Dr. FiTTON on the Strata below the Chalk. 331 



M. Constant Prevost*, and M. Passy t, on the coast of Normandy and the adjacent district ; 

 and more recently of M. Thirria, in the department of the Haute SaoneJ, have shown that many 

 of the beds between the chalk and the oolitic system in the different provinces of France, agree 

 with each other, and with those of England, perhaps as nearly as the more distant portions of the 

 same formations are found to do in our country : the Portland strata, which form the lower 

 boundary of the Wealden, occurring at several detached points along their course ; while above 

 them, the chalk affords a limit which cannot be mistaken. It may deserve the inquiry, there- 

 fore, of resident geologists, whether traces at least, of all the intermediate groups between the 

 Portland and the chalk may not be detected generally along the line of connexion; and I am not 

 without hope that the task of comparative examination may be assisted by the preceding pages. 



Among the points deserving of notice by those who examine the beds below the chalk in dis- 

 tricts imperfectly known, I would mention especially the characters of the Portland Sand ; — from 

 the facility with which that formation may be confounded with other sands, likewise charged with 

 green particles. I have reason to believe that a neglect of the distinctions between these dif- 

 ferent sands, and a consequent ignorance of the distance by which they are separated in the series 

 of strata, has produced difficulty, and led to much confusion in the arrangement of the groups be- 

 tween the chalk and the oolitic system. 



(169.) Beds below the Wealden. — The groups next below the Wealden, 

 in the tracts described in the preceding- pages^ are connected by several cha- 

 racters; the Portland sand forms a transition to the Kimmeridge clay; 

 and the latter, at its lower part becomes charged with sand and calcareous 

 matter, and passes insensibly into the Oxford oolite. The fossils, also, of 

 these associated groups are very much allied, and there is no reason to suppose 

 that the deposition of this part of the series was interrupted by any important 

 change of circumstances. 



Portland strata. — The space occupied by the Portland strata, so far as they have yet been dis- 

 covered in England, is bounded on the west by a line passing from Portland to the outcrop of the 

 Portland stone in the Vale of Wardour, and thence towards Swindon. On the east of that line 

 the Purbeck and Portland formations are visible only in the remaining portions of the once- 

 continuous sheet, which seems originally to have invested a great part of Oxfordshire and Buck- 

 inghamshire. Whether the Portland beds exist, or ever have existed, under the sands of tlie 

 Isle of Wight, Sussex, and Kent, it is now impossible to say ; though it is probable that they 

 did so, from their occurrence on the opposite coast of France, and their thickness in that 

 quarter. 



The peculiar circumstance relating to the junction of this formation with the Purbeck beds, is 

 the distinct character of superficial soil, which the bed in which the trees and Cycadeae are found, 

 exhibits in the Isle of Portland. Trees and plants have occurred in the upright position in 

 many other places, and in other parts of the series of strata; but the soil in those cases had lost 

 its recent character, and been in a great measure assimilated to the surrounding matter. Some 

 other thin beds alternating with the lowest strata of the Purbeck limestone, have likewise more 



* Mem. de la Soc. d'Hist. Nat. de Paris, t. ii. 1825. p. 389. 



t Sur le Departcment, Sfc. 4to, Paris, 1833. 



I Mem. de la Soc. d'Hist. Nat. de Strasbourg, 1830, t. i. 



