64:6 LOWER CEAG ON NORTH DOWNS. 



of the crag period, resting on the summit of the North Downs at vari- 

 ous points between Folkestone and Dorking. These ferruginous sands 

 include layers of iron sandstone, and of quartzose sand, with flint peb- 

 bles, and occasionally green earth, the whole deposit resembling pre- 

 cisely in mineral character the sands of Diest, in Belgium, which have 

 long been considered as of the same age as the older crag of Suffolk. 

 The same Terebratula grandis, which abounds in the English crag, and 

 in the sands of Diest ; and the casts of Astarte, Pyrula, and other fos- 

 sils, concur with the mineral character of the beds to prove the con- 

 temporaneous origin of these British and Belgian strata. At Paddles- 

 worth, 4 miles W.N.W. of Folkestone, the irony sands, above mentioned, 

 rest on an older flint gravel, at an elevation of between 600 and 700 

 feet above the sea, and near the edge of the chalk escarpment. Some 

 idea of their exact position may be gained by the reader by supposing 

 them placed on the heights marked by the strong black line above fig. 

 3, in the woodcut 321 (p. 272 of the text of this edition, and 4th edition 

 p. 243), or he may suppose the tertiary outlier b, fig. 329 (p. 282 of this 

 edition), to consist of Coralline crag, instead of being a mass of Eocene 

 clay and sand. 



It follows from such facts, that although the first elevation of the 

 Wealden took place, as shown in the 19th chapter, in the early Eocene, 

 or partly, perhaps, in the cretaceous period ; and although much denu- 

 dation was then effected, yet the same area was again submerged during 

 the Older Pliocene epoch. The latest denudation, therefore, as well as 

 the present escarpments, were brought about after the sea had become 

 already peopled with species of mollusca, half of which are still living. 

 The great upheaval of land in the Wealden area, thus proved to be 

 subsequent in date to the Lower Crag, may, as Mr. Prestwich observes, 

 help to explain the difference observed in the fauna and climate of the 

 several successive crag periods (see above, p. 636) ; for we may now with 

 more confidence assume that the sea of the Coralline Crag was open to 

 the south, so that shells of southern forms lived in it, until at length, 

 the bed of thit sea having upraised 650 or 700 feet, all communication 

 with warmer latitudes was cut off, and the fauna of the Pied Crag ac- 

 quired its more boreal character. 



We also learn from these recent discoveries how impossible it may 

 often be to demonstrate the former presence of the sea on any given area 

 by organic remains, or by sea-beaches. Long and diligent inquiries had 

 been made before the year 1856, for sea shells of recent or crag species, 

 and for the signs of old sea margins within the Wealden area, or on 

 Nos. 3, 4, 5, 6, and 7 of the map (p. 272 of this edition, and p. 242, 4th 

 edition), and on the chalk downs and tertiary area between the Weald 

 and the Thames (Nos. 1 and 2, ib.) ; but in vain, until at last a few 

 casts of shells prove incontestably the long sojourn of the Older Pliocene 

 sea in those very spaces. We must now, therefore, admit the retreat of 

 its waters to have been an event of times comparatively modern. It 

 follows that in many cases the l&nd may have sunk and have emerged 



