V GLACIAL EROSION OF LAKE BASINS 113 



On the theory of glacial erosion we have here cause and 

 effect ; on that of earth movements we have another mere 

 coincidence added to the long series already noticed. The 

 depth of over 2,500 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. Geologists will probably not 

 think thirty thousand years an extravagant estimate for 

 the duration of the glacial period, in which case an erosion 

 of only an inch in a year would be sufficient. Lago di 

 Garda, the largest Italian lake, had a still larger catchment 

 area in glacial times, but not nearly so much concentrated, 

 hence, perhaps, its comparatively moderate depth of about 

 1,000 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 whatever, since the deep-seated agencies pro- 

 ducing subsidence, upheaval, 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. 



5. The Contours and Outlines of the Lakes indicate Erosion 

 rather than Suhmergence. — 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 mode of action that they ought to pro- 

 duce 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 hnd, 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 characters which will be tolerably con- 

 stant in all lakes so formed. Now I find that there are 

 three such criteria by which we ought to be able to dis- 



VOL. I. I 



