236 PROCEEDINGS OP THE GEOLOGICAL SOCIETY. [Apr. 21, 



places around London, and, again, more especially near Poole in 

 Dorsetshire. But in the London district these clays are restricted to 

 the western portion of the tertiary area, and occur but in one division 

 of the series ; whilst the Poole clays belong, not to the beds beneath 

 the " London Clay," but form part of the *' Bagshot Sands" above it. 

 It is to be observed also, that these subordinate clays which give the 

 name to this formation are the unfossiliferous portion of the series, 

 and consequently they present only a mineral designation, whilst the 

 palseontological type has been taken from the local and subordinate 

 fluvio-marine group of Woolwich, Bromley, and adjacent districts, 

 which, owing to its more favourable exhibition and rich store of orga- 

 nic remains, has attracted a larger share of attention than its relative 

 development would warrant, and its characters have come to be con- 

 sidered the ruling, instead of the subordinate, although important, 

 feature of the " Plastic Clay Formation," For out of nearly 3000 

 square miles over which the " lower tertiaries" are spread, these fluvia- 

 tile and estuarine beds occupy really an area of only about 200 miles, 

 nor is their vertical development relatively more important. In fact, 

 so far from the more fluviatile conditions prevailing in the lower 

 tertiaries, this series exhibits, on the contrary, throughout a great part 

 of its range, proofs of a distinctly marine origin, whilst in another 

 part the absence of fossils renders the question of origin uncertain. 

 With regard also to their mineral structure, the conjoint use of the 

 terms " London Clay Formation" and " Plastic Clay Formation" is 

 apt to convey a wrong idea, inasmuch as they might be supposed to 

 be two argillaceous deposits, or at least in some measure related in 

 lithological characters, whereas they are, as is well known, totally 

 dissimilar, the one formation being strictly argillaceous, and the other 

 mainly arenaceous but with subordinate clays and conglomerates. 



For these reasons therefore it seems desirable to change the name 

 of this part of the Tertiary series, or at all events to restrict the pre- 

 sent one to that portion of the series in which the plastic clays pre- 

 dominate ; but still this name indicates a physical character so essen- 

 tially a part of the ordinary properties of clays, that even in this more 

 limited division their strongly marked mottled appearance would 

 afford a less general and better designation. I therefore propose for 

 the present merely to call the series between the London clay and 

 the Chalk " The Lower London Tertiaries," and to subdivide them 

 into the foUovdng groups in descending order : — 



^1. The Basement Bed of the London Clay. 



The Lower London 

 Tertiaries. 



2*. The Woolwich and the Mottled Clays, Sands, and 

 Pebble Beds. 



3. The Thanet Sands. 



* The order of subdivision of this part of the series wiU be given in the next 

 and concluding part of this Paper. 



With reference to the French tertiaries, M. d'Archiac has discontinued the 

 use of the term " Formation d'Argile plastique," and named the series of strata 

 beneath Calcaire Grossier, the " Sables inferieurs," subdividing them into six 

 groups. In his extremely valuable ' Histoire des Progres de la Geologic de 1834 

 a 1845' (note, p. 598), he observes, " Quelques geologues, trop preoccupes du role 

 que joue I'argile plastique dont on trouve des amas plus ou moins etendus, mais 



