Alpine Valleys and Alpine Lakes. 91 



estimate, to assert that the glacier must have moved through 

 that defile at a pace twenty times greater than its rate of motion 

 anywhere over the level of the lake ; yet we see how limited its 

 effect has been upon the containing walls of the valley. 



A very interesting paper by M. Mortillet, referred to by Pro- 

 fessor Ramsay, but not as generally known as it should be in 

 England, is mainly devoted to prove that the lakes of Lombardy 

 must have at one time been filled up by the post-pleiocene dilu- 

 vium, which forms so important a deposit throughout the entire 

 valley of the Po, and that they have been cleared out by the 

 glaciers which descended into them during the subsequent 

 glacial epoch. M. Mortillet is careful to admit that against 

 solid rock the glacier would be comparatively inoperative, but 

 he assumes rather than seeks to prove that the case would be 

 different in regard to the incoherent masses of rolled pebbles, 

 sand, &c. which make up the diluvium. M. Mortillet is a 

 shrewd reasoner, and he has made out a good prima facie case 

 for his hypothesis, which does not present such formidable diffi- 

 culties as that of Professor Ramsay ; but I own myself to be 

 sceptical as to the possibility of a glacier under the supposed 

 circumstances excavating, even in yielding materials, trough- 

 like basins which vary in depth from 900 to 2600 feet. I feel 

 convinced that the whole effective work must have been accom- 

 plished during the advance of the glacier by the front of the ice, 

 which might probably, to a limited extent, have ploughed out a 

 furrow in soft materials ; but from the moment when any portion 

 of the diluvium became covered over by the glacier, the enormous 

 weight of the ice would tend to consolidate it, and, for the rea- 

 sons already given, the grinding action produced by the advance 

 of the lower surface of the ice over its bed would disappear, or 

 be reduced within very narrow limits. 



An additional difficulty, fatal, as I think, to Professor Ram- 

 say's theory, and hard to reconcile with that of M. Mortillet, 

 arises from the presence in most alpine lakes of projecting 

 points of rock, and sometimes of rocky bays and coves facing 

 in the direction towards which the glacier formerly flowed. An 

 instance which may be familiar to some readers is the rocky 

 promontory, extending into the Lake of Como, south of Tremezzo, 

 whereon stands the Villa Arconati. The glacier, whatever its 

 efficacy as a tool may have been, worked in one direction only, 

 and cannot have scooped out hollows in a direction contrary to 

 its own current. Such irregularities as I speak of cannot be 

 accounted for on Professor Ramsay's theory ; and according to 

 M. Mortillet we should expect to find them at least partially 

 blocked up by diluvium, remaining in situ in those places where 

 it had not lain in the path of the glacier stream. 



