TOERENTS OF THE HIGH ALPS. 19 



look horizontally across the sweep of this thaiweg we see in most cases a 

 curve, evidently continuous, the inclination of which rises, — or, if the 

 expression be preferred, a curve the tangent of which, by degrees, approxi- 

 mates the vertical as we approach the neck. 



" The curve is convex towards the centre of the earth, and it may be 

 remarked that the changes in the tangent are more rapid towards the neok 

 than towards the base. In other words, the radii of the curve diminish in 

 approaching the neck. 



"This configuration," says he, " is remarkable. Why should the bed of the 

 water-courses be disposed in the form of a continuous curve ] Why is this 

 curve convex t Whj does the curvature vary more rapidly above ? The 

 answer is— All these peculiarities are combined in the exact curve which 

 best suits the flow of a liquid the volume of the current of which increases 

 with the length of the distance gone over. And he asks, — Does it not 

 seem that the forms which are so perfectly adapted to the laws regulating 

 the movement of water can be themselves but consequences of these laws 1 

 If it be supposed that the thalwegs have been brought into the state in 

 which they are now seen by the same general cause, whatever it may have 

 been, which created the mountains, why have they such regular forms, 

 while the outlines of the summits, which, according to the hypothesis, would 

 have been formed at the same time as they, show only capricious lines 1 

 By what chance, in an infinitude of possible forms, have they taken exactly 

 such an one as the waters would have themselves created had they not 

 found it already made 1 It is in these circumstances reasonable to conclude 

 that a regulated cause has operated in the formation of the thalwegs, whilst 

 the summits have been left to themselves ; and it is equally reasonable to 

 attribute this to the action of the waters as the cause. 



" It is true that this supposition attributes to the waters a prodigious 

 power, very different from the effects which they produce daily before our 

 eyes, and therefore it is necessary fully to understand the manner in which 

 they have been able to act in the formation of the curve of their bed, or in 

 other words that of the thalweg. 



" When we trace attentively the course of the Durance it is seen that the 

 valley successively expands and contracts in such a way as to produce a 

 succession of basins separated by connecting straits. These basins are 

 elongated in the line of the river's course. The bottom of them is very 

 level, and exhibits a clear and well-defined junction with the base of the 

 enclosing mountain, giving to it an appearance suggestive of its having been 

 in some measure reduced to level by water." 



According to a generally received opinion, such elliptical basins are the 

 basins, now filled up, of ancient lakes, and it may be that for a time the 

 place of the river was occupied by a succession of such lakes or sheets of 

 water appearing at different successive levels, communicating with each 

 other by waterfalls or rapids, through which the waters then poured from 

 the lakes, successively passing, as it were, from mill-race to mill-race. 

 Little by little the beds or basins have been silted up, the rocks by which 

 they were separated have been hollowed down, and the waters have at length 

 come to flow in a united bed, and over continuous slopes. We have in our 

 own day an example- of such action in the consecutive lakes in the north of 

 the United States, which seem destined to be lost one day in the Kiver 

 St Lawrence, and numerous illustrations of the same thing may be seen in 

 Finland in all directions throughout the country. 



