222 MOVEMENTS AND DEFORMATIONS 



sinking of the ocean basins, whose specific gravity is greater than 

 that of the continents. If the ocean basins and the continents, 

 respectively, be regarded as the surfaces of great segments of the 

 earth all of which are crowding toward the center, the stronger and 

 heavier segments may be conceived to take precedence, squeezing 

 the weaker and lighter ones between them. The consequent swell- 

 ing up of the lighter segments accounts for the relative protrusion 

 of the continents. 



The area of the depressed segments is almost exactly twice that 

 of the protruding ones, if we count the 10,000,000 square miles of 

 the continental shelves as parts of the latter. In millions of square 

 miles, the depressed segments are approximately as follows: the 

 Pacific 60, the Indian 27, the South Atlantic 24, the North Atlantic 

 14, leaving 8 for minor depressions. The elevated segments are the 

 Eurasian 24, the African 12, the North American 10, and the South 

 American 9, leaving 10 for the minor blocks. 



The downward movement of the larger segments and the 

 crowding of the smaller and lighter segments between them involves 

 deformation of the latter. The movements that spring from the 

 deeper crowding affect the continental protuberances generally, or 

 at least broadly, while the crowding of the more superficial parts 

 affects the lands more locally. According to this view, it is obvious 

 there should be special bowings on the borders of the continental 

 segments, and this tallies with the archings common on borders 

 of the continents, even where there is no folding. The shell of the 

 earth is free at the surface, and as a result, folding and faulting are 

 the modes of easiest accommodation there, while the deeper parts, 

 under great pressure, must be deformed throughout. 



The periodicity of the movements is assigned to the rigidity of 

 the thick, massive segments which must be deformed to effect 

 readjustment after shrinkage. Because of this rigidity, stresses 

 accumulate for a time until they are equal to the resistance opposing 

 them. A further increase of the stresses then causes yielding and 

 readjustment. When masses under stress once begin to yield in 

 the direction of their free surfaces, their attitudes for resistance be- 

 come less favorable, and hence the yielding continues until the stress 

 is eased. After this another period is required for stresses to ac- 

 cumulate sufficient to produce another general deformation. Mean- 

 time the minor stresses that may remain, or may be produced by 

 the great deformations, tend to ease themselves and thus give rise 



