CHAPTER III. 

 • LITHOLOGY. 



Section 1. 

 THE EOCKS OF THE WASHOE DISTRICT. 



Importance of lithology to the theory of ore-deposits. ThoUgh tlie prCSellt meiUOir is 



intended as a contribution to mining geology, the impoi'tance of the Hthology 

 of the district is certainly not less than it would be, were no economical 

 problems involved. The slightness of the advances which have been made 

 in the theory of ore-deposits is regarded by business men as a reproach 

 to geological science. But the influence of the inclosing rocks on the char- 

 acter and tenor, and to some extent upon the occurrence of ore bodies, was 

 recognized before geology became a science; and the fact of this influence 

 has received confirmation from more extended observation. Whatever, then, 

 may be the true theory of the genesis of ores, the indications are clear that 

 exhaustive studies of the nature of the inclosing rocks, and of the influences 

 to which these have been subjected, are essential to its elucidation; for even 

 if it should prove that ores are derived from immense depths, and are brought 

 to the surface under conditions which are wdiolly removed from observation 

 and study, the influence of the wall rocks on their deposition is still within 

 the accessible field of inquiry. The way to such investigations is already 

 paved. The microscopic analysis of rocks initiated by Mr. Sorby, and raised 

 to its present rank as a science by Messrs. Vogelsang, Zirkel, Rosenbusch, 

 Fouque & Levy, and their fellow workers, enables us to reach very definite 

 conclusions respecting the mineralogical composition and physical structure 

 of rocks; while Prof F. Sandberger and others have of late years made great 

 advances in proving the chemical relations which, in many cases, exist 

 between the wall rock and the contents of veins. On the other hand, the 

 mineralogical study of decomposed rocks under the microscope has made 



