18 RICHTHOFEN NATURAL SYSTEM 



There is plainly indicated, from a geological point of view, the existence of two large 

 groups within the limits of Beudant's " trachyte." The two existing names, " trachyte " 

 and " andesite," may conveniently be used for their designation, since those rocks, 

 for which either of the two names was first introduced, are indeed the types of the 

 two natural groups. 



Mode of Geological Occurrence. The trachytic rocks have had their independ- 

 ent epoch of eruption in every volcanic country. They preceded rhyolite and basalt 

 in age, and were posterior to the ejection of propylite and andesite. The trachytic 

 epoch was usually of long duration. In many localities, its later part blended with 

 the earlier of the rhyolitic epoch, which is manifest by the alternate emission of tra- 

 chytic and rhyolitic matter, during that time which was intermediate between the epochs 

 of the ejection of the principal bulk of either of them. As regards geographical dis- 

 tribution, trachyte is as much dependent upon, and as closely allied to, the preexisting 

 masses of propylite and andesite, as is the case with rhyolite. It towers up in peaks, 

 cones, and ridges, which are distinguished by their rugged outlines, and rest, in the ma- 

 jority of cases, upon the summits or the flanks of the ranges composed of those older 

 volcanic rocks. But there are also numerous instances when these may be seen to be 

 accompanied, at some distance, by trachytic outbursts, when a cursory examina-. 

 tion might make the latter appear to occupy an independent position. Trachyte does 

 probably not compose, by itself, any extensive mountain ranges, and it remains, in 

 general, greatly inferior in bulk to andesite. In* Europe, its outbreaks were scattered 

 and isolated, and, though they have been quite numerous, the aggregate quantity of 

 trachytic rocks is not considerable. They occur in Hungary and Transylvania, on the 

 Lower Rhine, in Central France, in the Grecian Archipelago, and in other parts of 

 that continent, as well as in the adjacent portions of Asia. Specimens of trachyte, 

 owing to their beauty and varied aspect, are usually much more numerous in geological 

 cabinets than those of andesite a fact which has frequently occasioned some miscon- 

 ception regarding the relative proportion and importance of trachyte and andesite 

 among volcanic rocks. 



In the structure of the North American Andes, trachyte takes a more import- 

 ant part than in Europe. A continuous range of it, at least ten miles in extent, and 

 forming rugged crests, encircles the Washoe Mountains to the east, in the shape of a 

 crescent. Trachyte rests there on propylite, and its ejection has probably had an 

 intimate connection with the formation of the Comstock Lode. Other accumulations 

 of similar extent may be noticed at Esmeralda, on the eastern slope of the Sierra 

 Nevada, around Red Rock Canon, south of Walker's Pass, in the surroundings of Lake 

 Tahoe and Sierra Valley, and at other places east and west of the Sierra Nevada. In 

 the northern provinces of Mexico, trachyte is known to occur quite extensively. At 

 all the places mentioned, the greater part of its bulk bears evidence of having been 

 brought into its present position by massive eruptions, while traces of extinct trachytic 

 volcanoes are scarcely wanting at any of them. Trachyte is still being ejected by a 

 number of active volcanoes. Among them may chiefly be mentioned those of Central 

 America, the greater part of the modern lava of which has been proved to have the 



chemical and mineral composition of trachyte. 

 (56) 



