16 RESUME OF SURELl's STUDY OF 



and as forming larger bodies of water which, when swollen, continue so for 

 a time more or less protracted ; the slope of their fall is constant throughout 

 long stretches, and does not exceed 15 millimeters per metre, or a fall of 

 15 in a thousand. They are in many places characterised by a water- 

 coiu-se in a level bed of very great breadth, a small portion of which only 

 is taken up by such a water-course, and this is liable to be forsaken and 

 left dry, while the waters flow in another channel which they have formed 

 for themselves, to be again changed for another, and that again after a time 

 for another ; by which constant changes there is frequently occasioned a 

 great waste of land, and this, if cultivated, must be cultivated at the risk of 

 the whole being swept away — crop and soil together. 



Elsewhere he mentions that traces of the former existence of ancient 

 lakes are frequent in these mountains, and that it is the constant rule for 

 a water-course, whatever may be the class to which it belongs, when it 

 enters one of these basins, to change its bed when traversing it ; but while 

 this happens once and again, perchance, with others of the different kinds of 

 Alpine water-courses which he has enumerated, it occurs so constantly as a 

 general feature of all the rivers, repeating itself unceasingly throughout the 

 whole of their course, while in the other forms of water-course its occur- 

 rence is only occasional and as it were accidental, that he considers this one of 

 the permanent and specific characteristics of the rivers. 



Torrents, on the contrary, is a name given to what may be called a dry 

 water-course, along which a tiny stream may be generally seen to flow, 

 but which from time to time is filled with a rushing, roaring, resistless 

 flood. They generally traverse very short valleys, which cut up the moun- 

 tains into buttress-like projections. Their faU throughout the greater part 

 of their coiubo exceeds six centimetres per mfetre, and it is never less than 

 two centimHres per mfetre, or two in the hundred. Changes in the slope 

 of their fall succeed one another very closely ; and there is given as a charac- 

 teristic of them that they constantly, if they have not previously done so 

 to a great extent, undermine the sides of their course at one place, and 

 sweep away the debris and deposit it at another, and subsequently change 



their course above the place at which the deposit has been made, ^giving 



occasion for the same process being again repeated at some other spot. By 

 the rapid fall, the rapid succession of changes in the degrees of this and 

 their destructive effects, they are distinguished from rivers, and also from 

 torrential rivers, in the technical classification of water-courses adopted. 



Of torrential rivers, rivieres torreTitiales, in the High Alps, Surell 

 enumerates five, but he intimates that there are many more. They are 

 affluents to the principal rivers. The valleys in which they flow are less 

 extensive and more compressed, and they cut up the mountain range into 

 spurs and lesser chains. Variations in the slope of their fall succeed each 

 other more closely than do those of the rivers. They do not change their 

 courses as do these, or they do so but little. Their fall is greater, but it does 

 not exceed six centimfetres per mfetre, or six in the hundred. They have 

 not the characteristics or specific characters assigned to rivers ; neither do 

 they present the characteristics or specific charactei-s assigned 'to torrents • 

 they present characteristics of both with characteristics peculiar to them- 

 selves ; and they are classed apart that the field may be clear for the studv 

 of what are known specifically as torrents. ^ 



While the distinctions thus drawn between torrents and other water- 

 courses is maintained in the treatise, it is stated that the difierent forms 



