374 GEOLOGY OF THE COMSTOOK LODE. 



The older diabase is porphyritic, and almost the whole of it is in a very 

 advanced stage of decomposition. When fresh, it considerably resembles 

 an augite-andesite; its groundmass, however, is thoroughly crj'stalline and it 

 contains no glass inclusions, but frequent fluid ones; the augites, too, show 

 both pinacoidal and prismatic cleavages, and a tendenc)^ to uralitic decora- 

 position. It is also manifestly older than the other diabase. An important 

 characteristic is the latli-like development of the porphyritic feldspars, for 

 in cases of extreme decomposition of the bisilicates this characteristic at 

 least serves to suggest whether the rock is dioritic or diabasitic. The older 

 diabase has been considered as propylite or andesite, according to the stage 

 of decomposition. The younger diabase ("black dike") is very highly 

 crystalline and not porphyritic. It is bluish when fresh, but in course of a 

 few hours turns to a smoky brown. It is identical with many of the dia- 

 bases of the New England and the Middle States. 



The older hornblende-andesite and the augite-andesite whei'e fresh are 

 typical rocks macroscopically and microscopically. When decomposed they 

 have been taken for propylite. The younger hornblende-andesite which 

 overlies the augite-andesite is a cross-grained, soft, often reddish or pur- 

 plish rock, with large glassy feldspars. It has always been supposed to be 

 trachyte; but when endeavoring to determine the different species of feld- 

 spar under the microscope, I was unable to include any satisfactorily deter- 

 minable sanidins in the list. Dr. G. W. Hawes was kind enough to under- 

 take the separation of the feldspars by Thoulet's method, and analyses of 

 the feldspars were made by Mr. F. P. Dewey. The specimen selected was 

 the most trachytic in appearance, that of Mount Rose, but no feldspar what- 

 ever was found corresponding either physically or chemically to orthoclase. 

 There is much reason to believe that trachyte occurs less often than had 

 been supposed in the Great Basin area. 



Apart from the effects produced by decomposition the Washoe rocks 

 are typical of their kind, and correspond to representative specimens of the 

 same species from other parts of the world, even in the minutiae of miner- 

 alogical composition and ph3'sical structure. This persistence of rock types 

 in minor features, which would seem to be fortuitous, or at least unessential, 

 is one of the most remarkable facts established by microscopical lithology, 



