THE CAMBRIAN PERIOD 481 



2. Adjustments between continental and oceanic segments, a 

 possible cause of submergence. It has been shown 1 recently that 

 there is a plane about 100 miles below the surface of the earth where 

 the pressure downward beneath the overlying continental areas, 

 is equal to the downward pressure beneath the overlying oceanic 

 areas, in spite of the fact that the surfaces of the continents are, 

 on the average, three miles above the ocean bottoms. The reason 

 is that the rock beneath the oceans is denser than that beneath the 

 continents. At all horizons above this isostatic plane at the depth 

 of about 100 miles, a column of average continental rocks measured 

 downward from the surface of the land, weighs more than a similar 

 column of sub-oceanic rocks. This tends to aid the spreading move- 

 ment noted above, and so tends to bring about submergence of the 

 land. A similar state of things probably prevailed in Cambrian 

 times. It appears however that notably elevated regions, like the 

 Cordilleran tract, stand higher than they would if they were in 

 isostatic balance. 2 This lack of balance may give rise to special 

 movements to secure a more perfect equilibrium, and hence to 

 regional or local deformations. During any great period of defor- 

 mation, like that at the close of the Proterozoic, it is probable that 

 some portions of the crust were pushed up above a position of 

 equilibrium, and later tended to settle back. Other portions may 

 have been depressed below the position of equilibrium, and later 

 tended to rise. Such movements would be unsymmetrically dis- 

 tributed, and might result in slow and quiet, but unequal warping. 



3. Other adjustments, as possible causes of submergence. Va- 

 rious other changes and adjustments of the surface of the different 

 parts of the lithosphere, such as the unloading of the continents by 

 erosion and the loading of the ocean basins by deposition, the out- 

 pourings of lava, and unequal additions or losses of heat in differ- 

 ent places, may also have helped to cause submergence or emer- 

 gence of the lands. 



Basis for the Subdivision of the Cambrian 

 We have now to inquire the means by which the Cambrian 



1 Hayford, Washington Acad. of Sci., Vol. VIII, 1906, pp. 25-40. 



2 Putnam and Gilbert, Bull. Phil. Soc. Washington, Vol. XII. pp. 31-75, 

 and Gilbert, Jour. Geol., Vol. Ill, p. 351. 



