276 PROCEEDINGS OF THE GEOLOGICAL SOCIETY. [JaU. 23, 



After a time, however, the inhabitants of deeper waters would 

 gradually immigrate into these parts of greater depths, and there 

 remain, until from some cause the sea became again sufficiently shallow 

 to allow of the incoming of shallow seatestacea, and then that original 

 fauna, which in the interval had been preserved in the same sea, in 

 the distant Hampshire tertiary area, might have extended itself and 

 reappeared in that part of it spreading over the more central and 

 easterly parts of the London tertiary district. 



This I consider to have been the succession of events at this period 

 of the tertiary epoch. On this supposition I account for the absence 

 or extreme scarcity of organic remains in the lower part of the London 

 clay in the central and eastern divisions of the district, aad for their 

 abundance in the western divisions. In the neighbourhood of London, 

 as we ascend in this formation, we meet with remains of Cephalo- 

 podous testacea in strata succeeding the lower unfossiliferous beds, 

 and we find them further eastward with the remains of Cephalopoda, 

 of Echinodermata, and of other denizens of deeper seas, in some 

 abundance. The lower beds of the London clay overlying its basement 

 bed, may therefore from this cause without difficulty be conceived to 

 present, although synchronous, considerable modifications in their 

 organic remains, whose presence or absence taken separately does not 

 consequently afford a test in this case to the determination of the 

 geological horizon. 



We now have to consider the physical changes indicated by the 

 structure of the basement bed itself, marking as it does the passage 

 from the arenaceous beds below to the argillaceous ones above it. 

 Of the conditions of the sea preceding this period we shall treat on a 

 future occasion. 



Indiscriminately over all the variable " Lower Eocene " deposits 

 spreads the basement bed of the London clay. It is the first brush 

 of uniformity, where previously all had been different. Extending 

 from the Isle of Wight to Woodbridge in Suffolk, this bed presents 

 some general characters of remarkable persistence. In the first place 

 it is evident that it does not form a sequence in structure conformable 

 to the beds which it immediately overlies. Yet no fresh element is 

 introduced at first into its composition. Although the materials 

 composing this bed are not in many cases found in any of the beds 

 immediately below, yet they all exist in the underlying series in 

 some part of their range, and are, I believe, derived from that source. 

 The great depositary of the rounded flint pebbles in the under- 

 lying beds, are the estuary and flmdatile strata of Woolwich and 

 Bromley, where they occur in remarkable abundance. Associated 

 with these pebble beds are thick beds of yellow sand and also several 

 subordinate beds of a strong coarse green sand (which become, how- 

 ever, much more important in East Kent), some beds of a deep 

 ferruginous character, and a few clay beds — the mottled clays of the 

 western districts also are interstratified with beds of yellow sands, but 

 without pebbles. Now the pebbles of the basement bed of the Lon- 

 don clay have been probably derived from those previously accumu- 

 lated locally in these underlying beds, and, if the pebbles, then also 



