542 A TREATISE ON METAMORPHISM. 



In the arid regions not only are the relatively insoluble compounds 

 precipitated in the belt of weathering, but the readily soluble compounds 

 may likewise be thrown down, such as the salts of the alkalies and alkaline 

 earths. These elements may be combined with any of the acids which 

 exist in the soils. Thus there may be precipitates of such soluble salts as 

 the alkaline carbonates, and even the alkaline chlorides and nitrates. The 

 more abundant salts precipitated are sodium, potassium, calcium, and mag- 

 nesium carbonates, sulphates, and chlorides. Nitrates and borates of the 

 bases mentioned are also locally abundant. In any given case the bases 

 and acids unite in such a manner as to })roduce the most insoluble com- 

 pounds. Therefore the carbonic acid first unites mainly with the alkaline 

 earths and is thrown down as calcium and magnesium carbonates. Where 

 sulphuric acid is present it unites mainly with calcium, producing gypsum. 

 The hydrochloric and nitric acids are left for the alkalies. Frequently the 

 salts and acids may not be present in proportion to combine as above. For 

 instance, after the alkaline earths are exhausted by the sulphuric and 

 carbonic acids there may be residual carbonic acid which will necessarily 

 unite with the alkalies. Or again, on the contrary, the sulphuric and 

 carbonic acids may not be sufficiently plentiful to exhaust the alkaline 

 earths, and under such circumstances chlorides of calcium and magnesium 

 may form. This is well illustrated by the calcium chloride deposits of the 

 borders of Great Salt Lake, where locally there is as much as 40 per cent 

 of this material." 



The compounds are likely to be precipitated in the openings of the 

 rocks, where evaporation may occur. Such openings are, of course, by far 

 the most abundant in the soils at and near the surface, and decrease in 

 abundance with depth. Therefore it is in the soils that the greatest amount 

 of precipitation takes place, but below the soils are found joint cracks, 

 bedding partings, caves, etc, and in such openings deposits of the various 

 compounds mentioned are built up. While in these openings the precipi- 

 tation is largely dependent upon the abundance of the solutions and their 

 evaporation, and also upon the abundance of the various salts and their 

 relative solubilities, in exceptional cases the reactions may not be simple 

 chemical ones, but be largely or wholly dependent upon animal life. This 

 is illustrated by certain caves where abundant phosphates and nitrates are 



"Gardner, F. D., and Stewart, John, A soil survey in Salt Lake Valley, Utah, Kept. Div. of Soils, 

 U. S. Dept. Agric, No. 64, 1899, p. 113. 



