THE HUMAN OR PRESENT PERIOD 933 



Falls which owe their origin to the deformations of the recent 

 deformative period abound on all the continents, but most of them 

 have canyons below, showing that the falls have been receding for 

 a long time. The falls and canyons are often so related to slack 

 water below as to show that the rejuvenating process stopped some 

 time ago. The Falls of the Columbia, Congo, Zambesi, Brahma- 

 putra, Yang-tse, and of a multitude of other rivers descending 

 from the elevated portions of the continents, are illustrations in 

 point. If the various criteria of topographic age are applied to 

 the face of the continents, it will be seen that, while they betray, 

 very generally, evidences of rejuvenation by deformation in rela- 

 tively recent times, there is very little to indicate rejuvenation now 

 in progress. The most declared evidences of topographic youth 

 are in regions recently abandoned by the ice-sheets of the last 

 glacial stage. 1 



If attention is turned to the borders of the continents, significant 

 evidence is found in the fact that the real edges of the continents 

 are almost everywhere submerged. They generally lie 100 fathoms 

 or so below sea-level; that is, continental shelves almost universally 

 border the continents. An area of 10,000,000 square miles, more 

 than 15% of the true continental surface, is thus submerged. The 

 submergence of the shelves took place so recently that they are 

 marked by trenches, valleys, and embayments referable to rivers 

 that formerly crossed them. These features imply that the con- 

 tinental shelves were out of water recently, and that rivers then 

 reached the true borders of the continental platforms. They also 

 imply a recent (if not present) general movement toward continental 

 submersion. 



The channels on the continental borders. The conclusion has 

 generally been that the coastal tracts affected by valleys once stood 

 high enough to allow streams to excavate the valleys. As com- 

 monly interpreted, this means that the tracts have in recent times 

 stood some thousands of feet higher than now, and the submerged 

 valleys have been much appealed to in support of the elevation 

 hypothesis of glaciation. The submerged valleys of the continental 



1 Jour. Geol., Vol. XII, p. 707. 



