THE CAMBRIAN PERIOD. 233 



fore a presumption in favor of gradation and against body defor- 

 mation during the period, and good practice in interpretation leads us 

 to inquire how far the former may have been the cause of the sub- 

 mergence of the continent, before we resort to the doubtful expedient 

 of invoking profound deformation. 



If gradation were the sole agency involved in the submergence of 

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

 not necessarily equal in rate or extent at all times and places, and 

 along this line a means may be found of judging how far this agency 

 was a real one. Without attempting to make a full application of 

 this test here, it seems certain that there were some shiftings of the 

 areas of deposition other than those which might have arisen from a 

 simple gradational advance of the sea upon the land, and that there 

 are specific evidences of warping, as implied by the differences in the 

 depth of the deposits and in their characters and relations to one another, 

 as brought out in the previous descriptions. None of these warpings 

 were, however, of the more intense and declared type, such as occurred 

 repeatedly in the Proterozoic and Archeozoic eras. Those which are 

 now known were of a relatively gentle type, and limited in amount 

 and in prevalence. These characteristics of the Cambrian deforma- 

 tions lead to inquiry under the second head. 



(2) How far may superficial deformation have contributed to the 

 submergence of the lands? — (a) Lateral spreading. — The continental 

 platforms now stand forth above the ocean bottom as great plateaus. 

 They may be roundly estimated to rise 15,000 feet above their true 

 sub-oceanic base. Their weight develops a pressure on their basal 

 parts that may safely be estimated at 15,000 to 20,000 pounds to the 

 square inch. 1 This pressure tends to cause the platforms to spread 

 by lateral creeping. This is opposed by the strength of the rock, and 

 by the hydrostatic pressure of the oceans against the sides of the plat- 

 forms. If this last be estimated at 5000 pounds per square inch at 

 the bottom, there remains an unbalanced pressure of 10,000 to 15,000 

 pounds per square inch. Even the lesser of these figures is equal to 



1 For rough computations where exact accuracy is immaterial, it is convenient 

 to remember that a cubic foot of rock weighs about as many pounds as there are 

 square inches in its base, though usually somewhat more. Hoskins, in his com- 

 putation of the strength of domes (Vol. I, p. 581), takes 180 pounds as the weight 

 of a cubic foot of firm crystalline rock of the granitic type. 180 -f- 144 = 1.25. At 

 2.7 sp. gr., a cubic foot weighs 167 lbs. 167-^144 =1.16. 



