294 THE ICE AGE AND ITS WORK. 



the valley walls sloped back on each side, perhaps to be again flanked 

 by precipices. Now, if sncli a valley were converted into a deep lake 

 by any foim of subsidence, these ravines would remain under water 

 and form submerged river channels. But neither in the lakes, which 

 have been sniveyed by the Swiss Government, nor in the Atlas des 

 Lacs Fraiicaises of M, Delebecque, nor in those of the German Alps 

 by Dr. Alois lieistbeck, nor in the lakes of onr own country, can I find 

 any indications of such submerged river channels or ravines, or any 

 otlier of the varied rock features that so often occur in valleys. Almost 

 all these lakes present I'ather stee[)ly sloping sides with broad, rounded, 

 or nearly level bottoms of sancer shape, snch as are certainly not 

 characteristic of sub-aerial valley bottoms, bnt which are exactly what 

 we might expect as the ultimate result of thousands of years of inces- 

 sant ice grinding. The point is, not that the lake bottoms may not in 

 a few cases represent the contonrs of a valley, but that they never 

 present peculiarities of contour which are not unfrequent in mountain 

 valleys, and never show submerged ravines or those jutting rocky pro- 

 montories which are so common a feature in hilly districts. 



The next point is, that alpine lake bottoms, whether large or small, 

 frequently consist of two or more distinct basins, a feature which could 

 not occur in lakes due to submergence unless there were two or more 

 points of flexure for each depression, a thing highly improbable even 

 in the larger lakes and almost impossible in the smaller. Flexures of 

 almost any degree of curvature are no doubt found in the rocks form- 

 ing mountain chains; but these flexures have been ])roduced deep 

 down under enorinoiisi)ressureof overlying strata, whereas the surface 

 beds which are supposed to have been moved to cause lakes are free to 

 take any upward or downward curves, and as the source of motion is 

 certainly deep-seated those curves will usually be of very gradual cur- 

 vature. Yet in the small lake of Annecy there are two separate 

 basins; in Lake Bourget also two; in the small lake of Aiguebellette, in 

 Savoy, there are three distinct basins of very different depths; and in 

 the Lac de St. Point, about 4 miles long, there are also three separate flat 

 basins. In Switzerland the same phenomenon is often found. 



The exceedingly irregular Lake of Lucerne, formed by the confluence 

 of many valleys meeting at various angles hemmed in by precipitous 

 mountains, has eight distinct basins, mostly separated by shallows at 

 the narrow openings between opposing mountain ridges. This is 

 exactly what would result from glacier action, the grinding ])ower of 

 which must always be at a maximum in the wider parts of valleys, 

 where the weight of the ice could exert its full force and the motion be 

 least impeded. On the subsidence or curvature theory, however, there 

 is no reason why the greatest depth should occur in one part rather 

 than in another, while separate basins in the variously diverging arms 

 of one lake seem most improbable. - - - 



The third point of difference between lakes of erosion and those of 



I 



