STATEMENT BY M. MARSCHAND, 217 



sides ; but the ditch on the side of the forest will remain filled with 

 water, proceeding from infiltration through the wooded soil long after 

 the other, contiguous to the open ground, has performed its office of 

 drainage and become dry. The ditch on the left will have discharged 

 in a few hours a quantity of water which the ditch on the right requires 

 several days to receive and carry down to the valley." And but for 

 this drainage into the ditch the water might have remained there for 

 an indefinitely longer time. Thus by an operation, distinct from all 

 that have been mentioned, the humidity of the soil is prolonged and 

 maintained by forests. By continuous exhalations of moisture 

 through the stomates of the leaves — the moisture being obtained 

 in part, and it may be in great part, froni what is stored up in these 

 superficial and deeper-lying reservoirs, replenished from time to time 

 by raiu — the humidity of the air in a forest is increased ; bnt it is 

 the other phase of the operation with which we have at present 

 more especially to do. 



It is this effect of forests which has led to the extensive replanting 

 of the Alps, the Cevennes, aud the Pyrenees with trees, herbage, and 

 bush, with a view to arresting and preventing the destructive conse- 

 quences and effects of torrents. In a separate volume I have given a 

 copious compilation of records of what has been done, of what led to 

 this being done, and of the successful results which have followed. 

 Here I may cite some of the statements made by one of the writers 

 on the subject, M. L. Marschand. I have already referred to 

 remarks by him on the hydroscopicity and capillarity of certain 

 minerals; as a property distinct and different from these which retain 

 moisture, he speaks of the permeability of soils, by which they give 

 passage to moisture and allow of its transmission ; aud he gives 

 cases of minerals which absorb and retain very little moisture in 

 their structure, but which are very permeable by water. 



In general, rocks which are highly hydroscopic are not very 

 permeable, for the molecules, once moistened, cohere and present the 

 appearance of a compact mass impermeable to water, as may be seen 

 in clay. 



On a permeable soil or subsoil, trees create and maintain on the 

 surface a layer of humus of considerable hydroscopic and capillary 

 properties, retaining water, and modifying the general permeability. 

 While, on an impermeable rock, the roots break up this and increase 

 the permeability. 



The^principle which he seeks to establish is, that forests have the 



