566 



CENOZOIC ERA— AGE OF MAMMALS. 



The feeble remains of some may still be found hidden away among the 

 coolest and shadiest hollows of the highest summits. 



Lakes. — A period of flooded lakes, marked by successive terraces 

 about the present lakes, is well shown, especially in the Basin region. 

 The period of the flooded lakes in this region seems to have corre- 

 sponded with the Glacial epoch, for the great glaciers ran into some of 

 them. 



About Lake Mono there are five or six very distinct terraces, the 

 highest being about 700 feet above the present water-level. Evidently 

 at that time the water washed against the steep slope of the Sierra, 

 and many of the glaciers in this region ran into it. About Salt Lake, 

 several terraces are very conspicuous, the highest being about 1,000 feet 

 above the present lake-level. Traced out by this highest level, the out- 

 line of the lake embraced an enormous area. Similarly about all the 

 saline lakes of the Nevada basin terraces have been traced up to more 

 than 600 feet above the present level. In general terms, we may say 

 that the Basin region at that time was occupied by two great lakes : 

 the one filling the Utah basin, the other the Nevada basin, the eastern 

 shore of the one washed against the Wahsatch, the western shore of 



Fig. 935.— Map of the Quaternary Lakes Bonneville and Lahontan (after Gilbert and Russell). 



the other against the Sierra. The former has been accurately mapped 

 by Gilbert and called Lake Bonneville — the latter, also accurately 

 mapped by King and Eussell, and called Lake Lahontan, in honor of 

 these early explorers. Lake Bonneville when at its 1,000-feet level, 

 emptied northward into the Snake and Columbia Kivers. It eroded its 

 outlet down to the 600-feet level ; there lost its outlet and dried away to 

 its present condition. Lake Lahontan when at its 600-feet level, had 



