68 LITERATURE ON TORRENTS. 



other visible changes, as well in the superincumbent rocks as in those that 

 were of an older formation. It is probable that the more a climate was 

 warm, and approached that of the tropics, the greater would be the evapor- 

 ation, and the quantity of rain; greater also would.be the intensity of 

 power of certain meteoric agents ; consequently, according to this hypothesis, 

 the different deposits ought to present indications of the influence of such 

 climates, more marked in proportion as the epoch in which they were formed 

 was more remote from the present. If rains, like to those of the tropics, have 

 fallen on high mountains such as the Alps — even supposing that many of them 

 had an elevation less than that of these — these rains would produce effects 

 very different from those which we see now in the same countries ; one may 

 see that these would form aU at once torrents of which the actual inhabi- 

 tants of these mountains have no idea ; such volumes of water would sweep 

 away quantities of detritus far greater than those which the actual torrents 

 of the Alps carry away, the volume of which, however, is pretty considerable. 

 " Thus, though admitting the correctness of this hypothesis in this, it is 

 necessary always to take into account the differences produced on the sur- 

 face of the earth by the action of meteoric agents, the which is more power- 

 ful as the climate is more warm. One ought especially to give attention to 

 this, when from the observation of a series of the layers of the same district 

 it appears evident that the temperature, under the influence of which they 

 were formed, has gradually diminished. Let us examine now to what 

 degree vegetation can, in warm climates, counterbalance the power of dis- 

 integration, and transport which atmospheric agents possess. It appears 

 that, all other circumstances being equal, the more warm a climate is, the 

 more vigorous is the vegetation which it produces. The question then 

 comes to this : Does the vegetation protect the soil against the destructive 

 action of the atmosphere ? It is impossible to answer this otherwise than 

 in the affirmative. If we want proofs of this fact we shall find them in the 

 artifical moimds, or harrows, which are so common in many parts of England ; 

 they had been exposed in that climate to the action of the atmosphere for 

 about 2000 years; and yet they have not undergone, in their form, any 

 perceptible change, although they have, during at least a considerable 

 portion of that time, only been covered by a light layer of turf. If now it 

 is admitted that the vegetation protects, to some extent, the ground which 

 it covers, it follows that the stronger the vegetation is the more efficacious 

 is the protection which it affords, and as a consequence the ground is always 

 defended from the destructive action of the atmosphere in proportion to the 

 need it has of such protection. Without this providential law of nature, 

 the softer rocks of tropical regions would be speedily carried away by the 

 waters, and the soil woiild no longer be able to sustain vegetables or 

 animals ; for, although in many tropical regions we meet with vast extents 

 of land which present the appearance of sterile deserts, but which one sees 

 suddenly start to life after two or three days' rain, and cover themselves, as 

 by enchantment, with a beautiful verdure, we should bear in mind that 

 the roots of the briskly vivacious plants from which moisture causes to 

 be produced so vigorous a vegetation— and even those of the annual plants 

 which have passed away, of which the seeds produce leaves so verdant— 

 interlace themselves in such a way in the soil that they oppose a consider- 

 able resistance to the destructive power of rain. In the Savannahs of 

 America it is frequently the case that there is little vegetation, and there 

 they experience considerable disintegration. 



