310 THEORY OP THE ORIGIN OF LAKE-BASINS chap.xv. 



contorted positions, must also have given rise to many super- 

 ficial inequalities, in some of which large bodies of water 

 would collect. M. Desor, in a memoir on the Swiss and 

 Italian lakes, suggested that they may have escaped being 

 obliterated by sedimentary deposition, by having been filled 

 Math ice during the whole of the glacial period. 



Subsequently to the retreat of the great glaciers, we know 

 that the lake-basins have been to a certain extent encroached 

 upon and turned into land by river-deltas; one of which, that 

 of the Ehone at the head of the Lake of Geneva, is no less 

 than twelve miles long and several miles broad, besides 

 which there are many torrents on the borders of the same 

 lake, forming smaller deltas. 



M. Gabriel de Mortillet, after a careful study of the glacial 

 formations of the Alps, agreed with his predecessors that the 

 great lakes had existed before the glacial period, but came to 

 the opinion, in 1859, that they had all been first filled up 

 with alluvial matter, and then re-excavated by the action of 

 ice, which, during the epoch of intense cold, had, by its 

 weight and force of propulsion, scooped out the loose and 

 incoherent alluvial strata, even where they had accumulated 

 to a thickness of 2000 feet. Besides this erosion, the ice had 

 carried the whole mass of mud and stones up the inclined 

 planes, from the central depths to the lower outlets of the 

 lakes, and sometimes far beyond them. As some of these 

 rock-basins are 500, others more than 2000 feet deep, having 

 their bottoms in some cases 500, in others 1000 feet below 

 the level of the sea, and having areas from twenty to fifty 

 miles in length and from four to twelve in breadth, we may 

 well be startled at the boldness of this hypothesis. 



The following are the facts and train of reasoning which 

 induced M. de Mortillet to embi-ace these views. At the 

 lower ends of the great Italian lakes, such as Maggiore, Como, 

 Garda, and others, there are vast moraines which are proved 



