( 397 ) 



CHAPTER XXII. 



MINERAL VEINS. 



Origin of Mineral Veins. Facts observed in mining districts have 

 strongly enforced the belief that the water which descends through 

 fissures to regions beneath the earth's surface, becomes so heated as 

 to dissolve and hold in solution a multitude of metallic and mineral 

 substances. Different kinds of mineral matter may be deposited suc- 

 cessively from heated water as its temperature becomes lower ; and 

 hence the belief that, as waters on their way downward dissolve the 

 minute particles of metallic substances which are scattered in the 

 rocks with which they come in contact, so those waters on their way 

 back towards the surface, traversing the fissure of some fault which 

 gives them passage, become cooled, so as to throw down in a small 

 portion of the fissure, ores or other mineral matter which had been 

 invisible while diffused in the rocks. This belief is strongly sup- 

 ported by the fact that in passing down in a lode, it is no uncommon 

 thing for the metallic contents to change. 



Mineral Veins now Forming. Many ores, like those of copper, 

 tin, zinc, and lead, have the metal combined with sulphur, and it is 

 well known that many hot springs, like those of Sicily and Lake 

 County in California, deposit sulphur at the surface; and in the 

 district of Lake County the sulphur contains a little mercury in the 

 form of cinnabar. The sulphur is here deposited upon volcanic rock, 

 and the sides of the fissures are frequently coated with chalcedony, in 

 which are both pyrites and cinnabar ; so that the sulphur deposit has 

 been worked as a quicksilver mine. In a sinter bed, precipitated from 

 a hot spring in the county of Colusa in California, Mr. Oxland dis- 

 covered deposits of silver ; and in this county Mr. Melville Attwood 

 discovered cinnabar on the surfaces of a fissure which had become 

 covered by a subsequent deposit of brilliant metallic gold. At Steam 

 Boat Springs in the State of Nevada, which are about seven miles 

 N.W. of the silver mines of the great Cornstock lode, heated waters 

 or steam are constantly given off. 1 The fissures appear to have been 

 subjected to repeated widenings, and have their walls lined, sometimes 

 to a thickness of several feet, with incrustations of silica containing 

 hydrated ferric-oxide, and occasionally crystals of iron pyrites. A 

 similar group of fissures occurs a mile to the west, but they now only 



1 J. A. Phillips, Phil. Mag, 1868, p. 321 ; also Q. J. G. S.. vol. xxxv. p. 390. 



