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Show 314 EOCENg PERIOD. [Ch. XXII, which have been removed by denudation, that hill, instead of rising to the height of 800 feet, would be more than trebled in altitude *, and be about 2700 feet high. It would then tower far above the highest outlyers of tertiary strata which are scattered over our chalk, for Inkpen Hill, the greatest elevation of chalk in England, rises ouly lOll feet above the level of the sea. Some geologists who have thought it necessary to suppose all the strata of the London and Hampshire basins to have been once continuous, have estimated the united thickness of the three marine Eocene groups before described, as amounting to 1300 feet, and have been bold enough to imagine a mass of this height to have been once superimposed upon the chalk which formerly covered the axis of the Weald t. Hence they were led to infer that Crowborough Hill was once 4000 feet high, and was then cut down from 4000 to 800 feet by diluvial action. We, on the contrary, deem it wholly unnecessary to suppose any removal of rocks newer than the secondary from the central parts of the valley of the Weald ; and we suppose the waste of the older rocks to have been caused gradually during the emergence of the country. The small strips of land which were first protruded in an open sea above the level of the waves, may have been entirely carried away, again and again, in the intervals between successive movements, until at last a great number of reefs and islands rising at once, afforded protection to each other against the attacks of the waves, and the lands began to increase. 'Ve do not conceive, therefore, that a mountain ridge first rose to the height of more than 2000 feet, and was then lowered to less than half that elevation ; but that a stratified mass, more than 2000 feet thick, was, by the con· tinual stripping off of the uppermost beds as they rose, diminished to a thickness of about 800 feet. It is not our intention, at present, to point out the applica- "' Phil. Mag. and Annals, No. 26, New Series, p. 117. t Martin, ibid, Cb. XXII.] VERTICAL STRATA OF ISLE OF WIGHT, 315 tion _of the above theory to the region immediately westward of the great line of chalk escarpment which runs through the central parts of England from Dorsetshire to Cambridgeshire. The denudation in that country has doubtless been on a great scale, and was, perhaps, effected during the Eocene period ; for we know of no reason why one line of movements should not have been in progress in a direction north-east and southwest, while others were heaving up the strata in lines running east and west. We may remark, at the same time, that if the chalk in the interior of England, in those tracts from which it has been extensively swept away, began to rise during the tertiary epoch, and before the emergence of the chalk which once extended over the central axis of the 'Veald, some tertiary deposits may, in that case, have been thrown down upon that central ridge. We have at present, however, no data to lend countenance to such conjectures. Vertical st·rata of the Isle of Wiyht.-A line of vertical and inclined strata running east and west, or parallel to the central axis of the Weald, extends through the isles of 'Vight and Purbeck, and through Dorsetshire, and has been observed by Dr. Fitton to reappear in France, north of Dieppe. The same strata which are elevated in the Weald Valley are upheaved also on this line in the centre of the Isle of Wiaht 0 t where all the tertiary strata appear to have partaken in the same movement*, From the horizontality of the fresh-water series in Alum Bay, as contrasted with the vertical position of the marine tertiary beds, Mr. Webster was at first led very naturally to conclude, that the latter had undergone great derangement before the deposition of the former. It appears, however, from the subsequent observations of Professor Sedgwick t, that these appearances are deceptive, and that at the eastern • See Mr · Webster's section, Geol. Trans., vol. ii. First Series, plate XI. t Anniv. Address to the Geol. Soc., Feb. 1831, p. 9. Professor Sedgwick in· forms me that his observations, made six years ago, have recently been confirmed by Professor Henslow. |
