THE CRETACEOUS PERIOD. 149 



and limestone) above. Both formations are of shallow-water origin, 

 as shown by the structure of the beds at some points, by the bird 

 tracks and remains of land animals at others, and by the species of 

 shallow-water mollusks which abound throughout both formations. 

 While clastic formations predominate in the Colorado series as a whole, 

 there are also beds of chalk comparable to those of Europe, which 

 gave the name Cretaceous to the corresponding system of the old world, 

 Chalk occurs in Kansas, 1 Iowa, 2 Nebraska and South Dakota. The 

 chalk is not only widespread, but its amount is very great, for it locally 

 (mouth of the Niobrara) attains a thickness of 200 feet. 



Beds of coal are of occasional occurrence in the Colorado series. 

 They were probably formed about the borders of the sea, or about the 

 islands which stood above it. Charred wood and even charcoal in 

 the series point to the existence of forest fires during the epoch. 



The aggregate thickness of the Colorado series is locally as much 

 as 3000 feet, as strata are measured, though its average thickness 

 on the plains is much less. It is between 400 and 500 feet in eastern 

 Nebraska, and thickens westward. 3 It has a thickness of about 2000 

 feet on the west slope of the Black Hills. 4 Its distribution is shown 

 in a general way on the map (Fig. 388). 



At the close of the Colorado epoch there was some deformation 

 of the beds of this and earlier series, as indicated by their relation to 

 the beds of the following epoch. 5 These movements changed the 

 relation of land and water somewhat, and the fossils of the succeed- 

 ing series indicate that the sea was then deeper, at least locally. 



The Origin of Chalk. 



There has been much difference of opinion concerning the origin 

 of chalk. Its resemblance to the foraminiferal ooze of the deep seas 

 long since led to the belief that it was a deep-sea deposit; but closer 

 examination has thrown doubt on this conclusion, for it appears that 

 the points of difference between the chalk and foraminiferal ooze 



1 Williston, Univ. of Kans. Geol. Surv., Vol. II, and Logan, Jour of Geol., Vol. VII, 

 p. 85. 



2 Calvin, Iowa Geol. Surv., Vol. Ill, pp. 213-235. A brief review of chalk in 

 North America; good bibliography. 



3 19th Ann. Rept. U. S. Geof. Surv. 



4 Darton, New Castle, Wyo.-S. D. folio, U. S. Geol. Surv. 



1 Emmons, the Denver Basin, Monograph XXVII, U. S. Geol. Surv. 



