Action of Streams on Soft Materials. 9 



channel, and by the renewal of this action during their passage 

 through great distances, they are converted into all the varieties 

 of detritus from vast boulder-stones to the finest sand or mud. 

 In these varieties we find detritus reposing in all those parts of 

 glens and valleys which are sufficiently level and expanded to 

 afford it a resting-place ; and, from these temporary resting- 

 places, it is liable to be removed whenever the volume of . water 

 is sufficient to overcome the obstruction*. >h yartJ 



The effects produced ^1^ a stream of water depend not only 

 on its depth, volume, and rapidity, and on the quantity of solid 

 materials with which it is charged, but also on the nature of 

 the channel through which it passes. A comparatively small 

 force being necessary to remove loose earth and stones, it will 

 chiefly carry forward the materials, furnished by the processes 

 which have been described in the former part of this paper, or 

 already deposited by the same stream in a previous period of its 

 action, 



A case of the first description I had an opportunity of ob- 

 serving last summer in Savoy. The cliffs on the right hand of 

 the valley of the Arve, going from Servoz to St Martin, consist 

 of stratified limestone, with numerous softer beds of black sandy 

 shale. At this spot one of the greatest landslips occurred in 

 1751. The ruin covers more than a square league, and is 

 crossed by a small torrent, which is called the Nant Noir, in 

 consequence of its blackish colour. This stream, seemingly in- 

 considerable, is continually undermining its banks. At the be- 

 ginning of last July a greater quantity of rain than usual had 

 caused it to act more vigorously, so that its channel was exca- 

 vated to the depth of 100 feet, and the surface of the ground, 

 on each side of it, marked by fearful rents. Only a week be- 

 fore I saw it, the water, pent up for a short time by fallen 

 masses, in an instant broke through this barrier, and unfortu- 

 nately carried away the supports of a bridge, while two men 



" Dr Hutton (Theory of the Earth, vol. ii. p. 154. Pare ii. ch. iv.) denies 

 that reunded pebbles can be "thus worn by travelling in the loncest river;" 

 and maintains that the attrition, which produce 1 their form, was that of the 

 waves of the sea upon some former coast. Nevertheless, we trace these peb- 

 bles through the valleys up to the rocks, from which they have fallen, and 

 every stream which rolls them exhibits an impelling force not inferior to that 

 of the ocean's waves. 1 



