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Show PROPYLITE AND ANDESITE. 109 ordinate orthoclase, and with proportions of augite and magnetite very-much smaller than is usual in the basaltic group. We have also varieties in which the orthoclase is much less though still notable, and the augite and magnetite, accompanied with glassy or slaggy material included in the groundmass, are very copious; and there are many intermediate varieties. It seems probable that Richthofen may have contemplated only the former in his expression of the characters of augitic andesite, while Zirkel, taking the entire range of variety as one sub-group, with the more augitic and vitreous ones as the type, did not find reasons for separating them, and, therefore, placed them together among the basalts, to which his types certainly most nearly approach. It must be admitted that a hard and fast line cannot be drawn within this range, nor can it be satisfactorily drawn between the more acid augitic andesites and the augitic trachytes. Nevertheless, it seems advisable to draw one arbitrarily, and place the more acid varieties among the andesites and the more basic among the basalts (dolerite), thus following Richthofen rather than Zirkel. Group III.â€"SUB-BASIC ROOKSâ€"PROPYLITE AND ANDESITE. Sub-groups. Characteristics. 1. HORNBLENDIC PROPYLITE .. Consisting of predominant plagioclase and subordinate orthoclase, the former especially, in large, well-formed crystals, abundantly disseminated throughout a compact, homogeneous base. The fracture is superficially like diorite or other medium-grained granitoid rocks. The varieties usually are olive or tawny green color, sometimes reddish, or the green and red are banded, the former greatly predominating. Hornblende is rarely conspicuous to the eye, but in the microscope is seen in abundance in small fragments, disseminated dustlike, or in spangles. It is pale green and with sharply-defined edges. Biotite and brown hornblende sparingly occur. The facies of the rock suggests that it has been more or less altered and the microscope and chemical analysis confirm it. 2. Augitic propylite (?)..... This rock is mentioned by Richthofen, but has not been recognized in the High Plateaus. A rock having the essential characters of hornblendic propylite, but with the addition of a notable amount of free quartz. It is generally a more siliceous rock than the latter and in most occurrences is fresher in appearance. |