176 GEOLOGICAL HISTOUY OF LAKE LAHONTAN, 



act as solvents for many minerals otherwise not easily dissolved by water. 

 Its solvent power is also augmented ))y the increase of temperature and 

 pressure which it undergoes as it descends into the earth. The waters 

 issuing as springs, frequentl}^ with a high temperature, are almost invaria- 

 bly found to have dissolved a great variety of mineral substances. In 

 many instances the less soluble minerals occurring in spring waters are 

 held in solution by the presence of carbon dioxide, or by high temperature 

 or pressure. When such waters reach the surface they lose a large part of 

 their dissolved gases, pressure is relieved, and they are rapidly cooled; 

 the result is that much of the mineral matter they contain is deposited 

 about the orifices through which they discharge. The substances most 

 commonly precipitated under such conditions are calcium carbonate, oxides 

 of iron and of manganese, calcium sulphate, and silica. Accumulations of 

 these substances are frequently of great extent as may be amply illustrated 

 in any of the hot-spring regions of the world. Only a portion of the dis- 

 solved matter brought to the surface by springs is thus deposited, however, 

 and in many cases no immediate precipitation takes place. The waters, 

 iifter losing their dissolved gases and becoming cooled, are usually much 

 richer solutions than ordinary river waters, and, on joining the surface 

 drainage, contribute large quantities of mineral matter to the neighboring 

 streams. The solvent action of subterranean waters is frequently indicated 

 by the porous and cellular character of certain rocks, as well as by the 

 caves, frequently of vast size, that occur, especially in limestones. 



The analyses of river waters in all ordinar}^ instances must exhibit the 

 combined result of the solvent action of both superficial and subterranean 

 drainage. Springs frequently rise in the bottom of lakes or beneath the 

 sea and thus contribute directly to the solutions with which rivers eventu- 

 ally mingle. In the case of inclosed lakes, the reaction of mineral waters, 

 rising in dense saline solutions, is followed by interesting results, some of 

 which will Ije considered in describing the tufa deposits of Lake Lahontan 

 {postea page 221). 



In illustration of the chemistry of natural waters we have compiled the 

 following table (B) showing the composition of a few of the better known 

 American springs and artesian wells; by comparison with Table A, the greater 

 richness of subterranean waters over surface streams is at once apparent. 



