CHAP. xv. BY THE EROSIVE ACTION OF GLACIERS. 319 



mountain areas namely, the central and the circumferential 

 is for the first time rendered visible. 



By adopting this hypothesis, we concede that there is an 

 intimate connection between the glacial period and a pre 

 dominance of lakes, in producing which the action of ice is 

 threefold ; first, by its direct power in scooping out shallow 

 basins where the rocks are of unequal hardness; an opera 

 tion which can by no means be confined to the land, for it 

 must extend to below the level of high water a thousand feet 

 and more, in such friths as have been described as filled with 

 ice in Greenland (see above, p. 236J. 



2ndly. The ice will act indirectly by preventing cavities 

 caused by inequalities of subsidence or elevation from be 

 coming the receptacles first of water, and then of sediment, 

 by which the cavities would be levelled up and the lakes 

 obliterated. 



Srdly. The ice is also an indirect cause of lakes, by heaping 

 up mounds of moraine matter, and thus giving rise to ponds 

 and even to sheets of water several miles in diameter. 



The comparative scarcity, therefore, of lakes of post-pliocene 

 date in tropical countries, and very generally south of the 

 fortieth and fiftieth parallels of latitude, may be accounted for 

 by the absence of glacial action in such regions. 



Post-glacial Lake-dwelling in the North of Italy. 



We learn from M. de Mortillet that in the peat which has 

 filled up one of the ( morainic lakes ' formed by the ancient 

 glacier of the Ticino, M. Moro has discovered at Mercurago 

 the piles of a lake-dwelling like those of Switzerland, together 

 with various utensils, and a canoe hollowed out of the trunk 

 of a tree. From this fact we learn that south of the Alps, as 

 well as north of them, a primitive people having similar 

 habits flourished after the retreat of the great glaciers. 



