120 LrTEEATURE ON TORRENTS. 



the flood, taking into account the quantity of water discharged, and all the 

 causes or sources of perturbation operating ; which is a much more difficult 

 problem. 



" And in resolving the whole question into the permeability of the soil, and 

 its capacity of absorption, it appears, importance is attached exclusively to 

 the reduction of the volume of water which flows away. It seems to be 

 forgotten in this that water-courses, if steadily supplied, constitute it may be 

 said the principal riches of a country, and the most potent of all instruments 

 of labour. 



" By their modulating power forests act as vast reservoirs, not only in 

 preventing sudden variations of delivery during a flood, but in feeding the 

 water-courses and raising their level during the period of exhaustion. In 

 what relates specially to the torrents of the Alps, it has been demonstrated 

 that the renewed devastating power which they have exhibited, and which 

 has assumed such portentous magnitude in the course of the last forty 

 years, is a consequence of the disappearance of the woods. When one goes 

 over these lands — cut into ravines and despoiled of all vegetation — he meets 

 with numerous stumps of pine and of larch, which testify that at a period 

 as yet still recent they were covered by vast forests. 



" M. Surell cites, as an example of the action of forests, the torrent of 

 Savines, now completely extinguished, and the basin of which is everywhere 

 adorned with a magnificent forest of firs and pines. The forest has 

 effectively contributed to the extinction of the torrent, but at this point the 

 following observations may be made : 



" This natural extinction of the torrent goes back to ages most remote. 

 The cone is of a perfect geometric regularity. At its base, opposite the 

 Durance, it presents a troncature or section, produced by the erosion of the 

 river, and the escarpment of which is about 30 mfetres (100 feet) in height 

 at its culminating point. This section of the ground lays open the interior 

 of the torrential deposit formed of rolled pebbles. 



" The whole surface of the cone is cultivated, and on one portion has 

 been built the large village of Savines, the chief place of the canton. 



"This enormous heap of deposit is situated at the foot of a high 

 mountain called Morgon, in the flanks of which are dug out a profound 

 gorge surrounded by a vast basin, the work of the water. AU the upper 

 slopes of the mountain are hung with a beautiful forest, producing firs 

 more than 30 mfetres (100 feet) in height, and 3 metres (10 feet) in girth. 



" The lower slopes are deeply ravined, but wooded to the very edge of the 

 thalwegs. A pretty strong stream rises from the principal gorge, but it 

 swells but little ; it carries down no materials, and it flows into the Durance 

 by a bed deeply enclosed in the left bank of the cone. Extinction and 

 stability are complete; but it is certain that if the forest should be made to 

 disappear, anew would disorder revive, and this with the same intensity as 

 before. 



" In going over the basin with attention, I satisfied myself that every- 

 where the bed of the thalwegs of the gorges and the ravines, formed of the 

 hard rock, were absolutely incapable of being undermined. From this it may 

 be inferred, that during the activity of the torrent, when the basin was 

 being deepened more and more, the surface could not have been wooded. 

 But from the time that the waters had everywhere reached the hard rock, 

 and that these could no more be undermined and washed deeper, their 

 thalwegs in the upper slopes tended to consolidate themselves, taking their 



