82 GEOLOGY OF THE COMSTOCK LODE. 



Failure of the search for propyiite. — Washoe presenting thc tjpical American oc- 

 currence of pi-opylite, a study of the rock necessarily formed a prominent 

 feature in the re-examination of the District; for while the structure and vein 

 formation of the Comstock are the objects of first importance and interest in 

 AVashoe, the first step toward their elucidation was manifestly to clear up the 

 lithological obscurities as far as possible. Since Baron v. Richthofen and Mr. 

 King- examined the District, the exposures of rock have been greatly inci'eased. 

 Not only have the mines on the Lode been deepened by a couple of thousand 

 feet, but innumerable roads, quarries, prospect-holes, and the like, have ex- 

 posed raoi'e than the mere weathered surface in thousands of spots. It soon 

 became apparent that the area of andesite, which to Baron v. Richthofen 

 seemed inconsiderable and to Mr. King quite subordinate to that of the 

 propyiite, had been undeirated. Fresh andesites were found exposed by 

 cuts in many localities which had been laid down as propyiite; and since 

 the latter was supposed to underlie the former, the upper portions of these 

 exposures furnished a safe study of decomposed andesites, the results of 

 which could be applied elsewhere. It was found that even where a high 

 degree of decomposition and a thoroughly propylitic character prevailed, 

 reasonably fresh rocks could be discovered by diligent search, either as 

 masses protected by some accidental arrangement of fissures, or as nodules 

 at the centers of concentrically weathered blocks; and to the east of the 

 Lode, wherever fresh rocks were discovered among the propylites, they 

 always proved andesitic. Where andesite dikes or overflows had been 

 recognized, and had been supposed to succeed propyiite, careful examina- 

 tion and excavation showed that the change was through a transition, not 

 by a contact. In short, the propyiite area to the east of the Lode was 

 reduced almost foot by foot, until it disappeared altogether. The propyiite 

 from the head of Ophir ravine, one of the type-localities, had a slightly 

 different character from the eastern rock, yet the difference was not greater 

 than seemed possible within the limits of a rock-species. Fortunately 

 there are many long tunnels penetrating the hills in the neighborhood; and 

 an examination, undertaken to establish contacts between propyiite and 

 diorite, resulted in a study of transitions between typical diorite-porphyries 

 and decom])osed porphyritic forms of the same rock. At last even in the 



