188 SILVER-LEAD DEPOSITS OF EUREKA, NEVADA. 



surface the quartz-porphyry is the only one which can have furnished the 

 metals of the ore bodies. 



The soifataric action and the ore formation. — The manner of occurrence of the ore 

 and its connection with the fissure system are consistent only with the sup- 

 position that whatever the source of the ore may have been, it reached its 

 present position in ascending solutions. The formation of the ore-bearing 

 solutions is almost certainly due to the soifataric action arising from the erup- 

 tion of the rhyolite. The intrusion of this rock was the last dynamical dis- 

 turbance on Ruby Hill, for the main fissure with the rhyolite dike faults all 

 formations with which it comes in contact, except the ore bodies, and is 

 itself nowhere faulted. The rhyolite dike also shows every evidence of 

 soifataric decomposition. There is no evidence of two -distinct periods of 

 solfatarism, and unless the ore formation and the alteration of the lava are 

 due to comparatively late volcanic agencies, which have left no other trace 

 of their existence, the ore deposition and the eruption of rhyolite must have 

 been related phenomena. 



MANNER OP THE DEPOSITION OP THE OEE. 



The deposition of the ores. — The solutions containing the ore penetrated the 

 limestone, passing through fissures and interstices in the broken rock, and 

 deposited the ore where conditions of temperature and chemical activity were 

 favorable to its precipitation. It is impossible to determine what may have 

 been the chemical composition of the solutions which carried the oi'e, but it 

 is not improbable that they consisted in great part of metallic sulphides dis- 

 solved in alkaline sulphides. These solutions were necessarily formed under 

 the influence of heat and pressure. Rising into the shattered limestone at a 

 diminishing pressure and temperature, the liquids lost much of their solvent 

 power and many of the metals that they contained were precipitated. This 

 precipitation could have occurred in only two ways: either through deposi- 

 tion in pre-existing large cavities or through a substitution of ore for country 

 rock. The manner in which the deposition took place has a very important 

 bearing upon the probabilities of finding ore at any considerable distance 

 below the water level. 



