14 PSOCEEDINGS OF THE GEOLOGICAL SOCIETY. [NoV. 22, 



in the London Clay, but comparatively rare in the Woolwich beds : 

 moulds of it have not been hitherto recorded in the first deposit, but 

 Messrs. Prestwich and De la Condamine noticed them many years 

 ago in the latter at Counter Hill*. 



However common these spaces may be, the facts still remain for 

 consideration, that a rather insoluble mineral has been deposited in 

 and removed from sedimentary marine and fluviatile deposits, and 

 that the mineral has not hitherto been formed artificially. The crys- 

 talline nature, optical properties, and chemical composition of selenite 

 are well known ; but it is a species which has either been much neg- 

 lected or has been treated as crystalline gypsum, being considered 

 to be influenced identically with it by reagents. 



The part of the Woolwich bed and tbat of the London Clay under 

 consideration, were deposited under diJfferent circumstances. Mr. 

 Prestwich's great essay has so exhausted the subject, that it simply 

 remains to state that the one was accumulated during the physical 

 changes incident to an estuary, and that the other is more or less a 

 deep-sea deposit. 



It is evident that neither the selenite nor the shells forming the 

 bulk of the deposit near Mottingham were rolled, for the impressions 

 are sharp, and the fragile shells are usually entire. The shells 

 belonged to individuals of all ages, they were all jumbled together, 

 and it is evident that the mollusca did not live quite on the same 

 area in which their shells are preserved. If it could be allowed that 

 these masses were washed together to die under some unusual cir- 

 cumstances, one of which must have been a sudden depression of the 

 surface, an immense amount of decomposition must have ensued, and 

 the decaying mass of mollusca, extending over a large area, would 

 have produced well-marked changes in the hthology of the district. 

 All the chemico-geologic facts, however, disprove such an amount of 

 decomposition, and indicate a feeble amount of organic decay. The 

 water-supply probably consisted of river-water, brackish water, and 

 occasionally of pure sea- water. The river probably worked its way 

 through the chalk, and its water was as well supplied with sulphate 

 of lime as similar streams now are. More of this salt it could not 

 have had, for the molluscous and plant-life was evidently abundant 

 and vigorous. The quantity of clay and sand which now forms the 

 matrix for the shells is but a small vestige of what was deposited 

 contemporaneously with repeated quantities of shells, and it is most 

 probable that during the slow depression of the surface of the land 

 and estuarine bottom, the layers of sheUs were deposited with great 

 quantities of silt and clay, which were more or less washed away 

 from time to time. That is to say, the moUusca were gently washed 

 into channels and formed a thin layer ; they were covered up with 

 silt and clay, and their decomposition was probably in progress when 

 another layer was formed above ; the greater part of the inorganic 

 deposit was then washed away, and the layers gradually merged one 

 into the other ; or the succession of layers may have been numerous 

 before the bulk of the sand and clay was removed. This explanation 

 * Prestwich, Quart. Journ, Geol. Soc. vol. x. p. 123. 



