Flints of the Upper or White Chalk. 183 



babllity, drifted by tidal or other currents into those areas in 

 which they became finally accumulated. This, coupled with 

 the almost entire absence of sponge-spicules, tends moreover 

 to prove that there were no siliceous sponges in those areas, 

 and, consequently, that the only substance which would have 

 ensured their solution, namely sponge or Foraminiferal pro- 

 toplasm (for in like manner no Foraminifera are observable 

 in the limestone), toas entirely wanting. Hence their im- 

 munity from destruction and perfect preservation in the 

 limestone. 



Of the existence of pure Diatomacean deposits at much 

 greater depths in the ocean and at vastly greater distances 

 from land than those just named, there is, as every biologist 

 knows, abundant evidence — for example, in the antarctic 

 regions, where they were discovered by Sir Joseph Hooker in 

 1843, and thirty years later by the naturalists on board the 

 * Challenger.' I have now before me sections of a Norwegian 

 limestone literally crowded with marine diatoms of the kind 

 already described, which are also in the same perfect state of 

 preservation, — the inference I draw from these facts being 

 that the unaltered condition of the organisms under notice 

 is due to the very limited power of sea-water at mode- 

 rate depths, and consequently under moderate pressure, even 

 when aided by abundant products of animal and vegetable 

 decomposition, to reduce silica to a colloidal state; and, con- 

 versely, that the superabundance, over the deep-sea calcareous 

 areas, of siliceous sponges and their concomitant protoplasmic 

 investment furnishes us with a highly probable and satisfac- 

 tory explanation why flint-formation has taken place under 

 one determinate set of conditions and has failed entirely to 

 take place where these conditions are absent. 



Reasons have already been given by me for regarding the 

 simple deposition of silica from an aqueous solution, whether 

 in the condition of flint which Mr. Sollas describes as " crys- 

 talline^'' or of pure rock-crystal, as furnishing no parallel 

 whatever to the process of the true chalk-flint formation as it 

 occurs in the chalk strata, in which I contend there is no 

 deposition of silica in tl^e ordinary acceptation of the term, but 

 a still more simple process of solidification of two gelatinous 

 colloids " more or less rich'''' (to quote an expression of Mr. 

 Graham's) " in combined ivater as at first produced,''^ but 

 which gradually part with their " combined water " to the 

 surrounding medium, under the dialyzing action of their own 

 gelatinous substance, and become more and more consolidated 

 until the period arrives when they have lost the whole of their 

 " hydration," and then " appear as a colloidal glassy hyalite " 



