480 GEOLOGY 



Cambrian period. There is therefore a presumption against much 

 diastrophism during 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 warpings of pronounced type like 

 those recorded in the rocks of the Proterozoic and Archeozoic eras. 



Deformation as a cause of submergence. The deformations which 

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

 either superficial, involving the rocks down to depths of a few 

 miles at most, or deep-seated, involving the rock to much greater 

 depths. One or two phases of deformation may be mentioned. 



1. Lateral spread or continental creep. The continents stand 

 about 15,000 feet above the ocean bottom. Their weight causes a 

 pressure of 15,000 to 20,000 pounds to the square inch on their 

 bases. This pressure tends to cause the continents to spread by creep 

 into the ocean basins, on the same principle that a great body of ice, 

 such as an ice-sheet, spreads. Spreading is opposed by the hydro- 

 static 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 than 

 this, the rock in mines is often observed to creep. It is not im- 

 probable, therefore, that such a pressure, constantly exerted for 

 a prolonged period, might cause some spreading of the great con- 

 tinental platforms, and hence (1) some lowering of their surfaces, 

 (2) some submergence about their borders, and (3) at the same 

 time some rise of the sea-level. Many phenomena which cannot be 

 cited here seem to lend support to this hypothesis of lateral 

 creep, 1 but its efficiency is not determined. 



1 Chamberlin and Salisbury, Earth History, Vol. II. 



