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Show 308 EOCENE PERIOD. [Ch. XXII. chalk in the counties of 'Vilts and Dorset, that, at. no great d1. stance f rom tl1 es e small elliptical valleys o. f elevatwn, there occur severa1 1 onl:Ol 'er and ]ar(lJ:l 'er valleys, formmg deep notches, · · t11as 1t were, 111 e lofty ed(oJ 'e of the chalk. These are of similar struct ure t o tlle smaller valleys we have been consideri.n g, and consist of green-sand, inclosed by chalk at ~ne extremity, a~d flanked by two escarpments of the same, facmg each other with an oppos1' te dip., but they diffe. r in .t he circumstance. of their other and broader extremity bemg without any such mclosure, ancl gradually widening till it is lost in the expanse of the adjacent country. The cases I now allude to are the Vale of Pewsey, to the east of Devizes, that of the vVily, to the east of Warminster, and the valley of the N adder, extending from Shaftsbury to Barford, near Salisbury; in which last not only the strata of rrreen-sand are brought to the surface, but also the still lower formations of Purbeck and Portland beds, and of Kimmeridge cla~ . It might at first sight appear that these valleys are nothmg more than simple valleys of denudation; but the fact of the strata composing their escarpments having an opposite and outward dip from the axis of the valley, and this often at a high ang1 e , as near Fonthill and Barford, i.n the Va.l e o.f the Nadder, and at Oare, near the base of Martmsell Hill, m the Vale of Pewsey, obliges us to refer their inclination to s.ome anteccde~t violence, analogous to that to which I have attnbuted the position of the strata in the inclosed valleys near Kingsclere, Ham, and Burbage. Nor is it probable that, without some pre· existing fracture or opening in the lofty line of the great chalk escarpment, which is here presented to the north-west, the power of water alone would have forced ~pen three such. de~p valleys as those in question, without causmg them to mam~am a more equable breadth, instead of narrowing till they end m a point in the body of the chalk 'X< . · , Rise and denudation of the secondm·y rocks gradual.-T.o "' Dr. Buckland, Geol. Trans., 2nd St!l·ies, vol. ii. P· 123. Ch. XXII.] ORIGIN OF TERTIARY STRATA. 309 return to the valley of the Weald, the strata of the North Downs are inclined to the north, at an angle of from 10o to 15°, and in the narrow ridge of the Hog's back, west of Guildford, in Surrey, about 45o; those in the South Downs dip to the south at a slight angle. It is superfluous to dwell 011 the analogy which in this respect the two escarpments bear to those which :flank the valleys above alluded to; and in regard to the greater distance which separates the hills of Surrey from those of Sussex, the difficulty may be reduced simply to a question of time. If the rise of the land and its de(J'radation 0 by aqueous causes was accomplished by an indefinite number of minor convulsions, during an immense lapse of ages, we behold in the ocean a power fully adequate to perform the work of demolition. If, on the other hand, we embrace the hypothesis of paroxysmal elevation, or, in other words, suppose a sub~ marine tract to have been converted instantaneously into high land, we may seek in vain for any known cause capable of sweeping away even those portions of chalk and other rocks which, all are agreed, must once have formed the prolongation of the existing escarpments. It is common in such cases to call in one arbitrary hypothesis to support another, and as the upheaving force operated with sudden violence, so a vast diluvial wave is introduced to carry away, with almost equal celerity, the mountain mass of strata assumed to have been stripped off. Materials ofthe te·rtia'ry strata whence derived.-If, then, we conclude that the wreck of the denuded district was removed gradually, it follows that it was deposited by degrees elsewhere. If any part of the sea immediately adjacent to the district which was then emerging, was of considerable depth, the drift matter would be consigned to that submarine region, since every current charged with sediment must purge itself in the first deep ca~ity which it traverses, as does a turbid river in a lake. Suppose that while the wave sand currents were excavating the longitudinal valleys, D and C (No. 81, p. 310), the deposits a were thrown down to the bottom of the contiguous <Jeep |