86 GEOLOGY OF THE OOMSTOCK LODE. 



the only resources of the Hthologist were a study of variations and transi- 

 tions, and a keen perception of the habitus characteristic of rock-species, 

 aided only by the feeble help of a lens and an occasional chemical analysis; 

 and how much can be accomplished in this way is evident from the fact that 

 the chief features of lithological classification are still much what they were 

 before the introduction of the microscope. Nor are the methods of the 

 older lithologists antiquated; on the contrary, the proper use of the micro- 

 scope greatly increases their applicability and efficiency. The microscope 

 enables lithologists of the present day to give greater precision to their ideas 

 of macroscopical habitus, and to distinguish in most cases between essential 

 and non-essential characteristics, and, with this advantage, they should 

 become even keener field observers than their predecessors. Indeed the 

 relations of lithological varieties, and of the causes on which they are depend- 

 ent, can be successfully studied only in the field. In the present investiga- 

 tion slides were ground and examined from day to day as the exigencies of 

 the field-work seemed to demand. The microscopical and macroscopical 

 appearances were also diligently compared (for grinding slides without ma- 

 chinery was a serious addition to the labor of days spent in the saddle or 

 under ground) ; and it became possible at length to recognize at a glance a 

 unity of origin in specimens of very diverse appearance and to detect litho- 

 logical diff"erences in spite of advanced decomposition and great apparent 

 similarity. It proved possible to make the proper allowance for decompo- 

 sition and to infer the original habitus when veiled by another of secondaiy 

 origin, as well as to identify the precise character of the change. 



Typical propyiite localities. — The thrcc most important propylite localities men- 

 tioned by Professor Zirkel in the Washoe region are the head of Ophir 

 Ravine, Crown Point Ravine, and Gold Hill Peak. The last is represented 

 in the map accompanying the present report, as the southern Twin Peak 

 (C.4). 



Head of Ophir Ravine. — The Upper portlou of Ophir Ravine presents a very 

 great variety of diorite-porphyries, which are not related as separate 

 flows or sheets, but pass over into one another as if the whole heterogeneous 

 mass had cooled at once. The character of the rock changes every few 

 feet, and the same varieties recur in spots. Among them are some so gran- 



