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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 Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/41/3/383/3414856/BUL41_3-0383.pdf by guest on 28 September 2021 384 F. K. MOBKIS----AMYGDULES AND PSETJDO-AMYGDULES 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. Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/41/3/383/3414856/BUL41_3-0383.pdf by guest on 28 September 2021 CLASSES OF CAVITIES AND CAVITY-FILLINGS 3 8 5 (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. Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/41/3/383/3414856/BUL41_3-0383.pdf by guest on 28 September 2021 386 F. K. MORRIS----AMYGDULES AND PSEUDO-AMYGDULES 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.

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