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Show BASALTIC GROUP. Ill an intermediate variety of texture (anamesite) has been named in which the crystallization is recognizable but not conspicuous. I fail to discover sufficient reasons for a subdivision on textural characters alone, but differences of habitude which are tolerably constant may, I think, be founded upon the mineralogical constitution. The basalts almost invariably contain olivin in abundance, while in the dolerites it is far less common though sometimes found. The dolerites are as a group more siliceous, though the true basalts sometimes have more than the normal percentage of that constituent. In the true basalts such minerals as augite, magnetite, olivin, leucite, and nephelin reach the extremes of their proportions; in the dolerites the same minerals are on the whole less abundant, and the predominance of the feldspathic ingredient is more emphatic. It has seemed to me, therefore, that the name dolerite should be fully recognized as applicable to a subgroup of the basalts, including those coarser-grained varieties in which the proportion of silica is notably higher than in the typical basalts, and also including the more basic of those rocks which Zirkel has called augitic ande sites Group IV.â€"BASIC BOCKSâ€"BASALTS. Sub-groups. Characteristics. Distinctly crystalline; plagioclase feldspar with (usually) subordinate orthoclase; augite always conspicuous and in large amount; much magnetite; a glassy base with pronounced fluidal texture; formless clots of black ferruginous material usually considered as amorphous augite. Color, dark gray to nearly black.. 2. Nephelin-dolerite....... Similar to the above but with nephelin replacing a part of the plagioclase. 3. Basalt________._________ Fine-grained; feldspar crystals distinguishable only by the microscope. Abundant augite and a glassy base; olivin usually present. Very dark colored, nearly black. 4. Leucite-basalt____..__. With leucite replacing a part of the feldspar and sometimes the whole of it. With nephelin replacing feldspar. G. Tachylite......___...... A vitreous obsidian-like lava, having the basaltic constitution. p |
