892 PROCEEDINGS OP THE CALIFORNIA 



Salt Spring Valley probably owes its existence, as such, entirely to inequality 

 of denudation ; the comparatively friable slates yielding much more readily to me- 

 chanical action than the harder and more highly metamorphosed rock on either 

 side, which has thus been left in the form of mountain ridges, projecting many 

 hundreds of feet above the adjacent region, while the intervening and surround- 

 ing rock has been swept away to the plains below, 



A partial description of the copper mines of Copperopolis will be found in 

 the " Geology of California," Vol. I, pp. 254-257. The depth of the main shaft 

 in the " Union" is now stated to be a little over five hundred feet, and the 

 greatest depth reached in the " Keystone," is said to be five hundred and sixty 

 feet. All the deposits of ore here worked lie parallel with the strike and dip of 

 the inclosing strata. The great ore mass of the " Union " Mine forks or divides 

 into two branches towards the northwest ; and at the lowest depth now reached, 

 its width or thickness, after having reached a maximum, is again diminishing. 

 In the " Keystone " Mine there have been two separate and nearly parallel bodies 

 of ore worked to a considerable extent, and a third one was struck last sprin°- 

 previous to the suspension of work in the mine. The two main bodies of ore 

 in this mine have " pinched out " or disappeared in various directions in their 

 lines of strike and dip. They seem to have an irregular lenticular form, and 

 together with the great mass of the " Union " appear to lie in what are called 

 "^hoots," which pitch at an angle of 50° or 60° in the direction of the strike 

 towards the northwest. The northwesterly prolongation of the strike of the 

 great " Union" deposit docs not coincide with either of the "Keystone" de- 

 posits, but passes east of them. There have been other and smaller deposits in 

 the " Union " ground, more or less worked, lying west of the main body, some 

 of which may possibly connect with the "Keystone" shoots, though the best 

 information I could obtain leads me to think otherwise, and that they were 

 probably isolated lenticular masses. The mass of the great deposit in the 

 " Union" Mine consists of an intimate mixture of chalcopyrite and iron pyrites, 

 containing on an average sixteen to seventeen per cent, of copper. Well defined 

 selvages are not to be seen at Copperopolis, and the country rock is impregnated 

 in all directions, sometimes to a considerable distance from the purer ore, with 

 more or less finely disseminated copper and iron pyrites. In Europe it would 

 pay to crush and work much of the wall rock itself for the copper which it con- 

 tains ; but here it is entirely worthless, as even ten to twelve per cent, ore is 

 not worth mining and shipping at present prices. 



It will be seen that the more recent and deeper developements in the Copper- 

 opolis mines have only served to confirm the opinion expressed two years ago by 

 the State Geologist (Geol. Vol. I, p. 225) that " the deposits of copper ore in this 

 regiou, like nearly all the others in California, do not appear to be included in 

 regular fissure veins, but rather to form independent masses [the italics are mine] 

 lying in the direction of the strike of the inclosing rocks, and dipping with them." 

 It seems, further, that they are here arranged in some sort en echelon. There 

 is no evidence whatever of the existence here of a regular and continuous vein of 

 copper ore, stretching for miles through the country, as some have supposed. 

 (See Ross Browne's Report, p. 144.) 



