218 Dr. FiTTON on the Strata below the Chalk. 



it ; intimating, however, that it will probably be found at the top of the chert 

 or flint in the upper part of the oolite. Some years afterwards Dr. Buckland 

 described specimens of a new family of fossil plants from the "Dirt-bed" above 

 mentioned; to which, at the suggestion of Mr. Brown, he gave the name of 

 Cycadeoideae*, but which M. Adolphe Brongniart, adopting a different system 

 of nomenclature, soon after formed into a genus which he called Mantelliaf. 

 Dr. Buckland was subsequently led to infer that the " Dirt-bed " was actually 

 the soil in which both the silicified trees and the Cycadeoideae had grown : 

 and in a paper read before the Society in 1830 |, now published in this 

 volume, he and Mr. De la Beche have stated several facts respecting that 

 remarkable bed; adding in a note§ that Professor Henslow had ascertained 

 the existence of two other beds of dirt (or of clay with carbonaceous matter) 

 below it; one of them about seven feet beneath, the other about two feet 

 still lower down. 



On visiting the Isle of Portland last summer, with a knowledge of these 

 facts, I found that the clay, or " dirt ", below the Cap, (the upper appa- 

 rently of the two additional clays described by Professor Henslow,) itself 

 contains Cycadeee, in an upright position, and to all appearance in the places 

 where they had grown ; and I obtained also some new evidence respecting 

 the character of the beds immediately above. 



(111.) The following are a sketch and enumeration of the strata exposed 

 at the time of my examination, in the quarries about the middle of the island, 

 and westward of that point ; which I was enabled to revise during a second 

 visit to Portland, when I had the satisfaction of being accompanied by 

 Mr. Brown. 



The whole mass of the strata which now form the Isle of Portland must have been subjected 

 to great disturbance long before the surface acquired its present configuration ; by which 

 the vertical rifts, from a few inches to some feet wide, were formed, which are now observ- 

 able in the face of the quarries, passing indiscriminately through both the Purbeck and 

 Portland strata, and having in many instances produced displacement in the separated portions : 

 and this at such distances from the present coast of the island, as to prove that no recent 

 disturbance or subsidence of the sea-clifFs has had any share in producing them. Yet it is 

 remarkable that the surface over these disjointed beds is now perfectly uniform ; the inequalities 

 occasioned by the sinking and fracture of the slaty Purbeck strata near the top having been com- 

 pletely filled up to a level by rubble and vegetable soil. In one place a fissure between two 

 and three feet wide cuts down, almost vertically, through about thirty feet of stony strata, 



* Geol. Trans., 2nd Series, vol. ii. p. 395 ; read June, 1828. 



-j- Prodrome d'une Histoire des Vegt'taux Fossiles, 1828, pp. 92 and 96. 



J Proceedings of the Geol. Soc, vol. i. p. 217. et seq. 



§ Page 16. of the present volume. 



