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Show 110 GEOLOGY OF THE HIGH PLATEAUS. Group III.â€"SUB-BASIC BOOKSâ€"PBOPYLITE AND ANDESITEâ€"Continued. Sub-groups. Characteristics. 4. HORNBLENDIC ANDESITE ... Consists of plagioclase, either wholly or with subordinate orthoclaso and with hornblende; the latter usually conspicuous; the crystals imbedded in a base which is usually moderately fine, sometimes a little coarse. The color is almost always green, from light to very dark. The fracture is peculiar, splintery or conchoidal, radiating from the point of impact. The hornblendes are mostly of the dark-brown variety ; in the thin section with a black, shaded border. The base shows fluidal structure, but not always. 5. AUGITIC ANDESITE......... Usually a more basic rock than the foregoing; feldspar almost wholly plagioclase; augite taking the place of hornblende; either gray or nearly black in color, never with greenish cast unless much altered; the more basic varieties merge into the dolerites and the less basic into the augitic trachytes by transition. Eesemblances to dolerito most frequent. 6. DACITE OK QUARTZ ANDESITE. Containing predominant plagioclase feldspar, with free quartz and almost always abundant hornblende. It has a somewhat rhyolitic texture and habit. Sometimes biotite replaces the hornblende. IV. BASALTS. The classification and subdivision of the basalts present some difficulty. In the basic lavas we have occurrences in which the minerals leucite and nephelin replace wholly or in part the feldspars, and a question arises as to the importance which is to be attached to this substitution. In the other great groups the subdivisions have rested upon texture and general habitus of the sub-groups as well as upon the occurrence of accessory and subordinate minerals in conspicuous quantity. In the acid and subacid rocks accessory minerals are relatively in small proportions and variations of texture and habit very strongly pronounced. In the basic rocks the reverse is trueâ€"the accessory minerals are more numerous, almost rivaling the primary ones, while the texture, though considerably varied, is far less so than in the acid rocks. These considerations would lead us to rest the subdivisions rather upon a mineralogical basis than upon a tex-tural one. Some authors separate dolerite from the so-called "true basalts" on textural grounds, the former being macroscopically crystalline while the basalts proper exhibit distinct crystals only under the microscope. Even |
