390 HISTORICAL GEOLOGY 



Expansion of Permian lands. The gradual withdrawal 

 of the epicontinental sea eventually left almost all of the 

 continental platform dry land. On studying other countries 

 we find evidence that there, likewise, the land was more 

 extensive than at any other period in the Paleozoic era. The 

 Permian was everywhere a time of expanded continents. 

 To explain such a general withdrawal of the seas the sugges- 

 tion has been made that the deep ocean basins sank slightly, 

 thus leaving the continents relatively higher than before. 

 So widespread and radical a change marks this as one of the 

 critical periods of geologic history. 



The Appalachian trough. - Throughout the Paleozoic era, 

 the thickest layers of sediments had been laid down in the 

 interior sea just west of the old Appalachian land, from New 

 England to Alabama, and even across to Oklahoma. This 

 curved belt had been a subsiding trough, sinking perhaps 

 because of the weight of sediments which were constantly 

 loaded upon it. Much of the time the sinking just kept pace 

 with the deposition of sediment, so that thousands of feet of 

 strata were formed in relatively shallow sea water, as we now 

 learn from the presence of such things as coral reefs. At 

 other times, the subsidence was more rapid, and deeper water 

 prevailed, or on the other hand was of a halting nature and 

 allowed the coastal rivers to build out the seashore with allu- 

 vial deposits. Finally, in the Permian, the sinking and the 

 sedimentation ceased, and the process was reversed into a 

 slow emergence. The sediments deposited in this trough had 

 then reached a thickness much greater than that of the 

 corresponding strata in the Mississippi Basin. 



Crumpling of the east flank of the continent. Near the 

 close of the Permian, whether as a result of a sinking in the 

 Atlantic basin, or from some other cause, the east side of 

 North America was subjected to powerful horizontal com- 

 pression. The Appalachian trough was a weak zone in the 

 crust, just as the bend in a crooked stick determines the 

 point at which it will break when pressure is applied at 



