BORDERLANDS OF NORTH AMERICA 159 



far these lands once extended beyond the present shorelines into the 

 oceans is unknown, but it is certain that much of their outer portions 

 have been fractured into the ocean depths. The present continental 

 shelves are therefore held to be of fairly recent origin — of late Cretaceous 

 making along the Atlantic and of late Cenozoic origin on the Pacific side 

 of the continent. 



• The topographically more or less high borderlands of Xorth America — 

 thie frame surrounding the inner basin — are periodically raised, and this 

 appears to be due to a shrinking earth. The earth is continually cooling, 

 though seemingly at an excessively slow rate, through internal magmatic 

 differentiations; it is also losing gases and water, while the centrosphere 

 is in molecular rearrangement, due to the great attraction pressures of 

 the earth's mass ; and because of these changes the earth shrinks in vol- 

 ume. The outer lithosphere, on the other hand, is stiff, rigid, and very 

 strong, and hence long resists the shrinking of the centrosphere. Peri- 

 odically, however, its resistance is overcome, and then crustal shortening 

 takes place, mainly in the areas of weakness, the geosynclines. 



Since the oceanic basins occupy about two-thirds of the earth's surface 

 and their mass averages about 3 per cent heavier than that of the conti- 

 nents, the areas of these basins are the main subsiding ones of the earth. 

 During the subsidence, there is, according to theor}^, also some deep- 

 seated rock flowage, and especially near the junction areas between oceans 

 and lands. This flowage differentiates out lighter masses, and these hot 

 magmas make their way, along with great pressures, tangentially upward 

 into the borderlands, raising them into the frame of the continents. It 

 is these periodically compensating and inwardly moving masses of the 

 more mobile lower part of the lithosphere that cause the thin, cold, and 

 rigid supercrust or stratosphere (also tectonosphere) to fold and over- 

 thrust toward the neutral areas. On the other hand, the rising granitic 

 magma may dominate in its movements, ascending like a vertical wedge 

 into the supercrust, thrusting it aside into a bilaterally symmetric moun- 

 tain chain. 



North America is margined on the east by Novascotica, Appalachia, and 

 Antillia. Each one of these borderlands has its own geologic structure and 

 history. Along the west coast of North America is the greatest of all the 

 borderlands, Cascadia, which later on divides into California and Char- 

 lotte masses (from Queen Charlotte Islands). Ywkonia, occupying a 

 great part of present Alaska, is not well enough known to say much about 

 it, other than that in this area the seaways ' seemingly show that there 

 often was a borderland here. Mexico, or Columbia, in Paleozoic time 

 appears to have been a lowland and apparently with the characteristics 



