OCR Text |
Show 198 CONSERVATIVE INFLUENCE OF VEGETATION. (Ch. XII. inferences n·g orous1 y scrutm· t·z ed ,· but we never supposed it possible that our adversaries would set up "as a vast coun-terpoise to all the agents of destruction," a cause so nugatory as " the single operation of vegetable 11'£ e *·" A s 1't WI'1 1 appear from what we bef.o re said, .t hat vegetation cannot act as an antagonist power amtd the mighty agents of change which are always modifying the ~urface of ~he globe, I t · u1et us nex tnq 'r·e how far its influence IS co.n servative,-ho. w far it may retard the levelling power of runnmg water, wh1ch it cannot oppose, much less counterbalance. It is well known that a covering of herbage and shrubs may protect a loose soil from being carried away by rain~ or even by the ordinary action of a river, and may prevent h1lls of loose sand from being blown away by the wind. For the roots bind together the separate particles int? a fi~m mass, and th~ leaves intercept the rain-water, so that 1t dnes up gradually mstead ·of flowing off in a mass and with great velocity. The old Italian hydrographers make frequent mention of the increased degradation which has followed the clearing away of natural woods in several parts of Italy. A ·remarkable example was afforded in the Upper Val d' Arno, in Tuscany, on the removal of the woods clothing the steep declivities of the hills by which that valley is bounded. When the ancient forest laws were abolished by the Grand Duke Joseph, during the last century, a considerable tract of surface in the Cassentina (the Clausentinium of the Romans) was denuded, and, immediately, the quantity of sand and soil washed down into the Arno increased enormously. Frisi, alluding to such occurrences, observes, that as soon as the bushes and plants were removed, the waters flowed off more rapidly, and, in the manner of floods, swept away the vegetable soil t. . '.rhis effect of vegetation is of high interest to the geolog~st, when he is considering the formation of those valleys whlch have been principally due to the action of rivers. The spaces "' Prof. Sedgwick's Anniv. Address, Feb. 1831, P· 24 •. t Treatise on Rivera and. l.'oneuts, p. 6, Ganton's translation. Ch. XII.] CONSERVATIVE INFLUENCE OF VEGETATION. 199 intervening between valleys, whether they be flat or ridgy, when covered with vegetation, may scarcely undergo the slightest waste, as the surface may be protected by the green sward of grass; and this may be renewed, in the manner before described, from elements derived from rain-water and the atmosphere. Hence, while the river is continually bearing down matter in the alluvial plain, and undermining the cliffs on each side of every valley, the height of the intervening rising grounds may remain stationary. In this manner a cone of loose scorire, sand and ashes, such as Monte Nuovo, may, when it has once become densely clothed with herbage and shrubs, suffer scarcely any farther dilapidae tion; and the perfect state of the cones of hundreds of extinct volcanos in France, Campania, Sicily, and elsewhere, may prove nothing whatever, either as to their relative or absolute antiquity. We may be enabled to infer from the integrity of such conical hills of incoherent materials, that no flood can have passed over the countries where they are situated since their formation ; but the atmospheric action alone in spots where there happen to be no torrents, and where the surface was clothed with vegetation, could scarcely in any lapse of ages have destroyed them. During a late tour in Spain I was surprized to see a district of gently undulating ground in Catalonia, consisting of red and grey sandstone, and in some parts of red marl, almost entirely denuded C?f herbage, while the roots of the pines, holm oaks, and some other trees were half exposed, as if the soil had been washed away by a flood. Such is the state of the forests, for example, between Orista and Vich, and near San Lorenzo. Being at length overtaken by a violent thunderstorm, in the month of August, I saw the whole surface, even the highest levels of some flat-topped hills, streaming with mud, while on every declivity the devastation of torrents was terrific. The peculiarities in the physiognomy of the district were at once explained, and I was taught that in speculating on the greater effects which the direct action of rain may once |