144 HISTORICAL GEOLOGY 



It will be seen that, in the Appalachian region, the rocks are 

 almost wholly clastic, while limestones are prominent in the Missis- 

 sippi River region. Also the system in the east has not been so 

 much subdivided, and detailed correlations with the subdivisions 

 farther west have not been made. Regarding the detailed classifi- 

 cation of subdivisions in America, much difference of opinion still 

 exists, and more or less local names are used in different regions. 



Distribution and Character of the Rocks 



General Distribution. — The accompanying map (Fig. 85) 

 shows the surface distribution of the Mississippian and Pennsyl- 

 vanian rocks together. In the western part of the continent these 

 two systems have not yet been satisfactorily separated, hence it 

 is impossible to delimit them separately upon the map. Also it 

 must be borne in mind that the large areas in British Columbia 

 and Alaska contain considerable amounts of other Paleozoic rock 

 as well as early Mesozoic rock, though the Mississippian and Penn- 

 sylvanian are abundantly represented. In the eastern part of the 

 continent, the two systems have been clearly separated, and map 

 Fig. 86 shows the surface distribution of Mississippian strata 

 there. A comparison with the Devonian surface distribution map- 

 (Fig. 68) shows that the Mississippian has a very similar distribu- 

 tion in eastern North America, and that the Mississippian generally 

 borders the Devonian areas. This is because the Devonian usually 

 passed so quietly into the Mississippian with continuous deposition. 

 A distribution feature of special importance as compared with the 

 Ordovician and Silurian, and to some extent with the Devonian, 

 is the complete absence of Mississippian strata from all of northern 

 North America east of the Rocky Mountains except around the 

 mouth of the St. Lawrence River. 



In the Appalachians, Rockies, and mountains still farther west, 

 the outcropping strata form long and short, narrow belts because 

 the rocks have been highly folded and only the eroded edges of 

 upturned strata are visible (Fig. 113). The eastern Mississippi 

 Basin shows a different type of distribution because the rocks are 

 there in nearly horizontal position and outcrop where the later 

 (overlying) Paleozoic strata have been removed from them by 

 erosion, or where later sediments were never deposited upon them. 

 The character of the rocks and distribution of outcrops, supple- 



