554 A TREATISE ON METAMORPHISM. 



the most insoluble compounds first to be thrown down, these relatively 

 insoluble compounds may be very sparse in quantity, and therefore more 

 soluble salts be first precipitated. While all the above complications 

 make the result very uncertain, still there is usually a marked tendency 

 to stratification, each stratum being' composed dominantly of one compound 

 or of two closely allied compounds. Thus a stratum may consist mainly of 

 marl, another mainly of gypsum, another mainly of common salt, and so on. 



By great changes in the humidity of a region and some changes 

 perhaps in the topographic conditions a basin lake may become smaller or 

 wholly disappear, thus baring its deposits to the ordinary forces- of erosion. 

 Such lakes are Lake Bonneville, described by Gilbert, and Lake Lahontan, 

 described by Russell. The chief chemical deposit of Lake Bonneville, 

 according to Gilbert, was a white calcareous marl. However, this marl is 

 very impure, containing a large amount of silicates and silica, in one case 

 as much as 74 per cent." The chief chemical deposits of Lake Lahontan, 

 according to Russell, are a calcareous white marl and tufa. The white 

 marl, like that of Bonneville, is impure, containing about 30 per cent of 

 silica, alumina, and iron, of which 22 per cent is silica. The tufa deposits 

 are mainly calcium carbonates, although small amounts of magnesium 

 carbonates and other constituents are contained.'' It appears that Lakes 

 Bonneville and Lahontan did not become saturated with the salts of the 

 alkalies. In the case of Bonneville this stage was not reached until it had 

 shrunk to Great Salt Lake. 



By still more radical topographic revolutions and changes in meteoro- 

 logical conditions than those experienced by Bonneville and Lahontan, 

 deposits of salt lakes may become deeply buried below other sediments, 

 and thus we have the explanation of the salt, the gypsum, and other similar 

 deposits of various parts of the world. 



DISTRIBUTION OF RESIDUAL MATERIALS. 



The distribution of the dissolved material in the belt of weathering is 

 of very great importance, but the distribution of residual material which 

 at no time goes into solution is of no less importance. The concentration 



"Gilbert, G. K., Lake Bonneville: Mon. U. S. Geol. Survey, vol. 1, 1890, p. 202. 

 >j Russell, I. C, Geological history of Lake Lahontan: Mon. U. S. Geol. Survey, vol. 11, 1885, 

 pp. 152-153, 203. 



