89 . 



he thinks it probable, that these insects were carried for some distance 

 down rivers before they reached the estuary of the Wealden*. 



Cretaceous System. — Passing from the Wealden, I must allude 

 to two communications which this session has produced from Dr. 

 Mantell, both relating to the Cretaceous system. In one of these 

 he describes three species of fruit from the lower greensand and 

 chalk of Kent and Sussex, thus offering the most important addition 

 to the flora of that period since the publication of the compendious 

 memoir of Dr. Fitton. The other memoir is upon certain small 

 darkbrownconcretions from the lower greensand of Maidstone, and to 

 which Dr. Mantell has given the name of Molluskite, in consequence 

 of their being supposed to be the carbonized remains of the soft 

 parts of Mollusca. This coriimunication is the result of the re- 

 searches of Mr. W. H. Bensted of Maidstone, who collected slabs 

 full of these concretions, previously supposed to be of coprolitic 

 origin, but he suggested that they might be the soft portions of tes- 

 taceous Mollusca. When free, and where the peculiar substance had 

 been formed in considerable quantity and in a pure state, it was in- 

 ferred by Dr. Mantell that the soft parts had floated away from the 

 shell after the death of the animals, as often occurs at present among 

 living testacea. Chemical examination by the Rev. J. B. Reade and 

 Mr. Rigg detected animal carbon in the substance of some of these 

 bodies. 



Tertiary Period. 



The session has not added much to our knowledge of British Ter- 

 tiary deposits. Mr. Trimmer, well known to us by his researches in 

 detrital phsenomena, has lately expressed his opinion that the pecu- 

 liar eroded surface of the chalk, in which pipes filled with sand or 

 gravel are of frequent occurrence, was produced by the action of the 

 sea during a period which preceded the deposit of the London clay. 

 There can be no objection to this view being applied to all those 

 corroded surfaces of the chalk, which are surmounted by the Eocene 

 deposits of plastic clay and sand and London clay, including, I would 

 add from the recent observations of Count Keyserling and myself, the 

 junction of the chalk and Lower Tertiary in Alum Bay. It would, 

 however, be manifestly wrong to suppose, that such a corrosion of 

 the surface of the chalk had not also been effected at other and sub- 

 sequent periods ; and as proofs of a still more recent corrosion, 

 the observer has only to examine the shore and cliffs near Brighton, 

 and see how similar cavities have been filled up by a breccia, in 

 which the bones of elephants are imbedded. 



Some remarkable concretions in the Tertiary beds of the Isle of 

 Man (where the newer marine Pliocene strata were first described 

 by Professor E.Forbes, and shown by him to occupy perhaps a larger 

 area than in any one locality of the British Isles), have elicited from 



* See observations on the Neocomian not being an equivalent of the 

 Wealden, p. 67. I may here also state, that Mr. Robertson of Elgin has 

 discovered some of the fresh water shells of the Wealden, intercalated in 

 the oolitic strata of Brora. His memoir will soon be read. 

 VOL. IV. PART I. H 



