1852.] PRESTWICH ON THE THANET SANDS. 245 



3. Organic Remains. — The fauna of this division of the Tertiary- 

 series is both Hmited in its species and confined in its range. In a 

 few locaUties only are its fossils at all abundant, and they occur in 

 patches and irregular layers, in which, although the number of indi- 

 viduals is sometimes great, the species are always few. Nevertheless 

 they form a well-marked and distinct group, a large proportion of 

 which is peculiar to this deposit. This scarcity of organic remains is 

 in part attributable probably to the mineral character of the strata, 

 for these being almost entirely composed of fine siliceous sands, with 

 an admixture of clay and carbonate of lime only (apart from a few 

 exceptional cases) in very small and variable proportions, they are 

 generally very porous and permeable. It is these beds which, as 

 before mentioned, constitute the main water-bearing strata beneath 

 the London Clay. They everywhere admit of the passage of water, 

 with which they are invariably charged when below a certain level. 

 The consequence of this is, that the substance of the shells has been 

 almost always removed, or, where it remains, it is generally in an 

 extremely friable state. Where the sands are sufficiently argillaceous, 

 the casts of the shells often remain ; but where this is not the case, 

 even traces of them are of very rare occurrence. Where, however, 

 in addition to the clay, carbonate of lime is present in appreciable 

 quantity, then the shells are preserved in considerable numbers, but 

 they are generally very tender and difficult to remove. 



By these observations I do not mean to imply that the " Thanet 

 Sands" may have been originally equally fossiliferous in West as in 

 East Kent, for it is probable that the causes which have favoured the 

 preservation of the shells when dead, may also have tended to their . 

 development when living. That it is at the same time a true cause to 

 a certain extent, is evident from the fact that in a few exceptional cases, 

 which we shall hereafter mention, where the fossil has by chance been 

 brought into a condition to be uninfluenced by the action of water, 

 then a few isolated proofs of the existence of animal life, in areas 

 otherwise barren, have been preserved. 



At the south end of the section at Richborough, the fossils are 

 numerous and perfect. A few yards further north, and still on the 

 same level, the external casts only are found. Many of these are in 

 a beautiful state of preservation, but are exceedingly soft and friable. 

 At the other end of the section the impressions are scarce, and the 

 shells still more so. At Pegwell Bay and Heme Bay the Thanet 

 Sands happen to be more argillaceous and calcareous than usual, 

 and there the shells are more numerous and more regularly distri- 

 buted, whilst the irregular calcareous concretions and the few layers 

 of semi-indurated marls have served in both places to preserve the 

 organic remains. In the upper bed, however, of loose incoherent 

 yellow sand at Heme Bay, the irregularity in the occurrence of the 

 fossils is again observable. At their highest level, near the Reculvers, 

 they are full of friable shells, which become scarcer, and finally dis- 

 appear, although numerous impressions remain, as the bed trends 

 westward and dips beneath the central beds of the Lower Tertiaries. 



A peculiar condition of the fossils sometimes occurs at Rich- 



