332 HISTORICAL GEOLOGY 



sion. Where the eroded surface of the older rocks has not 

 been deformed by later movements of the crust, it is nearly 

 level ; and from this fact it is thought that the denudation of 

 the continent, before the land was submerged by the Cam- 

 brian sea, must have continued for a very long time, suffi- 

 ciently long to allow the streams to reduce large areas to the 

 condition of peneplains (p. 147). On account of the great 

 duration of this interval of erosion, and because of the very 

 general presence of the unconformity in all continents where 

 the Cambrian has been studied, the interruption is regarded 

 as one of the greatest in the geologic record. 



Gradual submergence of the continent. Further light is 

 cast upon the geography of the times by the discovery that, 

 in North America, the layers which contain the oldest Cam- 

 brian fossils exist only near the eastern and western borders 

 of the continent. Farther inland it is the Middle Cambrian 

 that rests on the eroded pre-Cambrian surface; and in the 

 interior, from New York to Michigan, the strata above the 

 unconformity contain the Upper Cambrian fossils. From 

 this we infer that the sea encroached so slowly upon the 

 gently inclined land surface that nearly the whole of the long 

 Cambrian period was required to accomplish the submergence. 

 Not all of the continent seems to have disappeared beneath 

 the sea even at this time. A large area of ancient rocks in 

 eastern Canada, another occupying what is now the Atlantic 

 seaboard, and also some parts of the West seem to have 

 remained as land masses. These continued to be eroded and 

 hence to supply sediments to the seas of the time. The 

 name " Appalachia " is used to designate the large island 

 which then lay just east of the present Appalachian Moun- 

 tains, from New England to the Gulf states. Its influence 

 on the rocks formed in later periods will be mentioned in 

 succeeding Chapters. 



Where seas have encroached upon the land, it is often im- 

 possible to decide whether the ocean surface actually rose or 

 whether the lands sank. In the Cambrian, it is significant 



