1854.] PRESTWICH, LONDON CLAY AND BRACKLESHAM SANDS. 437 



Sables moyens) is now too well known, too palpable to all those who 

 will but look at their respective faunas, to require further proofs 

 from us. Of this it is easy to judge by the great number of similar 

 species found in both districts*." 



I merely mention these facts to show how very important differ- 

 ences, which if seen alone could not fail to attract attention, can, 

 when associated by long usage with a certain amount of resemblances, 

 be overlooked when correlating the whole with other strata in which 

 the resemblances are the positive and the differences the negative 

 phsenomena. 



Since the publication of my paper f " On the probable age of the 

 London Clay," &c., in which lists of the fossil shells of this deposit 

 and of the Bracklesham and Barton beds are given, the valuable 

 publications of the Palseontographical Society and Dixon's 'Geology 

 of Sussex ' have rendered practicable a comparison of the other 

 portions of the fauna, whilst the continued researches of Mr. Edwards 

 on the Bracklesham Molluscs, and the additions made by Mr. Wethe- 

 rell and Mr. Bowerbank to those of the neighbourhood of London 

 and of Sheppey, have materially extended the list of these fossils, at 

 the same time that it has rendered necessary a revision of some 

 supposed identical species. The result of these additions to our 

 knowledge has been not only to confirm the distinction of the 

 faunas of these three tertiary deposits, but also to place this distinc- 

 tion in a still stronger light, so much so as to warrant our assigning 

 to the London series a more definite and separate position than 

 before. \\\ the same paper (p. 377) I expressed an opinion that 

 " we had in this country an important and large 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 J." Of this fact I now feel 

 so assured, as I also am of the sufficient difference existing between 

 the London Clay and the Bracklesham beds, that I would suggest 

 that they should be considered as independent groups, and that the 

 London Clay, with its basement bed, the Woolwich and Reading series 

 and the Tbanet Sands, form of themselves a distinct and separate stage 

 in the Tertiary series. To this the term of " the London Tertiary 

 group," or merely the London Tertiaries, should be applied, as 



* Cours elem. de Paleon. et de Geol. vol. ii. p. 753. 



t Quart. Journ. Geol. See. vol. iii. p. 354. 



X Quart. Journ. Geol. Soc. vol. iii. p. 377. M. Alcide d'Orbigny has since 

 made a somewhat similar division of the Lower Tertiaries in France ; i. e. he has 

 separated from the " Calcaire grossier " the beds beneath it, forming of the 

 upper division his " Etage Parisien," and of the lower his " Etage Suessonien." 

 The London Tertiaries, however, include only part of this lower division : the 

 upper and more important part I am inclined to group with the Bracklesham 

 series, which is the true equivalent of the " Calcaire grossier." M. Dumont, in his 

 classification of the Belgian series, also groups together the " Sables et Argiles 

 Ypresiens," these representing, as I shall show in the next paper, the London 

 Clay and Lower Bagshot Sands, which latter I should, however, rather place in 

 the Bracklesham series. 



