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Show 100 GEOLOGY OF THE HIGH PLATEAUS. heavy, holding triclinic feldspar, augite, and magnetite. Upon analysis, the two groups were found to differ greatly in chemical composition; the lighter orthoclase rocks were found to be much richer in silica and much poorer in iron, lime, and magnesia, than the others. This led to the divis-• ion into the two well-known groups of acidic and basic rocks. To the former the name of trachytes was usually applied, while the latter were termed basalts. As knowledge of volcanic rocks increased and became more detailed, it was at length recognized (by Beudant) that the basic rocks were susceptible of further division. The study of the South American volcanoes convinced him that two types of basic rocks could be distinguishedâ€"one the typical basalts, characterized by an abundance of augite, magnetite, and usually olivin commingled with lime-feldspar; the other apparently a less basic rock, containing hornblende rather than augite, very little magnetite, and never olivin. The two types differed in appearance, the more basic being nearly black, the less basic being usually greenish, and certain tolerably constant differences of texture being easily recognized, though hard to describe; the name basalt being preserved for the more basic variety. Beudant called the other type Andesite. The name trachyte for a long time was used very vaguely, and it is now somewhat surprising to find what a vast range of variety it was made to cover. It was applied not only to the light-colored orthose and quartzose rocks, but was extended over varieties belonging well within the basic division, including Beudant's andesites, and hardly stopped short of anything except the extremely basic olivinitic basalts. The general sense of the more acute lithologists, however, was against such a sweeping use of the name, and in favor of confining it to the orthoclase-bearing varieties. Although in this restricted use of the name trachyte a considerable number of varieties had been noted by various writers, Biehthofen appears to have been the first to have clearly discerned that the trachytic group resolved itself into two members. Of these the most acidic division was characterized by the presence of free quartz and a general poverty in all minerals except quartz and orthoclase (sanidin); also by peculiarities of texture. The less acidic division rarely contained free quartz, and never in notable quantity; was richer in sanidin as well as in the accessory or subordi- |
