438 PROCEEDINGS OF THE GEOLOGICAL SOCIETY. [JunC 7, 



serving to designate the oldest division of the Tertiary series in this 

 part of Europe; — a series as much marked by its Reptiles and 

 Fishes, as the overlying group is by its Mammals and Molluscs ; whilst 

 the absence of Nummulites in this lower group constitutes of itself an 

 important distinguishing feature. 



In the first place, the physical features and lithological character 

 of the two groups will be observed to be extremely well defined. 

 The London Clay is persistent over a large area, and everywhere 

 maintains throughout its mass a nearly uniform mineral character. 

 Between this deposit and the overlying Bagshot and Bracklesham 

 series there is no decided passage. It is true that there is no 

 marked unconformability — no eroded surface — nothing in fact to 

 indicate that the London Clay had been elevated and existed as dry 

 land before it was submerged and covered by the Bagshot Sands, 

 but merely that, owing to some extensive alteration in the distribution 

 of land and water, the drainage from other lands and from new rivers 

 overspread the former sea-bed : for it is evident, from the completely 

 changed nature of the Bracklesham series, from the prevalence 

 throughout its mass of vast beds of sands with subordinate green 

 sands and shifting clays, and the entire cessation of the repetition in 

 that series of the mineral character of the Loudon Clay, that the 

 debris forming each deposit was derived from two distinct and separate 

 sources. Further, the immediate change was probably not marked 

 by any very violent action within this area, as the overlying group 

 does not show at its base coarser materials, nor exhibit a more 

 trenchant divisional plane, than prevail in the beds throughout their 

 mass generally. Here and there in the Bagshot area, a band of rolled 

 flint-pebbles occurs at the base of the sands, but this is an exceptional 

 case ; on the contrary, the admixture of the upper surface of the 

 London Clay with the first 2 or 3 feet of sands — a necessary con- 

 dition of a muddy clay sea-bottom — gives rise at places to some ap- 

 pearance of a passage between the two deposits. In the last paper* 

 it was shown that the London Clay is thickest in Kent and Essex, and 

 becomes much thinner as we proceed west and south-west ; but this 

 I do not believe to arise from the wearing down of the London Clay, 

 or from any unconformability of the strata, so much as from the larger 

 original accumulation of the London Clay in those areas, owing, as 

 I before stated, to the nearer proximity to the river mouth. 



With respect to the palseontological evidence, although the Lon- 

 don Clay forms but the upper division of the London Tertiary series, 

 and the Bracklesham beds are the lower part only of the overlying 

 series, still, as these two deposits constitute by far the most important 

 members of each series, offer the best known organic remains, and 

 present the nearest like terms both in physical and zoological condi- 

 tions, I will take their two faunas as the fittest for comparing the 

 life of these lower and middle Eocene periods. 



Mammalian remains are very limited in number. Prof. Owen, in 



* Loc. cit. 



