80 AQUEOUS AGENCIES. 



gether with the composition of its waters, is usually supposed to indi- 

 cate that it has been formed by the simple concentration of the waters 

 of a once fresh lake.* Yet there are some evidences, as we shall see 

 hereafter, of this sea having been once connected with the Black Sea 

 and with the Arctic Ocean. The composition of the waters of the Great 

 Salt Lake of Utah would seem to indicate its origin in the isolation of 

 sea- water ; but there are evidences of its once having had an outlet, in 

 which case it must have been fresh, or at least brackish, f 



Alkaline lakes can only be formed by the second way. Both salt 

 and alkaline lakes, therefore, may be formed by indefinite concentra- 

 tion of river-water in a lake without outlet. Whether the one or the 

 other is formed depends on the composition of the river- water. If al- 

 kaline chlorides predominate, a salt lake will be formed ; but if alkaline 

 carbonates, an alkaline lake. Such alkaline lakes are found in Hun- 

 gary, in Lower Egypt, and in Persia. In our own country, Lake Mono, 

 fifteen miles long and twelve miles wide, and Lake Owen, of at least 

 equal dimensions, are examples of alkaline lakes. The waters of Lake 

 Mono consist principally of a strong solution of carbonate of soda and 

 chloride of sodium, with a little carbonate of lime and borate of soda. { 



Conditions of Salt-Lake Formation. — Spring and river waters always 

 contain a small quantity of saline matter derived from the rocks and 

 soils. Suppose, then, we have a lake supplied by rivers: 1. If the 

 supply of water by rivers is greater than the loss by evaporation from 

 the lake-surface, then the water will rise until, finding an outlet in the 

 rim of the lake-basin, it flows into the sea. In this case the lake will 

 remain fresh , or the quantity of saline matter will be inappreciable. 

 But if, on the other hand, the loss by evaporation is greater than the 

 supply by rivers, the lake will decrease in extent, and therefore in evap- 

 orating surface, until an equilibrium is established. Now all the saline 

 matters constantly leached from the earth accumulate in the lake with- 

 out limit ; the lake, therefore, must eventually become saturated with 

 saline matter, and afterward begin to deposit salt. It is evident, then, 

 that whether a lake is fresh or salt depends upon whether or not it has 

 an outlet, and this latter depends upon the relation of supply by rivers 

 to loss by evaporation. Lakes are mostly fresh, because much more 

 water falls on continents than evaporates from the same surface, the 

 excess running back to the sea by rivers. It is only in certain parts 

 of the continents, where the climate is very dry, that there is no such 



* Bischof, Chemical and Physical Geology, vol i, p. 91. 



f Gilbert, Wheeler Report for 1872, p. 49. U. S. Geol. Surv. Monogr. 1, p. 171 et seq. 



X The probability of Great Salt Lake having been produced by simple evaporation of 

 river-water is increased by the difference in the composition of the waters of lakes in 

 this general region. Where sedimentary rocks prevail, as in Utah, they are salt ; where 

 volcanic rocks prevail, as about Mono and Owen, they are alkaline. 



