TRANSACTIONS OF THE SECTIONS. 63 



M. Deshayes in the third volume of the ' Memoirs Geol. Soc. of France,' from Ter- 

 tiary deposits in the neighbourhood of Kertcli ; the majority of them are, however, 

 from near Sevastopol. The classes represented, with the exception of two new species 

 of Amorphozoa, are entirely composed of species of Conchifera and Gasteropoda, in 

 nearly equal proportions, more than half of which are new. Of the peculiar forms of 

 Cardhim resembling the present Aralo-Caspian types, thirty-one species are tabulated, 

 including those described by M. Deshayes, as found associated with bands of iron-ore 

 before alluded to. Several of these are in the collection of the British Museum from 

 the same locality ; also twelve species of Trochus, some of them being in beautiful 

 preservation, and mostly collected by Capt. Cockburn from the Quarantine Harbour, 

 Sevastopol. Six of these are identified with species iigured in the fine work of M. 

 Hommaire, and described by M. d'Orbigny from the Tertiary of Kichinev in Bessa- 

 rabia, and contemporaneous deposits. 



On the Origin of Siliceous Deposits in the Chalk Formation. 

 By J. S. BowERBANK, F.R.S., F.G.S. 8fC. 



Some years since the author read at the Geological Society of London, a paper on 

 the origin and structure of the siliceous deposits of the chalk and greensand forma- 

 tions, and subsequently one on the spongeous origin of moss, agates, &c., in which he 

 advocated the doctrine of the derivation of nearly the whole of the flints and cherts 

 from various species of sponges that existed in the ancient oceans. 



The principal proofs adduced at that time in favour of the views then enunciated, 

 were to a great extent derived from the microscopical evidence afforded by sections 

 of such siliceous bodies. . The object of the present communication is to strengthen 

 and confirm those views by the production of evidence derived from information 

 recently acquired, regarding the habits and manner of growth of the recent Spon- 

 giadae. In the opinion of the author, the whole of the numerous strata of nodular and 

 tabular flints are derived from vast quantities of sponges that existed in the seas of those 

 periods ; the attraction of the animal matter of the sponges inducing the deposit of 

 the silex, which in the first instance is always in the form of a thin film surrounding 

 the skeleton of the sponge, and from which successive crops of chalcedonic crystals 

 proceed until the solidification of the whole is effected. 



The tabular beds of flint are accounted for on the presumption that the sponges 

 originating the deposit grew on a more consolidated bottom than the tuberous ones, 

 and that they therefore developed themselves laterally instead of perpendicularly, as 

 many species of recent sponges are in the habit of doing, and that approaching and 

 touching each other, they united and thus formed extensive and continuous beds 

 instead of numerous isolated specimens. The author illustrated this part of his 

 subject by producing four recent sponges of the same species, which having been 

 placed in close contact while in the living state, became firmly united to each other 

 within eighteen hours, and ultimately formed but one sponge. 



Th« occurrence of the shells of bivalves and of echinoderms filled with flint or chert, 

 was accounted for on the principle of their having been previously filled with living 

 sponges, and subsequently fossilized by the deposit in the spongeous tissue of silex 

 held in solution in the water ; in illustration of which the author produced specimens 

 of recent bivalve shells in a closed condition, which were completely filled with recent 

 sponges. 



The loose specimens of fossil sponges contained in the Wiltshire flints were ex- 

 plained on the same principle ; but their not adhering to each other, the author stated, 

 was in accordance with the law that always obtains among the recent Spongiadse, 

 that although individuals of the same species of sponge always adhere on being 

 brought in close contact, those of different species never unite under such circum- 

 stances, and specimens of recent sponges, one species completely enveloping the other, 

 but without the slightest adherence between them, were exhibited. 



The author concluded his paper by applying the same principles to the siliceous 

 deposits of the whole of the geological formations of aqueous origin, and by expressing 

 his opinion that the geological office of the Spongiada; in creation is that of inducing 

 the deposit of siliceous matter held in solution in the ocean, as the Corallidse assist in 

 the consolidation of the calcareous matter. 



