640 GEOLOGY. 



and Ordovician periods into land; but as the Silurian period pro- 

 gressed, the sea gradually crept back over much of the area from which 

 it had temporarily withdrawn. Of the larger part of this area the 

 sea retained possession through the later part of the Silurian and 

 the Devonian periods, and during the Mississippian it was greatly 

 extended. 



In the Coal Measures period great areas were converted into marshes, 

 and still later in the period, or at its close, a wide-spread withdrawal 

 of the sea made most of the area of the United States east of the 96th 

 meridian, land. The geographic changes may be best appreciated by 

 comparing the maps on pages 497 and 545. The movements which 

 resulted in these changes were probably slow, and for most of the 

 area not notably deformative. 



The great geographic changes which had been accomplished, or 

 far advanced, by the close of the Paleozoic, were (1) the development 

 of the Appalachian mountain system at the western border of Appa- 

 lachia; (2) the deformation of the surface of Appalachia; (3) the 

 development of the Ouachita mountains; (4) the final conversion 

 of the larger part of the area between the Great Plains and Appalachia 

 from an area of deposition to an area of erosion; and (5) the restriction 

 at the west of the area of sedimentation in the western interior. 



Such extensive geographic changes, involving the conversion of 

 such great areas from sea bottom into land, must have caused pro- 

 found changes in the circulation of water, in climate, and in the dis- 

 tribution of terrestrial and marine life. 



The Life of the Permian. 



The life of the Permian can carry its full meaning and receive its 

 full interpretation only when put into association with the extraor- 

 dinary physical conditions which formed its environment. These were 

 the most remarkable in the post-Cambrian history of the earth. 

 Between them and the life there must have been reactions and adapta- 

 tions of the utmost significance, if we could surely read them. At no 

 period, save our own, were the phenomena so pronounced, and hence, 

 with little doubt, so rich in possible instruction as to the adaptation 

 of life to extreme conditions. Their teachings have been less eagerly 



