424 HISTORICAL GEOLOGY 



considerable interval at the close of the Comanchean 

 period. 



The Cretaceous sediments were evidently derived from the 

 mountains which bordered the Pacific shore. These moun- 

 tains were probably higher in the preceding period, but had 

 been much reduced before Cretaceous times. The thick- 

 ness of the Cretaceous sediments is correspondingly less than 

 that of the Comanchean system. In explanation it may be 

 suggested that the mountains had been eroded down to a 

 subdued hilly tract before the Cretaceous period. 



A period of quiet throughout the world. We have 

 already seen how the eastern and western portions of the 

 continent had been reduced to lowlands, either before the 

 Cretaceous or by the end of that period. The only notable 

 elevations which seem to have remained in North America 

 were certain hills and mountains along the Pacific coast and 

 others in the Carolinas. The same condition may be traced 

 in Europe and in Asia, where peneplains of enormous extent 

 seem to have developed by this time. While the lands 

 were thus low and monotonous, comparatively little sedi- 

 ment was being worn from them, and even that was fine 

 mud and silt. In the flood plains of the broad river val- 

 leys, clays and silts were spread in very wide but thin layers 

 interspersed with marsh deposits ; while along seashores little 

 except mud accumulated, and the deposition of pure lime 

 ooze was permitted comparatively near the coast. Thus 

 in England and France, and in many other parts of the 

 eastern continents, the Cretaceous rocks consist largely of 

 chalk or limestone. 



Greater uniformity of climate than we now have seems 

 to have been another characteristic of this quiet period of 

 low lands and shallow seas, for plants much like those of the 

 Gulf states lived also in Greenland, where snow and ice now 

 prevail. 



Conditions favor the sea animals. The conditions of life 

 in such a period must have been somewhat more stable 



