76 SILVEK-LEAD DEPOSITS OF EUREKA, NEVADA. 



exhibit similar features as regards fissures and ore bodies if they were more 

 carefully examined. 



The main fissures which follow the axial plane of the fold show that 

 there was a zone of crushed rock produced in the country parallel to them. 

 The dissolving waters followed this zone and often penetrated to a consid- 

 erable distance laterally. When there is more than one fissure in such a 

 zone it is a matter of great difficulty to decide which one ought to be con- 

 sidered the true ore carrier or decisive factor in determining the present 

 arrangement of the deposits. Several fissures may have been instrumental 

 in determining the ore channels. 



Relative ages of fissures and ore bodies. AltllOUgll mOSt of the fisSUl'CS with which 



ore bodies are connected were unquestionably formed before the deposition 

 of the ore, yet there are some few which may possibly have been made 

 since its deposition. It is a very difficult matter, where there are no signs 

 of stratification in the country rock, to tell whether a fissure has faulted an 

 ore body or not. When two ore bodies are found at some distance from 

 one another and on opposite sides of a fissure, it by no means follows that 

 they were originally portions of a mass which has been faulted by the 

 fissure. The two ore bodies may always have been distinct. There can- 

 not have been much faulting since the deposition of the ore, for fissures, the 

 existence of which prior to the deposition of ore cannot be disputed, show 

 very few signs of any displacement. 



The partial falling in of caves and the mixing of bowlders of limestone 

 and ore near open fissures does not prove that there was any considerable 

 motion of the country. The roof of the big cave between the ninth and tenth 

 levels of the Eureka is falling in from time to time, but this is probably due 

 to the chemical action of water loosening blocks of limestone, and the mix- 

 ing up of ore and limestone in the northwestern portion of the Richmond 

 mine below the seventh level can be attributed to the mechanical action of 

 the same agent. 



sediment. — In connection with fissures it may be well to describe the 

 transported material or sediment which is often an accompaniment of fis- 

 sures and ore bodies. It consists of loose bowlders of gravel more or less 

 connected together, of large and small brecciated fragments of limestone, 



