LECTURE XI. 315 



Morlot, witli whose deductions, as regards two glacial epochs, 

 I do not agree, has, nevertheless, very cleai-ly shewn the 

 connection between the blocks and the glacial clay. Such 

 enormous masses of ice must, by their under surfaces, produce 

 a corresponding quantity of till, hence the glacial clay is found 

 in immense quantities near the Alps, for instance, near the lake 

 of Geneva, where it is forty feet deep. It is also clear, that at 

 a time when the ice masses reached the highest clifis of the 

 Jura, no angular blocks could have been deposited upon the 

 level land, and that the lower blocks on the Jura must belong 

 to the period of recession, during which the formation of the 

 glacial clay continued as long as the motion of the glaciers 

 upon the soil. But it is not less clear, that at the time of this 

 greatest extension, but few Alpine pinnacles rose above the 

 Swiss ice-sea; that, consequently, but comparatively few blocks 

 could be transported upon the ice, and therefore no perfect 

 moraines could be formed, as is the case with smaller glaciers 

 which receive the detritus from more extensive rocky tracts. 



Upon this glacial clay in Western Switzerland we find, in 

 many places, considerable beds of pebbles, gravel, and sand, 

 which are, by the infiltrated lime, so cemented together as to 

 form a species of gompholith. The pebbles frequently attain 

 the size of a man^s head, or even larger. They show no traces 

 of grooves or striea, but are simply rounded and clean ; nowhere 

 is clay or marl attached to them ; they are manifestly rounded 

 by the action of water. One of the finest specimens of such 

 deposits may be seen near Geneva, where the heights of St. 

 Jean and the woods of Lancy, through which the bed of the 

 Rhone runs, consist of such old alluvia, which are also found in 

 great extent in most other parts of Switzerland. Of particular 

 deposits of this kind in Eastern Switzerland I shall speak on 

 another occasion. 



It is clear that these old alluvia could only have been depo- 

 sited after the retreat of the glaciers towards the Alps. As 

 the retreat of a glacier is effected by the melting of its mass, 

 which necessarily produces an accumulation of water, it is 

 evident that the recession of colossal glaciers gave rise to tur- 

 bulent streams, which here and there excavated their beds, but 



