5 i8 HISTORICAL GEOLOGY 



larger sea gave place later to a somewhat more constricted one 

 (Montana), along the edges of which the conditions were favorable 

 for coal formation. This is shown by thick coal beds of this time in 

 Montana, Wyoming, California, Utah, and New Mexico. It is prob- 

 able that neither of these seas was of great depth. 



(3) The closing epoch of the western interior was the Laramie. 

 The evidence points to a land so low that a slight oscillation either 

 raised or submerged it. When the latter occurred, the sea overspread 

 the land, and marine sediments were deposited ; as the sea was 

 filled in by sediments or was partially drained by elevation, swamps 

 and marshes were formed and peat accumulated in sufficient quan- 

 tities to form later some workable beds of coal. There is probably as 

 much coal in the Cretaceous formations of the west as in the Carbonif- 

 erous of the eastern United States, though usually of a poorer quality. 



There is some disagreement as to where the line between the latest 

 Cretaceous (Laramie) and the earliest Tertiary (Eocene) should be 

 drawn. A formation (Lance) in Wyoming, containing dinosaurian re- 

 mains and other typical Mesozoic animals but with plants that have 

 a Tertiary aspect, is separated from the Laramie below by an uncon- 

 formity involving, it is thought, the removal of over 20,000 feet of 

 strata. This formation is placed by some in the Tertiary, although 

 others believe that it should be included in the Mesozoic. 



The maximum thickness of the Upper Cretaceous in the western 

 interior is about 24,000 feet, making it one of the great periods of the 

 earth. 



Upper Cretaceous of Other Continents. — Not only was the Upper 

 Cretaceous a period of great submergence in North America, but 

 in other continents as well. Large tracts of Europe were beneath 

 the sea at this time. 



Limestones were deposited in southern Europe, and chalk (p. 523), 

 to a thickness of several hundred feet, accumulated in France and 

 England ; the latter has, however, little development elsewhere on 

 the continent. In Asia the land area was much smaller than now, 

 the Himalaya region, as well as large tracts in India and elsewhere, 

 being covered with water. Australia and South America show a 

 similar extension of the seas. The summits of much of the eastern 

 Andes of South America, to a height of 14,000 feet or more, are 

 formed of Upper Cretaceous beds. 



The Cretaceous Peneplain. — Before the close of the era, not only 

 North America but Europe and Asia appear to have been reduced 



