460 HISTORICAL GEOLOGY 



its water, like that of Lake Bonneville, has largely evaporated, 

 leaving several small, salty lakes and dry, flat-bottomed de- 

 pressions covered with sand and crusts of lime. These deserts 

 were fatal to some of the early emigrants to California who 

 came overland from the East. 



Decline in volcanic activity. While volcanoes were less 

 numerous in the West during this period than in the Tertiary 

 they were still fairly common, as is attested by numbers of 

 small cinder cones among the western mountains and high 

 plateaus. In the bed of old Lake Bonneville several little 

 craters were formed after the lake shrank to nearly its pres- 

 ent size. In northern California there is another little cone 

 surrounded by a recent lava flow and a layer of ashes in. 

 which the stumps of trees killed by the last eruption are still 

 standing. The great cones of Mts. Shasta, Rainier, and others 

 along the Pacific slope, which were built largely during the 

 Tertiary period, probably increased somewhat in size in the 

 course of the Quaternary period. Some, indeed, are thought 

 to have had eruptions within the last few centuries. Except 

 in Alaska and Mexico, however, the present is not a time of 

 notable volcanic activity in western North America. 



ANIMALS OF THE GLACIAL EPOCH 



Mammals attain their modern state. The Glacial epoch 

 is so recent geologically that the animals of that time differ 

 but little from those which exist to-day. Add to the mam- 

 mals of to-day certain large forms which were common then, 

 but have since been exterminated, and we have essentially 

 the Quaternary fauna. 



Migrations caused by glacial fluctuations. Much more 

 striking peculiarities are found when we compare, from the 

 standpoint of their distribution, the animals of the present 

 day with those of the Glacial epoch just preceding. Obvi- 

 ously the effect of the slow expansion of the ice sheets was 

 to crowd all animals and plants away from the glaciated region, 



