500 Mr. J. Ball on the Formation of Alpine Lakes. 



tened out and accurately levelled at the top, in a way altogether 

 inconsistent with the idea of their being ordinary moraines left 

 in situ by the retiring glacier. It is not allowable to suppose 

 that the lake itself, when formed by the melting of the glacier, 

 can have done this work, unless we admit that at the period 

 when the moraine amphitheatre was formed the sea beat upon 

 its outer face at a level considerably above the present plain. 

 In that case the streams cutting through the terminal moraine 

 to give exit to the drainage of the glacier would have gone as 

 low as the then sea-level, but no lower ; so that at a later period, 

 when the sea had retired, and the lake was formed by the melt- 

 ing of the glacier, this might have been kept at a higher level, 

 being held in by the circuit of hills constituting the terminal 

 moraine, which would have been gradually cut through by the 

 Mincio. During the interval, the lake, acting on the materials 

 lying within the amphitheatre, may have modified their form and 

 arrangement, though I think the appearances agree better with 

 the supposition that the level-topped mounds were formed from 

 stranded ice-rafts, as already mentioned. If the glacier when at 

 its furthest had advanced into the plain, this being formed of 

 dry land, the action of the streams issuing from the glacier 

 would have kept a channel open, and no bar would have been 

 formed by the terminal moraine capable of holding up the level 

 of the lake. 



It is only by careful levelling along the banks of the lakes, 

 and the alpine valleys connected with them, that it will be pos- 

 sible to distinguish accurately between the traces of extinct 

 glaciers and those of floating ice. Many peculiarities in the dis- 

 tribution of erratic blocks and glacial drift in the valleys of the 

 eastern Alps would lead me to believe that the sea, during some 

 part of the glacial period, must have there reached a consider- 

 able height, approaching, if not exceeding, 1800 feet above the 

 present level; but without more complete and more accurate 

 observations I place no confidence in this conclusion. 



I think I have shown in the foregoing pages that the theory 

 of M. de Mortillet involves a chain of hypotheses each of which 

 is inadmissible. 



It is not admissible to assume that violent currents existed 

 in each of the valleys of the Alps, capable of bearing down such 

 an enormous amount of diluvium as would have filled up the 

 great lakes ; it is inadmissible to assume that, if such currents 

 did exist, they could have carried the diluvium along the level 

 floor of the lake-basins so as to reach the plain ; it is not admis- 

 sible to assume that, if it had reached the plain, it could have 

 been evenly spread out over the surface by the supposed cur- 

 rents ; lastly, it is an assumption entirely inadmissible that the 



