448 EOCENE PERIOD. CHAP. 



Thanet Sand, the lowest of all the groups, is little seen except 

 in the lower course of the Thames, where it is traceable from 

 London downward to the district from which it derives its name. 

 It is, however, discovered in wells under the Woolwich beds, in 

 a considerable tract west and north of London, seldom more than 

 50 feet thick. It is a light-coloured sand, with very slight trace 

 of mica, and very little carbonate of lime ; and only near the base 

 mixed with any sensible proportion of argillaceous matter. At 

 the base it is often greenish, and almost always holds (or is almost 

 composed of) chalk flints of various magnitude, sometimes extremely 

 large, not much worn by attrition, but often stained with a permanent 

 green tint on the surface. The fossils found in several situations, 

 as Reculver, Pegwell Bay, and Richborough, are mostly shells, 

 and all marine. 



The Woolwich and Reading series of sands, and mottled and 

 laminated clays, with fluviatile, estuarine, and marine shells, 

 succeeds and forms a narrow belt on the south side of the Thames 

 above the Thanet sands, about 50 feet thick. The fossil shells 

 lie in bands, and are on the whole estuarine, or a mixture or 

 alternation of truly marine and truly fresh-water mollusca. The 

 genera Cyrena, Cardium, Melanopsis, and Cerithium, with Ostrea 

 bellovacina, are common. In the great pit south of Erith, at the 

 base of these beds, are pebble bands with abundance of shells, 

 and an alternation is remarked of one such band in the upper 

 part of the Thanet sands which rest on the chalk. The Cyrense 

 are often found with valves united. Remains of fishes (chiefly 

 sharks and rays) occur. 



What may be regarded as the uppermost member of the series 

 under consideration is a capping of well-rolled flint pebbles, usually 

 of small size, and in some places, as at Blackheath, of considerable 

 thickness. These are sometimes loose as the pebbles on a sea- 

 beach, and almost unmixed with sand, but elsewhere aggregated 

 to a conglomerate. Possibly to this horizon should be referred 

 the flint pebble bed or pudding-stone of Hertfordshire, which has 

 what is called a siliceous cement, in which an evident granular 

 texture remains to indicate its arenaceous origin. 



On this point I may remark that in the Chiltern Hills, especially 

 about Nuffield, large angular, irregular blocks of this kind of stone 

 are found in the fields on the slope of the chalk, much as near 



