CONCENTRATION OF DISSOLVED MATERIAL AT SURFACE. 551 



Such waters were capable of taking more calcium carbonate and silica into 

 solution than ordinary cold underground waters. An interesting 1 fact in 

 connection with these deposits is the correlation of their character with the 

 rocks adjacent. As pointed out by Hague, the calcareous deposits of 

 Mammoth Hot Springs and Terrace Mountain are adjacent to sedimentary 

 rocks which contain abundant calcium carbonate to serve as a source of 

 supply." The geyserite deposits of siliceous sinter, on the other hand, are 

 in a region of volcanic igneous rocks, mostly rhyolite, and where there are no 

 sedimentary rocks. Here calcium carbonate is not available, but silica is 

 very abundant in. natural glass and in crystallized minerals. These sili- 

 cates are decomposed by the processes of weathering. Colloidal silicic 

 acid is thus formed, which is readily taken into solution by the hot waters 

 and abundantly brought to the surface. There dehydration, partial or 

 complete, takes place and the geyserite deposits are built up. 



In these cases it is clear that the major portion of the material deposited 

 was derived from the solid rocks through which the hot solutions circulated, 

 rather than from the magmas which heated the water, as maintained by 

 some. (See pp. 1033-1034, 1071-1072.) 



CONCENTRATION BY OVERGROUND CIRCULATION. 



The waters of streams and lakes always contain soluble material 

 derived from the soil dissolved by the run-off and from the lower part of the 

 belt of weathering and the belt of cementation by the issuing spring waters. 

 The material dissolved by the run-off may form in the soil by the processes 

 of weathering or be brought to the surface by means of life or by the 

 underground circulation. The amount of this material is comparatively 

 small in humid regions, but is relatively large in arid regions. In some arid 

 regions the amounts held by the surface waters may be so great as to unfit 

 them for domestic purposes, or even for purposes of irrigation, since, as 

 already noted, a comparatively small amount of alkali is sufficient to 

 prevent plant life. 



The best illustrations of overground concentration are furnished by 

 basin regions, where there is no permanent run-off. In the United States the 

 Great Basin is the one of dominant importance. Southern California also 

 contains basins of great extent. Into the Great Basin a large number of 



" Hague, Arnold, Geological history of Yellowstone National Park: Trans. Am. Inst. Min. Eng., 

 vol. 16, 1888, pp. 795-796. 



