PHYSICAL GEOGEAPHY. Ixi 



rat 



Lli 



condition, caeli with its stream and its lake at the end of it, 

 and because the separate streams have never had force enough 

 to break through the barriers which all streams have at first 

 to encounter, and to unite their waters, so as to form a 

 complete drainage system. There is no doubt that formerly 

 the atmosphere was more humid, and that more rain fell, for 

 the remains of fresh-water shells of the present epoch, 

 covering large tracts of desert, prove the existence at one 

 time of lakes much greater in extent than any which can 

 now be foimd ; but instead of being filled to overflowing, 



and breaking tlirough their barriers to the sea, these lakes 

 lost more water by evaporation and percolation than their 

 tributary streams supplied, and thus were gradually dried up. 



The drainage, then, of the Great Basin is in a primi- 

 tive stage of existence, and will probably always remain so. 

 Wherever in this region there are lofty mountains, there 

 we are pretty certain to find a lake proportionately great. 

 If the lake has no outlet, it of necessity contains salt water, 

 which becomes Salter and salter as time advances, from the 

 concentration, by means of evaporation, of the salts washed 

 into it from the decomposed rocks of the moimtains. But 

 when the lake has an outlet, the water is, as usual, fresh. 

 Great Salt Lake is an example of the former class ; Utah Lake 

 of the latter. Most of the lakes, however, are not perma- 

 nent ; they form broad sheets of water after rain, but are 

 perfectly dry and barren during the greater part of the year. 

 They vary greatly in elevation and size. Great Salt Lake 

 exceeds 4,000 feet above the level of the sea; Sevier, 

 5,000; Lake Tahoe, 6,250; Monro Lake, 6,454; Pyramid, 

 3,940 ; Williamson's, 2,388 ; Morongo Sink, 1,500 ; Mojave 

 Sink, 1,000 ; and Perry Basin, 530. 



Two depressions, at least, ai-e below the level of the 



