PRESTWICH BRITISH AND FOREIGN TERTIARIES. 12/ 



warmer seas. A further but minor change afterwards took place, 

 marking the commencement of the Bracklesham Sands and the Cal- 

 caire grossier period : the former sea-bed seems to have been en- 

 larged and further extended, and a fauna of a still more southern 

 facies introduced — which fauna, fixing upon the favourable localities 

 in the French area, multiplied exceedingly *. 



In reviewing the general characters of that portion of the Paris 

 group which succeeds at this last change, we cannot fail to be 

 struck with the distinctive physical features of the French and En- 

 glish formations, and by the less independent nature of their Belgian 

 equivalent. That the three areas, however, were at the Paris geo- 

 logical period closely connected and partially continuous, seems to be 

 proved both by the occurrence of some intermediate tertiary out- 

 liers t, and by a certain amount of common mineral elements and 

 structural peculiarities. At the same time there were, as we have 

 before noticed, important specific differences in the organic remains 

 and in the volume of the deposits ; but these zoological and physical 

 differences are such as would apparently have resulted more from the 

 variable character of the sea-bed and from the different rate of sub- 

 sidence or elevation affecting each district, than from the isolation of 

 any of these sea-areas. 



The period of the Lower Bracklesham Sands and of the two lower 

 divisions of the Calcaire grossier was, in the Paris area, after the in- 

 troductory somewhat sudden disturbance, one of comparatively little 

 change — there was a calcareous sea-bed and an absence of muddy 

 sediment, consequently conditions peculiarly favourable for the exist- 

 ence of a rich and varied testaceous fauna, such as then became 

 there developed. Whilst this comparative tranquillity prevailed in 

 the French area, in England a more rapid subsidence was, by giving 

 greater depth to the sea, tending to increase the vertical dimensions 

 of the strata ; and at the same time causing, either by too sudden a 

 change of depth or by too rapid an accumulation of sediment, occa- 

 sional intervals during which particular parts of the sea-bed were 

 depopulated of those Molluscs which flourished in the same area 

 under other more favourable conditions that intervened from time to 

 time. At the same time the more argillaceous character of the silt 

 in the seas of the English area was calculated to foster the exist- 

 ence of many local species ; whilst, on the other hand, such a sea-bed 

 was not favourable to the immigration of a greater proportion of the 

 French species. These were causes which must necessarily have 

 stamped the animal life of the two areas with certain peculiarities 

 distinctive of each, and may, in great part, account for so large a 

 number of species being confined to the French and English districts 

 respectively. 



Notwithstanding the extent of this Tertiary sea over the north of 



* As the Glauconiemoyenne overlaps the aestuarine and freshwater beds of the 

 " Argile Plastique," so does the Calcaire grossier overlap the Glauconie moyenne, 

 extending further south and south-east than the latter. 



t Of the lower beds principally, but also of the Lower Bagshot Sands and 

 Glauconie moyenne when the hills are high enough. 



