THE MISSISSIPPIAN PERIOD 147 



Upper Mississippian Rocks in the East. — In the northern 

 Appalachian district, the Mauch Chunk formation, consisting 

 mostly of red sandy shales, directly overlies the Pocono, while in 

 Maryland and West Virginia the lower portion of the Mauch 

 Chunk gives way to the Greenbrier limestone. The Mauch Chunk 

 shows a maximum thickness of 3000 feet in eastern Pennsylvania, 

 but this diminishes notably to the north, west, and south. It is 

 considered to be either a great flood-plain or a delta deposit. 

 Farther west, in the Mississippi River states, the Upper Mississip- 

 pian is represented by the St. Louis and Chester series. The 

 former is made up almost wholly of limestone of very widespread 

 extent, while the latter is rather variable lithologically, and is 

 more restricted in distribution. 



The Mississippian of Nova Scotia and New Brunswick has not 

 been so carefully subdivided, but it is largely sandstone below and 

 limestone, with some red beds and gypsum, above. Its thickness 

 is about 2500 feet. 



Mississippian of the West. — This system is very widely dis- 

 tributed in the West as proved by the numerous exposures, but it 

 has not been carefully studied and subdivided as in the east. 

 Throughout the system, which is commonly several thousand feet 

 thick, limestone greatly predominates. Thus in the Canadian 

 Rockies the system measures over 6000 feet thick with over 5000 

 feet of limestone. 



Thickness of the Mississippian. — The Mississippian system 

 in eastern North America ranges in thickness from about 5000 

 feet in eastern Pennsylvania to only some hundreds of feet in the 

 western part of the same state. In the Mississippi River states the 

 maximum thickness is 1500 feet, though it is generally less than 

 1000 feet. In the western part of the continent thicknesses of 

 several thousand feet (maximum over 6000 feet) have been 

 observed at several places, while in other localities, as in the Black 

 Hills and parts of Colorado, it measures only a few hundred feet 

 thick. Eighteen hundred feet are known in the Grand Canyon of 

 the Colorado River. 



Igneous Rocks. — Igneous activity appears to have been wholly 

 confined to the region from Alaska to northern California, where 

 vulcanism occurred on a large scale. Mississippian rocks are there 

 often largely made up of igneous materials. 



