220 ACTION OF FORESTS ON THE FLOW OF UIVERS. 



this and is carried off, feeding springs and fountains, and streams, the 

 saddest thing that can be said is that it is lost, thoiigii this can only 

 be said with a show of truth of that portion of it which is lost in the 

 sea ; but of what is canned off by ruisseUeni''nt comparatively little is 

 utilized ; a much greater proportion of it finds its way to the sea, and 

 this in rushing thither often carries with it devastation and destruc- 

 tion. 



According to the statements I have cited from Surell, evaporation, 

 infiltration, and ruissellement, are equal in their sum to the quantity 

 of the rainfall, and if after evaporation has taken up its part the in- 

 filtration be complete, the ruissellement will be nil. 



To the negative good which might thus be done, or damage which 

 might thus be prevented, and the effect of trees in doing these, by 

 equalizing to some extent the flow of rivers, would I next di'aw 

 attention. 



In connection with a statement made in the preceding section, 

 relative to the effect of the destruction of forests upon the Durance, 

 reference was made to the important circumstance that while the 

 river is confined to a current little more than thirty feet in width, 

 the bed in some places exceeds a mile and a quarter in breadth, and 

 so far back as 1789 it was computed that it had covered an area of 

 not less than 130,000 acres with gravel and pebbles; and it was 

 intimated that thus was brought before us another of the effects of 

 forests, or rather of the desti'uction of forests, which would be -sub- 

 sequently brought under discussion, 



Mr Marsh, writing on this subject, says : " The traveller who visits 

 the depth of an Alpine ravine, observes the length and width of the 

 gorge, and the great height and apparent solidity of the precipitous 

 walls which bound it, and calculates the mass of rock required to fill 

 the vacancy, can hardly believe that the humble brooklet which 

 purls at his feet has been the principal agent in accomplishing this 

 tremendous erosion. Closer observations will often teach him that 

 the seemipgly unbroken rock which overhangs the valley is full of 

 cracks and fissures, and really in such a state of disintegration that 

 every frost must bring down tons of it. If he compute the area of 

 the basin, which finds here its only discharge, he will perceive that 

 a sudden thaw of the wintei-'s deposit of snow, or on-e of those terrible 

 discharges of rain so common in the x\lps, must send forth a deluge 

 mighty enough to sweep down the hugest masses of gravel and of rock. 

 The simple measurement of the cubical contents of the semicircular 



