90 EFFECTS OP FORESTS ON MARSHES. 



because it is the effect on the soil of the meteorological effects of 

 forests which chiefly concerns us at present in the discussion of these. 



Sect. IV. — On the Desiccating Effects produced on Marshes hy Forests 

 in Prolonged Periods. 



Reference is made by Marsh to a recuperative operation, whereby 

 even the bog produced by the destruction of a forest regains, after a 

 time, its former aspect, and in this the desiccating effect of vegetation 

 comes again into play, pi'omotes the work, and expedites the result. 

 In Denmark we see what may thus be accomplished. We there 

 meet with a land, which, at one or more periods of its existence as 

 land, must have been over a great extent of its surface covered by 

 morasses and marshes, and peat bogs, but which is now so dry as to 

 be habitable, and cultivated by man, giving him in return for his 

 labours fields of increase ; and the peat bogs supply extensive 

 indications that forests must have played an important part in 

 producing the change. 



From Bogens Indrandring i de BansJce Skove, by Chr. Vaupell, we 

 learn that a careful examination of the peat mosses in North 

 Sjselland, which are so abundant in fossil wood that within thirty 

 years they have yielded above a million of trees, shows that these 

 trees have generally fallen from age and not from wind. They are 

 found in depressions on the declivities of which they grew, and 

 they lie with the top lowest, always falling towards the bottom of 

 the valley. 



The origin of these bogs dates from pre-historic times ; but the 

 origin of other bogs has been studied, and the result has been given 

 in part in the preceding section. 



Marsh, in his valuable treatise on " The Earth as Modified by 

 Human Action," which I have already cited, says, " Bogs generally 

 originate in the checking of water-courses by the falling of timber or 

 of earth and rocks, or by artificial obstructions across their channels. 

 If the impediment is sufficient to retain a permanent accumulation 

 of water behind it, the trees whose roots are overflowed soon perish, 

 and then by their fall increase the obstruction, and, of course, 

 occasion a still wider spread of the stagnating stream. This process 

 goes on until the water finds a new outlet, at a higher level, not liable 

 to similar interruption. The fallen trees not completely covered by 

 water are soon overgrown with mosses ; aquatic and semi-aquatic 

 plants propagate themselves and spread till they more or less com- 



