Thanet Sand. ' 239 



other seems to be more arbitrary, being founded on con- 

 siderations of a purely palseonfcological kind. It will 

 at all events be most easy in this book to treat of the 

 strata as consisting of a lower fresh- water and estuarine, a 

 middle marine, and an upper fresh-water and estuarine 

 series. 



The Thanet Sand, absent in the Isle of Wight, is 

 so named by Professor Prestwich because it is so well 

 developed in the Isle of Thanet on the Thames. It 

 lies at the base of the Eocene strata of England, and 

 consists of fine, light-coloured, quartzose sands, partly 

 mixed with clayey matter. It usually lies on a layer 

 of Chalk flint, of an olive-green colour externally, and 

 which probably represents the effect of the waste of the 

 carbonate of lime of the chalk which was carried away 

 in solution as bicarbonate, through the infiltration of 

 rain-water after the deposition of the sands, the associated 

 silica having been concentrated and deposited in this 

 band. These sands range from the Isle of Thanet 

 westward to the neighbourhood of London, varying 

 from about 50 feet thick, in parts of Kent, to 4 feet, 

 at East Horsley, where they disappear, being over- 

 lapped by the Woolwich and Eeading beds. They are 

 quite unknown in the Hampshire basin. 



The fossils of this subformation are entirely marine, 

 and embrace about 70 known species. Among these are 

 a shark of the genus Lamna, Pisodus, and others ; a 

 Nautilus ; Gasteropoda, such as Fusus tuberosus, 

 Scalaria Bowerbankii, Natica, Aporrhais, &c. ; a con- 

 siderable number of Lamellibranchiata, such as Nucula 

 Thanetania, &c.; Pholadomya Koninckii &c.; Corbula, 

 Cardium, Ostrea Bellovacina, &c. &c. ; Crustacea, 

 Hoploparia, andPalceocorystesi spines of Echini (rare), 

 a coral, a few Foraminifera, and land-plants. In the 



