332 Dr. FiTTON on the Strata below the Chalk. 



or less of the character of a mechanical agglomerate ; but none of them approach to the gravel- 

 like texture and composition of the " Black-dirt " of Portland and the adjacent coast. 



The small extent which the Portland strata occupy with reference to the Kimmeridge clay, is 

 a remarkable fact ; showing either that the former, like the Coral-rag, was originally deposited in 

 detached portions; — or that the portions of the Portland group that we now see, are only the re- 

 maining and eroded borders of one continuous coating originally lodged upon the clay. 



It is remarkable, also, that the Wealden has not yet been found beyond the limits of the Port- 

 land group ; never reposing, as the green-sands are frequently seen to do, on any of the 

 formations beneath the Portland stone : but before this exclusion be adopted, it will be necessary 

 to examine more completely the counties on the north of Buckinghamshire. Reciprocally, in all 

 the places where the Portland beds have hitherto been found, one or more members of the 

 Wealden group are connected with it. It is possible, therefore, that the Portland strata formed the 

 only land, at the period when the freshwater beds of the Wealden group began to be deposited. 



The Kimmeridge clay thus more widely diffused than the Portland and the Wealden, forms 

 the continuation upwards, of a long series of alternate strata of oolitic limestones, sand, and 

 clay. The extent, thickness, and development, of this formation, vary much in different places, 

 the upper beds alone being visible at the point from whence the name has been derived. The 

 relations of the groups are fully seen on the coast near Weymouth, and still more completely on 

 the shore of the Boulonnois, and near Scarborough : but, although the clay extends, without 

 interruption, from Dorsetshire to Norfolk, it is seldom seen to advantage in the interior of En- 

 gland. Near Oxford, the lower members, which form the transition from the bituminous clays 

 of Kimmeridge to the Oxford oolite, are evidently wanting ; and the surface of the calcareous 

 reestone beneath the clay, is deeply worn and eroded. 



(170.) Composition of Strata. — Among- the circumstances relating to the 

 composition of the strata above mentioned, below the chalk in the South-East 

 of England, the following are deserving of attention. 



1. Proofs of the formation of stone, in the midst of strata of gravel, sand, and clay, distinctly of 

 mechanical origin. This fact, which is of universal occurrence throughout the series of secondary 

 strata, is especially remarkable in the Lower green and the Hastings sands of Kent and Sussex : 

 sex : the former containing masses of siliceous grit, chert, and chalcedony, and concretions of 

 chert, evidently formed subsequently to the division of the beds which inclose them, yet traversed 

 by the same lines of false stratification which pervade the looser matter : — (22.) and (23.) 



2. The change, in the fossil trees of Portland, of the original woody substance into silex ; and 

 the contrast of their composition with that of the surrounding matter in the *' Dirt-bed", in which 

 fragments of soft limestone are abundant ; the strata, both above and below, likewise, consisting 

 principally of carbonate of lime. In the Lower green-sand also, at Woburn, the petrifying 

 petrifying matter of the coniferous wood is silex*. 



3. The shells preserved in the green-sands of Kent, consist of carbonate of lime ; but in the 

 sands of Blackdown, though shells occur in still greater numbers, the calcareous matter has almost 

 entirely disappeared, and with very few exceptions has been replaced by chalcedony ; and the 

 surrounding sand, instead of effervescing with acids, imbibes them tranquilly. 



* The fact remarked by Mr. Brown, that all the fossil wood hitherto discovered in the strata 

 mentioned in these pages, is either monocotyledonous, coniferous, or cycadcous, has been 

 already adverted to : (112.) p. 225. 



