1852.] PRESTWICH ON THE THANET SANDS. 243 



The upper part of these sands near London, and throughout the 

 greater part of Kent, appears to be almost perfectly free from car- 

 bonate of hme, and in the middle and lower parts usually mere traces 

 of it are present. A rough analysis of six specimens from various 

 localities yielded only from 1 to 2 per cent. At Heme Bay and Peg- 

 well Bay, however, carbonate of lime is present in greater abundance, 

 so as to be readily detected by the ordinary tests. It forms, a few 

 feet from the top of the deposit, at these places, a tabular bed of 

 large, hard, flat, concretionary masses, effervescing strongly with dilute 

 acid. At Wingham, near Canterbury, these beds form a consider- 

 able thickness of semi-indurated fossiliferous marls. As this division 

 approaches London, the carbonate of lime almost entirely disappears* : 

 owing to this cause, and the perfect permeabihty of the strata, even 

 the carbonate of lime of the shells, of whose existence we have some 

 evidence, has been entirely removed. 



A distinctive feature of this division is, that it never contains layers 

 or beds of those rounded black flint-pebbles so common in the over- 

 lying divisions, nor does it ever exhibit subordinate beds of those mot- 

 tled clays which so well mark the middle division. A few rounded 

 flint-pebbles, some few even as large as a cannon-ball, have been occa- 

 sionally found in the mass of the lower sands, but they are usually 

 dispersed singly, and are of very rare occurrence. 



Mica is sparingly disseminated in the sands. At Pegwell Bay, 

 however, it occurs in a thin band of sandstone, in quantity sufficient 

 to divide it into fine laminae. The peroxide of iron slightly tinges 

 some of the beds, and forms occasionally small nodular sandy concre- 

 tions and casts of the interior of the shells. It is sometimes present 

 in the form of veins traversing the sands diagonally, and also in 

 patches as stains in the sands, but it never forms solid masses or layers 

 of ferruginous sandstone. With the exception of the calcareous 

 sandstones of Pegwell Bay and the Reculvers, the Thanet Sands very 

 rarely, if ever, contain blocks or layers of concretionary sandstone. 

 At Erith, such masses of flat, tabular, very hard sandstone have, 

 it is true, been found near the top of the pit f ; and in the lane lead- 

 ing from Gad's Hill to Shorne Ridgeway, a block of this description 

 was to be seen, apparently in situ, a short time since in a cutting 

 through either the upper part of these sands or the lower part of the 

 middle division. These sands may possibly have furnished some of 

 ►the large masses of sandstone which so commonly occur in the drift 

 on the edge of the chalk downs in Kent, but I believe that the bulk 

 of them were derived from the- next division of these Lower Tertiaries, 

 and more especially from the " Basement-bed of the London Clay." 



Small grains of selenite are, according to Mr. De la Condamine, 

 frequently mixed in some abundance with these sands in the neigh- 

 bourhood of Blackheath. 



A very marked feature of the ^^ Thanet Sands" is the constant oc- 

 currence at the very base of the deposit, and immediately reposing on 



* At Faversham even the few fossils wliicli are preserved are almost all in a 

 silicified condition, and only traces of carbonate of lime are present. 



t It is uncertain, however, whether these do not belong to the middle division. 



