1854.] PRESTWICH, LONDON CLAY AND BRACKLESHAM SANDS. 449 



"We thus see that, while the London Tertiaries form a group 

 distinctly separable from the Bracklesham series (which belongs to 

 the Paris group), with which however it possesses a certain com- 

 munity of species and a generally analogous/«cze*, although in many 

 of the classes of organic remains there is nothing, or almost nothing 

 in common, yet the former maintain almost unimpaired their 

 Tertiary attributes. Still the one or two cretaceous species which 

 have been found in the London Tertiaries, combined with the more 

 antique cast of some of the fishes, corals, and echinoderms, and a cer- 

 tain cretaceous fades in some of the molluscs, may form a link con- 

 necting them distantly with the Cretaceous period. But many inter- 

 mediate links are yet wanting, probably of the Tertiaries, and 

 certainly of the Cretaceous series ; for it must not be forgotten that 

 in England the Tertiaries repose upon the Middle Chalk, to which 

 belongs the chalk with flints of Gravesend and the neighbourhood, 

 usually termed the Upper Chalk, but which I have elsewhere* shown 

 to be the middle beds of that deposit. To have just terms of 

 comparison, we need a Cretaceous series with a varied marine 

 sestuarine and fluviatile fauna, such ag flourished during the succes- 

 sive Tertiary periods f. We have already in the Maestricht beds 

 a change in the fauna — a dying-out of many old forms, and the 

 appearance of many genera common in the Tertiary series, and I 

 look forward with much interest to the important addition likely to 

 be made before long by M. Hebert to our knowledge of the molluscs of 

 the Caleaire pisolitique, which beds carry us probably still higher in 

 the series. Still in these we want the mud-banks, the river-courses, 

 and the brackish water estuaries of the Tertiary seas. 



In some respects the London Tertiaries, taken as a whole, are how- 

 ever even more tertiary (meaning by that, that they present, in many 

 of the generic forms, a closer approach to those now existing in our 

 climate) than the Bracklesham Sands ; for, with a greater per-centage 

 of older forms, they nevertheless contain a large, and probably a larger, 

 proportion of forms such as now flourish on the land and in the sea in 

 these latitudes. The greater number of the common fishes frequenting 

 our seas have their types at this early Tertiary period, — the Reading 

 plants are such as our existing woods might furnish the analogues of, — 

 and the Molluscs, the Lamellibranchiates especially, have many repre- 

 sentative forms in our present seas. In considering all these singular 

 vicissitudes, and in contemplating the extent to which certain more 

 northern influ3nces operated in giving to a large portion of the fauna 

 of the London Tertiaries an aspect much more closely resembhng that 

 of the present day than is found to exist in many more recent 

 deposits, the question suggests itself — of how far that law, enunciated 

 by Prof, E. Forbes, and according to which the distribution of Molluscs 

 in depths of southern seas is equivalent to their appearance at lesser 

 depths or at the surface in parallels of latitude of more northern seas, 



* " The Water-bearing Strata of London," p. 139. 



f Can those singular and anomalous beds beneath all the known Tertiaries at 

 Mens, and which contain freshwater shells, belong to any such Upper Cretaceous 

 period ? 



