THE ICE AGE AND ITS WORK. 247 



of earth movements we have another mere coincidence added to 

 the long series already noticed. The depth of over twenty-five 

 hundred feet undoubtedly seems enormous, but that depth exists 

 just at the point where the two great valleys which have collected 

 the converging streams above referred to unite together. Geolo- 

 gists will probably not think thirty thousand years an extrava- 

 gant estimate for the duration of the Glacial period, in which 

 case an erosion of only an inch in a year would be suflQcient. 

 Lago di Garda, the largest Italian lake, had a still larger catch- 

 ment area in glacial times but not nearly so much concentrated ; 

 hence, perhaps, its comparatively moderate depth of about one 

 thousand feet. We see, then, that on the theory of erosion, the 

 size, depth, and position of the chief lakes are all intelligible, 

 while on that of earth movements they have no meaning what- 

 ever, since the deep-seated agencies producing subsidence, up- 

 heaval, or curvature of the surface would be as likely to act in the 

 small as in the large valleys, and to produce deep lakes in other 

 places than those where, at a later epoch, the thickest glaciers 

 accumulated. 



The Contours and Outlines of the Lakes indicate Ero- 

 sion RATHER than SUBMERGENCE. While Collecting facts for 

 the present articles, it occurred to me that the rival theories of 

 lake formation erosion and submergence were so different in 

 their modes of action that they ought to produce some marked 

 difference in the result. There must be some criteria by which 

 to distinguish the two modes of origin. Under any system of 

 earth movements a valley bottom will simply become submerged, 

 and be hardly more altered than if it had been converted into a 

 lake by building an artificial dam in a convenient situation. We 

 should find, therefore, merely a submerged valley with all its 

 usual peculiarities. If, however, the lake basin has been formed 

 by glacial erosion, then some of the special valley features will 

 have been destroyed, and we shall have a distinct set of charac- 

 ters which will be tolerably constant in all lakes so formed. Now 

 I find that there are three such criteria by which we ought to be 

 able to distinguish the two classes of lakes, and the application 

 of these tests serves to show that most of the valley lakes of gla- 

 ciated countries were not formed by submergence. 



The first point is that valleys in mountainous countries often 

 have the river channel forming a ravine for a few miles, after- 

 ward opening out into a flat valley, and then again closing, while 

 at an elevation of a hundred or a few hundred feet, at the level 

 of the top of the ravine, the valley walls slope back on each side, 

 perhaps to be again flanked by precipices. Now, if such a valley 

 were converted into a deep lake by any form of subsidence, these 

 ravines would remain under water and form submerged river 



