CHAPTER XVII 



The Carboniferous divided. The Mississippian, Pennsyl- 

 vanian, and Permian periods were formerly combined under 

 the name of Carboniferous. Evidence is accumulating, 

 however, which indicates that the three divisions are really 

 quite as distinct from each other as are such periods as the 

 Devonian and the Silurian ; and so it is thought best to make 

 three separate periods out of the old Carboniferous. Each 

 is named for a region in which the rocks are well exposed and 

 well known. 



Transition from the Devonian. The transition from the 

 Devonian into the Mississippian period was not marked by 

 abrupt changes in most parts of the North American conti- 

 nent. The chief event which characterizes the Mississippian 

 is the further expansion, over the greater part of the United 

 States and the Northwest, of the epicontinental sea which, 

 even in the late Devonian, was fairly extensive. This expan- 

 sion of the sea was followed later in the period by a corre- 

 sponding retreat. For the eastern interior, it was the last 

 period of purely marine conditions. 



Clastic sediments in the East. Over what is now the 

 Mississippi Basin, as far east as Ohio, and as far west as 

 Nevada at least, was the open sea. In that portion of 

 this vast region which the Devonian ocean had also covered 

 the strata of the two systems are generally conformable. In 

 much of the West, however, the Mississippian extends be- 

 yond the Devonian and rests directly upon more ancient 

 rocks, in some places even on the Archaean. About the 

 northeastern border of this sea, notably in Pennsylvania and 

 Ohio, coarse sands and muds were accumulating rapidly. 



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