Flints of the Upper or White Chalk. 177 



viewed in connexion with the fact tliat they rendered it neces- 

 sary for me to show on what grounds I reject those conclusions ; 

 and I mean from henceforth in this paper to discuss only the 

 replacement theory of Prof. Rupert Jones, although compelled 

 to dissent* from it to the extent of maintaining that it performs 

 no part whatever in the production of the true or black flint of 

 the Upper Chalk, which, apart from its imbedded pseudomorphs 

 of Foraminifera and other organisms, and their comminuted 

 debris, I regard as having, from first to last, passed through 

 the following stages : — first, in the state of inorganic, and pro- 

 bably some organic, silica held in solution in sea-water under 

 the special conditions prevailing at the deep-sea bed ; then, 

 in the shape of sponge-skeletons and spicules* ; next, of 

 silica in its gelatinous and perfectly colloidal condition ; and, 

 finally, in the form of the flints. 



In short, every imbedded pseudomorph, without exception, 

 consists not of pure but of impure flint ; in other words, it 

 becomes cherty, and ought to be in the strictest sense regarded 

 as an " inclusion.'''' Moreover the whole of these pseudo- 

 morphs included in, but not forming part and parcel of, the 

 pure flint (of course omitting the large foreign bodies, such as 

 Echinoderm and other shells, which so frequently form a 

 nucleus, around or within which the colloidal silica has col- 

 lected), if consolidated into compact masses, would rarely, if 

 ever, occupy a space that would not be insignificant in com- 

 parison with the bulk of the remaining mass of pure flint in 

 which they had been imprisoned. In their case replacement 

 of carbonate of lime by silica must undoubtedly have taken 

 place, precisely as it takes place when large masses of shell, 

 as, for example of Inoceramus, have been accid«ntally en- 

 trapped in the still plastic and viscid colloid. But inasmuch 

 as it would be a palpable error to regard such foreign bodies as 

 integral portions of the flinty matrix, even though occurring in 

 the highest stage of silicification, so long as there is the slightest 

 trace of the opalescence resulting from the combination of the 

 silicic acid with a mere remnant of the calcareous or fibrous 

 tissues, so it would be a palpable error to regard the minute 

 organisms which are almost invariably imprisoned in the flint, 

 like insects in amber, as constituting integral portions of the 

 imprisoning material. Or, per contra^ if these are regarded as 



• For the purposes of the present inquiry, I have deemed it inexpedient 

 to inchide the Poljcystina, and other minute siliceous-shelled structures, 

 the silica of which, though undoubtedly contributing their quota to 

 the general volume of the flints, exists in such a comparatively small 

 proportion as not to deserve mention in discussing the general question 

 of the flint-formation. 



