T0EEBNT8 OF THE HIGH ALPS. 25 



Two distinct causes concur in the formation of these deposits. First, the 

 torrents proceeding from a confined channel in the mountain come into a 

 valley, in which, being all at once deprived of the side support of sustaining 

 banks, they diffuse themselves, losing velocity and depth. And then the 

 passing from the steep declivity of the mountain to the gentle declivity of the 

 plain proves a second and an additional cause of loss of velocity and of 

 depth. The two causes are distinct and altogether independent of each 

 other, and importance is attached to this circumstance. The tendency is 

 to form a continuous curve from the caned d'ecoulement, corresponding to 

 the angle of stability. Where this has been done the first cause alone wiU 

 make additions to the bed. Where this limit of slope has not been created 

 deposits will be continued in virtue of the operation of the second cause. 

 From which it follows that some torrents may be confined by artificial 

 structures, but not others ; and that in the former case, other things being 

 equal, the eifects will be probable in proportion as the diminished slope may 

 be continuous with that from the gorge, as this continuity is a presump- 

 tive proof that the curve of the bed has been definitely taken to such an 

 extent that the dejected matters have reached the limit of their slope, 

 which is to them in the circumstances the angle of stability. 



Detailed information is given in regard to the effect of the current in 

 giving to the bed of dejection its peculiar form, with such variations as have 

 been noted, and in regard to the effect of this upon the current. 



There are next described the materials brought down by torrents— clay, 

 gravel, shingle, and blocks of stone. The laws regulating the deposit of 

 these are noticed ; and the injuries which are thus done are detailed. 



Every thing connected with the phenomena of the bassin de reception, the 

 canal d'ecoulement, and the lit de ddjection, having been discussed, attention 

 is given to the phenomena of the flood of water by which the dam^e and 

 devastation are occasioned. This he traces to two sources — first, the "melting 

 of snow towards the beginning of June, and second, storms of rain occurring 

 towards the end of summer. Those occasioned by the latter are by far the 

 most awful, and by far the most injurious 



In general, says he, the rain of such a storm gives rise to a much more 

 terrible swelling of the torrents than does the melting of the snow. Rains 

 are rare in these mountains ; but when they do fall they fall in tremendous 

 showers, like waterspouts. Their action is instantaneous and cannot be 

 foreseen. The snows never melt so suddenly and quickly as come the 

 deluges of rain, and they produce more prolonged but less sudden sweUinga 

 of the torrents. Besides this, they may be foreseen and anticipated, for 

 'they come at known times. The torrent de V Ascension owes its name to 

 the regularity with which it flows about the time of Ascension day. And 

 the melting of the snows produces a general swelling of the torrents and 

 rivers, which causes all to overflow at the same time. The swellings caused 

 by storms of rain are local ; one torrent becomes furious, while another quite 

 near to it remains dry. The time of the melting of the snow is that for the 

 highest floods in aU the water-courses in all the department ; and for all, 

 without exception, the time for low-water is towards the end of autumn. 



The phenomena which accompany the swelling of torrents are very varied. 

 It may be said that each torrent in its manner of flooding has something 

 which is peculiar to itself, and which is not found in any of the others. 

 It must be so, for aU the torrents have not the same distribution of elopes; 



