318 THEORY OF THE ORIGIN OF LAKE-BASINS CilAf. xv. 



eumferenee 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 deejjest 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 the^^ cross such vallej^s 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 necessaiy that the rate 

 of depression should be sufflcientl}* fast to make it imj^ossible 

 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 vallc}', all the conditions of the problem 

 are altered. Instead of the mud, sand, and stones dx'ifted 

 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 



