CHAPTER XXV 



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. Cretaceous 

 formations are found in (i) the Atlantic Coastal Plain; (2) the 

 Coastal Plain of the Gulf; (3) the Great Plains, from the Gulf of 

 Mexico to the Arctic Ocean; (4) at many points in the western 

 mountains; and (5) over considerable areas along the Pacific coast. 

 While its distribution has much in common with that of the Coman- 

 chean, it is much more widespread (Fig. 451), and unlike the 

 Comanchean, this system is chiefly marine. 



Atlantic border region. Cretaceous formations come to the 

 surface in a belt near the western margin of the Atlantic Coastal 

 Plain (Fig. 451), just east of the outcrop of the Potomac series. 

 The beds have been but little disturbed, and still dip, as when 

 deposited, slightly to seaward, and in that direction pass beneath 

 younger formations. They are largely of unindurated clay and 

 sand, with some greensand marl, which is rather characteristic of 

 the system. The distinguishing constituent of this marl is glau- 

 conite, primarily a hydrous silicate of potassium and iron, 1 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 

 the conditions necessary for its development on such a scale as to 

 make considerable beds, are the following: 2 (i) Water of moderate 

 depth, 100 to 200 fathoms being the most favorable; (2) a meager 

 supply of land-derived sediment; and (3) the presence of forami- 

 nifera. The production of the glauconite seems to be effected by 



1 Most Glauconite is impure, and, as it occurs in nature, contains several 

 other ingredients. 



2 For brief summary concerning the origin of greensand marl, see Clark, Jour. 

 Geol., Vol. II, p. 161. For a fuller account, see Challenger Report on Deep Sea 

 Deposits. 



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