SUMMARY OF PALEOZOIC HISTORY 195 



highly folded and eroded Appalachians, while the maximum 

 thickness of strata there must be between 40,000 and 50,000 feet. 



The Paleozoic group of rocks in the interior of the Mississippi 

 Basin measures only a few thousand feet in thickness, and lime- 

 stones are there relatively more abundant than clastic deposits, 

 because of the generally greater distance from the eroding lands. 



In the western United States, Paleozoic strata usually show a 

 thickness of many thousands of feet, and limestones are there 

 also prominently developed. 



The only large masses of igneous rocks of Paleozoic age are 

 Mississippian or Pennsylvanian or both in the northern Cordilleras 

 and in parts of New England. 



Physical History 



Relations of Land and Sea. — Throughout the Paleozoic era the 

 most persistent lands were large portions of northeastern North 

 America; Appalachia, the large area which occupied the eastern 

 side of the United States; and islands and land areas of varying 

 positions and sizes in the Cordilleran region. Most of the Missis- 

 sippi Basin and much of the western side of the continent were 

 submerged under epicontinental seas during the greater portion 

 of the time. There were many oscillations of level of the 

 land, or rising or falling of the sea-level, or both, causing re- 

 peated emergence and submergence of small and large areas. In 

 this summary only the most salient geographic changes will be 

 mentioned. The paleogeographic maps should be reviewed. Also 

 the accompanying generalized map (Fig. 118) should be studied. 



The era opened with much of North America a land area, but 

 early in the Cambrian marine waters extended across the southern 

 portion of the United States and northward by estuaries through 

 both the Appalachian and Rocky Mountain districts. Throughout 

 Cambrian time and to mid-Ordovician, a progressive submergence 

 took place reaching a climax with all the continent submerged 

 except two or three land areas around Hudson Bay, Appalachia, 

 and some islands in the west. In late Ordovician time came emer- 

 gence of considerable areas followed by extensive submergence 

 which reached a climax toward the middle of the Silurian with 

 nearly as much of the continent under water as during the mid- 

 Ordovician. Similar extensive emergence at the beginning, and 



