274 PROCEEDINGS OF THE GEOLOGICAL SOCIETY. [Jan. 23, 



in a few points, but they all present a general resemblance. This 

 may not however be considered sufficient for our object — such thin 

 and ordinary beds might be subordinate to some other portion of the 

 Eocene series, and not peculiar to this part of them, and therefore 

 some other proofs of their position may be thought necessary. 



In the first place, all the sections in the tertiary district show, by 

 evidence of the clearest kind, that the London clay forms a nearly 

 homogeneous mass, several hundred feet thick, of tough clay of a 

 predominating brown colour — that throughout its whole body it 

 nowhere presents any subordinate beds of a mineral character 

 essentially different from that of its ordinary argillaceous type — and 

 that its organic remains are very irregularly dispersed, abounding in 

 some parts and being entirely wanting in others. This clay occupies 

 an area which is very well defined. Now wherever, without a single 

 exception that I am aware of, the lower beds of this clay outcrop, 

 there is found underlying them a basement bed of a conglomerate 

 character and with or without organic remains — and these, if present, 

 invariably belong to one and the same group of fossils. Further, if we 

 go more into the chalk district, we shall find that whenever the out- 

 liers of the lower tertiary sands and mottled clays without organic 

 remains, attain a thickness on an average of from 50 to 100 feet, 

 the basement bed of the London clay invariably sets in. Again, this 

 deposit always exhibits a peculiar mineral character, the chief 

 feature of which is the presence of rounded flint pebbles, mixed with 

 yellow, green, or ferruginous sands in variable proportions. Inter- 

 mingled with the conglomerate bed, or in the thin sandy layer above 

 it, are frequently found numerous organic remains belonging to a 

 fauna of about forty species, many of which are persistent throughout 

 the greater part of the range of this stratum. Now although the 

 London clay does not always contain organic remains, nor is the 

 basement bed always fossiliferous, neither is the mineral character 

 of one or the other always exactly alike, nevertheless the concurrent 

 testimony afforded in each case, either by position, or by organic re- 

 mains, or by lithological structure, although the force and value of 

 one or the other class of evidence may vary materially, is I consider, 

 in all the instances I have adduced, sufficiently strong to prove the 

 position assumed. I cannot admit, as has been urged, that the 

 absence of organic remains in the lower beds of the London clay, at 

 New Cross, Upnor, Gestingthorpe, Kyson, and elsewhere (see Sections 

 8, 10, 17, 20), is an argument against such beds forming part of the 

 London clay. It is not possible to take up a position upon a mere 

 negative fact — to use as substantive evidence that which of itself is but 

 a difficulty arising from variable, and not from conflicting, conditions. 



If the " tnassif of the adjoining district consists of London 

 clay, and the dip and position of the strata, as well as their mi- 

 neral characters, lead us to suppose that these beds crop out in 

 the position which should be occupied by the lower beds of the 

 London clay, then I hold that as such they must be considered, 

 unless they can be proved to be something else. Otherwise, in trying 

 to avoid one difficulty a more formidable one will be raised, in having 



