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Show CLASSIFICATION OF ERUPTIVE EOCKSâ€"RHYOLITES. 103 culty in assigning them to their places in accordance with all their natural affinities. Leucite rocks will fall readily among the basalts. Nephelin, when associated with other minerals common to the basic rocks, may be considered as replacing labradorite, and the rock containing it may be assigned to the basaltic group. When associated with orthoclase, as in phonolite, the rock will fall among those trachytes which contain notable percentages of plagioclase. It yet remains to speak of those lavas which contain no distinct minerals, but which are wholly glassy or amorphous, like obsidian, pumice, &c. Here chemical constitution becomes the sole criterion, and although the external or macroscopic facies may often indicate to the trained eye the approximate constitution, the only safe guide to determination is a chemical analysis. I. RHYOLITES. The rhyolites are distinguished by their high percentage of silica and by the presence of orthoclase and free quartz. The number of varieties of texture found in this group is immense. We find some which have an outward semblance to granite; others containing large, beautiful, and perfect crystals of glassy feldspar an inch or more in length, and large grains of quartz imbedded in a compact matrix; others having the coarse, irregularly granular aspect of trachyte; very many with a groundmass full of elongated vesicles like drawn-out glass and holding small crystals; very many which are so vitreous or slag-like that the crystals are discernible only with the microscope, and many which exhibit no determinable crystals. So protean are the forms, that the lithologist may well feel discouraged in attempting to resolve the group into intelligible or rational subdivisions. Richthofen has attempted it, however, but it seems to me with very partial success. While he has no doubt divided the more prominent sub-groups, cases are often encountered which neither of them appear to satisfy, and microscopic research indicates that many of the characters he has seized upon are less distinctive than the external appearances might at first suggest, and Ibrings to light many others which are of high importance, and which the external appearance does not suggest at all. Considering external characters alone, however, his subdivisions may represent a convenient temporary grouping of the greater part of the rhyolites. |