20 THE POPULAR SCIENCE MONTHLY. 



sands swept out from underneath the advancing glacier, and 

 therefore to be no older, geologically, than the moraine matter 

 which overlies it. The Swiss geologists, however, do not appear 

 to hold this view, since they have recourse to a very remarkable 

 hypothesis in order to overcome what they evidently believe to be 

 a real difficulty in the way of the pre-glacial origin of the lake. 

 The suggested explanation is as follows : At the beginning of the 

 Ice age the glacier of the Rhone crept on down its valley past 

 Martigny and St. Maurice till it reached the lake ; it is then sup- 

 posed not to have marched on with an ice wall, say five hundred 

 or more feet high, but to have at once spread out like so much 

 soft pitch, and to have filled the lake to its present water level or 

 thereabouts. Then, over this great plain of ice, the subglacial 

 torrent of the Rhone is supposed to have flowed, carrying with it 

 and depositing at the end of the lake that ancient alluvium which, 

 somehow, has got to be accounted for ! * 



Having thus filled the lake with ice instead of water, the main 

 body of the glacier is supposed to start afresh and to travel over 

 the ice, and thus obviate the imaginary difficulty of a glacier 

 moving up hill, though every student of glaciers now admits that 

 they did so, and though it is universally admitted that this very 

 glacier of the Rhone moved over higher, steeper, and more irregu- 

 lar hills on its way to the Jura and to Soleure. 



Now this extraordinary theory involves two difficulties which 

 are passed by in silence, but which seem to entirely contravene 

 all that we know of the nature of glaciers, and to be entirely un- 

 supported by facts. The first is, the glacier ceasing to move on- 

 ward as a glacier, but spreading out to fill up a lake basin, as if 

 the lake were simply frozen to the bottom. Is this conceivable or 

 possible ? I think not. When glaciers come down to a fiord or to 

 the sea they do not spread out laterally, but move on till the water 

 is deep enough to buoy them up and break off icebergs, and no rea- 

 son is given why anything different should have happened in the 

 case of the great Swiss and Italian lakes, supposing they existed 

 before the Ice age came on. That the glacier should afterward 

 slide over this level plain of ice is equally inconceivable, in view 

 of the property of regelation of ice under pressure. Owing to 

 this property the glacier and the lake ice would become one mass, 

 and would move on together under the law of decreasing velocity 

 with depth. This, however, is of little importance if, as I con- 

 ceive, the supposition of the formation of an ice-sheet at the water 

 level for fifty miles in advance of the glacier is an impossible one. 

 The only other theory is, that the lake was filled up by alluvium 

 before the Ice age, and that the glacier re-excavated it. I have, 



* A. Falsan, La Periode Glaciaire, pp. 135, 137. 



