28 Reviarks on the Formation ()f Alluvial Depoiites. 



the stream, spreading itself out, shot the rocks over every part 

 of the plain, where we now find them dispersed. 



Where the channel is moderately deep, the effect of reverbe- 

 ration is often very apparent in the curvature of the upper sur- 

 face of the stream. A section of one of the branches of the 

 Arve at Chamonix, as I saw it last summer, would exhibit the 

 form represented in Fig. SI *. The elevation of the ir-ddle 

 above the sides appeared to be about 3 feet ; and the accele- 

 ration of the current down the middle^ arising agreeably to the 

 law of the composition of forces from the union of all the minor 

 currents reflected transversely from the sides and bottom, was 

 perfectly manifest to the eye. These appearances seem to war- 

 rant the inference, that the minor oblique currents conveyed all 

 the transportable materials towards the middle, and that, if we 

 could see the interior of such a stream, when in vigorous action, 

 we should observe boulder-stones tossed along the bottom, peb- 

 bles shoved with them, and in a great measure suspended above 

 them, coarse sand floating still higher, and other particles in- 

 creasing in fineness in proportion as they reach the surface. 

 Even at the surface, where alone they are subject to inspection, 

 many of the particles are so large as to be visible to the naked 

 eye ; and if, after the subsidence of the stream, we go to the ex- 

 panse, where its contents are deposited, we find all the varieties 

 distributed at different distances from the embouchure according 

 to their size and weight, the large stones being carried a very 

 little away, the pebbles forming a zone around them, masses of 

 coarse sand removed still farther, and the finest sand deposited 

 in bays and recesses, where the water must have been reduced 

 nearly to a state of quiescence. 



The same mode of action is conspicuous both in the tongues 

 of alluvium above described, and in the slopes which form the con- 

 vex bank, wherever the stream, being diverted from its direct 

 course, destroys the high concave bank upon which it impinges. 

 In all these cases the reverberation from the steep bank carries 

 the detritus in the same direction, and causes its deposition as 

 soon as the force of the stream is overcome. 



In proportion as the banks become lower or more remote, 



• Buffon describes the same appearance in the Arveiron ; but Prolessor 

 Robison's explanation of it is far from satisfactory. 



