THE MISSISSIPPIAN PERIOD. 511 



a thickness of only 300 to 600 feet. In Maryland the thickness is 

 about 1300 feet. 1 In Ohio the sedimentary beds ( Waver ly shales) 

 are rather thicker than in the adjacent parts of Pennsylvania. In 

 the region of the Mississippi, where the system is chiefly limestone, 

 it reaches a maximum thickness of about 1500 feet, being thinner to 

 the north and thicker to the south, within the area of its outcrop . 

 Even this great thickness is exceeded in West Virginia, where the lime- 

 stone is more than 2000 feet thick. In Indian Territory (Sycamore 

 limestone and Caney shale 2 ) the Mississippian has a thickness of about 

 1800 feet, in the Black Hills of South Dakota 275 to 525 feet, in Colo- 

 rado (Anthracite-Crested Butte region) 400-525 feet, and in the regions 

 of the Grand Canyon of the Colorado in northern Arizona (Red Wall 

 formation) 1800 feet. At any probable rate of accumulation, such 

 great thicknesses of limestone call for very long periods of time. The 

 duration of the period was probably as great as that of some of the 

 preceding Paleozoic periods. 



As in preceding periods, the thickest beds of the Mississippian 

 system are in the Appalachian mountains. Here too, where the beds 

 are thickest, they were accumulated in shallow water, suggesting 

 either that subsidence was in. progress pari passu with the sedimenta- 

 tion, or that inclined deposition took place, as the shallow-water zone 

 crept seaward, pari passu with the sedimentation. 



The distribution of the uncovered portions of the Mississippian 

 beds in the eastern part of the continent is shown in black in Fig. 228. 

 The beds themselves are of course much more extensive than their 

 outcrops. Within the area of North America they are believed to 

 be concealed by younger beds, largely Pennsylvanian (the lined areas 

 of Fig. 228). They have probably been removed from considerable 

 areas (dotted areas Fig. 228) where they were originally deposited. 

 Like all preceding systems, the Mississippian doubtless has wide dis- 

 tribution beneath the sea, where it is probably thin. 



The Lower Carboniferous 3 of Other Continents. 



Europe. — The post-Devonian Paleozoic systems of Europe resemble 

 the corresponding systems of North America in some ways, and are in 



Grosser, Jour, of Geol., Vol. IX, pp. 422-4. 

 2 Taff, Professional Paper 31, U. S. Geol. Surv. 



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

 it is the term in common use in Europe. 



