OCR Text |
Show 318 EOCI<:NE PERIOD. [Ch. XXII. alluviums which cover the surface of Scotland, a country which probably became land long before the commencement of the tertiary epochs. Elevation of land gradual.-As we have assumed, throughout this and the preceding chapter, that the elevatory force was developed in a succession of minor convulsions in the southeast of England, we may seem called upon to answer an objection which has been drawn from the verticality of the strata in the Isles of Wight and Purbeck. Mr. Conybeare has remarked, that the vertical strata are traced through a district near] y 60 miles in length, so that 'if their present position were the effect of a single convulsion, no disturbance in the least comparable with it has occurred in modern times *.' As we can by no means dissent from this proposition, we only ask where is the evidence that a single effort of the subterranean force, rather than reiterated movements, produced that sharp flexure of which we suppose the vertical strata of the Isle of Wight to form a part, the remainder of the arc having been carried away by denudation. It appeat·s extremely probable that the Cutch earthquake of ] 819, so often alluded to by us t, may have produced an incipient curve, running in a linear direction through a tract at least 60 miles in length. The strata were upraised in the Ullah Bund, and depressed below the level of the sea in the adjoining tract, where the fort of Sindree was submerged. It would be impossible, if the next earthquake should raise the Bund still higher, and sink to a lower depth the adjoining tract, to discriminate, by any geological investigations, the different effects of the two earthquakes, unless a minute survey of the effects of the first shock had been made and put on record. In this manner we may suppose the strata to be bent, again and again, in the course of future ages, until parts of them become perpendicular. To some it may appear, that there is a unity of effect in the * Phil. Mag. and Annals, No. 49, new Series, p. 21. f Vol. i.1 Second Edition, p. 465, and vol. ii.1 First Edition1 P· 265, Ch. XXII.] EXCAVATION OF VALLEYS. 319 Jine of deranged strata in the isles of Wight and Purbeck, as also in the central axis of the Weald, which is inconsistent with the supposition of a great number of separate movements recurring after long intervals of time. But we know that earthquakes are repeated throu!!hout a lonO' series of aooes in ...... 0 0 ' the same spots, like volcanic eruptions. The oldest lavas of Etna were poured out many thousand, perhaps myriads, of years, before the newest, and yet they have produced a symmetrical mountain; and if rivers of melted matter thus continue to flow in the same direction, and towards the same points, for an indefinite lapse of ages, what difficulty is there in conceiving that the subterranean volcanic force, occasioning the rise or fall of certain parts of the earth's crust, may, by reiterated movements, produce the most perfect unity of result? Exca·vation of Valleys.-In our attempt to explain the origin of the existing valleys in the south-east of England, it will be seen that we refer their excavation chiefly to the ocean. We are aware that we cannot generalize these views and apply them to the valleys of all parts of the world. In Central France, for example, rivers and land-floods, co-operating with earthquakes, have deeply intersected the lacustrine and volcanic deposits, and have hollowed out valleys as deep, perhaps, as any in our Weald district. In what manner these effects may be brought about in the course of time, by the action of running water, even without the intervention of the sea, may be understood by what we have said of the removal of rock by aqueous agency during the Calabrian earthquakes*. Those geologists who contend that the valleys in England are not due to what they term r modern causes,' are in the habit of appealing to the fact, that the rivers in the interior of England are working no sensible alterations, and could never, in their present state, not even in millions of years, excavate the valleys through which they now flow. We suspect that a false theory is involved even in the term ' modern causes,' as "' Vol.i. chap. xxiv. |