OCR Text |
Show 476 THE WHOLE AMOUNT OF SUBSIDENCE high. 'l'he superior altitude, then, of a district may often be due to the transportation of matter at a former period to lower levels. It would probably be more consistent with the natural course of events, if, instead of a succession of clevatory movements, we were to suppose considerable oscillations before the district attained its full height. Let there be, for example, three hundred instead of two hundred shocks, each separated from the other by intervals of about fifty years. Let the mean alteration of level produced by each earthquake be ten feet, two hundred and fifty shocks causing a rise, and the other fifty a sinking in of the ground; although more time will have been consumed by this operation than by the former, we shall still have the same result, for a tract will be raised to the height of about two thousand feet. The chief difference will consist in the superior breadth and depth of the valleys, which will be greater nearly in the proportion of one-third, in consequence of the number of landslips, floods, opening of chasms, and other effects produced by one hundred additional earthquakes. It should be borne in mind, moreover, that some of the lowering movements, happening towards the close of the period of disturbance, may have given rise to strange anomalies, should an attempt be made to reconcile the whole excavation in various hydrographical basins to the levels finally retained. Perhaps, for example, the middle portion of a valley may have sunk down, so that a deep lake may intervene between mountains and certain low plains, to which their debris had been previously carried. But to return to the consideration of the proportion between the elevation and depression of the earth's crust, which may be necessary to preserve the uniformity of the general relations of land and sea, on the surface. The circumstances are in truth more complicated than those before stated, for, independently of the transfer of matter by running water from the continents to the ocean, there is a constant transportation of mineral ingredients from below upwards, by mineral springs and volcanic vents. As mountain masses are in the course of ages created by the pouring forth of successive streams of lava, so others originate from the carbonate of lime and other mineral ingredients with which springs are impregnated. The surface of the land, and parts of the bottom of the ~ea are thus raised, and if we conceive EXCEEDS TIIAT OF ELEVATION. 477 the dimensions of the planet to rem . . pose these external ace . am umform, we must sup. of an opposite kind eAs siOns .t o be counte rae t ed b Y some acti.O n may sink down int~ fis considerable quantity of earthy matter cannot be deemed suffic~ur;st caused by earthquakes, but this mountain masses by the en obc~unterbalancc the addition of causes ewre adverted to d therefore supp?se, that the subsidences of th ' a~ we must ceed the elevations caused by subt e earth s crust ex~ b d erranean movements It is to e cxpccte ' on mechanical principles th t th . traction of matter from the interior :11 a e co~~tant sub~ 1 f. WI cause vacmtles so that t 1e sur ace undermined will fall in durin 1 . ' . 1 k h h, g convu s1ons wh1eh s 1a e t e eart s crust even to ()'reat depths d th . k' '11 b . o 'an e sm 100' down WI e occasiOned partly by the hollows left h l? the solid crust are heaved up and partly wh wthen portiOns of . d b I . ' en ey are under~ mme y t 1e subtraction of lava and the ino- d' t fd d I Th . 0 re 1en s o ecom-pose roc <:s. e geoloo-1eal consequences 1 · 1 '11 e 11 · b b W 11C 1 WI 10 OW If we em race the theory now proposed are ve · 'f 1 b ry Important, for 1 t 1ere e upon the whole more subsidence th 1 · h . an e evation t en we must constder the depth to which .It! ormer sur1ea ces' have sunk down beneath their orio-inal level t d h h · h h · 1 · . o , o excee t e eig t w Ic 1 ancient marme strata have attained above the sea. If, for example, marine strata about the f I age o our chal ( and gree. n-sand have been lifted up i.n E m·ope t o an extreme elevatiOn of more than eleven thousand e t d I . .tee , an to n mean leight of some hun~reds above the level of the sea, w~. 7ay ~ondcludhe t 1 hat certam parts of the earth's surface, w 1c 1 ex1ste w ~t 1er above or below the waters when those strat8: were deposited, have subsequently sunk down to an extreme depth of more than eleven thousand feet b 1 1 . . . II I e ow t 1e1r or1gma eve , and to a mean depth of more than a few hundreds. In regard to ~aults, also, we must, according to the hypothesis now proposed, mfer that a greater number have arisen from the smkmg down than from the elevation of rocks. If we fin~, therefore, ancient deposits full of fresh-water remains wl11ch evidently originated in a delta or shallow estuary, covered subsequently by purely marine formations of vast thickness we shall not b.e .surprised ; for we must expect that a greate; number of extstmg deltas and estuary formations will sink below, than those which will ri~e above their present level, |