Proceedings of the United States National Museum . cus-sion based upon the investigationof anhydrous systems. It is prob-able that the differentiation ofany natural magma is speeded upand that the reactions are facili-tated by the presence of volatileconstituents. The behavior of the water re-leased after crystaUization of thefinal magmatic product is furtherdiscussed below. MIAROLITIC CAVITIES. Under the term miarolitic cav-ities are comprised smaU openspaces which occur in the coarseralbitic rocks and are lined withquartz and albite crystals. In mostcases in the rocks rich in micropeg-matite
Proceedings of the United States National Museum . cus-sion based upon the investigationof anhydrous systems. It is prob-able that the differentiation ofany natural magma is speeded upand that the reactions are facili-tated by the presence of volatileconstituents. The behavior of the water re-leased after crystaUization of thefinal magmatic product is furtherdiscussed below. MIAROLITIC CAVITIES. Under the term miarolitic cav-ities are comprised smaU openspaces which occur in the coarseralbitic rocks and are lined withquartz and albite crystals. In mostcases in the rocks rich in micropeg-matite these cavities are at thejunction of several areas of micro-pegmatite, and the albite andquartz crystals on the walls are as arule continuous with the same min-erals of the adjacent rock and haveonly formed euhedral crystals because of space being available fordevelopment. Entirely similar cavities occur also in coarse albiterocks considered as possibly having originated through hydrothermalalteration of normal —24 4. Fig. 2 —Quartz—SHOWING highly modified DEVELOPMENT OF QUARTZ CRYSTALS OCCURRINGIN MIAROLITIC CAVITIES. 42 PEOCEEDINGS OF THE NATIONAL MUSEUM. VOL. 66. The minerals of these cavities have been referred to as: First gener-ation, those which are the constituents of the surrounding rock andseem to have only accidentally abutted agginst the cavity, namely,albite, quartz, diopside, and titanite; and second generation, which areconsidered to have been deposited on the others by hydrothermalsolutions, namely, epidote, fibrous hornblende, chalcopyrite, and chlo-rite. In one thin section, however, typical fibrous hornblende wasseen incorporated in an albite crystal which was itself a later prod-uct than the albite of the adjacent rock. The cavities vary from very minute or microscopic to some whichare 3 cm. across. The minerals in them may be described individu-ally as follows: QUARTZ. The quartz is in the form of transparent slightly smoky
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Keywords: ., bookauthorun, bookcentury1800, bookdecade1870, booksubjectscience
