10 Remarks on the Formation of Alluvial DeposHes. 



were passing over it. Ten years ago the passage of the Nant 

 Noir was interrupted by a similar accident. 



Next to debris and detritus, the softer strata, which are in 

 place, yield to the action of flowing water. Beds of shale, si- 

 milar to those which occasioned the slip at Servoz, always give 

 to alpine torrents that traverse them the colour of the Nant 

 Noir ; and in various instances, where quantities of water are 

 retained in the soft strata of clay or shale, they burst forth in 

 the state of thick mud, carrying v^ously-sized fragments of 

 rock. Even the softer kinds of gray wacke and clayslate are very 

 quickly eroded, so that in the Eifel I have seen deep gullies 

 worn in such slate by the side of a newly made road, where 

 it could have been exposed but a few months to the action of 

 the rain-water. 



The same law, which has been mentioned as regulating the 

 disintegration of rocks, independently of the action of streams, 

 also modifies the action of running water. The fragments 

 which it removes are very frequently portions, the forms and 

 boundaries of which are determined by the structure of separa- 

 tion, which characterizes the parent rock. The ravine in the 

 annexed sketch (Fig. 6.) shows, on the one hand, a smooth 

 highly inclined plane, which is either a seam or a cleavage in 

 the mountain mass ; while, on the other, the distinct concretions 

 are gradually worn away and cut through in every direction. 

 In the falls of Imatra, as represented by Mr Strangways *, the 

 same principle is well illustrated, the sides of the chasm being 

 formed by highly inclined sti-ata of gneiss. In nearly all cases 

 the action of streams appears to be directed to those parts of the 

 rocks exposed to them, which have a natural tendency to yield 

 to their action. Hence we not only find that ravines follow the 

 course of pre-existent rents, but we observe valleys hollowed at 

 the junction of distinct formations, where, generally, the rocks 

 of each formation are more subdivided, more indeterminate in 

 their character, and more prone to disintegration. Thus, in 

 the long valley which forms the western boundai-y of the group 

 of Mont Blanc, extending from the Col de Bon Homme to St 

 Gervais, the Bon Naut separates the schistose from the calca- 

 reous mountains ; the same general fact is exhibited on a sub- 

 • Geological Transactions, vol. v. Plate xviii. 



