16 LONDON DISTRICT. 



first instance was derived from the sea-water by those organisms 

 which have siliceous structures, certain sponges, diatoms and 

 radiolaria. The decay of these organisms led to the diffusion of 

 much siliceous matter in the soft calcareous ooze, and it appears 

 to have become concentrated in certain fairly regular layers, 

 from which it segregated into nodules or into thin bands pro- 

 bably in the soft calcareous mud during the process of consoli- 

 dation. It formed around many kinds of nucleus a tooth of 

 fish or saurian, an echinoid or brachiopod and some of the 

 fossils became entirely replaced by flint. Sponges themselves 

 more frequently than other forms of life were enveloped, because 

 they contributed to the siliceous covering. Hence we often find 

 thin-shelled flints enclosing sponges and with them bryozoa, 

 foraminifera, &c. 



There are also veins of flint which traverse the strata 

 obliquely, and these must have been produced after the Chalk 

 was consolidated and the flint bands and nodules (which they 

 intersect) were formed. 



The decay of flint gives rise to a white siliceous crust, and 

 pebbles wholly made of such material are sometimes found in 

 Tertiary and Drift deposits. What are called ' banded flints ' 

 occur both in the Chalk and in the form of pebbles derived from 

 it. They appear to have been due to infiltration or the deposi- 

 tion of impure siliceous matter in cavities in the flints. 1 



The nature, formation and varieties of flint have been fully 

 discussed by William Hill, whose paper gives many references 

 to detailed work on these questions ; 2 more recently these 

 questions have been studied by Mr. W. A. Richardson in the 

 light of new knowledge concerning the chemistry and physics 

 of colloids. 3 



CHAPTER IV. 



EOCENE ROCKS. 



The Lower Eocene strata rest on the Chalk in apparent 

 conformity, the flint-layers in that rock being approximately 

 parallel, when seen in sections, with the bedding in the over- 

 lying formations. Nevertheless there is a great break between 

 the Cretaceous and the Eocene. This is marked by the change 

 in physical conditions, from the deep water of the Chalk to the 

 comparatively shallow water of the Thanet Beds, by the absence 

 in this country of any passage beds, by the occurrence of tiny 

 fragments of flint in the Thanet Beds and of flint -pebbles in 



X S. P. Woodward, Geol. Mag., 1864, p. 145. 



2 ' Flint and Chert,' Proc. Geol. Assoc., vol. xxii, 1911, pp. 61-94. 



3 Geol. Mag., 1919, pp. 535-547; see also Brydone, R. M., ibid., 1920, pp. 

 401-404. 



