198 MINERAL DEPOSITS FROM HOT SPRINGS. 



of the various felspathic and micaceous minerals, which, enter into 

 the composition of igneous and metamorphic rocks. 



We thus observe that springs differ from volcanoes in bringing to 

 the surface soluble substances, one of which usually predominates in 

 a particular spring ; but if we regard hot springs, geysers, and mud 

 volcanoes as a group of phenomena closely connected, we shall find 

 in their waters all the materials which go to form the minerals that 

 build up igneous rocks, or which are differentiated in lavas. 



Relation of Hot Springs to Mineral Veins. It is impossible in 

 examining the analyses of the waters of springs not to be struck with 

 the frequency with which they contain small percentages of copper, 

 tin, zinc, iron, and other metals ; and since it is well known that the 

 salts which they commonly bring to the surface are deposited as the 

 temperature decreases, so it has been inferred that the heated waters 

 have dissolved the metallic substances which were contained in large 

 masses of rock, and making their way from great depths into channels 

 which communicate with the surface, have lined the walls of these 

 fissures with crystalline deposits of metals, which have become sepa- 

 rated from each other in consequence of the differences of temperature 

 at which the different crystals are deposited. These are presumed to 

 have formed on the walls of the fissure in successive layers, because 

 the rock would always be colder than the water it contained ; and 

 thus it is conceived that the minute traces of metallic substances 

 which sometimes come to the surface, are but indications of larger 

 deposits of the same mineral, which the water of the spring parts 

 with at greater depths beneath the earth's surface. And it deserves 

 to be remembered that in regions where mineral veins are found to 

 be divided by fissures of a later date, which also contain the ores of 

 metals, the newer veins yield different metals and minerals to the 

 older ones, as though the plane of denudation of the present surface 

 of the earth had cut these deposits transversely at different distances 

 from the source, or in positions where the temperatures of deposition 

 were different. The deposits from hot springs at the surface are 

 chiefly alkaline salts, and salts of magnesia and lime ; but in mining 

 districts, where mineral veins occur, we have in almost all cases 

 evidence, not only of igneous action, or metamorphism and disruption 

 of strata, but also proofs of enormous denudation. So that it is 

 reasonable to conclude, especially with the experience of important 

 metalliferous deposits occurring in gravels, that the upper portions of 

 many mineral veins have been removed by denudation ; and it is 

 thus that we come upon the ores of metals in veins, which have 

 neither been erupted nor volatilised from below, but simply segre- 

 gated under the influence of water and volcanic heat, from the rocks 

 in which they were previously diffused. 



Denudation of Volcanic Regions. Any area in which volcanic 

 phenomena are manifested with the greatest intensity, always shows 

 indications that the region has undergone extensive denudation. "We 

 need but to examine such sections as have been drawn across the 



