313 THEORY OF THE ORIGIN OF LAKE-BASINS CHAP. xv. 



cumference of the mountains the rate diminishing about an 

 inch per mile, in a distance, say of forty miles this might 

 convert many of the largest and deepest valleys at their 

 lower ends into lakes. 



We have no certainty that such movements may not now 

 be in progress in the Alps ; for if they are as slow as we have 

 assumed, they would be as insensible to the inhabitants, as 

 is the upheaval of Scandinavia or the subsidence of Green 

 land to the Swedes and Danes who dwell there. They only 

 know of the progress of such geographical revolutions, because 

 a slight change of level becomes manifest on the margin of 

 the sea. The lines of elevation or depression above supposed 

 might leave no clear geological traces of their action on the 

 high ridges and table-lands separating the valleys of the 

 principal rivers ; it is only when they cross such valleys, that 

 the disturbance caused in the course of thousands of years in 

 the drainage becomes apparent. If there were no ice, the 

 sinking of the land might not give rise to lakes. To accom 

 plish this in the absence of ice, it is necessary that the rate 

 of depression should be sufficiently fast to make it impossible 

 for the depositing power of the river to keep pace with it, or, 

 in other words, to fill up the incipient cavity, as fast as it 

 begins to form. Such levelling operations once complete, 

 the running water, aided by sand and pebbles, will gradually 

 cut a gorge through the newly raised rock, so as to prevent 

 it from forming a barrier. But if a great glacier fill the 

 lower part of the valley, aM the conditions of the problem 

 are altered. Instead of the mud, sand, and stones drifted 

 down from the higher regions being left behind in the 

 incipient basin, they all travel onwards in the shape of 

 moraines on the top of the ice, passing over and beyond the 

 new depression, so that when, at the end of fifty or a thousand 

 centuries, the glacier melts, a large and deep basin repre 

 senting the difference in the movement of two adjoining 



