520 GEOLOGY. 



the earth is that of a planet in the midst of deformation, or that of 

 one recently deformed, and now returning to a more quiescent state. 

 On critical examination every stream should tell whether it has just 

 been rejuvenated, or has done some notable work since it was reju- 

 venated, and whether the amount of rejuvenating influence is stir 

 being increased, or is static, or is being diminished. Every coast 

 should show whether the continental border stands forth in the manner 

 typical of an earth-segment just crowded up by a deformative thrust, 

 or whether it has made some notable progress in settling back, or in 

 being cut back, to an inter-deformative state. 



The streams of the continents almost universally show that since 

 they were rejuvenated they have had time to do some appreciable 

 work, except in the case of small streams entering the deepened valleys 

 recently occupied by glaciers, and the limited work of these only 

 emphasizes the time implied by those streams that have done appre- 

 ciably more work. Falls which owe their origin to the deformations 

 of the recent deformative period abound on all the continents, but 

 they are almost universally attended by canyons below, that show a 

 period of activity of appreciable duration. These falls and canyons 

 are often so related to slack water below as to show that the rejuvena- 

 ting process was stopped some time ago; indeed it has often been 

 reversed, as illustrated by the falls of the Potomac, and the rapids of 

 the " Fall line " of the Atlantic border generally, and the depressed 

 valleys below. The Falls of the Columbia, Congo, Zambesi, Brah- 

 maputra, Yang-tse, and of a multitude of other rivers descending from 

 the elevated portions of the continents, are also illustrations in point. 

 If the various criteria of topographic age set forth in Volume I be 

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

 betray, very generally, evidences of rejuvenation by deformation in 

 relatively recent times, there is very little to indicate rejuvenation in 

 progress, except in features that are obviously local and special. While 

 evidences of various degrees of aging are nearly everywhere displayed, 

 the areas that bear the most declared evidences of topographic youth 

 are those recently abandoned by the ice-sheets of the last glacial stage, 

 and the ice-invasions seem to be the youngest of the rejuvenating 

 agencies. 1 



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

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



