1850.] PRESTWICH ON THE LOWER TERTIARY STRATA. 253 



of the London tertiary district*. Further descriptions, in which I 

 must necessarily go over again part of the same ground, may ap- 

 pear in some measure superfluous ; but although the relative position 

 which the whole of the series of beds known as the plastic clay for- 

 mation bears to the London clay and the chalk, has long been well 

 established, and is sufficiently apparent in several sections, the exact 

 grouping and subdivisions of these lower Eocene deposits in their 

 entire range, by which alone the precise co-relations of the strata 

 can be determined, have not yet I think been clearly shown. 



These beds have in fact been viewed as one deposit irregularly in- 

 terstratified, and the sections at Heme Bay, Upnor, Lewisham, 

 Woolwich, and Reading have been co-related " en masse,'' but not in 

 detail. It has of late even been considered doubtful whether the 

 larger original divisions into London clay and Plastic clay could be 

 maintained ; whether the latter were not merely the subordinate beds 

 of the former formation. 



The object therefore of this paper is both to describe several new 

 sections, and also to show, that the variable series of deposits form- 

 ing the lower tertiaries can be divided into distinct and separate, yet 

 not altogether independent subdivisions, each marked by different 

 conditions, indicating ancient hydrographical and palseontological 

 changes of some importance. 



The main body of the London clay presents throughout its whole 

 range a uniformity of mineral structure so well marked and distinct, 

 that either by this character alone, or else by its organic remains, 

 when present, it can almost always be readily recognized. But the 

 case is far otherwise with the more varied deposits which intervene 

 between the London clay and the chalk. This series is not large, yet it 

 exhibits in different places variations in its structure and in its fauna, 

 which render the determination of the exact parallelism between di- 

 stant sections difficult. Thus below the London clay in the Isle of 

 Wight f we find almost exclusively beds of compact mottled clays 

 without organic remains. In the neighbourhood of Newbury and 

 Reading are mottled clays, interstratified with beds of sand, and ge- 

 nerally underlaid by a bed abounding with the Ostrea Bellovacina, 

 At Woolwich, Charlton, and Bromley the chalk is overlaid by unfos- 

 siliferous sands, succeeded by a mixed series of clays and sands with 

 flint pebbles, and containing numerous organic remains of freshwater 

 and estuary origin ; whilst at Heme Bay and in the Isle of Thanet 

 there exists a thicker and more important series of sands, sometimes 

 in part very argillaceous, at other times much mixed with green sand, 

 and many of the beds of which abound with marine fossils, — the 

 fluviatile beds of Woolwich, and the mottled clays of the Western 

 districts, having in these places completely disappeared. 



Amongst the first questions therefore which arise, are, — with which 

 portion of the Woolwich series are the mottled clays of the Reading 



* See the early volumes of the Trans. Geol. Soc. from 1811 to 1825, and 

 Phillips and Conyheare's Geology of England, The separate references are made 

 in the course of the paper, wherever the sections have been previously described. 



t See Section of Alum Bay and White Cliff Bay, in the 2nd volume of the 

 Quart. Jour. Geol. Soc. pi. 9, strata 1 and 2. 



