EVILS Following DE^EtrctiON of f oresI's. 71 



storm ; and we may assume the. occurrence of a succes- 

 sion of very long continued rains to be necessary to 

 accomplish the complete saturation of the bed of humus. 

 Be this as it may, in any case the flowing sheet of water 

 on the surface of the ground is diminished by an immense 

 body of water absorbed arid retained by the soil of the 

 forest; and the unnumbered difficulties which it en- 

 counters in flowing over the surface, come to complete 

 the first beneficial effect of the forest.* 



' Under the humus we come upon vegetable soil pro- 

 ceeding from the disintegration of the rocks, under the 

 combined action of water and the atmosphere. We cannot 

 enter here on the consideration of the mechanical opera- 

 tions and chemical actions which occasion and expedite 

 this destruction of the rocks, but we may look at the 

 influence of the physical properties exercised by the vege- 

 table soil of the forests. This matter has been treated 

 thoroughly by M. Marchand, Sub-Inspector of Forests, in 

 the first chapter of his interesting study, Sur les Torrents 

 des Alps. The experiments of Thurmann which he reports 

 show that the absorption of water by earth is proportional 

 to their condition of sub-division, and as the roots of trees 

 have evidently the effect of breaking up by division ad 

 infinitum, and in every way, the layers which they traverse, 

 it naturally follows that the vegetable soil of forests is 

 eminently hydroscopic. But in addition to the augmented 

 hydroscopicity imparted to the vegetable ■soil by the" 

 ultimate ramification of ligneous roots ; this endows it in 

 sub-dividing it with a qonsiderable permeability. This 

 latter property is one which must not be confounded with 

 the former, as the ene is the faculty of absorbing and 

 retaining water, while the other is the faculty of allowing 

 it to infiltrate and pass beyond itself, or be retained in 



* The {allowing statement by M. Cezanne may give some idea ot the importance o{ 

 this power of absorption ; — • The earth of forests with a density equal to 1-225 retains 

 1'99 of its weight of water (Gasparin) ; it follows that if there be a layer of 10 centi- 

 metres — i inches — of earth saturated and swollen by moisture, it might retain a rain* 

 sheet of 21 centimetres, or half of the rain which falls in a year in Faris.'— 'iSuite a 

 L'Mude mr Us Tarrentf <Im aaMe Alps. 2 Ed. T. ii. p. 177. 



