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Show CLASSIFICATION OF EEUPTIVE ROCKSâ€"TRACHYTES. 105 rhyolites, and the same difficulty is experienced in finding a suitable system of subdivision. In attempting to divide them, Eichthofen has given two subdivisions, sanidin-trachyte and oligoclase-trachyte. The admission of an oligoclase-trachyte involves a dilemma. If (as appears from his language) he contemplates a rock in which oligoclase is the dominant feldspar, it cannot, according to ordinary conceptions and definitions, be a trachyte at all, but rather an andesite. If it means that it is abundant, though subordinate to orthoclase, then the same is true of by far the greater portion of the whole trachytic group. Again, sanidin-trachyte also seems objectionable as a characteristic name of a subdivision of the trachytes, since sanidin is the predominant mineral of the entire trachytic group. And yet my own limited studies have led me to the conviction that Richthofen, with his rare insight into the real nature of the subjects he has investigated, has hit upon a valid distinction, which we may safely follow. Among the older trachytic eruptions we find rocks into which plagioclase largely enters; indeed, to such an extent that we are often doubtful whether it may not preponderate over the sanidin, or at least be very nearly equal to it. In these same rocks we also find an abundance of hornblende and magnetite, giving them the dark iron-gray aspect which is presented by many andesites. These bornblendic trachytes, however, are usually coarser and rougher in fracture than the andesites, and the hornblende crystals are rarely found in such perfection and full development-as in the andesites, and macroscopic inspection will generally enable us to form a very good opinion as to which of the two we are dealing with, though sometimes we are deceived. It is evident that such trachytes are not far removed from the andesites, both in chemical and mineral constitution, and they sometimes blend with them. On the other hand, we encounter among the later trachytes a different series of macroscopic characters. They are very deficient in hornblende, and more often contain mica (biotite). They are usually light-colored, pale-gray, or red, or light brown, and almost never dark gray. In texture they vary widely, but in no case do they ever suggest any affinity to andesite, but rather to rhyolite. Some of the varieties, indeed, approach rhyolite so closely that we often have still greater difficulty in separat- |
