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Show 106 GEOLOGY OF THE HIGH PLATEAUS. ing them from it than we encounter in separating extremely hornblendic trachytes from andesites. In these trachytes sanidin is the only important mineral, and though plagioclase and hornblende are not uncommon, they are never conspicuous, and never seem to exert any notable effect upon the character or aspect of the rock. In seeking for purely descriptive names, it seems to me that the older trachytes will be sufficiently discriminated if we call them simply hornblendic trachytes. It occasionally happens that the other group requires to be spoken of collectively, and I shall in such cases employ the term sanidin trachytes, rather than coin a new name. But for precision it may be necessary to subdivide them rather more minutely, since these so-called sanidin-trachytes embrace very wide variations of lithological aspect. The time has not yet come to divide the immense trachytic group according to definite and final principles. To accomplish that will require the careful study of an enormous range of materials. Although my own observation is far too limited to encourage the hope of finding a complete and satisfactory arrangement, I am tempted to give provisionally and tentatively a subdivision embodying such a grouping as will embrace the facts within my knowledge. Group ILâ€"TRACHYTES OR SUB-ACID ROCKS. Sub-group A.â€"Sanidin Trachytes. • Characteristics. 1. Granitoid trachytes..... Trachytes having a superficial resemblance to granitic rocks; holding much orthoclase ana less plagioclase, with few other minerals; a very little Motite and hornblende; crystals conspicuous; a somewhat porous base, containing little ferritic matter. Usually very light-colored rocks; seldom dark gray. 2. PORPHYRITIC TRACHYTE____ A base resembling that of porphyrite, with very conspicuous and perfect crystals of orthoclase (usually the turbid or milky variety), often large. The base very fine, compact, and non-vesicular; more or less ferritic, sometimes showing a feeble aggregate polarization. The groundmass shows none of that coarse, rough texture so common in other trachytes. |