190 SILVER-LEAD DEPOSITS OF EUREKA, NEVADA. 



posits, which in many respects resemble those of Eureka, the ore has been 

 substituted for limestone, according to Mr. Emmons." 



Preponderance of evidence in favor of the substitution theory. \V exiling 1 the evidence On 



both sides of the question, it appears that a large part of the ore was brought 

 into its present position by substitution, while it seems impossible to demon- 

 strate that any part of it was deposited in pre-existing cavities. It is highly 

 probable that all the ore was deposited by substitution, and that future 

 developments will effectually establish the fact. 



ASSAYING. 



object of assaying country rock. — With a view to discovering, if possible, the 

 source of the ore in the mines of Eureka District, numerous and careful 

 assays of all the different kinds of country rock in the neighborhood of the 

 ore bodies were made. As the quantity of the precious metals contained 

 in any of these rocks is extremely small, it was necessary to take unusual 

 precautions in order to determine with any degree of exactitude the amounts 

 of gold and silver present. Assavers do not ordinarily attempt to estimate 

 with accuracy any value of either gold or silver less than one dollar to the 

 ton (0.0026518 per cent.), and as the country rock of this district never 

 contains so much as this, particularly delicate methods were required in the 

 determination of the actual quantities of these metals. 



Accuracy of the process of assaying. — The process by which the results given in 

 this report were obtained have been fully explained in the chapter on 

 assaying, and it is only necessary to state that it was found possible to de- 

 termine the value of silver in any country rock within a cent and a half. 

 It was not possible, however, to determine the gold value with equal accu- 

 racy, as the quantity of it was extremely small ; and it was neglected in most 

 cases. 



use of assays. — While various purposes maybe subserved by assays of 

 country rocks, the main objects of those described in this report were, first, 

 to ascertain in which of the rocks the precious metals could be detected, 

 and, second, to trace the variations of tenor in different occurrences of the 

 same rock. As a qualitative method, exception can scarcely be taken to the- 



"Second Annual Report of the Director of the U. S. Geological Survey. 



