310 GEOLOGY OF TUE EUliEKA DISTltlOT. 



tliis is true, the action of percolating waters in underg-round drainage chan- 

 nels would be influenced in like manner. During the present period of 

 excessive dryness these channels in the limestone carry no water, and con- 

 sequently exert but little solvent power. If, however, subterranean cham- 

 bers can be worn in the limestone since the deposition of the ore, it seems 

 but logical to assume that on the identical ground, under nearly similar 

 conditions, caves should have been formed before the deposition of 

 sulphides. 



Since the formation of these more recent water com-ses nothing of any 

 moment occured on Ruby Hill until historical time, when man, in his eager 

 search for wealth, excavated in a few years, by means of modern mechan- 

 ical appliances, the enormous mineral product which required untold ages 

 to deposit h\ natural process. 



Conclusions.— The couclusions reached after an investigation of the 

 Eureka Mountains with regard to the geological position, age, and origin of 

 the ore deposits may be briefly stated as follows: 



The rocks in which the ores occur are sedimentary beds belonging to 

 the Cambrian, Silurian, and Devonian periods. 



The ores were deposited after the eruption of the rhyolite, and conse- 

 quently they are of Pliocene or post-Pliocene age. 



In their mode of occurrence the ores are closely associated with the 

 dikes of rhyolite, although there is no evidence to show that they were 

 derived from them. 



The ores came from below. 



They were for the most part deposited as sulphides in preexisting caves 

 and cavities. 



They were oxidized by atmospheric agencies, mainly surface waters 

 percolating through the x'ocks. 



