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Show 432 EARTHQUAKE IN CALABRIA, A•D• 1783, the succeeding earthquake will be unable to exert its full power. Barriers must be worn through and swept away, and steep or overhanging cliffs again left without support, before another shock can take effect in the same manner. If a single convulsion be too 'violent, and agitate at once an entire hydrographical basin, o: i.f the shocks .follow ea~h. other too rapidly, the previously-ex1stmg valleys Will be anmlul.ated, instead of being modified and enlarged. Every stream ~1ll be mpelled to begin its operations anew, and to open for Itself a co . d f . passage through strata before undisturbed, ~nstea o contmu-ing to deepen and widen channel~ already ~n great p~rt excavated. On the other hand, if, consistently With all that IS known from observation of the laws which regulate. subte.rranean movements, we consider their action to have been mtermittentif sufficient periods have always intervened between the severer shocks to allow the drainage of the country to be nearly restored to its original state, then are both the kind and degree of force supplied which may enable running water to hollow out a val~ey of any depth and size consi~ten.t w~th the ~egree of elevatiOn above the sea which the district m questiOn may happen at any time to have attained during a succession of physical revo-lutions. NothwithstandinO' the O'reat derangement caused by violent 0 b • • earthquakes, there is an evident tendency ~n runmng water to remain constant to the same connected series of valleys. The softening of the soil is invariably greatest i~ the chann~ls of rivera and in alluvial plains. The water 1s a?sorbed m .an infinite number of rents, and when the ground IS swelled With water it is reduced almost to a state of mud by the vehement agitation of the ground in every direction, and oft~n for s~veral years consecutively. The erosive and transportmg actiOn of running water · is, therefore, facilitated in the tracts already excavated. When we read of the drying up and desertion of tl~e channels of rivers the accounts most frequently refer to the1r deflection into so~e other part of the same alluvial plain, perhaps several miles distant. Under certain circumstances a change of level may undoubtedly force the water to flow over into some distinct hydrographical basin; but even then it will fall i~mediately into valleys already formed. Pro .. EXCAVATION OF VALLEYS. 433 vided, ~heref?re, we suppose the elevation and subsidence of ~ountai~-.chams to be a. gradual process, there is no difficulty m explaim.ng h?w the r1vers draining our continents have converted ravmes mto valleys, and enlarged and deepened valleys to an enor~ous extent: 0~ the contrary, the signs of slow and gradual actiOn so mamfest m the sinuosities and other characters of valleys are admir~bly r~concileable with the great width and depth of the exc~vahons, If we are content not only to suppose a great successiOn of ordinary earthquakes but 1 th l . 1 f . , a so e usua mterva s o time between the shocks. . We may observe that earthquakes alone could never O'ive r1se to ~ regular ~ystem of valleys ramifying from a ~ain trunk hke the vems from the great arteries of the human body. On the contrary, they would, in the course of time, destroy .every system of valleys on the globe, were it not for the a~ency of aqueous causes.. We learn from history that ever smce th: first Greek colomsts, the Bruttii, settled in Calabria, that regiOn has been subject to devastation by earthquakes and fo.r the last century and a half, ten years have seldom el~pscd Without a shock; but the severer convulsions have not only been separated by intervals of twenty, fifty, or one hundred years, but have not affected precisely the same points when they rec.ur~ed. Thus the earthquak~ of 1783, although confined Within the same geographical limits as that of l(j38 and not very inferior in violence, visited, accordinO' to Grimaldi' very differen~ localiti~s. Th.e points where the local intensit; o.f th~ force Is developed, bemg thus perpetually varied, more time Is allowed for the removal of separate mountain masses thrown into rivet· channels by each shock. When chasms and deep h?llows open at the bottom of valleys, they must often be filled w1th those "mud lavas'' before descri? ed; and these must be extremely analogous to the enormous ancient deposits of mud which are seen in many countries as in !he basin of the Tay, Isla, and North Esk rivers, for ex:mple, In S~otland-alluvions hundreds of feet thick, which are neither stratified nor laminated like the sediment which subsides from water. Whenever a landslip blocks up a river, these currents of ~ud will be arrested, and accumulate to an enormous depth. 1 'I he transport~tion for several miles at a time, of masses as arge as great edifices by the momentum of these floqds of mud Vot. I. 2 F |