only exceptionally in the lower part of the Kiu Lung Shan and never in any higher horizon. We may therefore conclude that its intrusion must have taken place after the formation of the Men Tou Kou series, and during the sedimentation of the upper part of the Kiu Lung Shan series.” 7 The Kiulung Shan is Lower Jurassic and includes 700 meters of shale, sandstone and conglomerate, above which comes the Tiaochi Shan formation, 1,500 meters thick, of surface flows, conglomerates, shales and coals.I do not quite follow Doctor Wong’s reasoning that had the Upper Jurassic beds been in place, the sheet would have intruded them; but I agree that the cover was not very thick above the amyg- daloidal sill. If the Kiulung Shan series, or a part of it, was in place at the time of the intrusion, the vesicles would have had to expand against the downward compression exerted by the weight of the forma tion. The pressure would be of the order of 1,000 pounds per square inch for 300 meters of overlying rock, or 2,400 pounds per square inch for 700 meters. These considerations suggest that the gases which made the vesicles were introduced from outside the magma; for at such depth and pressure, magmatic volátiles should have rendered the magma more freely fluid and should have promoted differentiation and the growth of larger crystals. At such depths and under such pressures, it is physically and chemically improbable that volátiles would separate from solution in the magma, and expand to produce bubbles, and yet leave a rock of uniformally felsitic texture. A further suggestion of viscosity is given by what Doctor Wong calls the scaly character of the diabase, so that it is “easily confounded with
7 Loc. cit., p. 42.
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sedimentary rocks to the inexperienced eye.” I examined this scaly structure near Mentoukou, and suggest that movement in a very vis cous magma may develop strained zones which would not be perceptible until the rock had been exposed to weathering. In the Palisade sill of New Jersey, there is a pronounced sheeted jointing near the base, in the fine-grained chilled zone of the body, suggesting that this zone, when viscous, was dragged by the eastward movement of the main mass of more liquid magma, and that the strain then initiated caused joints to open when the rocks were exposed to weathering. In the Hsi Shan the shaly parting appears as much as 100 feet above the contact and is very prominently developed. It should imply high viscosity of the magma.
T h e F l o w n e a r N ia n g n ia n g M ia o An interesting association of cavities and fillings was observed in a weathered rock from near the temple called Niangniang Miao, near Hoshengchou, Chihli province. Dr. F. F. Mathieu did the field work, and I am indebted to him for the specimens and for information con cerning the field relations. There is an anticlinal valley, developed upon the easily weathered Archeozoic granites and gneisses. Flanking the valley are walls of conglomerate, arkose and minor shale members be longing to the Proterozoic Nankou series. The flow lies conformably between two arkosic sandstone members and appears on both sides of the valley, though different thicknesses intervene between it and the basal conglomerate on the two limbs of the anticline. The rock is a badly weathered red-brown felsite containing dark green amygdules. Under the microscope the feldspars are seen to form a felted mass of long slender laths, all Carlsbad twins. Between these are angular interstitial patches of a green chlorite, representing an al tered ferromagnesian mineral, and also a large amount of dense clayey matter stained dark brownish red with iron-oxide. I suppose that the original rock was a trachyte. The amygdules are round and as nearly smooth walled as these bodies commonly are—that is, the margins are crenulated a little in places where they have slightly bitten into the walls. They are filled with a green fibrous delessite, which is in some places arranged in countless incomplete spherulitic growths, crowded to gether; in other places the mineral grows inward from the walls of the cavity in graceful radiating tufts. The groundmass includes also irregular shaped bodies which ramify among the feldspars. They contain platy serpentine and a fibrous chlorite of the deep green color seen in the centers of some of the amyg-
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dules. Locally quartz appears as a late mineral filling the center of the mass. These bodies range in shape from thin triangular wedges, molded against the feldspar rods, to larger irregular branching masses. Some are connected with each other by irregular veinlets whose sides are not at all parallel; and some are connected by similar veinlets with the round amygdules. I trace a series of stages between the serpentine masses, which certainly represent pyroxene crystals, and the more com plex masses of fibrous serpentine, chlorite and quartz. Here we have amygdules and pseudo-amygdules in the same rock, an association which Pumpelly and Irving observed in the copper-bearing basalts of Keweenaw Point. The ferromagnesian minerals were probably altered when the primary vesicles were filled; and both the alteration and the filling were due to the same hot-water solution. Revived cavities, due to recent weathering, are seen in this rock, and every stage can be traced from green fillings containing small irregular cavities lined with yellow limonite, to completely restored vesicles whose walls are thinly crusted with limonite. Revived cavities are common in many weathered amygdaloids.
O t h e r E x a m p l e s o f P i l l i n g a n d P s e u d o -A m y g d u l e s Whitman Cross 8 writing of the petrography of the dike rocks of the Apishapa quadrangle says “with few exceptions the dike rocks of the Apishapa quadrangle are dark gray or black aphanites . . . a number of the specimens are characterized by white grains of analcite, which appear like primary phenocrysts but are believed to be the secondary filling of small vesicles or irregular cavities. This analcite is commonly associated with calcite and chlorite, and in one specimen round pores are partly filled with a fibrous zeolite.” Doctor Cross classifies these rocks as olivine-bearing augite vogesite, and, referring to the analcite, says, “its frequent association with calcite and chlorite suggests a second ary origin in such rocks, but it seems possibly primary in others.” Under the heading “Augite hornblende vogesite” he adds, “analcite occurs in rather notable grains, in some specimens being, however, open to the suspicion that it is secondary.” As some of these rocks have a glassy groundmass and all are very fine grained, they must have cooled quickly. The exposures lie in sedimentary rocks ranging from the Dakota sand stone to the Apishapa shale, and are assigned by Doctor Stose to the Tertiary, probably post-Eocene. Even if, at the time of intrusion, no
8 Whitman Cross: Petrographic description [of rocks of the Apishapa quadrangle, Colo.] in the Apishapa Folio, Colo., by George W. Stose; TJ. S. Geol. Survey, Geol. Atlas Folio, 186, 1912, pp. 9-10.
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Eocene sediments covered the region and the Upper Cretaceous had been no thicker than that now exposed, there would have been more than 1,200 feet of covering sediments counting up from the base of the Dakota, and more than 900 feet counting up from the top of the Greenhorn lime stone. Certainly these figures err in being too small; yet they indicate a notable thickness of cover for dikes that have glassy matter and vesi cles. The cover is comparable to that of the Hsi Shan amygdaloid, and the cover of the Liukiang and Linsi dikes was probably thicker. If the dikes sustained the weight of the overlying sediments, the vesicles must have been formed against pressures of from 900 to 5,000 pounds per square inch. It is quite possible that volátiles were distilled from the surrounding rocks and entered the viscous magma with pressure enough to create the bubble cavities. The same volátiles may have taken part in depositing the fillings. Pseudo-amygdules in lava flows and intrusive rocks have been noted by other authors, though the term is not always used. The classical example is found in the amygdaloidal basalts of Keweenaw Point, de scribed by Pumpelly and Irving.9 Irving says (page 62) : “These vesicular upper portions [of the diabase flows] have always under gone great internal changes, both in connection with the deposition of minerals in vesicles, and in the formation of pseud-amygdules, or minerals replacing primary constituents in such a way as to present macroscopically very much the appearance of true vesicular fillings.” Pumpelly indicates that pseudo-amygdular alteration took place even prior to the filling of true vesicles, and that the two processes are inti mately related. Barrell,10 writing of the Marysville district, Montana, says: “The dikes . . . are both dark basic intrusives containing solution cavi ties, filled with sky-blue secondary mineral. . . . Of the femic minerals, small fragments of still unaltered pyroxene, embedded in a matrix of pale green or yellow hydromica, indicate that there has formerly been an abundance of clear colorless pyroxene in the rock. In addition there are numerous amygdaloids from 2 to 4 mm. in diameter, filling polygonal or rounded cavi ties, showing a geodal lining of calcite succeeded by chalcedony, which has completed the filling. No indication of the original matter remains.” Barrell has distinguished what appear to be secondary pseudo-amygdules from true cavity fillings. Barrell11 writes of the hornblende microdiorite
9 Raphael Pumpelly: Lithlogy of the Iieweenawan system. Geology of Wisconsin, vol. Ill, 1880, pp. 27-49. Roland Duer Irving: The copper-bearing rocks of Lake Su perior. U. S. Geol. Survey, mon. v, 1883, pp. 87-90. 10 Joseph Barrell: Geology of the Marysville district. U. S. Geol. Survey Prof. Pap. 57, 1907, p. 63. “ Loc. cit., p. 46.
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of Bald Butte. “A few rounded cavities 1 to 2 mm. in diameter now filled with hornblende, some feldspar and chlorite may possibly have once contained augite.” He gives a photomicrograph (Plate IV) show ing one of these bodies, which looks like a pseudo-amygdule. The “geodal” character of the filling does not of itself prove that an open cavity once existed, as Pumpelly shows that the secondary products in pseudo-amygdules are quite commonly arranged in concentric, parallel bands. Irving12 quotes him in part :
“Very often one of these bodies has a large central area filled with closely- packed spheres, surrounded by fragments of a once continuous band with cross-fibrous structure, which evidently once formed the outer limit. Out side of these fragments is an outer chloritic area, resembling that in the cen ter, and generally bordered on its outer limits by a narrow cross-fibrous band which adapts itself closely to the primary constituents. The greater number of these bodies seem to have resulted from a gradual change of the primary minerals into chlorite by progress from molecule to molecule. At first glance, the structure does not seem to confirm this view, for the narrow outer band inclosing a large central filling seems to suggest either, 1st, a pre-existing cavity, on thè walls of which the thin outer layer was deposited, as the older member, and within this the central filling as the younger, or 2nd, the replace ment by chlorite of a former secondary mineral, which was attacked at the same time around its circumference, producing the outer band (shell), and throughout the interior. Amygdules resulting from both these processes are abundant in the amygdaloids proper ; but they betray their origin in a marked manner, and differ essentially from these pseudo-amygdules. Whatever the chemical nature of the process resulting in these pseudomorphs, the central area is the oldest member, while the outer band is the younger.”
. Rogers 13 describes a rock in the Headquarters Mountains, at the ex treme western border of the "Wichita Range, Green County, Oklahoma, as follows: “A short distance east by northeast of Mt. Walsh a fine-grained, black basic dike about 4y2 feet wide occurs. It has an east-west strike and almost verti cal dip. The rock is exposed in a prospect shaft and is much altered. It consists of lath-shaped plagioclase, amygdules filled with a chlorite-like sub stance (representing original ferromagnesian mineral) and abundant mag netite. The rock has the typical ophitic structure, and so is a diabase.” Here again, the filling is regarded by Professor Rogers as secondary, and is a pseudo-amygdule in Pumpelly’s sense.
12 Loc. cit., p. 65. 13Austin F. Rogers: Aegirite and riebeckite rocks from Oklahoma. Jour. Geol. 15, 1907, pp. 283-287.
XXVI—B u l l . G e o l . S o c . A m ., V o l . 41,1930
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Blackwelder,14 writing in 1907, suggested the possible presence of pseudo-amygdules in “an altered basalt, Rock No. 43 . . . an in trusive in the Man-t’o shales (Lower Cambrian) near Lau Kia p’u,” in western Shantung. He writes :
“Decomposition products, such as calcite and fibrous greenish and yellowish minerals contribute largely to the formation of this groundmass. The larger bodies in the rock are of variable size, and, regardless of the question as to whether they are phenocrysts or amygdules, they now consist almost entirely of calcite, with in some cases a fibrous chloritic substance and chalcedony. The majority of these bodies are more or less round or irregular in outline, but others have fairly definite crystal forms which are identical with those of feldspars. These crystals were probably plagioclases which have been re placed by calcite. . . . Throughout the rock there is an abundance of cal cite in formless plates and clusters. It is doubtless a result of weathering of the feldspars and pyroxenes of the original basalt.” I agree with Doctor Blackwelder that the changes cited result from katamorphic processes in the rock, and undoubtedly weathering has taken place; but the replacement of feldspar phenocrysts by calcite, chloritic substances and chalcedony, suggests the action of more deep-seated solu tions, of higher temperatures than those of ordinary weathering. Here, too, the bodies are probably pseudo-amygdules. Fenner15 has described alteration on a far larger scale than that in volved in the making of pseudo-amygdular bodies. He says in his summary16: “It is believed that the basal sheet represents a surface flow of lava poured out over a continental area which had been depressed by crustal movements of deformation (either warping or faulting) and in whose lower lying por tions a lake or series of lakes occupied shallow basins. The pahoehoe struc ture of the basalt is believed to have been developed over or immediately adjacent to the lake beds through quicker cooling of the flow.” He thinks the alterations were brought about by meteoric waters in troduced into the lava immediately after its extrusion, and that the succession of minerals accompanied the falling temperature of the cir culating waters as the lava cooled. He describes a remarkable succession of secondary minerals beginning with the high temperature types— albite, garnet, amphiboles, specularite, sulphides and quartz—which were later largely destroyed as the temperature of the circulating waters continued to fall; prehnite, pectolite, datolite were succeeded by a series
14 Bailey Willis, Charles D. Walcott, Eliot Blackwelder, R. H. Sargent and others : Research in China. Carnegie Inst. Pub. no. 54, vol. 1, 1907, p. 394. 15 C. N. Fenner: The Watchung basalt and the paragenesis of its zeolites and other secondary minerals. Ann. N. Y. Acad. Sci., vol. 20, no. 2, pt. 2, 1910, pp. 93*187. 16 Loc. cit., p. 185.
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of zeolites, and the latest minerals were calcite and gypsum. His studies clearly demonstrate one method in which pseudo-amygdules, and true amygdular fillings may be formed, though it is clear than in intrusive rocks the volatiles must be other than meteoric waters.
F il l in g s i n t h e N o e it e s o f T s in a n f u In western Shantung, northwest of Tsinanfu, about a dozen small cone-shaped hills rise above the alluvium of the Great Delta, like islands above the sea. The hill most easily reached is crowned with a temple and is called Piao Shan. From the summit one sees that the knobs lie within a somewhat elliptical area, 6 or 8 miles wide. Piao Shan was visited by Yon Bichthofen, who called the rock diorite. A fuller description is given by Willis and Blackwelder 1T, and Blackwelder gives a petrographic description of the rock which my own studies confirm. In 1922 when Professor Barbour 18 became interested in the Tsinan area, I turned over to him my notes, specimens and thin-sections. He has since published a paper on the area. The rock is a hypersthene norite invading an Ordovician limestone. It has uniform, evenly granitoid texture throughout its extent except at one locality, where a zone of coarse pegmatite surrounds a mass of garnet-pyroxene-mica-calcite rock, which we have interpreted as a xeno- lith of the Ordovician limestone. Miarolitic cavities are found in the pegmatite, and cavities bordered by euhedral inward-pointing crystals are seen in the carbonate rock. The problem of the origin of the cavities alone concerns us here. In the top of one of the norite hills there is a mass of coarse-grained rock whose weathered outcrop measures about 150 by 100 feet. The margin of the area is a true pegmatite which surrounds a core of contact- metamorphic minerals. The pegmatite consists of light gray crystals of andesine, 10 to 20 mm. long, in and between which are long black crystals of pyroxene. Next in abundance is titanite in dark yellow crystals as much as 4 mm. long. Accessory minerals include: apatite, magnetite, chalcopyrite and a little biotite which borders some of the pyroxenes. There are small interstitial spaces between the feldspars. A dark lining marks the original boundary of each cavity, but projecting into it from the margin is a druse of long, slender rods of a mineral which Barbour identifies as andalusite. Outside of the drusy cavity the same mineral is seen in more massive crystals, some of which are in
17 Loc. cit., p. 47. 18 George B. Barbour: The Tsinan intrusive. Bull. Geol. Soc., China, vol. 2, no. 1*2, 1923, pp. 35-78.
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optical continuity with the comb crystals of the druse, despite the dense margin line that separates them. The larger crystals form a zone 0.1 to 0.3 mm. wide about the cavity, and on the outer margin they dovetail into the feldspars, fitting themselves in long, delicate fibre-like proc esses into the feldspars parallel to the multiple twinning. The andalu- site (?) is the latest mineral of the magma and was formed under pneumatolytic conditions. Probably its formation was not continuous,
F i g u r e 4 .— Filled miarolitic Cavity in Pegmatite near Tsinanfu
Thin-section of filled miarolitic cavity in the norite of Tsinanfu. Two successive growths of a rod-like mineral are shown, alternating with a black dense mineral (pyrolusite?). The center is quartz.
but was interrupted by periods of balanced equilibrium between the solu tion phase and the crystals. Such interruptions are shown by several boundary lines, one of which marks a considerable halt in the growth of the crystals, and possibly a partial reabsorption. The center of one such cavity is filled with fibrous silica arranged in tufted, centripetal growth, and fitted in among the rod-shaped crystals. The cavities are apparently true miarolites, filled later with silica. Other interstitial spaces are filled with white, fibrous albite of radiating, spherulitic growth,
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which to the naked eye looks like pectolite. Some of these interstitial spaces are 4 mm. long. The feldspars bounding them are perfectly euhedral against the albite mass but are not geodally arranged. The cavities in the partly replaced xenolith were found only in the weathered zone, and are due to removal of the soluble calcite; but the centripetally arranged silicates that project into the calcite mass suggest two alternative explanations. If the limestone had been dissolved in the magma, forming a syntectic, the interstitial calcite would have been the last to crystallize; but if the limestone was never dissolved, the carbonate is residual and is the oldest mineral in the rock. It has been thoroughly recrystallized, so that all trace of the finely granular structure of the original rock has been lost, and the carbonate has slowly attained its present coarsely crystalline form under the influence of heat, pressure and magmatic volatiles. The centripetal crystals have merely replaced the calcite, and the rock contained no cavity until it was exposed to weathering. The structural relations of the mass convinced Barbour and me that the explanation just given is correct. The cavities can be classified only after the nature of the rock itself has been determined. It is necessary to know whether the rock should be called igneous or syntectic, and whether the carbonate was an introduced cavity-filler or an unreplaced residuum whose removal by weathering made the cavities. This dis cussion bears in turn upon the true pegmatite of the border and upon the filled cavities found in it. I suggest the possibility that the solvent volatiles of this pegmatite were derived partly or wholly from the en closed block of limestone because of the paucity of pegmatite in the norite mass and because of the close relation of the pegmatite to the pyroxene-garnet-carbonate rock. If these ideas be admissible, the miaro- litic cavities in the pegmatite would come under class I 6 (b) of the; proposed classification.
A mygdaloidal D i k e i n G a s p e Prof. William A. Parks of the University of Toronto has very kindly written the following note upon an amygdaloidal dike which he has studied: “In the ‘Report of Progress of the Geological Survey of Canada to 1863,’ page 402, Sir Wm. Logan describes a dyke of ‘greenstone’ at Tar Point, Gasp6, as follows: ‘This dyke has a breadth of 10 or 12 yards. Its color is dark grey, weathering to a rusty red, and abounds in large and small druses; which, as well as the joints, are often lined with chalcedony; sometimes, in the case of the druses, presenting botryoidal surfaces, and at others, incrusted with
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crystals of quartz and calcite. These cavities, as well as others which are not thus lined with chalcedony, are filled with petroleum; this, in some cases, has hardened to the consistency of pitch.' “The dyke cuts the upper part of the Grande Gréve limestone and the lower part of the Gaspé sandstone. It is a fairly coarse diabase in which amyda- loidal cavities would not be expected. Petroleum occurs in joints and fissures of the limestone as well as in the dyke. It has been thought that subsequent infiltration would account for both occurrences; it is possible, however, that the cavities in the dyke are due to the absorption by the magma of hydro carbons derived from the neighboring petroliferous rocks.”
C o n c l u sio n s A successful classification of cavities and fillings must be based on a complete history of the rock, including the size and type of the igneous body; the quality of the rocks traversed by the magma, and their capacity to deliver volatiles and dissolved minerals to the magma; the depth of cover over the intrusive body, and the probable pressure that it could exert, remembering that tensional earth-movements may open a dike chamber so as to counteract the pressure of overlying rocks; the nature of the cavity itself, remembering that a given cavity may be in part jprimary, in part secondary; the sources of the filling minerals, and the manner of their emplacement. In choosing criteria the need for quantitative data is urged. The volatiles of many intrusive magmas are derived in part from the rocks which the magma invades. The derived volatiles may either become dissolved in the magma, or they may merely inflate cavities, and aid in altering the primary minerals, in redistribut ing the alteration products, and in introducing mineral matter from without.
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