THE CAMBRIAN PERIOD 63 



that such areas were submerged under the early Cambrian sea, 

 because Lower Cambrian strata could have formed only during 

 that time. To these areas must be added still others from which 

 the rocks have been removed by erosion. Further, since the later 

 Cambrian strata almost invariably rest in perfect conformity 

 upon the earlier, we can be sure that any region occupied by later, 

 but not earlier, Cambrian rocks was never covered by the earlier 

 Cambrian sea because the conformity proves that there was no 

 erosion interval during which any of the earlier Cambrian strata 

 were removed before the deposition of the later. Again, some other 

 areas were almost certainly dry land during early Cambrian time 

 because there is not the slightest evidence of any sort that deposi- 

 tion went on over those areas during that time. The principles 

 here set forth are of fundamental importance in constructing a 

 paleogeographic map of North America for early Cambrian time, 

 and the same principles must be kept in mind in considering the 

 paleogeography of any given region during succeeding time. 



In accordance with the above principles, we can be sure that 

 the general outlines of the continent of North America in early 

 Cambrian time were much as indicated upon the accompanying 

 map (Fig. 29). The great central region, between what are now 

 the Rocky Mountains on the west and the Appalachian Mountains 

 and St. Lawrence Basin on the east, was dry land. 



Bordering this central mass on the west and on the east were 

 two long, narrow sounds or mediter anean seas, the one on the west 

 covering the present site of the Rocky Mountains, and the one on 

 the east the present sites of the Appalachian Mountains and the 

 Champlain and St. Lawrence Valleys. Lower Cambrian sediments 

 were accumulated in these sounds. 



Along the Atlantic side, and bordering the eastern sound, an 

 important land mass, called "Appalachia, " extended from New 

 Brunswick southward to the Gulf of Mexico and doubtless 

 included much of the present continental shelf area. As we shall 

 learn, Appalachia apparently persisted, though with more or less 

 changing outlines, throughout the Paleozoic era. 



Along the Pacific side, and bordering the western sound, there 

 were considerable (possibly mostly continuous) land masses which 

 separated the sound from the Pacific ocean. 



Middle and Late Cambrian. — During middle and late Cam- 

 brian time more and more of the continent gradually became sub- 



