Mr Murchison on ihe Glacial Theory* Wf 



posit, which had been accumulated in the radiating valleys during a pe* 

 riod of great disturbance, anterior to the existence of glaciers in that lati- 

 tude. Describing (like M. Necker) one of these '^ trainees" as having a 

 continuous length of fifteen leagues, he infers that such a mass could 

 never have been deposited by a glacier proceeding from mountains of no 

 greater altitude than the Alps. Arguing that glaciers arc merely the con- 

 densed or central portions of vast accumulations of snow, forced down- 

 wards into the gorges by increasing volume from above, the chief novelty 

 of M. GodefFroy's work is contained in the opinion, that in advancing, 

 these bodies of ice cut through the ancient diluvium or drift, just as a 

 plough-share cleaves the soil (" presso tellus consurgit aratro" being his 

 motto), and threw up some portions into lateral moraines, as well as 

 pressed before them others to form terminal moraines. To the crystalline 

 and mechanical changes which the snow has undergone in its passage 

 int« solid ice, is attributed much of the confusion and irregularity of out- 

 line so visible in the " aiguilles" and other icy masses of the Alps ; and 

 to the same disturbing action is referred the rounded and worn exterior 

 of the boulders in moraines, as contrasted with comparatively angular 

 blocks of the pre-existing drift which have not been in contact with the 

 glacier. I refer you to the work of M. Godeffroy for the explanation of 

 the manner in which he supposes the surface of the advancing or retreat- 

 ing glacier was subjected to lateral overflows or " ecroulemens" of stones, 

 gravel, and earth, and also for his theory of medial moraines ; but I now 

 bring to your notice his ingenious effort to solve one of the very difficult 

 climatological problems in the Alps. Having shewn how the lower val-: 

 leys must, from year to year, become more and more encumbered with 

 detritus, he seizes this fact to explain by it alone, both the well-known 

 retreat of the glaciers and the fact brought forward by Venetz and other 

 observers ; viz. that roads which existed in certain former passes of the 

 high Alps are now quite choked up with snow and ice — a fact which 

 has been supposed to indicate a sensible decrease of temperature within 

 the historic eera. M. Godeffroy contends, that in ancient times, when 

 the gorges were more open, and the heaps of detritus at the entrance into 

 the lower valleys were less in size and fewer in number, and when conse- 

 quently the glaciers easily extended to greater distances, the continual 

 and unrestricted supply of snow and ice from many affluents more than 

 couBtervuiled the loss through atmospheric action ; but that as the ob- 

 stacles increased at some distance above the terminal moraine, the lower 

 ends of the glaciers not being so fed as to regain in one season the melt- 

 ing losses of the previous year, the inevitable result was a successive 

 shrinkage and retrocession of the mass. The increase of snow and ice in 

 the upper passes, and the blocking up of the roads, are explained by the 

 same agency ; for as soon as the descent of the glacier from the higher to 

 the lower Alps was impeded, it would follow, that the frozen matter of 

 the higher regions, deprived of its previous exit, must find its way into 

 the adjacent upper depressions, and there form those mers de glaet which 



