10 SILVER-LEAD DEPOSITS OF EUREKA, NEVADA. 



which it occurs would indicate that it was an intruded mass of igneous rock, 

 and this theory is in a measure sustained by the fact that specimens closely 

 resembling rhyolite have been found in the mass. Other specimens are com- 

 posed almost entirely of calcium carbonate and have unquestionably the 

 structure of limestone. In the upper levels of the mine this rock is so much 

 ■decomposed and mixed with quartzite that its boundaries are not distinct. 

 It is very possible that it was originally an intercalated bed of limestone and 

 has been metamorphosed by an intruded mass of rhyolite and the attending 

 solfataric action. 



