OF THE EARTH'S SURFACE. 141 



Before mentioning my own observations, I beg leave to lay 

 before the Society the state in which I found the question. 

 The central ridge of the Alps, of which Mont Blanc is the 

 principal mountain, consists, as is well, known, of Granite. 

 Nearly parallel to it, and at a distance of fifty-four miles to the 

 north-west (by Sir George Shuckburgh's measurement) lies 

 the mountainous ridge of Jura, fifty or, sixty miles in length, 

 composed entirely of calcareous matter. Between these two 

 ridges are interposed the valley and lake of Geneva, and other 

 vallevs and inferior hills. The native place of the granite is 

 confined to the central ridge ; but over the surface just de- 

 scribed, innumerable detached granitic blocks are very irregu- 

 larly scattered, which have originated, as we must presume, 

 in that central ridge, and have been transported to their pre- 

 sent position by some mechanical power. 



Upon the southern side of the Baltic, a multitude of similar 

 blocks are found, also scattered irregularly over a sandy di- 

 strict, of which a particular and detailed account is given in 

 M. de Luc's Geological Travels, vol. i. 



The origin and history of such blocks, which occur in va- 

 rious other places,, have given rise to considerable discussion j 

 and the question is inseparably connected with other points 

 of magnitude in geology, — such as the formation of valleys 

 and lakes, and the distribution and arrangement of various 

 beds of clay and gravel, and of all the loose and alluvial as- 

 semblages which occur under various forms in all parts of the 

 globe, and constitute its most valuable districts in point of fer- 

 tility. 



M. de Saussure ascribes the transportation of the granitic 

 blocks on the Alps to the action of an immense torrent of wa- 

 ter, or Debacle, as he terms it, which at some remote period, 

 flowing over the summit of the Alps, had carried these blocks 

 along with it. 



Dr 



