TOBRBNTS OP THE HIGH ALPS. 17 



may be considered as passing, by intermediate gradations, into one another, 

 and that the same body of water may in one part of its course appear in 

 one of these forms, and in another part of its course it may appear in another. 



The torrents thus specified he classifies under three heads, those of each 

 category presenting characteristics by which they may be distinguished 

 from the others. Torrents of the first class take their departure from a 

 col in the mountains and flow through a valley. Those of the second class 

 flow from the mountain-top and follow the line of greatest declivity. Those 

 of the third class take their origin from the flank of the mountain at some 

 distance below the summit. 



Of these also there are intermediate varieties, and varieties assimilating 

 them to some of the other forms of water-courses. The first class approxi- 

 mate in some of their features to those of torrential rivers ; in the second class 

 all the characteristics of the torrent are prominent, and to this type most 

 of the torrents in the High Alps are conformed ; and the third class often 

 show ravines, with all the secondary characteristics of these. 



The washing away of earth, and stones, and blocks of rock being one of 

 the constant ejffects of torrents in the High Alps, and the deposits of the 

 detritus presenting certain constant features whereby they may be distin- 

 guished at a glance, not only from the moraines of a glacier, but from the 

 shot-heap of a land-slip, and from all other earthen moimds whatever, 

 Surell has fixed upon the bed of deposit as the most characteristic indica- 

 tion of the previous action of a torrent, and makes the study of these beds 

 of deposit, or lits de dejection, the point of departure in his study of 

 toiTents. 



Of these torrents, he says, in the introduction to his work, " The depart- 

 ment of the High Alps presents us with water-courses of a singular form. 

 There is given to them in the locality the name of torrents, but with the 

 term, as thus used there, there are associated peculiar characteristics which 

 do not manifest themselves in the torrents of other countries. 



" The sources of the torrents are hid in the recesses of the mountains, 

 thence they descend to the valleys, on arriving in these they spread 

 themselves out over an immensely extended convex bed, the convexity of 

 which establishes a marked distinction between these torrents and most 

 other water-courses, 



" In these the waters always flow in a hollow which encloses them in 

 such a way that a section of the ground in a direction perpendicular to 

 their course would give a curve concave towards heaven, the lower portion 

 of which was occupied by the waters. In the torrents, on the contrary, 

 when they reach the plain, a similar section would show a curve convex 

 towards heaven, and the waters confining themselves in their course on the 

 summit of this. With the water flowing in a slight depression on the 

 summit of a convex torrent bed, it may be imagined that there can be but 

 little stability in the current ; and such is the case. The most trifling rise 

 or swelling of the torrent throws the water out of the depression, and it is 

 scattered right and left, flowing away in streams which, however, still 

 follow the line of the course of the bed. 



" This instability renders the torrents very damaging, for they are ever 

 breaking bounds at new points, and subjecting to their ravages immense 

 areas of ground. Beds of torrents are to be seen exceeding 3000 metres, 



B 



