BEMBRIDGE MARLS. 171 



Another measurement of the Marls, made near the same place 

 in 1888, gave a total visible thickness of 93 feet; but about 15 

 feet of the upper part of the cliff are overgrown and hidden. 

 Possibly there may be on outlier of Hamstead Beds here, but 

 if not, the Bembridge Marls must be at least 106 feet thick, with 

 the top not reached. This computation agrees with the thickness 

 proved at St. Helen's. 



The marine base of the Bembridge Marls is so variable that 

 the following detailed notes of the beds seen on the coast will be 

 useful,* especially as portions are often entirely hidden by beach 

 sand or talus. The account is that given by Forbes, with some 

 additions from notes made in 1888. 



The blue septarian limestone strikingly resembles in mineral 

 character the harder insect-bearing limestones of the Purbeck 

 beds. It is thickest (about 1 foot) and finest about half way 

 between Whitecliff Point and the Foreland, where its upper 

 surface forms part of the floor of the shore. Everywhere it 

 preserves the same pecuhar mineral character. Near the same 

 place the finest display of the oyster bed is seen, the surface of 

 which also, for some distance, forms the floor of the shore. There 

 it is underlain by a pale concretionary blue marl, containino- occa- 

 sional pebbles, and abounding in casts of shells, especially of 

 Ccritliium (probably C. mutahile), occasionally mingled with casts of 

 freshwater shells {Limncea longiscata, Pknwrbis discus and P. ohtu- 

 sus), CyrcncB of more species than one, a small angulated Corhula, 

 Murex Forhcsii, a curious pupa-like Bulimus ?, occasional Mytili^ 

 HydrohicB, a Tellinoid bivalve, occasional examples of Melania 

 muricata, and traces of fish. Between this blue marl and the 

 oyster band is a thin sandy bed, filled with comminuted shells, 

 and on this rest numerous individuals of Cythcrea incrassata, with 

 their valves closed, but the shells are in so exceedingly decayed a 

 condition that, after many trials, Forbes was unable to remove 

 any entire. The internal casts, however, are fine and transportable. 

 Then come the oysters, mostly, but not all, single valves, here and 

 there mingled with good double specimens. They are thinly dis- 

 tributed, but occasionally occur in clusters of considerable number, 

 bristling the surface of the shore. Individuals vary much in 

 shape even in the same cluster. With them are Mytili (31. affinis), 

 Nucula similis, a Solecurtus-Yi^Q bivalve, and Forbes once met with 

 a Natica. The Mytili and Nuculce. retain the substance of their 

 shells perfectly. Occasional pebbles are mingled with the oysters. 



In a few places interesting indications can be found that 

 marine conditions lingered for some time. (7/?V>??a-bored oysters 

 occur, on and in Avhich Serpnla and Balanus have grown, and the 

 dead Sei-pula and Balani have been subsequently covered by a 

 growth of Polyzoa. The best preserved marine shells will be 

 found at about half-tide level, a short distance south of the Fore- 

 land Inn, where even the Cythcrea may occasionnlly be obtained 

 in a perfect condition, though fragile. Nucidu similis is abundant 



