CHAPTER XXVI 

 THE CRETACEOUS PERIOD 



FORMATIONS AND PHYSICAL HISTORY 



The Cretaceous period was ushered in, so far as North America 

 is concerned, by a notable encroachment of the sea. In the United 

 States, the system is found in (1) the Atlantic Coastal Plain; (2) 

 the Coastal Plain of the Gulf, both east and west of the Mississippi ; 



(3) the Great Plains, from the Gulf of Mexico to the Arctic Ocean; 



(4) at many points in the western mountains, and (5) over consid- 

 erable areas along the Pacific coast. While the distribution of this 

 system has much in common with that of the Comanchean, it is 

 much more wide-spread (Fig. 504). Unlike the Comanchean, the 

 system is chiefly marine. 



The Atlantic Border Region 1 



The^ Cretaceous beds come to the surface in a belt near the 

 western margin of the Atlantic Coastal Plain, just east of the out- 

 crop of the Potomac series (Fig. 504). The beds have been but 

 little disturbed, and still dip, as when deposited, slightly to seaward, 

 and in that direction pass beneath younger formations. The for- 

 mations are chiefly of unindurated clay and sand, with not a little 

 greensand marl, which is rather characteristic of the system. 

 There is also some limestone. 



The distinguishing constituent of the greensand marl is gluu- 

 conite, primarily a hydrous silicate of potash and iron, 2 which 

 occurs in grains. Glauconite is now making in some parts of the 

 sea, and from the situations in which it is formed, it is inferred that 



1 Besides the State Reports referred to under the Comanchean, sec Clark. 

 Bull. Geol. Soc. of Am., Vol. I, 1897, pp. 315-358, and Weller, Jour. Gc.,1.. 

 Vol. XIII, p. 71. 



2 Glauconite is usually impure, and, as it occurs in nature, contains several 

 other ingredients. 



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