CENOZOIG ERA. AGE OF MAMMALS. 



395 



100 miles wide and 300 miles long. The traveler on the 

 Union Pacific Railway can hardly fail to observe the old 

 terraces, rising up to 1,000 feet above the present lake- 

 level. It drained at that time into the Snake and Co- 

 lumbia Rivers, then lost its outlet, and dried away to the 

 remnants Great Salt Lake, Utah Lake, and Sevier Lake 

 which we now have. The lake which filled the Nevada 

 basin Lake Laliontan was of nearly equal size, and its 

 dried-away residues are seen in numerous salt and alkaline 

 lakes, such as Pyramid, Winnemucca, Humboldt, Carson, 

 Walker, etc., which overdot this great area. 



Rivers. The old or preglacial river-beds, on the 

 eastern side of the continent, as we have seen (page 391), 

 underlie the present river-beds i. e., are in the same 

 place, but deeper. In middle California the relation is 

 quite different and peculiar. Here the old river-beds 

 overlook the new i. e., they are in a different place, and 

 higher. The old river-beds are on the divides between 

 the new. The reason is this : In middle California, at 

 the beginning of the Glacial epoch, the old river-beds .had 

 already been filled up, first with gravel, and then, by 

 igneous outbursts, with lava. The rivers were thus dis- 

 placed, and began to cut new beds. But at the same 



FIG. 350. Ideal section through two modern river-beds and table-mountain divide : 

 r', old river-bed ; r, r, present river-beds ; s, slate ; gr, new gravel ; /, lava ; 

 gr', old gravel under the lava. 



time there was a considerable lifting of the whole moun- 

 tain-region, and consequently the rivers now cut deeper 

 than before (Fig. 350). Thus it has come to pass that 

 the new river-beds occupy the places of the old divides, 



