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Show ACTION OF RUN' NlNG WATER. 168 · mine the influence tt ropt to exa . 1 dan O'er, therefore, when we a ~ okin()' the modifications .whlC 1 cxet~ed singly by each, of lov~~ :nd cliese are so comphcated, . d uce on one a not 1ei ' s forces co-operate to they pro . l i()'neous and aqueou ided b that somettmes t 1e o l . 1 neither of them una . y pro d uce a J. oint ef. fect,. to wa ns cw 11 1 en repe ated earthquakes umt.e t11 eo ther could glVe rlseid,-enava1 1 cy. Sometimes the orgamcd w1' tll running wate.r to. wm .e causes ., as when a reef, compose 1 d . com bI·n e with the mmgat ts one ]'m e of coast from tIe. estray-f shells and corals, pro .ec . d turns them agamst some o. power of t·la es or current.s , ba n floated into a l ak e, fill mg d 'ft urn er ff1i ·s a otl 1 er. p oint; ho. r hw thheen strrei am wou ld not have had su Cient hollow to w IC h sediment. . velocity to convey eart y to divide our obsei'vatiOns on these It is necessary, however,.f them systematically, endeavour• varJ•O tl S causes, and 'tbol clasksie eyp m. vi.e w that the effects in n.a tu.r e in()' as much as posst . e to he may appear in an artdicta1 arbe m1• xe d , and not simple, as t y . arrangement. a ueous causes, we may cons1~er I n treating, first, of the q h which are connected wlth d. . . s. first, t ose h' h tllem under two IVISlon . 1 1 Ild to the sea, under w JC . f ter from t le a . dl the circulatiOn o wa f rivers and sprmgs; secon y, re included all the phenomena o nts of water in lakes, seas, a . f the moveme f ·a tl Se which arise rom . d tlle phenomena o tJ es 10 h · re compr1se c and the ocean, w erem a. our first attention to the J.Ormer and currents. In turnm~ effects of rivers may be sub-dI. V I.S I.O n , w. e find thfa t t d e troy.m g an d those of a re.n ovat·f dI. v 1' ded into those ho at mese r are m. e luded the erosiOn o 1 · in nature. In t e or. f matter to lower leve s; m rogc k s and the transp. ortatfi ona no d -b ars a nd deltas.' the shallow· the latter, the formatiOn o s in()' of seas, &c. 'b-b We shall begm• , t h en, by descr1 Action of Running Wate·r.- . ower of running water, as . the destroying and tra~sportl~g .P ell known that the lands mgh'b'ted by torrents and nvers. t lS w t'on to their volume ex I 1 t · n propor 1 h · h elevated above the sea attr~c I that a ueous vapour w lC and density a larger quantlt! ofll abso;bs from the surface the heated atmosph;e ~~:st:~:n;, ;he higher redgi?n~ ~:o;: of lakes and seas. y 1 . h descend an Irng perpetua~ reservoirs of water' w nc ACTION OF RUNNING WATER. 169 lowet• valleys and plains. In consequence of this provision, almost all the water is first carried to the highest regions, and is then made to descend by steep declivities towards the sea; so that it acquires superior velocity, and removes a greater quantity of soil than it would do if the rain had been distributed over the low plains and high mountains equally in proportion to their relative areas. Almost all the water is also made by these means to pass over the greatest distances which each region affords, before it can regain the sea. The rocks in the higher regions are particularly exposed to atmospheric influences, to frost, rain, and vapour, and to great annual alternations, of moisture and desiccation-of cold and heat. Among the most powerful agents of decay may be mentioned the mechanical action of water, which possesses the remarkable property of expanding during congelation. '\Vhen water has penetrated into crevices and cavities, it rends open, on freezing, the most solid rocks with the force of a lever, and for this reason, although in cold climates the comparative quantity of rain which falls is very inferior, and although it descends more gradually than in tropical regions, yet the severity of frost, and the greater inequalities of temperature, compensate for this diminished power of degradation, and cause it to proceed with equal, if not greater rapidity than in high latitudes. The solvent power of water also is very great, and acts particular! y on the calcareous and alkaline elements of stone, especially when it holds carbonic acid in solution, which is abundantly supplied to almost every large river by springs, and is collected by rain from the atmosphere. The oxygen of the atmosphere is also graduaJly absorbed by all animal and vegetable productions, and by almost all mineral masses exposed to the open air. It gradu .. ally destroys the equilibrium of the elements of rocks, and tends to reduce into powder, and to render fit for soils, even the hardest aggregates belonging to our globe *. And as it is well known that almost every thing affected by rapid com. bustion may also be affected gradually by the slow absorption of oxygen, the surface of the hardest rocks exposed to the air may be said to be slowly burning a way. When earthy matter has once been intermixed with running • Sir H. Davy, Consolations in Travel, p. 271. |