THE HUMAN OR PRESENT PERIOD. 527 



tinents. A very wide-spread movement that concentrated the folding 

 along a few lines seems best to accord with the observed results, and 

 to involve the least shrinkage, although it involves the most shear. 

 But however distributed, if all the crustal wrinkling that took place 

 in the late Tertiary is to be accounted for by lateral movement of the 

 outer shell, its amount cannot have been inconsiderable, and the thrust 

 of the shell up the bordering incline of the sub-shell body of the 

 continents, must have been competent to carry a zone of the sub- 

 merged portions of the shell obliquely out of the water, and permit 

 erosion to channel its surface. 



The reverse movement of the shell. — The squeezing-up of the con- 

 tinents by the lateral crowding of the heavier sub- oceanic sectors 

 increased the difference in height between the continental surfaces 

 and the bottoms of the ocean basins, and hence increased the ten- 

 dency of the continental mass to creep laterally. So, also, the push- 

 ing of the shell up upon the more elevated continents, and the bowing 

 of it up in wrinkles on their borders, furnished the conditions for a 

 slow reverse movement. It is therefore reasoned that, following the 

 great deformative movements, there would have been a much slower, 

 glacier-like creep, both of the uncler-body of the continental platform 

 and of the superficial shell, whose movement was facilitated by the 

 supposed shearing zone between them. The movement of the shell 

 is presumed to have been much the greater, because its previous move- 

 ment and its distortion had been much more considerable, and because 

 whatever movement took place in the mass below would carry the 

 shell with it, while the independent motion of the shell would be added 

 to this. The reversed movement of the shell, at the borders of the 

 continent, would carry the surface next the coasts, now affected by 

 valleys, down the slope, and submerge it. The body of the earth, 

 meanwhile, had undergone little change besides shrinkage. 



(3) The movement of sediments on the continental edges. — The sedi- 

 ments of the late periods are generally soft. There is good reason 

 to suppose that the muds and sands which chiefly formed the sedi- 

 ments built out at the edge of the continental shelves usually remained 

 incoherent for long periods, except where there were special cementing 

 agencies. Now the attitude of these ivas changed by the deformative 

 movement both of the earth-body and of the shell, and in so far as they 

 were pushed above the sea-level, their weight was increased some 70%. 



