Vol. 4] 



Lawson. — The Robinson Mining District. 



347 



RIIYOLITIC LAVAS AND TUFFS. 



In the general statement of the geology of the district it has 

 been said that the latest rocks with which we have to deal are 

 certain rhyolites and tuffs which were erupted after the geo- 

 morphic evolution of that region was well advanced toward its 

 present condition. In the district as mapped there are two con- 

 siderable areas of such rocks besides some outlying patches. The 

 first of these is at Quarry Hill which occupies the western medial 

 portion of Ocher Valley. The hill has a north and south extent 

 of about 4,000 feet and a width of 1,500 to 2,000 feet. The vol- 

 canic rocks which compose it clearly rest upon the eroded sur- 

 face of the Arcturus limestone and have a thickness of about 

 180 feet. The lava is not now connected with any eruptive vent, 

 and is evidently the remnant of a much more extensive sheet 

 which once occupied the valley, and doubtless was once contin- 

 uous with an area of similar rocks about two miles down the 

 valley near the old Ely stage road. One may completely encircle 

 the hill without leaving the exposed Arcturus shale limestone ; 

 but on the northeast side of the highest part of the hill the limit 

 of the lava is a fault which has dropped it down against the 

 limestone. It is evident from the situation of this lava that, 

 although, as will be shown later, the region has been deformed 

 and faulted since its eruption, the valley which it occupies must 

 have had locally pretty much the same general configuration as 

 is now presented to us by resurrection due to removal of the 

 greater part of the lava by erosion. 



Three Members of Volcanic Series. — The section afforded by 

 the abrupt faces of Quarry Hill shows that there are three dis- 

 tinct members in this volcanic accumulation. The first of these 

 is a white tuff exposed at the base of the hill, on its east side, 

 where the wagon road from the Ruth Mine to Copper Flat passes 

 it. The rock is composed of fragments of pumiceous glass and 

 rhyolite with broken crystals of quartz and feldspar. The frag- 

 ments are usually small but some of them are a few inches in 

 diameter. The tuff is rather firmly cemented. The full thick- 

 ness is not exposed but it was estimated to be not less than 10 

 feet. To the north and northeast of Quarry Hill beyond the 

 limits of the territory mapped, this tuff formation is well ex- 



