OCR Text |
Show 47:t RENOVATING EFFECTS OF EARTHQUAKES. in 1751, or . Jamaica 'in 169fl, may have compensated by increasing the capacity of the great oceanic basin. No river can push forward its delta without raising the level of the whole ocean, although in an infinitesimal degree; and no lowering can take place in the bed of any part of th~ ocean, without a general sinking of the water, even to the antipodes. If the separate effects of different agents, whethet· aqueous or ]. crneous are insensible, it is because they are continually coun-o ' teracted by each other, and a perfect adjustment takes place before any appreciable disturbance is occasioned. How .many considerable earthquakes there may be upon an average m the course of one year, throughout the whole globe, is a question that we cannot decide at present ; but as we have calculated that there are about twenty volcanic eruptions annually, we shall, perhaps, not overrate the earthquakes, if we ~stimate their number to be equal. A large number of eruptions are attended by local earthquakes of sufficient violence to modify the surface in some slight degree, and there are many earthquakes on the other hand, not followed by eruptions. Even if we do ~ot assume, as many have done, that the submarine convulsions exceed in number and violence those on the land, in spaces of equal area, we nm~t, nevertheless, rec~on about three shocks exclusively submarme, for one exclusively confined to the continents. We have said in a former chapter >K• that the aqueous and igneous agents may be regarded as antagoni~t fore~~' the aqueous labouring incessantly to reduce the mequahties of the earth's surface to a level, while the igneous are equally active in restoring the unevenness of the crust of the globe. But an erroneous theory appears to have been entertained by many geologists, and is indeed as old ~s the time of Lazzoro Moro, that the levelling power of runnmg water was oppose.d rather to the elevating force of earthquakes than to the1r action general1y. To such an opinion the numerous wellattested facts of subsidences must always have appeared a serious objection, but the same hypothesis would. lea~ to other assumptions of a very arbitrary and .improbable k1~d, masmuch as it would be necessary to imagme the magmtude of our * Chap. x. p. 167. PROPORTION OF ELEVATION AND SUBSIDENCE. i!75 planet to be always on th · . ·r .. h, e Increase 1 the elevation of tne eart s . surface, by s.u bt erranean movements exceeded the clepr~ssiOn.. 'I he sediment carried into the depths of the sea by rivers, tides, and currents, tends to diminish the height of the land; but, . on the other hand , I.t t en d s, m. a d egree, to augment the he1ght. of the ocean , si'nce wa t er, eq ua1 m. vo 1u me to the matter carried in ' is displaced . The mean dI' stance, therefore, of the surface, whether occupied by land or water from the centre of the earth, remains unchanged b th . t' f . 'd d y e ac Ion o rivers, ti es, an. c~1rrents. Now suppose that while these agents are destroymg Islands and continents, the restoration of land should take place solely by the forcing out of the e th' 1 . 'll ar s enve ope--It WI be seen that this would imply a contin 1 distension of the whole mass of the earth. For the grea~:r num her of earthquakes would be submarine, and they would cause the sea to rise and submerge the low lands even in a greater degree than would the influx of sediment. 'l.'wo causes would, therefore, tend to destroy the land; submarine earth· quakes, and the destroying and transporting power of water. and in order to counterbalance these effects, shallow seas mus; be upraised into continents, and low lands into mountains. If we first consider the question simply, in regard to the manner whereby earthquakes may prevent running water from altering the relative proportion of land and sea, or the height of the land and depth of the ocean, we shall find that if the rising and sinking be equal, things would remain upon the whole in the same · state : because rivers, tides, and currents, add as much to the height of lands which are rising, as they take from those which have risen. Suppose a large river to carry down sediment into a certain part of the ocean where there is a depth of two thousand feet, and that the whole space is reduced by the fluviatile depositions to a shoal only covered by water at high tide: then let a series of two hundred earthquakes strike the shoal, each raising the ground ten feet; the result will be a mountain two thousand feet high. But suppose the same earthquakes had visited the same hol1ow in the bottom of the sea before the sediment of the river had filled it up, their whole force would then have been expended in converting a deep sea into a shoal, instead of changing a shoal into a mountain two thousand feet |