26 Remarks on the Formation uf Alluvial Depositee. 



casioned by the breaking down of the side of a glacier in the 

 valley of the upper Doron in Savoy. The Lake of Aletsoh, 

 Canton of Valais, stood formerly at the height of sixty or seventy 

 feet, being hemmed in by a mountain ridge on three sides, and 

 by the glacier of the same name on the fourth. The water, 

 having undermined the glacier, ran out with such violence as to 

 become very destructive. A canal has been recently formed 

 across the mountain ridge to prevent the lake from rising above 

 a certain elevation. 



In the course of the Arve above Chamonix, we see proofs 

 that the two great glaciers of Bois and Argentiere have former- 

 ly blocked up that river so as to form two deep and extensive 

 lakes. The sketch, No. 19, is a view of the lower termination 

 of the glacier de Bois. A ridge, distinguished by all the pecu- 

 liarities of a moraine, but of great size, and of such antiquity as 

 to be now clothed with a forest of firs, is seen immediately be- 

 yond it. The breach in the ridge, through which the Arve 

 flows, is also very conspicuous. Above this barrier, we find an 

 assemblage of terraces, which indicate three different levels in 

 the water, and three successive breaches in the dyke. The 

 sketch. No. 20, represents part of these terraces on the north 

 side of the valley. 



On examining any recent moraine, we find that the glacier 

 itself so far penetrates into it, that it consists of ice as well as of 

 gravel, sand, and rocks. It is manifest how easily such a mass 

 must give way to the pressure of water from above, as soon as 

 a very hot summer arrives. Of all causes now in operation, 

 glaciers and moraines probably occasion the most sudden and 

 the deepest inundations. 



Such are the causes by which rivers are liable to be swollen, 

 and which operate in different elevations, in different c'imates, 

 and in different seasons. The general effect of all these aug- 

 mentations in the depth of streams is, that the greater the depth, 

 the more abundant the whole quantity of detritus, and the larger 

 the single masses which are carried along. 



OilB 



It being proved that the carrying force at the bottom of 

 streams increases with their depth, and causes being assigned, 

 which alternately and in various degrees augment and diminish 



