1847.] PRESTWICH ON THE LONDON CLAY. 377 



It may however be observed that the Lits Coquilliers bear a much 

 closer relation to the Calcaire grossier than the London clay does to 

 the Bracklesham sands and Barton clays ; suice the proportion of 

 species common to the two former is about 90 per cent., whereas in 

 the latter it is only about 40 per cent. 



We can thus for the present only approximate to the relative su- 

 perposition of the London clay, and place it below the Calcaire gros- 

 sier ; for, notwithstanding we cannot assign it an exact equivalent, 

 it is evident that it contains a rudimentary fauna (see lists, ante, 

 p. 366-371), which may be considered, as a group, to indicate a syn- 

 chronism with the lowest eocene division of M. d'Archiac (Groupe 

 des Sables inferieurs). 



We must not, however, at present draw too close a parallelism be- 

 tween the tertiary formations of the two countries. We appear in 

 this country to have an important and larger development of strata 

 of the age of the lower portion of the French series, and that to an 

 extent which would constitute them the type of the period rather 

 than a subordinate variation thereof. 



A few of the more general conclusions derivable from the fore- 

 going observations may here be mentioned. 



It is probable that the tertiary sea at first extended uninterruptedly 

 over the London, Hampshire and Paris tertiary areas ; — that, at a 

 period coeval with the change of conditions, both in structure and 

 fauna, which is evident at the base of the London and Bognor clays^ 

 a separation took place between the Paris and Hampshire areas, 

 leaving the latter one however still connected with that of London 

 during the deposition of the London clay ; — that, after this period, 

 the communication between the French and English tertiaries was 

 restored, as evinced by the introduction in the Bracklesham beds of 

 so many French species, and that this connexion was probably in 

 part prolonged until the completion of the Isle of Wight series, — the 

 London district in the meantime assuming a more isolated position, 

 and emerging sooner from beneath the sea. 



This leads to an inquiry into the age and structure of the Bagshot 

 Sands, which will be found to be in perfect conformity with, and to 

 corroborate, the hypothesis advanced in this paper. I hope shortly to 

 have the honour of laying some remarks on this subject before the 

 Society. 



of these deposits. Possibly the London clay may have been formed during a 

 period unrepresented, or only very partially so, in the French series. Perhaps 

 the presence of the typical group of Bognor fossils in the Lits Coquilliers is the 

 result of migration, at the conclusion of the London clay period, into more re- 

 cently formed seas wherein the Lits Coquilliers were deposited. I am almost 

 inclined to adopt this view, and to consider that the London clay period imme- 

 diately preceded that of the Lits Coquilliers ; — that it synchronises with some 

 older portion of the Sables inferieurs. 



The list of fossils, tables and sections accompanying this paper are placed at 

 the end of the following one. 



