104 SILVER-LEAD DEPOSITS OF EUREKA, NEVADA. 



The internal structure of the ore masses in no way resembles those of Raibl. 

 Where the ore is not oxidized there are no signs of a banded or concentric 

 structure, and the phenomena observed point entirely to substitution of the 

 sulphurets for country rock. 



some of the ore bodies formed by substitution. — The unoxidized ore bodies have not 

 yet been sufficiently explored to establish the fact that they were formed in 

 toto by substitution, but sufficient evidence has been obtained to prove that 

 a considerable portion of the ore at least was deposited in this manner. 

 In the cases of the Upper Mississippi, and those of Missouri, the galena is 

 found in the form of stalactites and stalagmites, which proves the pre- 

 existence of the openings, but in Eureka no such case has been noted. 



Evidence against the substitution theory. — There is one argument to be advanced 

 against the theory that the ore bodies were formed exclusively by substitu- 

 tion, namely, that some of the ore chambers are far removed from what 

 seems to have been their most natural course. It has been remarked that 

 in the mines southeast of the "compromise line" the ore bodies are of rare 

 occurrence near the Ruby Hill fault-fissure, except when the two fissures 

 approach in the deeper workings. The ground in the neighborhood of this 

 fault presents all the conditions necessary to the fulfillment of the phe- 

 nomena of substitution; it is crushed, shattered, and broken in various 

 ways, and is traversed by cross-fissures. The lines on which ore is found 

 gradually approach the quartzite foot wall, and correspond almost exactly 

 with what would have been the natural channels of surface-waters descend- 

 ing through fissures. If the caves were not formed before the ore, why did 

 the ore solutions not follow other channels apparently offering equal facili- 

 ties for the substitution of ore? No satisfactory answer has been found for 

 this question, but it is manifest that mine-workings, however extensive, 

 never fully expose the system of underground fissures, and it is entirely 

 possible that a barrier to the passage of solutions in this direction existed 

 which has not been brought to light. Even had ore been deposited only in 

 pre-existing openings, traces of lead minerals should have been precipitated 

 in the interstices of this broken ground if it was accessible to metalliferous 

 solutions, but none such could be discovered. 



