CHAPTER III. 



Effects or Foeests os Mabshbs. 



Ix studying the effects of forests on maiBhes there come under con- 

 sideration the drying up of marshes by the growth of forests, the 

 occasonal appearance of marches on the destruction of forests, and 

 also the occasional destruction of foreste by the creation of marshes. 



The combined consideration of all of these phenomena may be 

 necessary to a satisfactory view of the first ; but this is susceptible 

 of satisfiictory explanation by itself, and the second may be con- 

 sidered to be confirmatory of the first, and the third not otherwise 

 than accordant therewith. 



And by the consideration of these several phases of the correlation 

 of forests and marshes, we may be prepared for the consideration of 

 the effect of forests, in drying up marshes upon a grand scale through 

 protracted ages, in rendering what was a marshy land so dry as to be- 

 come a fit habitation for man, of which Denmark supplies an illus- 

 tration. 



Sect. I. — On the Drying up of Menkes on the Groxuih of Trees. 



We have latterly been considering the effect of arborescent v^eta- 

 tion in preserving the humidity of the soil ; but we had previously 

 been considering the effects of forests in increasiiig the humidity of 

 the atmosphere ; and previously to that we had under consideration 

 the phenomena of vegetation on which meteorological effects of forests 

 affecting humidity of soil and atmosphere depend : moisture being 

 withdrawn &om the soil by the spongioles of the root, transmitted by 

 the trunk to the branches and passed off into the atmosphere by the 

 stomates of the leaves. 



It may seem paradoxical to speak of forests both drying up marshes 

 and keeping the soil moist ; but it is not more so than to speak of a 

 man blowing both hot and cold. There is a great difference between 

 moist soil and marsh. AU moisture in excess of what can be retained 

 by shelter and shade and humus in the soil is firee to be otiierwise 



