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Show 320 EOCENE PERIOD. [Ch. XXII. if it could be assumed that there were ancient causes differing from those which arc now in operation. But if we substitute the phrase, existing causes, we shall £nd that the argument now controverted amounts to little more than this, ' that in a country free from subtet·ranean movements, the action of running water is so trifling that it could never hollow out, in any lapse of ages, a deep system of valleys, and, therefore, no known combination of existing causes could ever have given rise to our present valleys!' The advocates of these doctrines, in their anxiety to point out the supposed absurdity of attributing to ordinary causes those inequalities of hill and dale, which now diversify the earth's surface, have too often kept entirely out of view the many recorded examples of elevations and subsidences of land during earthquakes, the frequent fissuring of mountains, and opening of chasms, the damming up of rivers by landslips, the deflection of streams from their original courses, and more important, perhaps, than all these, the denuding power of the ocean, during the rise of our continents from the deep. Few of the ordinary causes of change, whether igneous or aqueous, can be observed to act with their full intensity in any one place at the same time; hence it is easy to persuade those who have not reflected long and profoundly on the working of the numerous igneous and aqueous agents, that they are entirely inadequate to bring about any important fluctuations in the configuration of the earth's surface. Recapitulation.-We shall now briefly recapitulate the conclusions to which we have arrived respecting the geology of the south-east of England, in reference to the nature and origin of the Eocene formations considered in this and the two pre· ceding chapters. 1. In the first place, it appears that the tertiary strata rest exclusively upon the chalk, and consist, with some trifling exe ceptions, of alternations of clay and sand. . . 2. The organic remains agree with those of the Paris basin, but the mine·ral character of the deposit is extremely different, Ch.XXII.] RECAPITULATION. 321 those rocks in particular, which are common to the Paris basin and Central France, being wanting, or extremely rare, in the English tertiary formations. 3. The Eocene deposits of England are generally conformable to the chalk, being horizontal where the beds of chalk are horizontal, and vertical where they are vertical; so that both series of rocks appear to have participated in nearly the same movements. 4. It is not possible to define the limits of the ancient borders of the tertiary sea in the south-east of England, in the same manner as can be frequently done in those countries where the secondary rocks are unconformable to the tertiary. 5. Although the tertiary deposits are chiefly confined to the tracts called the basins of London and Hampshire, insulated patches of them are, nevertheless, found on some of the highest summits of the chalk intervening between these basins. 6. These outliers, however, do not necessarily prove that the great mass of tertiary strata was once continuous between the basins of London and - Hampshire, and over other parts of the south-east of England now occupied by secondary rocks. 7. On the contrary, it is probable that these secondary districts were gradually elevated and denuded when the basins of London and Hampshire were still submarine, and while they were gradually becoming filled up with tertiary sand and clay. 8. If, in illustration of this theory, we examine one of the districts thus supposed to have been denuded, we find in the Valley of the Weald decided proofs, that since the emergence of the secondary rocks, an immense mass of chalk and subjacent formations has been removed by the force of water. 9. Vv e infer from the existence of large valleys along the outcrop of the softer beds, and of parallel chains of hills where harder rocks come up to the surface, that water was the removing cause; and from the shape of the escarpments presented by the harder rocks, and the distribution of alluvium over different parts of the surface of the Weald district, we VoL. III. y |