324 Dr. FiTTON on the Strata below the Chalk. 



The places beyond the limits of the tract described in the preceding pages, in which the 

 existence of the Wealden has been ascertained, or rendered probable, are mentioned hereafter, 

 (168.)p. S26. 



The sudden change from the marine fossils of the Lower green-sand to the freshwater pro- 

 ductions of the Wealden is remarkably exhibited at Atherfield in the Isle of Wight (100.) p.l96 ; 

 where the former stratum contains nearly thirty species, of about twenty genera ; while the Weald- 

 clay, not more than ten feet vertically beneath, contains scarcely any other shells than Cypris, 

 Cyclas, and Paludina*. The contrast, in descending from the Wealden group, is equally striking 

 and abrupt ; — the Purbeck strata near their junction with the Portland affording scarcely any 

 fossil but Cypris, while the *' Roche" at the top of the Portland group, within a few inches of the 

 freshwater limestone, is full of marine remains. (111.) p- 224. 



The Wealden strata, though differing so widely as to their fossils, appear to be conformable to 

 those of the adjoining formations, both below them and above. On the Dorsetshire coast, in 

 the Vale of Wardour, and in Buckinghamshire, the Purbeck beds correspond exactly in dip and 

 direction to the Portland stone beneath ; and at Hythe in Kent, and in the Isle of Wight, the 

 lowest beds of the green-sand are perfectly continuous with the upper beds of the Weald-clay, 

 (98.) p. 189. The only place with which I am yet acquainted, where a want of conformity is in- 

 dicated between these formations, is at Stopham in West Sussex, adverted to in (74.) p. 156. 

 But the eroded cavities, filled with what seem to be green-sand and Fuller's earth, in the upper 

 part of many of the quarries in Oxfordshire and Buckinghamshire, (of which those at Great 

 Hazeley and Dinton are good examples,) indicate an interval and some intervening disturbance, 

 between the periods of their production. (114.) p. 276 ; and (151.) p. 286. 



(166.) Theorij of the IVealden. — The mode in which the Wealden is 

 disposed^ in the South-east of England, accords with the hypothesis of its 

 having- originated in a lake of fresh water, or in the estuary of a large river. 

 It remains to be determined whether similar deposits are found to occupy 

 a corresponding situation in other parts of the great European basin, of 

 which England is but a small portion. 



The chalk and green-sands in England are so frequently prolonged be- 

 yond the limits of the beds below, concealing their outcrop, that no certain 



* Woodward, so long ago as in 1702, pointed out the resemblance of the Wealden Paludinae, 

 to some recent freshwater species': and Mr. Sowerby, about 1812, after mentioning their oc- 

 currence at Bethersden in Kent, remarks that " from these different localities of shells, appa- 

 *' rently of the same genus, we must conclude, either that analogy is not sufficient to prove that 

 " these fossils are of freshwater origin ; or else that there are more freshwater formations than 

 " are generally supposed-". Mr. Webster, about the same period, distinctly mentioned the 

 existence of freshwater shells in the Purbeck strata, and the probability that a part of that 

 formation had been deposited from fresh water^. In 1822, Mr. Mantell reasoned with great 

 sagacity upon the inferences derivable from the fossils, discovered by himself at Tilgale Forest*: 

 but the geology of the strata connected with that group was then undetermined ; and the diffi- 

 culties, I believe, were not removed till 1824. 



' " History of Fossils " ; quoted by Webster, — Letters, &c., pp. 142, 192. "^ Min. Con. vol. i. 

 p. 7(). tab. 31. ' Letters, &c. to Sir Harry Englefield, pp. 192, and 237. ^ Illustrations 



of the Geology of Sussex, 4to, 1832. 



