THE MISSISSIPPIAN PERIOD 371 



probably not filled by the sea. Thick sandstone and conglom- 

 erate are there succeeded by shales with gypsum. 



Decreasing seas at the close. The uppermost strata 

 of the Mississippian in the middle states are shaly and even 

 sandy, like the beds which immediately followed the Devonian. 

 Above these sandy beds there is usually a distinct unconform- 

 ity, which separates the Mississippian from the overlying 

 Pennsylvanian system. The lower shaly beds we have inter- 

 preted as the mud banks along the borders of an expanding 

 sea, in which the deposition of mud was gradually being re- 

 placed by that of limy ooze. Toward the end of the period 

 the sea was evidently being restricted in eastern United States. 

 As the shore line migrated west and south, the mud and sand 

 which are usually deposited near shores were spread out over 

 the limestone that had been deposited in the clearer sea 

 earlier in the period. Finally by withdrawal of the sea the 

 eastern part of the country became land. The erosion of this 

 low land was attended by slight warping of the surface, and 

 even a few faults and gentle folds were produced. The result 

 of the disturbance and the erosion together is the unconform- 

 ity between the Mississippian and Pennsylvanian systems. 



In the far West, changes of land and sea at the close of the 

 Mississippian period were less pronounced. No distinct line 

 of separation between the two systems has been recognized 

 in the Arizona-Nevada region ; but in the Rocky Mountains 

 a widespread unconformity indicates the emergence of the 

 sea bottom. 



The Paleozoic Alps. During the four preceding periods 

 sediments were being deposited rather generally over western 

 Europe, much as in eastern United States. It is noteworthy 

 that Britain and Germany were also volcanic districts through 

 much of this time. 



After the close of the Mississippian period, these deposits 

 were locally folded up (Fig. 388) into two great mountain 

 chains, one extending from Ireland into Germany and the 

 other from southern France into Bohemia. These folds now 



