THE CAMBRIAN PERIOD 479 



the Cambrian period in North America was the progressive sub- 

 mergence of the continent. Theoretically, this submergence may 

 have been brought about by a rise of the sea or by a lowering of 

 the land, or by both together. Both the lowering of the land 

 and the rise of the sea may be due to gradation, to diastrophism, 

 or to the two combined. 



Gradation a possible cause of submergence. Gradation is per- 

 petual and inevitable where land and sea exist. The waves attack 

 the land along its borders, and the agents of land degradation lower 

 its surface. The former is a direct cause of encroachment of sea 

 upon land, and the latter is an indirect cause, since all sediments 

 transported from land to sea displace an equal volume of water, 

 and raise the surface of the sea correspondingly. Small as this 

 rise is for any brief period, its effect is to cause the sea to advance 

 on the land; and the lowering of the land by degradation at the 

 same time, increases the area of the advance. // continued long 

 enough, shore-cutting about the borders of the lands, down-cutting 

 over the whole surface, and the accompanying rise of the sea-level, 

 must inevitably cause the water to cover the continents, and to 

 spread deposits over all but the last remnants of them, provided there 

 is no deformation of the body of the earth in the meantime. 



It has been computed that if the earth, in its present condition, 

 were to remain without deformation long enough for the continents 

 to be base-leveled, the deposition of the sediments thus derived in 

 the sea would raise the sea-level about 650 feet. This would sub- 

 merge a large part of the base-leveled land. The evidence of grada- 

 tion in the Cambrian period is clear and firm. Most of the sedi- 

 ments which make up the Cambrian system of rocks were eroded 

 from the land and deposited in the sea. This lowered the land and 

 raised the sea. Gradation was, therefore, a factor in the submer- 

 gence of the continent, and there is evidence that great progress was 

 made toward the base-leveling of America and other continents, 

 before the close of the Cambrian period. Base-leveling implies a 

 nearly undisturbed attitude of the land and sea, and hence in itself 

 favors the view that no great deformation affected the continent 

 while it was going on. In harmony with this view, there is an 

 absence of direct evidence of profound deformation during the 



