48 THE POPULAR SCIENCE MONTHLY. 



terranean 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 no doubt reduced to nearly 

 level plains by subaerial denudation before the ice began its 

 work. The basin of these valleys comprises about two hundred 

 square miles and the watershed to the north is moderately high ; 

 but there can be no doubt that a large overflow from the Como 

 Glacier poured into it ; and the difficulty seems to me to be purely 

 imaginary if we simply recognize the fact that an essential pre- 

 liminary to lake erosion is a pre-existing nearly level valley 

 bottom. 



Another difficulty is said to be the frequent presence of islands 

 in the lakes; but here again the answer is easy. The islands, 

 always ground down to roclies moutonnees, were craggy hills in 

 the pre-existing valleys, and such hills existed because they had 

 for ages resisted the subaerial denudation which had hollowed 

 out the valleys. The same characters of density or toughness 

 that enabled them to resist ordinary denudation, enabled them 

 also, to some extent, to resist destruction by ice erosion ; just as 

 the character of the rocks which enabled ordinary denudation to 

 bring them down to a nearly level surface in the valley bottom, 

 also facilitated the ice erosion which converted the level valley 

 floor into a rock basin and, after the ice left it, into a lake. 



Every writer brings forward the well-known fact that the 

 ends of glaciers pass over beds of gravel or moraine matter, with- 

 out destroying or even disturbing it. But there is no reason why 

 they should do more than compress such beds of loose material 

 and roughly level their surfaces. It is the old delusion of a gla- 

 cier acting like a scoop or plow that leads to the idea that if it 

 can erode rock slowly it must altogether demolish gravel or bowl- 

 der clay. But if we turn to the description I have given of 

 how a glacier erodes a rock basin and apply this to its passage 

 over a bed of gravel or bowlder clay, we shall see that in the lat- 

 ter case the erosion would be much more difficult, because each 

 ice-imbedded stone or rock would press into the yielding material, 

 which would close up instantly behind it under pressure of the 

 ice and thus leave no result. Where the subglacial water accu- 

 mulated, channels would be cut in the gravel or clay, but else- 

 where there would probably be no erosion at all. Some writers 

 maintain that the lakes were all filled up with alluvium previous 

 to the Glacial epoch, and that the ice cleared out this incoherent 

 matter ; but it is almost certain that no such clearance would 

 have taken place, because the glacier would pass over such a sur- 



