THE CRETACEOUS PERIOD 243 



remarkable variation in thickness of from 8700 feet in Colorado 

 to only 200 feet in the Black Hills of South Dakota. The Laramie 

 formation is quite certainly mostly of non-marine origin, with 

 fresh-water and land deposits (including much coal) common. 

 The formation shows a variable thickness possibly up to several 

 thousand feet, and it occurs only in the western interior region. 



Pacific Coast. — On the Pacific Coast, the Lower Cretaceous 

 is remarkably developed, where it shows a maximum thickness. 

 According to Diller, the older or Knoxville 1 series, comprising 

 nearly 20,000 feet of shales with some interbedded sandstones 

 and limestones, is overlain conformably by the Horsetown series 

 of sandstones and shales about 6000 feet thick. This enormous 

 thickness of sediments is clearly of shallow-water origin, the rocks 

 now nearly always being distinctly folded or tilted. In southern 

 California, too, the system is pretty thick and there contains some 

 volcanic rocks. Lower Cretaceous strata, usually folded and some- 

 times metamorphosed, also are widely developed in British Colum- 

 bia and Alaska with some coal in both regions and some volcanic 

 rock in the former. 



The Upper Cretaceous on the Pacific Coast is represented by 

 the single great Chico formation, but both the oldest and the young- 

 est portions of the series are often not represented at all. The 

 Chico is a marine deposit from a few hundred to several thousand 

 feet thick. It is prominently developed in the Coast Range 

 Mountains from Lower California to British Columbia. It consists 

 mostly of sandstones, shales, and conglomerates, and is, in some 

 places, conformable, and in others unconformable, upon the Lower 

 Cretaceous. 



Thickness of the Cretaceous. — The system shows a maxi- 

 mum thickness of fully 1700 feet on the north Atlantic Coast; 

 3000+ feet in the eastern Gulf region; 3500 to 7500 feet in the 

 western Gulf region; 10,000 to 15,000 feet in the western interior 

 region, though usually much less in any one locality; and 26,000 

 feet on the Pacific Coast. 



Igneous Rocks. — Volcanic rocks in the Lower Cretaceous are 

 known only in British Columbia and in the Coast Range of south- 

 ern California. Igneous rocks (chiefly lavas) of late Cretaceous 

 and Tertiary ages occur in vast quantities over great areas in 



1 According to some workers the Knoxville is now regarded as partly Ju- 

 rassic in age. 



