24:8 



STRUCTURE COMMON TO ALL ROCKS. 



212°, 350° F., and even more. Again, there can be no doubt that 

 the associated metallic sulphides were deposited from the same solu- 

 tions as the vein-stuffs, for they are completely inclosed in the latter. 

 But the gold, as already stated (p. 240), exists as minute crystals and 

 threads of metal inclosed in the sulphide of iron, and must therefore 

 have been deposited from the same solution as the iron. It seems 

 possible that the gold was dissolved in a solution of sulphate of iron, 

 and that the sulphate was deoxidized, and became insoluble sulphide 

 and precipitated ; and that the gold thus set free from solution was 

 entangled in the sulphide at the moment of the precipitation of the 

 latter. Or else, and more probably, the gold was dissolved as sulphide 

 along with iron sulphide in an alkaline sulphide solution and deposited 

 by reactions 1 or 4 given on page 245, the gold, on account of its feeble 

 affinities, giving up its sulphur at the moment of its deposit.* 



There are some phenomena connected with the occurrence of gold 

 in the iron sulphides of the deep placers which seem to prove the truth 

 of this view.f The deep placers of California are gravel-drifts in an- 

 cient river-beds, covered up by lava-flows 100 to 200 feet thick. These 

 placers are worked by running tunnels beneath the basaltic lava until 



N S 



Fig. 218.— Section across Table Mountain, Tuolumne County, California: i, lava; G, gravel; 8, S, 

 slate; R, old river-bed; B', present river-bed. 



the river-gravel is reached. Now, the waters percolating through these 

 lava-flows and reaching the subjacent gravels are charged with alkali 

 from the lava. These alkaline waters are also charged with silica 

 from the same source. Hence, the drift-wood of these ancient rivers 

 has all been silicified by these siliceous waters. The gravels are also in 

 many places cemented by the same material. These percolating waters 

 have evidently also contained iron ; for in contact with the silicified 

 wood is often found iron sulphide. There are two ways, in either of 

 which we may imagine the gold to have been deposited. It may have 

 been in solution in the iron sulphate ; or else, along with the iron in 

 alkaline sulphide. Following out the process on the first supposition, 

 while the wood decayed it was partly replaced by silica and partly by 

 iron sulphide produced by deoxidation of the sulphate by organic mat- 

 ter (p. 193). The gravel has also in some places been cemented by 



* Gold is soluble in sodium sulphide, probably as gold sulphide. — Becker. 

 f Arthur Phillips, ibid. 



