EFFECTS OF FORESTS ON SPBINGS. 171 



sion and thp diminution of the woods where they originate and 

 through which they run, is as well established as any proposition is 

 in the science of physical geography. 



" Of the converse proposition, namely, that the planting of new 

 forests gives rise to new springs, and restores the regular flow of 

 rivers, I find less of positive proof, however probable it may be, that 

 such effects would follow. A reason for the want of evidence on the 

 subject may be, that, under ordinary circumstances, the process of 

 conversion of bare ground to soil with a weU-wooded surface is so 

 gradual and slow, and the time required for a fair experiment is con- 

 sequently so long, that many changes produced by the action of the 

 new geographical element escape the notice and the memory of 

 ordinary observers. The growth of a forest, including the formation 

 of a thick stratum of vegetable mould beneath it, is the work of a 

 generation, its destruction may be accomplished in a day ; and hence, 

 while the results of one process may, for a considerable time, be 

 doubtful, if not imperceptible, those of the other are immediately 

 and readily appreciable. Fortunately, the plantation of a wood pror 

 duces other beneficial consequences, which are both sooner realised 

 and more easily estimated ; and though he who drops the seed is 

 sowing for a future generation as well as for his own, the planter of a 

 grove may hope himself to reap a fair return for his expenditure and 

 his labour." 



We do not always find all facts in manifest accordance with con- 

 clusions towards which we may be advancing, or to which we may 

 previously have come ; and though every fact must necessarily be in 

 accordance with every other fact we may often find it necessary to 

 take into account conflicting influences, the eiFect of which must be 

 determined ; and to the preceding statement Mr Marsh has appended 

 the following note : — 



" Some years ago it was popularly believed that the volume ot the 

 Mississippi, like that of the Volga and other rivers of the Eastern 

 Hemisphere, was diminished by the increased evaporation from its 

 basin and the drying up of the springs in consequence of the felling of 

 the forests in the vicinity of the sources of its eastern affluents. The 

 boatmen of this great river and other intelligent observers now assure 

 us, however, that the mean and normal level of the Mississippi has 

 risen within a few years, and that in consequence the river is navi- 

 gable at low water for boats of greater draught and at higher points 

 in its course than was the case twenty-five years ago. 



" This supposed increase of volume has been attributed by some to 



