PALEOGEOGRAPHIC MAPS OF NORTH AMERICA 43 



probably was not at any time actually reached. The middle Cam- 

 brian sea extended further in certain areas than the upper Cambrian 

 and retreated while the upper Cambrian sea spread over other regions. 

 These details are not well worked out, though in part recognized. 

 The map truly presents, however, the general fact that North America 

 was to a great extent submerged and the land areas very markedly 

 reduced. The prevailingly fine and calcareous sediments of the 

 wide seas and the siliceous coastal plain sediments of the littoral 

 deposits indicate that the relief of the land was low. 



The conditions of marine circulation had apparently been modified 

 by the expansion of the interior sea, and the climate conditions 

 incident to widespread seas and low lands had become so ameliorated 

 that similar habitats prevailed throughout a very wide range of 

 latitude. 



