CONCENTRATION IN BASIN LAKES. 553 



lakes. In journeying across the desert of Nevada in summer or autumn a 

 person is rarely out of sight of one or more areas, small or great, varying in 

 size from a fraction of an acre to hundreds or even thousands of square 

 miles, covered with the almost snow-white chemical deposits of the evapo- 

 rated lakes, which glare with intense light under the brilliant sun. 



The permanent lakes, like Great Salt Lake, continually receive supplies 

 of material in solution and are constantly being evaporated. When such 

 lakes were first formed the salts steadily accumulated until the water became 

 saturated. After a state of saturation is reached each year at the flood season 

 chemical deposition may and usually does cease and solution even may 

 take place to some extent. After the flood season the mechanical sediments 

 quickly subside. During summer and autumn evaporation greatly exceeds 

 the influx of water ; the lakes shrink ; the solutions become supersaturated and 

 chemical precipitation follows. Along the borders of the permanent lakes 

 chemical and mechanical precipitates are interstratified the same as in the 

 ephemeral lakes; but in the central areas of these lakes, beyond the depth to 

 which plentiful mechanical sediments are cai'ried, the chemical precipitates, 

 comparatively little contaminated with mechanical material, are steadily 

 built up. It is well known that the annual rainfall is not uniform, but that 

 there are periods extending over a number of years of more than and less than 

 the average rainfall. It is in the dry parts of the cycles that the thick deposits 

 of chemical sediments are largely built up. The formation of such deposits 

 is now also promoted in many lakes by the use of the water in irrigation. 



As already noted, the abundant chemical sediments are the alkalies 

 and alkaline earths, carbonates, sulphates, and chlorides. The order in 

 which the materials are deposited depends upon the relative amounts of 

 these substances and upon their relative solubilities. In consequence of 

 difference in solubility there is a more or less marked tendency for the 

 bases first to unite with the acids so as to produce the most insoluble com- 

 pounds. Thus the calcium and magnesium unite with carbonic acid to 

 produce marl and tufa. The calcium unites with sulphuric acid to produce 

 gypsum. But always before one substance is completely precipitated one 

 or more others begin to be thrown down, and consequently there is a min- 

 gling of precipitates. This is especially true of those compounds which are 

 closely allied chemically, such as the calcium and magnesium carbonates, 

 the sodium and potassium chlorides, etc. While there is a tendency for 



