294 GEOLOGY OF THE EUKEKA DISTRICT. 



stone chambers and cavities, it is quite likely to have undergone some 

 movement by earthquake shocks during Quaternary time, but this is quite 

 another matter from profound orographic displacement of beds. To-day 

 there is absolutely nothing positively known as to the source of the rhyolite 

 material nor the deep-seated centers from which it originated, and this 

 is equally true as to the source of the heavy metals. With our lack of 

 knowledge on these matters it seems out of place to speculate in the present 

 volume as to their ultimate source. Probably no geologists, however, 

 would question the statement that the volcanic products came from below. 



The writer, after a careful study of the facts observed at Ruby Hill 

 and Prospect Mountain, as well as of the entire Eureka region, is forced to 

 the conclusion that there exists the closest relationship between the rhyo- 

 lites and the formation of the ore deposits, although they have been 

 observed in actual contact in only one or two localities in the larger mines. 

 In almost all cases where mineral deposits are found, rhyolite intrusions are 

 known to penetrate the limestone in close proximity to the ores, and it is 

 presumable that in many instances the presence of such ore bodies might 

 be detected without the discovery of any intrusive rock. No theory of the 

 ore deposits seems applicable to this region that does not carry with it the 

 fundamental proposition that the ores came from below, as the result of sol- 

 fataric action which accompanied volcanic energy. Evidence shows that 

 the centers of greatest deposition of ores were not the same as those of 

 greatest eruptive energy, but that the latter are associated with the secondary 

 dikes and offshoots rather than with the great lines of volcanic outbursts. 

 Solfataric action may have continued and probably did continue for a long 

 period after the rhyolite eruptions had altogether ceased, during which 

 metallic sulphides filled certain preexisting fissures, cracks, chambers, and 

 crevices in the limestone. After the deposition of the sulphides came the 

 period of oxidation which, so far as can be told, lasted throughout the 

 greater part of Quaternary time. 



Ores of the Cambrian. In regard to the distribution and geological position 

 of the ores in the Paleozoic strata all evidence shows that although the 

 most remunerative mines and those explored to a great depth occupy some- 

 what restricted limits, ores of similar mode of occurrence and composition 



