AUSTEN — TERTIARY DEPOSITS OF THE SUSSEX COAST. 49 



the platform of consolidated nummulitic strata on which it rests. 

 (See fig. 4, g.) 



Fig. 4. — Diagram-section showing the general relations of the newer 

 Tertiary deposits of the Sussex Levels. 



'■iAWMM£M-:^!iSMEMi, 



9 9 



a. Vegetable mould, b. Brick-earth, c. Gravel, with occasional seams of sand 

 and pebbles, and with shells, d. Yellow clayey gravel, with large blocks. 

 e. Marine mud-deposit, with Lutraria rugosa, &c. f. Lower red gravel, g. 

 Older tertiary strata. 



The thickness of this deposit, and the probability of its increase 

 seawards, can only be properly estimated when seen at low- water 

 spring-tides ; on one such occasion I measured thicknesses of from 18 

 to 20 feet at places where the mass was partly above water, and at the 

 same time intersected by deep channels ; in these positions it is seen 

 passing away beneath the sea-bed of this part of the Sussex coast ; 

 or, in other words, the area of its accumulation had its main extension 

 in that direction. 



From low-water landwards the deposit diminishes in thickness. 

 This is in part due to ordinary coast-line destruction ; but it will be 

 seen that where it passes beneath the higher deposits of the Sussex 

 levels, as in the coast-sections, it has a furrowed uneven surface, 

 showing that it experienced considerable denudation in this direction 

 either before or at the time those newer beds were accumulated. It 

 is not found far inland. It evidently, however, had at one time a 

 greater continuous extension in this direction, from the small patches 

 which occasionally occur. 



The dark colour of these beds seems to be partly due to diffused 

 vegetable matter ; their composition, in the uniform minuteness of 

 their particles, indicates tranquil deposition ; the condition of the 

 water beneath which this took place must, however, be mainly de- 

 rived from the habits of the included Testacea. 



The whole of this deposit, so far as the limits within which it can be 

 observed extend, is seen to rest on a portion of that broad platform of 

 chalk and nummulitic strata which has been already noticed, with the 

 occasional intervention of patches of gravel (fig. 4,/") ; this platform, 

 when denuded, exhibits a surface which is of some interest in the his- 

 tory of the change the area has undergone. On the coast near 

 Medmeney, on the Bracklesham Ba^^ side, the surface is occupied by 

 the remains of a colony of the Pholas crispata which has burrowed 

 into it ; and the first peculiarity which strikes the attention is the 

 enormous size which this species here attained. This is not confined 



VOL. XIII. — PART I. E 



