Alpine Valleys and Alpine Lakes. 87 



brief reference to the ingenious suggestion of Professor Tyndall, 

 who perceives in the operations of the glaciers upon the rocky 

 framework of the Alps, not merely the agent that has hollowed 

 out the existing valleys, but one competent, by its destructive 

 power, to bring about the change from the present climate of the 

 Alps to that which prevailed when the glaciers descended to the 

 plains. If I could have the pleasure of standing beside him on 

 the Superga, or any other central position from whence he could 

 survey the outline of the main chain of the Cottian and Pennine 

 Alps, serrated with peaks and intervening depressions, I would 

 beg of Professor Tyndall to consider in detail whether, and by 

 what means, glaciers, however extensive, or endowed with what- 

 ever mechanical power, could have determined the form of the 

 topmost crest of that great range, or can hereafter reduce it 

 below its present level. It will certainly not escape him that the 

 grinding power of ice is exerted to an appreciable extent only by 

 considerable ice-streams flowing through defined channels, when 

 they have reached the level at which the temperature of the whole 

 mass is brought near to the melting-point, and that the snow 

 and ice which cover the highest parts of the Alps, so far from 

 tending to hasten their downfall, act as a protection from other 

 agents of destruction ; so that in truth no rocks, except those at 

 a considerable depth below the surface of the sea, are so little 

 exposed to degradation as the tops of high snowy mountains. 

 Whatever effect glaciers have produced in reducing their own 

 limits, must have been effected, not by lowering the general level 

 of the ridges of the Alps, but by deepening the hollows, and 

 unfitting some portions of the area from serving as reservoirs for 

 the accumulation of snow. Professor Tyndall, if I may venture 

 to say so, has thrown additional light on the recent history of 

 the Alps and other high mountain countries by directing atten- 

 tion to one of the causes that must have contributed to reduce 

 within narrower limits the action of glaciers, but has not gone 

 near to proving that the deepening of the main channels suffices, 

 without other climatal changes, to account for the disappearance 

 of glaciers that formerly reached to the level of the plains of 

 Piedmont and Lombardy, to a distance of 30, 40, and even 80 

 miles from the limits of the existing ice-streams. 



I pass over many facts of secondary importance which seem to 

 me to confirm the views above advocated, merely glancing at one 

 which is familiar to most persons who have studied the working 

 of the ancient glaciers of the Alps — the record, namely, which 

 is preserved by the harder rocks of the former limits of glacier- 

 action. In scanning the rocky sides of the higher alpine valleys, 

 the contrast between the surfaces that have once been subject to 

 the passage of a glacier, and those that were not reached even by 



