204 J. S. GARDNER ON THE LOWER LONDON TERTIARIES. 



290 yards further west the Corbula-\)e&is in grey sand near the base of 

 the cliff, and Dentalia occur ; and 150 yards west of this, at the Gap, 

 it is on a level with the shingle : 22 yards from the west corner of 

 the Gap, the Corbula-hed appears on the beach below high-water 

 mark and 10 feet below the base of the cliff, and soon after dis- 

 appears out to sea. A very rich zone of fossils occurs at this spot, 

 just below the Corbula-'bed and between it and the Gap. The great 

 Cyprina, 5^ inches in diameter, occurs here singly or in pairs of 

 valves, and nowhere else. Astarte tenera very common and per- 

 fect, Cucullcea decussata, Pectuncidus terebratularis, Sanguinolaria 

 Edwardsii, and Dentalium are other shells whose range in the Thanet 

 Beds seems almost limited to this zone, and which can only be col- 

 lected at this one spot in Heme Bay. It will be observed that the 

 fauna is much richer here than lower down ; the unrolled silicified 

 wood is replaced by rolled and bored lignitized wood ; and a few very 

 small quartz pebbles occur. None of the shells are in a living po- 

 sition, though the valves are very generally closed ; a few valves are 

 broken, and they seem to have been more under the action of moving 

 water, and therefore deposited at a less depth than towards the base 

 of the Thanet Beds. Seventy feet of muddy sand had in fact accu- 

 mulated in what was originally shallow water ; and unless this was 

 wholly counterbalanced b}' subsidence, the bottom would naturally 

 be raised nearer the surface. 



Prof. Prestwich includes this zone with its rich group of fossils in 

 the Woolwich-and-Eeading Beds. He, as well as Mr. "Whitaker, ac- 

 knowledge the extreme difficulty of separating them from the under- 

 lying beds, both here and at Bichborough, where the Corbula-zone 

 also occurs. The former says: — " This want of clear divisional sur- 

 faces, and the occurrence of several of the same species of shells in 

 the two series, might be considered an objection to their being thus 

 separated. The fossils, however, taken as a group, are different from 

 those of the Thanet Sands, whilst the sands are more siliceous and 

 contain a larger proportion of green sand and some disseminated 

 flint-pebbles — two mineral characters deriving some importance from 

 their breadth and constancy"*. 



These distinctions are in themselves very slight as bases of separation 

 between two marine formations ; but even such as they are, they 

 have since been materially lessened. Taking the marine bivalve 

 mollusca, the only group at all perfectly known from these 

 formations, we find, according to Prestwich, twelve species 

 occurring in the Thanet Sand and not in the "Woolwich-and-Beading 

 Beds. On the other hand, fourteen species from the latter do not 

 occur in the Thanet Beds. In fact six species only are cited as 

 common to the two formations. 



The Survey list, however t, gives very different results ; and as all 

 but two of the species have been also found at Pegwell Bay, and. 

 only admittedly Thanet Beds are present there, no errors as to the 



* Prestwich, Quart. Journ. Geol. Soc. vol. x. p. 112 (1854); see also Whitaker, 

 Quart. Journ. Geol. Soc. vol. xxii. pp. 409, 410, 419. 

 t Mem. Geol. Surv. vol. iv. 1876. 



