43 



MISSISSIPPIAN PERIOD 



General Considerations 



Thickness and outcrops. In keeping with the variations in 

 the sediments, the thickness of the Mississippian system varies 

 greatly. In Pennsylvania, there is a thickness of 1,400 feet of 

 sandstone (Pocono), with 3,000 feet of shale (Mauch Chunk) above 

 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 

 it reaches a maximum thickness of about 1,500 feet. 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 Colorado), 1,800 feet. 



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

 interior sea was contracted to narrow limits if not obliterated. 

 Great changes took place in the western half of the continent too, for 

 there is a widespread unconformity above the Mississippian system. 

 In 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. 



This great unconformity, and the great changes in life which 

 accompanied the emergence which it records, is the basis for 

 regarding the Mississippian a distinct period. 



Lower Carboniferous of Other Continents l 

 In western Europe, two great series, or systems, are included 

 under the Carboniferous, (i) the Lower Carboniferous, chiefly of 

 marine origin, and (2) the Coal Measures or Carboniferous proper, 



Fig- 375- Composite diagrammatic section, showing the unconformity between 

 the Mississippian and Pennsylvanian systems in Iowa. (Keyes, la. Geol. Surv.) 



1 The term Lower Carboniferous is here used, instead of Mississippian, because 

 it is the term in common use in Europe. 



