12 S1LVEK-LEAD DEPOSITS OF EUREKA, NEVADA. 



ther disturbing effect upon the structure of the country. There are no 

 large outbursts of this rock on Prospect Mountain itself, but it appears as 

 dikes in several places, and large masses of it, and hornblende-andesite, occur 

 in the immediate neighborhood. Many fissures and faults have unquestion- 

 ably been caused by the eruption of rhyolite, and as it is among the latest 

 disturbing agents which have entered into the formation of the country, 

 it is worthy of attention. It is also extremely probable that the eruption 

 of rhyolite and the solfataric action consequent upon it had an intimate 

 connection with, if they did not actually cause, the deposition of the ore. 

 Although the evidence found in the mines that the rhyolite preceded the 

 deposition of ore is not absolutely conclusive, it is strong enough to make 

 this order of succession almost certain. Where found in the mines the rhy- 

 olite is very much decomposed, being in places wholly changed to clay, but 

 still retaining enough of its original characteristics to permit of its determina- 

 tion with certainty. At a distance from the ore bodies this rock, although 

 somewhat weathered, is much fresher. • 



Relations of the granite to the other formations. It is not likely that the granite of 



Mineral Hill, which is the only known occurrence of granite in the district, 

 broke through the quartzite and limestone, but that it originally formed a 

 submarine hill in the bed of the ocean upon which the quartzite, limestone, 

 etc., were laid down, and that its exposure in its present position is due to 

 erosion. Quartzite containing bowlders of a rock which was probably gran- 

 ite has been taken from the bottom of the Richmond shaft, which has 

 attained a depth of 1,230 feet, and is the deepest in the district. These 

 bowlders consist of granular quartz, mica, and a substance that appears to 

 be decomposed feldspar. It has not been possible to determine the nature 

 of this rock with certainty, but it is very probable that, it is an altered 

 granite. Such being the case, it would indicate that the body of that rock 

 was at no great distance. 



Direction of the dip of the various formations. Tile Strata of the formations which 



compose Prospect Mountain do not always dip away from the axial plane 

 of the fold. There is a notable example of this occurrence in the Ruby- 

 Dunderburg mine, which is situated at the head of Goodwin Canon. The 

 principal shaft is sunk in the Hamburg limestone, but at a depth of 450 



