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 I'ocks were found to be nmch 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 

 fonner 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 distin- 

 giiished — 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 recog- 

 nized, 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 any- 

 thing 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, Richthofen 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 charac- 

 terized by the presence of free quartz and a general poverty in all min- 

 erals except quartz and orthoclase (sanidin); also by peculiarities of texture. 

 The less acidic division rarely contained free quartz, and never in nota- 

 ble quantity ; was richer in sanidin as well as in the accessory or subordi- 



