174 FORMATION OF LAKE-BASINS. [Ch. XII. 



ice would move slower and exert less friction on the "botton in propor- 

 tion to the depth of the cavity which it fills, for the motion of a gla- 

 cier resembles that of a river — the upper strata moving faster than the 

 lower ; and if there be a depth of 2600 feet, as in Lago Maggiore, it 

 is difficult to conceive, when the principal discharge of ice is almost 

 entirely effected in the upper part of the mass, that the movement at 

 the bottom would be sufficiently energetic to enable the ice to pene- 

 trate deeply into the rocks below. A still more serious objection to 

 the ice-origin of lake-basins is deducible from absence of such basins 

 of the first magnitude in the plains of the Po at certain points where 

 the greatest of the extinct glaciers once came down from the Alps, leav- 

 ing their gigantic moraines in the low country. Of this absence, the 

 finest example occurs at Ivrea and south of it, where we observe a 

 moraine more than 1500 feet high in its northern part, consisting of 

 mud, stones, and large erratic blocks, evidently brought down from the 

 two highest of the Alps, Mont Blanc and Monte Rosa. This old mor- 

 aine, when it issues from the mountains and spreads over the plains of 

 the Po, reposes on marine strata of the Pliocene age, so unconsolidated 

 that the glacier might have scooped out of it a deep cavity had mov- 

 ing ice possessed such an excavating power. 



Another example of the absence of a great lake where we ought to 

 have found one, according to the glacier-erosion hypothesis, occurs in 

 a contiguous region on the other side of Turin, between that city and 

 Susa, where the moraine of the Dora Riparia extends far and wide. 



If, in surveying a mountain-chain, lengthwise or transversely, we 

 observe a capricious distribution of lake-basins, we have no reason 

 to feel surprise, so long as we conceive the origin of the basins to be 

 due to subterranean movements in the earth's crust, for these may be 

 partial in their extent, or may vary in their direction in a manner 

 which has no relation to the course of the valleys. But if, rejecting 

 the aid of changes of level, we invoke a superficial agency, like that of 

 glaciers, we are then utterly at a loss to explain why they should scoop 

 out a hollow in one valley and perform no similar feat in an adjoining 

 one. 



We have shown that rivers are doubly instrumental in preventing 

 the formation of lake-basins ; first, by labouring incessantly to silt up 

 an incipient cavity, and secondly, by deepening their channels, or cut- 

 ting out new ones through the rocks, which may have been slowly 

 raised up so as to interfere with the regular drainage. There is no 

 analogous agency at work at the bottom of the sea except partially, 

 where marine currents deriving sediment from wasting shores, or from 

 rivers, deposit it at the bottom. With the exceptions of such areas 

 of submarine deposition, every partial subsidence will cause a perma- 

 nent depression, ready to become the receptacle of fresh water when- 

 ever the tract emerges or is turned into land. As to the extent of such 

 lake-basins, we should have no right to wonder if they equal in size 

 Lakes Erie and Ontario, or even Lake Superior itself, provided the 



