490 Mr. J. Ball on the Formation of Alpine Lakes, 



2. Towards the opening of the great alpine valleys the 

 diluvium, which extends for a greater or less distance into each 

 of them, is found to be composed of rocks derived from the 

 mountain-district drained by the valley. At a distance from 

 the base of the Alps the materials of the diluvium spread over the 

 plain exhibit a mixture of the rocks derived from different valleys. 



3. There is no appearance of any local disturbance of the 

 surface throughout the region in question since the deposit of 

 the diluvium. The latest changes arising from upheaval of the 

 surface must have been completed before that period. 



4. All the main valleys descending from the Alps exhibit 

 unequivocal traces of the action of ice. The glaciated surfaces 

 of hard rock, the presence of erratic blocks perched upon steep 

 slopes, and the remains of great moraines at the lower extremity 

 of the valleys, demonstrate the former presence of glaciers, some 

 of which must have been of enormous dimensions, perhaps 

 exceeding 100 miles in length, and 2000 feet in thickness. 



5. The deposit of the terminal moraines of these ancient 

 glaciers must have been subsequent to that of the diluvium, as in 

 many places the former may still be seen resting on the latter. 



6. Throughout the central portion of the chain, extending 

 from Domo d'Ossola to Riva in the Italian Tyrol, the principal 

 valleys of the Alps contain narrow and very deep lakes, all of 

 which lie within the area marked by the former action of ice. 



Starting from these data, and rejecting Professor Ramsay's 

 theory of the excavation of the lake-basins by glaciers, MM. 

 Mortillet and Gastaldi argue that, as the materials of the 

 diluvium are derived from the upper valleys of the Alps, the 

 lake-basins must have been filled up when these materials were 

 borne down to the plain of northern Italy. Refusing to admit 

 that ice may have been the material that filled up the lake-beds, 

 they hold that the diluvium must itself have performed this 

 office, and so bridged over the space that must have been 

 traversed before the great masses of diluvial matter can have 

 been discharged into the plain. To account for the subsequent 

 clearing out of the lake-beds, the glaciers are held to have 

 descended into the lower valleys, and scooped out the incoherent 

 masses of diluvium from the hollows which, on the subsequent 

 retirement of the glaciers, became filled by the existing lakes. 



This theory leads to two separate branches of inquiry — the one 

 mainly physical, the other mainly geological. It may be asked, 

 first, whether glaciers as mechanical agents are competent to do 

 the work which has been attributed to them ; secondly, whether 

 the evidence fairly interpreted leads us to seek their agency in 

 order to explain the phenomena. 



In discussing the first question — that as to the competency 



