350 CAMBRIAN PERIOD 



provided there is no deformation of the body of the earth in the mean- 

 time. 



If the earth were to remain without deformation long enough for 

 the continents to be base-leveled, the deposition in the sea of the 

 sediments thus derived would raise the water about 650 feet. This 

 would submerge a large part of the base-leveled land. The evidence 

 of gradation in the Cambrian period is clear and firm. Most of the 

 sediments 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 

 submergence of the continent, and there is evidence that great 

 progress was made toward base-leveling before the close of the 

 period. 



If gradation were the sole agency involved in the submergence 

 of the lands, the advance of the sea should have been steady, though 

 not necessarily equal in rate at all times and places. Without 

 going into details, it seems certain that there were changes in the 

 areas of deposition other than those which can be accounted for by 

 gradation, but none of these changes imply notable warpings such 

 as are recorded in the rocks of the Proterozoic and Archeozoic eras. 



Deformation as a cause of submergence. Deformations which 

 may cause submergence of land (and emergence of sea-bottom) are 

 of various sorts. Any deformation which causes the land to sink, 

 or the sea to rise, leads to submergence. Such movements and their 

 causes have been discussed briefly (chapter VIII). One special 

 phase of movement which may have especial significance here is 

 noted at this point. 



Continental creep. The continents are about 15,000 feet above 

 the ocean bottom. Their weight causes an average pressure of 1 5,000 

 to 20,000 pounds to the square inch on their bases, 15,000 feet down. 

 This pressure tends to cause the continents to spread by creep into the ocean 

 basins, on the same principle that an ice-sheet spreads. Spreading 

 is opposed by the hydrostatic pressure of the oceans against the sides 

 of the continental platforms. This is some 5,000 pounds per square 

 inch at the bottom, so that there remains an unbalanced pressure of 

 10,000 to 15,000 pounds per square inch, tending to cause creep. 

 Is this enough to overcome the strength of the rock, which opposes 

 creep? Even the lesser of these figures is equal to the crushing 

 strength of some of the weaker rocks, and is a notable percentage of 

 the crushing strength of even the strongest. Under less pressure 



