126 



THE AUEIFEROUS GEAVELS OF THE SIEEEA NEVADA. 



Unlike the counties to the north, of which the geology has just been de- 

 scribed, Calaveras does not extend so far up on the range as to take in any 

 portion of the summit. Alpine County has been formed out of the territory 

 on both sides of the dividing ridge at the expense of Amador, Calaveras, and 

 Tuolumne. This was done at the time the mines near Silver Mountain were 

 expected to be very productive, which they have not proved to be.* This 



* 4 



peak is the centre of a grand eruptive region, from which the lava flows have 

 spread themselves far down the slope of the range. Nearly all the crest of 

 the Sierra, through Alpine County, is covered with volcanic materials, which 

 are sometimes accumulated to a great thickness, as in the mass of Silver 

 Mountain itself; but it is everywhere evident that granite forms the basis on 



* 



which the volcanic materials have been piled up, the lava occupying only the 

 crests of the spurs. One of the grandest, and perhaps the most picturesque, 

 of all the volcanic ranges in the State is the one about six miles northwest 

 of Silver Mountain, called on Holt's map the a Cliffs of Nonatore." f From 

 here seem to have originated great flows of lava, which have spread down 



the slope of the Sierra into Calaveras County, and which are so conspicuous 

 to one looking from Murphy's towards the north, and over which the trav- 

 eller passes in going from the Calaveras Big Tree Grove to West Point or 

 Eailroad Flat. The last-named place is quite surrounded by volcanic ridges 

 and tables, which are elevated from 600 to 800 feet above the Flat, and 





capped by basalt which weathers into rounded, boulder-like masses. The 

 volcanic materials continue almost or quite uninterruptedly from Railroad 

 Flat to Mokelumne Hill and far down on the divide between the Calaveras 



m 



and the Mokelumne, into the very foot-hills. From the appearance of the 

 low flat-topped elevations which occur all along the base of the Sierra be- 

 tween Dry and Little John Creeks, a very large amount of volcanic material 

 has been carried down the range by currents of water, and deposited at its 

 very base in an almost or quite continuous mass of considerable extent and 

 thickness. This, however, has been so extensively and irregularly eroded 

 away, that, without maps on a very large scale and a long course of detailed 

 exploration, it will be impossible to lay them down with accuracy. The ma- 

 terials of the different flows have, in all probability, been much mixed 

 together by shifting currents, so that it would, under any circumstances, be 



* See Geology I. p. 449. Hardly anywhere in the country have more persistent efforts been made to 

 work worthless mines than here. 



t See Geology of California, I. p. 446. 



* 



