140 LIFE ON THE EARTH. 



rivers which enter them, and the truncation of hills 

 sloping to the sea by cliffs not far from the point 

 where the slope once met the sea-level, may be men- 

 tioned as data of this kind. Undoubtedly they all 

 tend to convince us of the comparatively short period 

 of time which has elapsed since the sea began to 

 waste, or to be filled up, and the rivers began to wear 

 away the upper parts and to fill up the lower parts 

 of the valleys ; but it is difficult to translate this con- 

 clusion into centuries or thousands of years. Per- 

 haps such a case as that of the Derwent River 

 flowing into the Lake of the same name in Cumber- 

 land, and augmenting year by year the mass of sedi- 

 ments at the upper end, may be found worthy of 

 special attention, because of the abundance of rains 

 on the slaty mountains around, the shortness of the 

 course of the river, and the very slight degree in 

 which cultivation, quarrying, and mining, have, till 

 within a few years, altered the natural character of 

 the surface. The requisite measures of the lake, the 

 delta, and the sediment brought by the river, would 

 present no great difficulty. 



Perhaps it may be sufficient to take one example, 

 the best at present known, to which computation has 

 been applied, for determining the number of years 

 in which a river has been running and excavating 

 for itself a channel in rocks of one definite charac- 



