470 



HISTORICAL GEOLOGY 



finer than in the East, and limestone becomes increasingly abundant 



until, west of Ohio, it constitutes the greater mass of the sediments. 



The presence of gypsum and salt in portions of the strata of this 



age in Michigan shows that the climate, for a time at least, was dry, 



Fig. 446. — Map showing the probable outline of North America during a portion 

 of the Lower Mississippian. Continental deposits are shown in solid black. (Modified 

 after Schuchert.) ** 



and that the sea or seas of this region were isolated. The Mississip- 

 pi;m gypsum of Nova Scotia implies a similar condition for that re- 

 gion, and, as has been seen, an arid or semiarid climate was present 

 over the lands contiguous to the Appalachian trough. 



The Mississippian seas spread over a large area of the Cordilleras 

 of the West, from Mexico to the Arctic, the strata of this period being 

 1 al thousand feet thick in certain places. 



Close of the Mississippian. — The extensive seas of the early 

 Mississippian were gradually drained, so that before the close of the 



