THE MAMMALS 137 



includes the present), has been a period of irregular 

 continental elevation and mountain building to which 

 may be attributed all the rugged profiles of the present 

 world. In the Middle Cretaceous the Rocky Mountain 

 area had been submerged in a marine trough loo miles 

 wide and extending from Alaska southward and east- 

 ward into the Gulf of Mexico; in the late Cretaceous, 

 continental upHft had drained the salt water from this 

 trough to leave swamps that made many a rich coal 

 bed from Alberta to Mexico. Then in the Laramide revo- 

 lution (named after the Laramide Range in Wyoming), 

 which ushered in the Eocene, the North American con- 

 tinent buckled along this trough to give birth to the 

 basal structures of the present Rocky Mountain System 

 —over 3000 miles long and, at its maximum extent across 

 eastern Colorado to central Idaho, 500 miles wide, with 

 the Front Range at Longs Peak in Colorado reaching a 

 height of perhaps five miles— the most severe disturbance 

 to be experienced by this continent since pre-Cambrian 

 times. Another range extended from Trinidad southward 

 through Colombia to beyond Cape Horn, a distance of 

 nearly 5000 miles. In Europe, the Eocene saw the first 

 decided upthrust of the Alps, though these, like the 

 Laramide Rockies and Andes, have subsequently suf- 

 fered extensive erosion, and their present altitude is at- 

 tributable to continued uplift in the Pliocene and 

 Pleistocene— that is, within the last two million years 

 or so. 



The present Sierra Nevada are also relatively yoimg 

 mountains, and represent a colossal block 100 miles wide 

 and 300 to 400 miles long, the eastern edge of which 

 was tilted in the Pleistocene to an elevation of 13,000 

 feet, the western edge depressed perhaps 25,000 feet 

 below sea level in what is now the 'great trough' of cen- 

 tral California, between the Sierra and the Coast Range. 



Also dating from the last geologic period are the Hi- 

 malaya, the foothills of which have been Hfted 6000 feet 

 since the middle of the Pleistocene so that marine forma- 



