OCR Text |
Show 312 EOCENE PERIOD· [Ch. XXII. Eocene deposits originated while the chal~ and other secondary 1·ocks were rising from the deep and wastmg away. Earthquakes dwring the Eocene period.-We have pointed out in a former chapter, our reasons for concluding that the Par'i s basin was a theatre of subterranean convul s1. ons during the Eocene period, the older beds of the calcaire grossier having been raised and exposed to the action of the waves before, or at least during, the deposition of the upper or second marine group *. These convulsions were doubtless connected with that depression which let in the sea upon the second freshwater formation, and gave rise to the superposition of the upper marine beds. We have also demonstrated, in a preceding chapter, that some of the earlier volcanic eruptions in Auvergne happened before the Eocene species of animals were extinct, and we suggested that the great lakes of Central France may have been drained by alterations of level which accompanied the outbreak of those earlier Eocene volcanos of Auvergne. We ought not, therefore, to be surprised if we discover proofs that the south-east of England participated in the earthquakes which seem to have extended at that time over a considerable part of the neighbouring continent; and we may refer the alternation of marine and fresh-water beds in the Isle of Wight and coast of Hampshire, to changes of level analogous to those which gave rise to the intercalation of the upper marine formation in the Paris basin. Why the English Eocene st·rata rise nearly as high as the denuded secondary dist·ricts.- Those geologists who have hitherto regarded the rise and denudation of the lands in the south-east of England as events posterior in date to the deposition of the London clay, will object to the foregoing reasoning, that not only certain outlying patches of tertiary strata, but even the central parts of the London and Hampshire basins, attain very considerable altitudes above the level of the sea. Thus the London clay at Highbeach, in Essex, reaches the height of 750 feet, an elevation exceeding that * Sre above, p. 248. Ch. XXII.] AMOUNT OF DENUDATION. 313 of large districts of the chalk and other denuded secondary rocks. But these facts do not, we think, militate .against the theory above proposed, for we have assumed a long-continued series of elevatory movements in a region where the degradation and reproduction of strata were in progress. If this be granted, it is evident that the great antagonist powers, the igneous and the aqueous, would, throughout the whole period, be brought into play in their fullest energy, the igneous labouring continually to produce the greatest inequality of surface, by uplifting certain lines of country and depressing others; the aqueous no less incessantly engaged in reducing the whole to a level, by cutting off the summits of the upraised tracts, and throwing the materials thence removed into the adjoining hollows. If the volcanic forces eventually prevail, so as to convert the whole region into land, we must expect that some of the materials drifted into the hollows, and fortnin()' 0 the newer strata, will be brought up to view, while the de-nuded districts are raised at the same time. If these last continue, in general, to occupy a higher position above the level of the sea, it is all that can be expected after the levelling operations before aUuded to. Now the tracts occupied by our Eocene formations are low, not so much with reference to the secondary rocks which remain, as to these masses which must be supposed by our theory to have disappeared, having been carried away by denudation. Let the portions removed from the space intervening between the North and South Downs, and which are expressed by faint lines in our section, wood-cut No. 63, be restored, and we may readily conceive that those masses may have formed shoals and dry land for ages before any part of our tertiary basins emerged. 'I'he estimate of Mr. Martin is not, perhaps, exaggerated, when he computes the probabl~ thickness of strata removed from the highest part of the Forest ridge to be about 1900 feet. So that if we restore to Crowborough Hill, in Sussex, the beds of 'Veald clay, Lower green-sand, Gault, and chalk, |