Chalk and Flint of the South-east of England. 79 



spongeous tissue, and of spicula, in our flints ; in the presence of 

 polythalamia and infusoria, particularly o{ Xanthidia, in the canals 

 of sponges, and their frequent suspension throughout the mass 

 of a siliceous nodule ; as if the spongeous tissue had retained its 

 form sufficiently long to allow of the silicification of the animal- 

 cules, and had subsequently perished. At the same time I must 

 express my conviction, that the facts he so faithfully portrays do 

 not warrant the hypothesis that all the nodules, veins, dikes, and 

 sheets of flint, are to be ascribed to the silicification of sponges ; 

 neither can I admit that the cavities of the shells of echinoderms 

 and moUusks, now found filled with flint, were previously occu- 

 pied by sponges. The theory of M. Ehrenberg, that the compact 

 nodules of flint are the consolidated pulverulent siliceous parti- 

 cles of infusoria, I conceive to be equally untenable. Nor do the 

 facts hitherto brought before us seem to warrant the inference, 

 that the abundance of siliceous spicula in any of the porifera 

 rendered those bodies more favourable for silicification ; on the 

 contrary, the soft gelatinous animal matter, as Mr. Bowerbank 

 has suggested, does appear to have exerted such an influence by 

 some species of elective affinity or attraction : hence the frequent 

 silicification of the bodies of mollusks, while the shell retains its 

 calcareous character, as in the specimen of an oyster figured in 

 the ' Medals of Creation,' p. 363. 



In many of the silicified fossils of the chalk, the minerahzation 

 is simply that of incrustation and infiltration ; such is the state of 

 numerous sponges, which are, as it were, invested by the flint, and 

 have their pores and tubes filled with the same substance ; but the 

 spongeous tissue is in the condition of a brown friable earthy sub- 

 stance. In other examples the sponge has been incrusted by a mass 

 of liquid silex, and its tissue has subsequently perished ; in this 

 manner have been formed those hollow nodules, which, on being 

 broken, present a large cavity containing only a little white pow- 

 der, or some loose fragments of silicified sponge ; while in other 

 specimens the cavity is hned with quartz and chalcedony, probably 

 introduced by subsequent infiltration through the nodule. It fre- 

 quently happens that the zoophyte is only partially invested with 

 silex, while the other portion is imbedded in the chalk, and is a 

 friable calcareous substance. The Choanites and Ventriculites are 

 often found in this condition, and hence the protean forms assumed 

 by the flints that have been moulded in the cavities of these orga- 

 nisms. These specimens appear to demonstrate that the organic 

 bodies became permeated with flint, only when they happened to 

 be exposed to the current or stream of liquid silex, which pene- 

 trated such portions of structures, or entered the cavities of such 

 shells, and echinoderms, as were lying at the bottom of the ocean 

 over which it flowed, or were immersed in the calcareous detritus 



