234 ORIGIN OF METALLIFEROUS DEPOSITS. [XII. 



earth are very various. There is scarcely a spar or an ore to 

 be met with in the stratified rocks that is not also found in 

 some of these vein-stones, which are often very heterogeneous 

 in composition. In certain veins we find the elements of lime- 

 stone or of granite, and these often include the gems, such as 

 tourmaline, garnet, topaz, hyacinth, emerald, and sapphire ; 

 while others abound in native metals or in metallic oxides or 

 sulphides. The nature of the materials thus deposited depends 

 very much on conditions of temperature and of pressure, which 

 affect the solvent power of the liquid, and still more upon the 

 nature of the adjacent rocks and of the waters permeating 

 them. The chemistry of mineral veins is very complicate -d. 

 Many of these fissures penetrate to a depth of thousands of 

 feet of the earth's crust, and along the channels thus opened 

 the ascending heated subterranean waters may receive in their 

 course various contributions from the overlying strata. From 

 these additions, and from the diminished solubility resulting 

 from a decrease of pressure (ante, page 204), deposits of different 

 minerals are formed upon the walls, and the slow changes in 

 composition are often represented by successive layers of unlike 

 substances. The power of these waters to dissolve and bring 

 from the lower strata their contained metals and spars is 

 probably due in great part to the alkaline carbonates and 

 sulphides which these waters often hold in solution ; but the 

 chemical history of the deposition of the ores of iron, lead, 

 copper, silver, tin, and gold, which are found in these veins, 

 demands a lengthened study, and would furnish not less beau- 

 tiful examples of nature's chemistry than those I have ahvady 

 laid before you. 



The process of filling veins has been going on from the earli- 

 est ages ; we know of some which were formed before the 

 Cambrian rocks were deposited, while others are still funning, 

 as the observations of Phillips have shown us in Nevada, wln-n- 

 hot springs rise to the surface and deposit silica, with metallic 

 ores, which incrusts the walls of the fissures. These thermal 

 waters show that the agencies which in past times gave rise to 

 the rich mineral deposits of our western regions, are still at 

 work there. 



