Handbook of Paleontology 215 



tinent into a northern (Canadian Shield) and a southern 

 (United States and Mexico) plain. These mountains 

 were reduced almost to sea level (peneplane) near the 

 close of the Ordovician, making of the interior of the 

 continent one vast plain which was transgressed by seas 

 again and again during Paleozoic and in late Mesozoic 

 time. 



The Paleozoic era opened with the continent much as 

 in the Proterozoic. Both the Atlantic and Pacific oceans 

 were in existence though probably much shallower than 

 today and broader despite the fact that the continent ex- 

 tended for some distance into the oceans on the east and 

 west. The Arctic ocean was also in existence at this 

 time though fossils indicate that the waters were at times 

 much warmer than now. This era was a time of oscilla- 

 tory movements, that is, certain areas were at times ele- 

 vated and then again depressed, giving the extensive 

 development of seas over the continent which was such 

 a characteristic feature of the Paleozoic. There were 

 also certain dominant land masses or positive elements, 

 quite distinct from the dominant land areas of today, 

 which remained land more or less constantly, and certain 

 areas of more or less continuous subsidence, interrupted 

 in some cases, and in general parallel to the principal land 

 masses from which was carried to them a more or less 

 constant supply of clastic material or sediments. 



The several seas which transgressed the continent due 

 to alternating depression and elevation of the land were 

 such as are seen today in Hudson bay and the Baltic sea. 

 For the most part they were shallow and covered large 

 areas of the continent as it exists today, and hence have 

 been variously termed continental, epicontinental (Greek 

 epi, upon) or epeiric (Greek epeiros, mainland) seas. 



