602 GEOLOGY 



it; but so rapidly do the formations thin westward, that in the 

 western part of the same state the equivalent formations have a 

 thickness of only 300 to 600 feet. In the region of the Mississippi, 

 where the system is chiefly limestone, it reaches a maximum thick- 

 ness of about 1,500 feet, being thinner to the north and thicker to 

 the south. In Oklahoma, the thickness is about 1,800 feet, in the 

 Black Hills, 275 to 525 feet, in Colorado (Crested Butte region) 

 400-525 feet, and in northern Arizona (Grand Canyon of the Colo- 

 rado), 1,800 feet. 



The distribution of the uncovered portions of the Mississippian 

 beds in the eastern part of the continent is shown in Fig. 426. 

 The beds themselves are of course much more extensive than their 

 outcrops, being extensively concealed by younger beds. Like all 

 preceding systems, the Mississippian doubtless has wide distribu- 

 tion beneath the sea, where it is probably thin. 



Close of the period. At the close of the period, the eastern 

 interior sea was contracted to very narrow limits if not completely 

 obliterated. Great changes took place in the western half of the 

 continent too, but they are less fully determined. Evidence of 

 their existence is found in the wide-spread unconformity above 

 the Mississippian. In some parts of the west, however, so far as 

 now known, marine conditions prevailed uninterruptedly from 

 the early Mississippian period to the later part of the Pennsylvanian. 



Reasons for regarding the Mississippian a distinct system. The 

 withdrawal of the sea from a large part of the eastern interior at 

 the close of the Mississippian period exposed the newly deposited 

 sediments to erosion. The exposure was long and the erosion con- 

 siderable. At the opening of the Pennsylvanian period, as will be 

 seen in the sequel, a large part of this area was again the site of 

 deposition, and the new system rests unconformably on the Missis- 

 sippian over wide areas, from Pennsylvania and Tennessee on the 

 east, to Utah and Montana on the west (Fig. 426) . 



The wide-spread emergence, erosion, and subsequent sub- 

 mergence recorded by the unconformity between the Mississippian 

 and the Pennsylvanian systems is just the sort of change which is 

 held to separate periods, not epochs. Nowhere else in the whole 4 

 course of the Paleozoic era are so great physical changes embraced 



