THE REVOLUTIONS OF THE CRUST OF THE EARTH. 333 



ters. How much must the meim temperature of Europe be lowered ia 

 order that the extension of the ghiciers attain such proportions ? The 

 estimation is not difiScult. In the first place, experiment shows that the 

 temperature diminishes one degree for 188 meters (G16^ feet) of height. 

 The glaciers descend 1,150 meters (3,772 feet). The mean temperature 

 must then be lowered 6^ or 7° centigrade in order that the glaciers may 

 descend to the level of the sea. This is actually the case in the island of 

 Georgia and in Patagonia, where, according to Darwin, the glacier reaches 

 the level of the sea at 40° 40' of south latitude.* According to M. Ch. 

 Martin, a lowering of 4'=> centigrade would be necessary in the mean tem- 

 perature of Switzerland in order that the glaciers of the valley of 

 Chamouuix should extend to the environs of Geneva.t 



Another proof of the action of ice is given by the abraded and pol- 

 ished rocks. The glaciers scratch the rocks in the bed of their course 

 with the pebbles and stones contained in their lower strata; they polish 

 the salient points they encounter first by their passage around them, 

 and afterwards as their mass increases by their movement over them 4 

 This phenomenon is not only observed in Switzerland, but also in Sweden, 

 in Norway, in Lapland, and in Finland. In the latter province, the 

 striped and polished surfaces are especially remarkable, because the 

 country is not mountainous ; now the phenomenon in this case can only 

 be produced by the action of permanent glaciers, which would imply a 

 considerable cooling of the mean temperature of Europe. It is true 

 that the scratches and polish upon the rocks has been attributed to the 

 action of floating icebergs, which plowed the bottom of the sea with 

 the stones attached to their lower surfaces. This opinion is difficult to 

 sustain, iu view of the, for the most part, parallel character of the stripes 

 in Finland, and the direction almost always north and south of the 

 stripes. If floating ice had produced these marks, the scratches would 

 not have been parallel, nor in the same direction ; for, supposing the 

 current constant in its course, the rotation of the floating mass should 

 be taken into account, and also the obstacles the current would meet, 

 causing it to deviate, and producing whirls and rapids. Moreover, 

 Greenland, which, at an equal latitude, is now permanently covered 

 with ice, shows the possibility of a similar cooling in Southern Europe. 



The dissemination of erratic blocks is also attributed to the action of 

 glaciers. We observe these in the plains of Switzerland and of Pied- 

 mont ; it is ice which has detached them from the sides of the Alps, 

 and ice which has transported them to the place where they now rest. 

 Some naturalists have introduced water as an agent in this matter, 

 which according to them must have covered certain parts of Europe to 

 explain the transportation of these bowlders. This hypothesis has, 

 however, been abandoned even by its illustrious autho r ;§ for had the 



* Cotta, Brief e iiber Ah v. HumboMfs Kosmos, vol. 1, p. 216. 

 tAlph. Favre, Eeclierches geologiques, t.i, p. 183. 

 t E. Desor, La Limite Sup. des Polls Glaciaires, etc., 1855, p. 7. 

 $ Lyell, Elements oj Geology, 6th edit., t. i, p. 229. 



