Dr. FiTTON on the Strata below the Chalk 321 



5. Deposition upon the "skull-cap" of a bed of "dirt", (110.) p. 223. ; and exposure of it as 



dry land. 



6. Growth upon this land, of the Cycadeoe now found in the Dirt-bed, in situ, and in the upright 



position. 10 : p. 223-4. 



7. Second submersion (at least), of the land, in fresh water ; proved by the deposition and exist- 



ence of the "cap". (111.) 9, p. 222, 3; and (113.) p. 226. 

 S. Exposure of the surface of this "cap" as land ; and growth upon it of coniferous trees, and of 

 Cycadeae. (111.) 8, p. 220-22. 



9. Ravages (from the effects of currents of wind, or water, — or from earthquakes), by which the 



trees were overthrown, or broken off at short distances above the roots. 



10. Third submersion, in fresh water ; and deposition of slaty limestone, containing freshwater 

 fossils only, but near the bottom alternating with clay*. (111.) p. 219, 220.; and (113.) 

 p. 226. 



11. Alternation of Oysters with freshwater fossils in the slaty Purbeck limestone; proving 

 access of the seat, apparently in an estuary (104.) p. 208 ; and (113.) p. 226. 



12. Continuance of the estuarine condition during the whole remaining epoch of the Wealden ; 

 proved by the occasional presence of oyster-shells, throughout the Hastings-sands and 

 Weald-clay. (98.) p. 190. ; and List of Fossils, p. 178. 



13. Sudden, or rapid, depression of the entire Wealden, to such a depth as to be covered with 

 salt water ; proved by the sudden and exclusive appearance of marine productions and 

 fossils, above and in immediate apposition with strata containing only freshwater fossils, 

 though mineralogically of the same composition, (100.) p. 196-7. 



(165.) Local Distribution of the Strata. — If a line drawn from the coast at 

 Folkstone^ touching- London on the north, and thence towards Newport 

 Pagnell; and another hne from about Atherfield in the Isle of Wight 

 towards Faringdon in Berkshire, the tract intervening will comprise the 

 space throughout which the beds immediately below the chalk are most 

 fully developed : and if the standard be taken from the condition of the strata 

 when possessed of their greatest bulk, that of the Green-sands and Wealden 

 may be said to occur in Kent, Sussex, and the Isle of Wight; from which 

 central tract these groups appear to thin out in every direction. 



The whole series immediately below the chalk is finely developed between 



* The alternations, in this case, are distinguished from those of the coal formation, (in which 

 also the remains of terrestrial and freshwater productions alternate — in some instances repeatedly — 

 with those of marine origin), by the growth of plants upon the surface of some of the alternating 

 strata ; a proof of their exposure, as dry land, between successive immersions. Mr. Prestwich, 

 in a valuable paper on the coal-field of Coalbrook Dale, has justly remarked, that the repeated 

 alternations in that district are no proof that the tract itself was raised above the sea and again 

 depressed, as many times as the freshwater remains alternate with the marine ; — since the fhcts 

 may be accounted for by supposing that the strata were accumulated in an estuary liable to 

 freshes from a river of considerable size. — Proceedings of Geol. Society, 1836, vol. ii. 



t Lamarck (Animaux sans Vertebres, vol. vi.) mentions altogether 81 species of the genus 

 Ostrea; 48 of which are of existing species, 33 known only in the fossil state. Of the 48 recent 

 species, 40 are marine, and 8 uncertain, but also probably marine. In the present state of our 

 knowledge therefore, the occurrence of Oysters may fairly be considered as proof of the former 

 access of the sea. 



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