KANGE OF ORE DEPOSITS. 299 



without any indication of the precious metals that they may be wholly 

 discarded. The same may be said of the entire area of the Carboniferous 

 block lying between the Hoosac and Pinto faults. It is somewhat remark- 

 able that in this latter block, which lies in the very center of volcanic 

 action, no mineral occurrences of any importance are known. Along the 

 two great meridional faults enormous masses of igneous rocks have been 

 poured out, notwithstanding which no ore deposits have been reported 

 either on the east side of the Hoosac fault or on the west side of the 

 Pinto fault. 



Geological Range of Ore Deposits. It will be seen from these facts that the 

 ore deposits of Eureka are found throughout a wide vertical range, extend- 

 ing from the base of the Prospect Mountain limestone to the summit of the 

 Nevada limestone, occurring in every grand division of the Cambrian, 

 Silurian, and Devonian periods, with the exception of the two great shale 

 belts the Secret Canyon and Hamburg shales. From the base of the 

 Prospect Mountain limestone to the top of the Hamburg shale it is esti- 

 mated that there are 6,200 feet of strata; the Siluiian rocks measure 5,000 

 feet and the Nevada limestone of the Devonian 6,000 feet. This gives 

 from the base to the summit of the included strata over 17,000 feet of 

 sedimentary rocks, through which argentiferous lead ores have been 

 deposited on a sufficiently extensive scale to encourage more or less 

 expensive outlays for mining exploration. 



From the rapid review of these facts it is evident that within the area 

 of the Eureka District the ores are by no means restricted to any definite 

 geological horizons and have been deposited in siliceous as well as calcareous 

 strata. Notwithstanding that the ore . bodies occur through a great thick- 

 ness of rock, it still remains true that the greater part of the mineral depos- 

 its and probably all those which have proved remunerative to the investor, 

 lie within restricted limits. The most productive mines, those carrying 

 the largest and richest bodies of ore, are found in Cambrian strata. This 

 is owing to orographic and structural conditions rather than to the 

 geological age of strata or the chemical nature of sediments. A study of 

 the structural features of the mountains together with the mode of occur- 

 rence of the rhyolite eruptions shows that the age of the rock has but little, 



