104 STUDIES, SCIENTIFIC AND SOCIAL chap. 



carried away the whole of the many cubic miles of solid 

 rock that once filled up the valle}^ But the geologist 

 knows that these apparently insignificant forces have done 

 the work, through their continuous action always in one 

 direction for thousands, or even for millions of years ; and, 

 therefore, having before him so many proofs of the eroding 

 power of ice, in planed and rounded rocks and in the 

 grooves and furrows which are the latest marks left by the 

 ice-tool ; and bearing in mind the long duration and 

 possibly recurrent phases of the ice age — to be measured 

 certainly by tens, perhaps by hundreds, of thousands of 

 years — he can have little difficulty in accepting the erosion 

 of lake-basins as the most satisfactory explanation of their 

 origin. 



3. Ohjedions of Modern Writers considered. — Professor 

 Bonney and many other writers ask, why lakes are so few 

 though all the chief valleys of the Alps were filled with 

 Ice ; and why, for instance, there is no great lake in the 

 Dora Baltea valley whose glacier produced the great 

 moraines of Ivrea opposite its outlet into the plains of 

 Italy, and which form a chain of hills fifteen miles long 

 and fifteen hundred feet high ? The answer, in the case 

 of the Dora Baltea is not difficult, since it almost certainly 

 has had a series of lake-basins at Aosta, Verrex, and other 

 places where the broad level valley is now filled with allu- 

 vial gravel. But the more important point is the extreme 

 narrowness of the lower part of the valley above Donnas 

 and again near its entrance into the valley of the Po. The 

 effect of this would be that the great glacier, probably 

 two thousand feet thick or more, would move rapidly in 

 its upper layers, carrying out its load of stones and debris 

 to form the terminal moraine, while the lower strata, 

 choked in the defiles, would move very slowly. And once 

 out in the open valley of the Po, then a great inlet of the 

 warm Mediterranean Sea, the ice would rapidly melt away 

 in the water and in the warm moist atmosphere, and 

 therefore have no tendency to erode a lake-basin. 



The Lake of Lugano, with its curious radiating arms, is 

 said to be another difficulty. But each of these arms is 

 the outlet of a valley or series of valleys, which were 



