478 THE CRETACEOUS PERIOD 



islands, to the eastern base of the Rocky Mountains. The Coast 

 Range of California formed a chain of islands and reefs. In the 

 Sierra Nevada occurs an unconformity between the Lower Creta- 

 ceous and the uppermost Jurassic, but it does not imply the lapse 

 of a very long period of time. 



The older division of the Californian Lower Cretaceous is called 

 the Knoxville, and has an estimated maximum thickness of 

 20,000 feet, laid down upon a slowly subsiding sea-bottom. At 

 the end of the Knoxville age, the subsidence became more rapid 

 and the sea began to encroach upon the land, for the Horsetown 

 beds, which have a thickness of 6000 feet, overlap the Knoxville 

 shoreward and extend over upon the underlying Jurassic and other 

 pre-Cretaceous systems. Although the two stages of the Califor- 

 nian "Lower Cretaceous are entirely conformable throughout, and 

 appear to have been formed by a continuous process of sedimen- 

 tation, yet there is a very marked faunal change between them. 

 The Knoxville beds have a northern fauna, allied to that of Russia, 

 showing that the connection with Russian seas, which had been 

 established in late Jurassic times, was still kept up. With the 

 beginning of the Horsetown age, however, this northern commu- 

 nication was interrupted, and a connection was formed with the 

 waters of southern Asia, and in that way with central Europe. 

 The decided contrast which we find between the Lower Cretaceous 

 faunas of California and of Texas points to the existence of a land 

 barrier between the seas of the two regions. 



In the southern region the Lower Cretaceous was terminated 

 by a great upheaval, which over most of Mexico and Texas caused 

 the ocean to retire nearly to its present position, raising at the 

 same time a long ridge of land which became connected with the 

 Great Basin land. This mid-Cretaceous land epoch must have 

 continued for a considerable time, permitting extensive denuda- 

 tion and a complete change in the fauna. Wherever the Upper 

 Cretaceous is in contact with the Comanche limestones, the two 

 are unconformable, and no species of animal is known to pass 

 from one to the other. These limestones form the principal mass 

 of the Mexican mountains, where the force of compression has 



