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Show 310 ,EOCENE PERIOD. [Ch, XXII. No. 81. ~I water E, the sediment being drifted through transverse fissures, as before explained. In this case, the rise of the formations Nos. 1, ~' 3, 4, 5, may have been going on contemporaneously with the excavation of the valleys C and D, and with the accumulation of the strata a. There must be innumerable points on our own coast where the sea is of great depth near to islands and cliffs now exposed to rapid waste, and in all these the denuding and reproductive processes must be going on in the immediate proximity of each other. Such may have been the case during the rise of the Valley of the Weald, and the deposition of the beds of the London and Hampshire basins. The theory above proposed requires that the deposits a should be composed, for the most part, of a mixture of such mineral ingredients as would result from the degradation of the secondary groups, Nos. l, 2, 3, 4, 5. Now the tertiary strata answer extremely well to these conditions. They consist, as we have before seen, of alternations of variously-coloured sands and clays, as do the secondary strata from the group No. 5 to No. 2 inclusive, the principal difference being, that the latter are more consolidated. If it be asked, where do we find the ruins of the white chalk among our Eocene strata? 'Ve reply, that the flint pebbles which are associated in such immense abundance with the sands of the plastic clay, are derived evidently from the destruction of chalk; and as to the soft white calcareous matrix, we may suppose it to have been reduced easily to fine sediment, and to have contributed, when in n state of perfect solution, to form the shells of Eocene testacea; or when mixed with the waste of the argillaceous groups, Nos. 2 and 4, which have been pecu· liarly exposed to denudation, it may have entered into. the composition · of the London clay, which contains no slJght Ch. XXII.) FURROWS ON THE CHALK HOW CAUSED. 311 proportion of calcareous matter. In the crag of Norfolk, undoubtedly, we find great heaps of broken pieces of white chalk, with slightly-worn and angular flints; but in this case we may infer that the attrition was not continued for a Jon time; whereas the large accumulations of perfectly-roUe~ shingle, which are interstratified with our Eocene formations, proves that they were acted upon for a protracted period by the waves. We have many opportunities of witnessino- the entire demolition of the chalk on our southern coast, as at Seaford, for example, in Sussex, where large masses are, year after year~ detac~ed from the cliffs, and . soon disappear, leaving nothmg behmd but a great bank of flmt shingle*. Valleys and furrows in the chalk how caused.-The furrows which occur on the surface of the chalk, filled with sand and pebbles of the plastic clay, may be easily explained if we suppose the English Eocene strata to have been formed during a period of local convulsion. For if portions of the secondary rocks emerged from the sea in the south-east of our island during that period, it is probable that the chalk underwent many oscillations of level, and that certain tracts became land and then sea, and then land again, so that parts of the surface, first excavated by currents or rivers, were occasionally submerged, and, after being covered by tertiary deposits, upraised again. We must also remember, that almost every part of the chalk must have been exposed for some time to the action of the wav. es, if we assume the elevation to have been slow and b y successive movements. The valleys seen everywhere on the surface, and the layers of partially-rolled and broken flints which very generally oV'erspread it, may be referred to the sea breaking upon the reefs and shoals when the rocks were about to emerge. We apprehend, indeed, that no formidable difficulty will be encountered in explaining the position of the tertiary sand which sometimes fills rents and furrows in the chalk, ~r the occurrence of banks of shingle at the junction of the tertiary strata and the chalk, if we once admit that the * Vol. i. p. 279, aD.d ~econd Edition, p. 319. |