IN THE VALE OF THAMES. 443 



Another curious mark of the proximity of this old chalk surface 

 to the level of the sea at the time of the beginning of the eocene 

 deposits, is a bed of oyster shells lying on the chalk. These shells 

 (O. bellovacina) have been long known and often considered since 

 Dr. Plot, in his Natural History of Oxfordshire, suggested that 

 those found in abundance at * Cat's-grove' (now Katesgrove) near 

 Reading may be the reliquiae of oysters supplied for food by the 

 Danish navy while the army held a fortified post at the junction 

 of the Kennet and Thames. 



Among important memoirs on the cainozoic deposits near Read- 

 ing, may be mentioned Dr. Buckland's notices of the section at 

 Katesgrove; Mr. Prestwich's comprehensive Essay on the Lower 

 Eocene Beds in the whole Basin of the Thames ; Professor Jones's 

 Notices of the country about Newbury ; and Mr. Whitaker's 

 Memoir on the Geological Map of the district. 



Only the lower portions of this great system of strata appear 

 in the south-eastern slopes of the chalk ranges of "Wilts, Berks, 

 Bucks, and Beds. Among these the 'Sarsen' stone, which lies 

 in the chalk valleys near Marlborough, the * Pudding-stone' 

 of St. Albans, and the oyster beds and other shelly layers at 

 Reading, may be remarked as among the most interesting. 



The highest points of land occupied by any of the cainozoic 

 strata within the area embraced in the General Map, Plate L, are 

 on the line of the old coach road over the Chiltern Hills, at 

 Nuffield, 696, and Nettlebed, 692 feet above the mean sea-level. 

 From these and other elevations at many points, but not over 

 large surfaces, the eocene beds are noticed in descending from the 



Diagram CC. Eocene beds in the Vale of Thames. 



C. Chalk. T. Thanet sand. W. Woolwich beds. &. Blackheath pebbles. 

 L. London clay. B. Bagshot beds. G. Gravel. 



downs of Herts, Oxon, and Berks toward the Thames and the 

 Kennet, and they give favourable sites for a considerable number 

 of thickets and woods which are rarely seen on the less genial and 

 drier chalk. 



