TORRENTS OP THE^HIGH ALPS. 37 



cut into the dry and bluish-tinted hills, which give to the mountaius of 

 Embrun their peculiar aspect. These hills are crumbled to such an extent 

 that in trying to climb them one sinks often to the knees in the detritus. 

 And this valley is situated in what may be called the point of intersection 

 of the atmospheric and geological causes of the formation of torrents. To 

 the north we travel over similar formations, but under a different atmo- 

 sphere ; to the south we travel under a similar atmosphere but come upon 

 soil of a different character, — and in both directions the number of the 

 torrents is diminished, as are also their effects. Other illustrations of the 

 same fact are given. 



Studying thus the natural history of torrents, he attributes their appear- 

 ance to the simultaneous operation of several causes in combination. There 

 appears to be (1) a geological cause — the nature of the soil; (2) a topo- 

 graphical cause — the superficial aspect assumed by the country ; and (3) a 

 meteorological cause — the rainfall in the locality. And the question next 

 raised is — Is the second of these seen in the existence and form of the bassin 

 de rSception, or basin drained by the torrent, to be considered a primary, or 

 only a secondary cause of the torrent ? 



Surell maintains it is a secondary cause — itself a consequence, effect, or 

 product of that to which it ministers. Were it otherwise, he says, in order 

 to this being the case, it would be necessary that the cause which created 

 these mountains should have moulded and shaped at one stroke these basins, 

 according to the characteristic figure which they present to-day ; it would be 

 necessary that this form, shape, and outline should have preceded all the 

 action of the waters collected from them ; that these, from the first, should 

 have found all the ground so moulded and prepared for them ; and that 

 they should have produced, from the first day, all the phenomena which 

 they continue tiU to-day to present before us. 



But it is impossible, says he, to admit such a supposition. The hassins 

 de reception are evidently the result of the violent and long-continued action 

 of the water collected at first in a simple recess of the ground, and flowing 

 over a soil deprived of coherence and consistency. 



What proves this decidedly is the presence of the larger Uts de dejection, 

 which have been formed entirely and exclusively at the expense of the 

 lower-lying lands, whence the torrents issue. Every day, moreover, we see 

 the hassins de reception increasing in magnitude. These effects follow on 

 with such rapidity that a limited number of years should have suflSced to 

 have produced enormous modifications in the original outline of the land. 

 We have then only to carry back, so to speak, into olden times the action 

 going on to-day under our eyes, supposing that present phenomena are the 

 continuation of an action begun some centuries ago, and the digging out of 

 the basin finds a ready explanation. And he refers to the facts already 

 cited, that there are torrents of quite recent formation ; that new ones are 

 being formed constantly ; and that these then aid in the formation of basins 

 in the midst of grounds in which there was nothing of the kind previously 

 existing. 



He goes on to say, — " I know well that I may seem to have exaggerated 

 this action when there is considered the vast extent presented by the basins 

 of certain torrents, and the profound depths of their declivities forming 

 veritable valleys. But there should be taken into account, ou the other 

 hand, and at the same time, the enormous cubical contents of the deposits 

 produced by them, which can have been obtained only from the erosion of 



