] 72 SILVER-LEAD DEPOSITS OF EUREKA, NEVADA. 



two mines in the lower levels before the two fissures came together, this will 

 not prove that no ore bodies are to be expected in the limestone which un- 

 derlies the lower belt of shale. It will only prove that the limestone or the 

 fissure system in this part of the hill was for some unknown reason unfavor- 

 able to the deposition of ore. It will be a very discouraging circumstance 

 to the companies interested, but it may be expected. Large zones of barren 

 ground have been known in the upper levels as well. 



Conditions of ore deposition in the lower wedge of limestone. If Plate V III. is referred tO 



it will be seen that the lower mass of limestone is gradually widening out, as 

 would inevitably be the case if the ideal section of Ruby Hill (Plate IV.) be 

 true. The main fissure below the great limestone wedge has a hanging wall 

 of crushed limestone which is overlain by a belt of shale. If the theory of 

 the source of the ore stated in Chapter VII. is correct, the ore solutions 

 passed upward through this fissure along the contact with the limestone, 

 which offered all the conditions necessary to a deposition of the ore, pro- 

 vided the ore substituted itself for limestone. If such was not the case, and 

 the ore was deposited in caves previously formed, there is little likelihood 

 that this cave formation could have taken place in the lower bed of limestone, 

 for the following reasons: It is certain that the caves in the upper lime- . 

 stone were formed after the faulting occurred which broke up this mass of 

 rock and formed the main fissure. This main fissure with its wall of clay 

 and the lower belt of shale effectually excluded any great flow of surface 

 water into the lower belt of limestone, nor does the cave formation even in 

 the upper limestone seem to have extended to the lowest points of this 

 wedge of rock, probably because percolating surface waters became satu- 

 rated with calcium carbonate upon reaching this depth. It is therefore 

 improbable that caves of any great extent could have been formed at the 

 depth at which this limestone lies. It will be seen, therefore, that the 

 chances of finding any considerable bodies of ore in the lower limestone, 

 if the ore deposition was dependent upon the prior formation of caves, are 

 very few. But the evidence that the ore bodies were formed, at least in 

 part, by substitution is very conclusive. There seems to be no probability 

 that such a manner of deposition should be limited to a few hundred feet 

 or to the upper mass of limestone. 



