HISTOEY OF THE CRETACEOUS FLINTS. 71 



reasons for doubting the possibility of cretaceous rock, with inter- 

 calated flint-beds, being to this day in process' of formation, inas- 

 much as the requisite materials are still forthcoming, and, as already 

 urged, the physical conditions observable in the abyssal waters, after 

 a certain depth is reached, have in all probability never altered to 

 such an extent as to render a flint-bearing Cretaceous formation 

 even improbable. 



So far as I have been able to discover from the writings of the 

 most recent authors who have discussed the mode of formation o 

 the flints, their explanations have stopped short just where, in 

 reality, the unique and by far most important and interesting points 

 in the history of these structures may be said to commence. Thus 

 we find it stated that, " by some means or other, the organic silex, 

 distributed in the shape of sponge-spicules and other siliceous 

 organisms in the Chalk, has been dissolved or reduced to a colloid 

 state, and accumulated in moulds formed by the shells or outer walls 

 of imbedded animals of various classes." We do not precisely know 

 how the solution of the silica has been effected, though, when " once 

 reduced to a colloid condition, it is easy enough to imagine it may 

 be sifted from the water by a process of endosmose, the chalk 

 matrix acting as a porous medium, and accumulated in any con- 

 venient cavities"*. 



But it must be obvious at a glance that this furnishes no expla- 

 nation whatever of the mode of production of the flints properly so 

 called, but only of the fossilization or mode in which siliceous casts 

 of organisms of various kinds imbedded in the chalk have been 

 formed — the question of the mode of formation of the flint-beds, and 

 their alternation with the strata of chalk (which is, in reality, the most 

 remarkable and unaccountable of the whole series of phenomena), 

 being left just as intact as before, no attempt having been made to 

 show, even as regards the quantity of the siliceous material contributed 

 from every recognized and available source, that, independently of 

 the colloid-producing substance which constitutes an indispensable 

 factor of the operation, there was enough silica present to meet the 

 requirements of the case. Whence, then, did all the silica come ? 

 Why is it almost invariably found existing in layers parallel to 

 the stratification of the Chalk ? And what has really been its 

 history, from first to last ? 



It is to these questions that I hope, on the present occasion, to 

 be able to furnish such answers as shall, at all events, form the 

 groundwork of a good working h3'pothesis, and one capable of 

 further elaboration as time and opportunity- permit. Meanwhile I 

 may be allowed to state that the conclusions arrived at by me have 

 their origin in the assumption that, in the nearly total elimination 

 of the organic silica from the organic carbonate of lime, in the 

 almost constant aggregation of the colloid silica around some 

 foreign body, in the ultimate consolidation of the colloid material 

 into nodular masses or more or less continuous sheets, in the 

 stratification of these masses and sheets, and, collaterally, in the 

 * 'The Depths of the Sea,' by Sir WyviUo Thomson, 1872,;p. 482. 



