Tlie Glacial Theory. 581 



great pre-existing diluvial deposit, which had been accumulated in 

 the radiating valleys during a period of great disturbance, anterior 

 to the existence of glaciers in that latitude. 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 alti- 

 tude than the Alps. Arguing that glaciers are merely the condensed 

 or central portions of vast accumulations of snow, forced downwards 

 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 thi'ough 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 into solid ice, is attributed much of the 

 confusion and irregularity of outline so visible in the "aiguilles" 

 and other icy masses of the Alps ; and to the same disturbing ac- 

 tion 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 re- 

 treating glacier was subjected to lateral overflows or " ecroulemens " 

 of stones, gravel, and earth, and also for his theory of medial mo- 

 raines ; but I now bring to your notice his ingenious effort to solve 

 one of the very difficult climatological problems in the Alps. Having 

 shown how the lower valleys 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 Higli Alps are now quite 

 choked up with snow and ice — a fact whicli has been supposed to 

 indicate a sensible decrease of temperature witliin the historic sera. 

 M. Godeffroy contends, tiiat 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 consequently 

 the glaciers easily extended to greater distances, the continual and 

 unrestricted supply of snow and ice from many affluents more than 

 countervailed the loss tiu'ough atmospheric action ; but tiiat as the 

 obstacles increased at some distance above tlie terminal moraine, the 

 lower ends of the glaciers not being so fed as to regain in one sea- 

 son the melting losscjs of the previous year, the inevitable result 

 was a successive shrinkage and retrocession of the mass. Tiie in- 

 crease of snow and ice in tlie upper passes, and the blocking up of 

 the roads, are explained by the same agency ; for as soon as the de- 

 scent of the glacier from the higher to the lower Alps was im- 

 peded, it would follow, that tlu? frozen matter of tlu; higher re- 

 gions, dejjrived of its previous exit, must find its way into thejid- 

 jacent upper dejjressions, and there form those mcrs de ylucc which 



