PREVIOUS INVESTIGATIONS. 13 



but no quartz. It forms the prouiinent topographical feature of tlie dis- 

 trict, Mount Davidson. Adjoining the syenite to the north and south are 

 nietamorphic rocks, the most recent of which Prof. J. D. Whitney has shown 

 to be Triassic. Overlying a portion of the nietamorphic strata is quartzose 

 porphyry.^ 



The foregoing form the ancient series. Of tlie Tertiary rocks only 

 two^ have an}- close relation to tlie Comstock Lode. Propylite has this 

 remarkable peculiarity, namely: that it resembles many ancient rocks 

 exactly in appearance, and yet is among the most recent in origin. It is 

 prominent among tlie inclosing rocks of the Comstock vein and, besides, 

 incloses several, perhaps most, of the largest and most productive silver veins in 

 the tvorld, as those in the Karpatliiau Mountains, of Zacatecas and other 

 places in Mexico, and probably several veins in Bolivia. Mineralogically 

 it consists of a tine-grained paste of ordinarily greenish, but sometimes 

 gray, red, and brown color, with embedded crystals of feldspar (oligoclase), 

 and cohunns of dark-green and fibrous, seldom of black, hornblende, which 

 is also the coloring matter of the base. A peculiarity of the rock is its 

 ferruginous character when decomposed. I'robably it contains other metals 

 besides ii-on. Geologically it is an eruptive rock, but it is accompanied by 

 vast accumulations of breccia, which is sometimes regularly stratified. The 

 flats of Virginia City, Gold Hill, American City, and Silver City consist of 

 propylite It lies, in general, east of the mountains consisting of the ancient 

 formations, and contains several mineral veins besides the Coji.stock Lode. 

 Its distribution in other countries of the world is not very general. Sev- 

 eral different kinds of volcanic and eruptive rocks followed the outbreak of 

 propylite, but onl)- one of them demands attention in reference to the Com- 

 stock vein, as it probably caused its formation, besides taking a prominent 

 part in the structure of the country. This is sanidin trachvte. 



'lu liis memoir on The Natnral System of Voltauie Rocks, p. 41, Barou von Kichthofen says: 

 "Quart/.ose jjoiphyry occurs to some extent iu Wasboe under circumstances which make the exact 

 <letermination of its age difficult, but render it certain that it is intermediate in this respect between 

 granitic and volcanic rocks." Tliis rock was later regarded by Mr. King as quartz-propylite, and 

 determined Ijy Professor Zirkel as dacite. (v. Quartz-porphyry.) 



-In his Natural System of Volcanic Rocks, p. 31, Barou von Richthofen, speaking of the 

 Washoe district, says: "Andesite is insiguilicant in bulk in that region. It composes a few hillocks on 

 the propylitic plateau, and in some cuts and tunnels andesitic dikes may be seen." 



