1847.] PRESTWICH ON THE BAGSHOT SANDS. 397 



With regard to the Alum Bay section, the difference in mineral 

 composition between the Bagshot sands and these their presumed 

 equivalents at iVlum Bay, is not so great as might be supposed. They 

 both consist in by far their larger part of loose siliceous sands with 

 some green sands and clays. At Alum Bay these are grouped in 

 many distinct strata ; in the Bagshot district on the contrary, with 

 the exception of the central green sands and brown clays, the clays are 

 more intimately mixed and interstratified with the sands. At the 

 same time the clay also probabl}^ exists in rather less total quantity. 



With regard to the bright and brilliant reds of the Alum Bay section, 

 that is a local condition produced by the more abundant presence of 

 the peroxide of iron. Of this we have some traces even in the Bag- 

 shot sands, for at West End near Farnham some thin beds of the 

 upper sands are of a bright red colour, whilst the general prevalence 

 of various tints of yellow, ochreous and brown, indicates a like general 

 diffusion of the hydrated peroxide of iron. 



The silicate of iron, forming the mass of the beds of green sands, 

 occurs in both ; so also do we find in both thin layers of small round 

 flint pebbles and beds of lignite. All these are points of evidence, which 

 though of little importance separately, yet together form, from their 

 general and undivided agreement, a corroborative proof of a like origin 

 in the strata so co-related. For, where the distances are not exces- 

 sive, and the general evidence points to a probable contemporaneity 

 and continuity, the persistent prevalence of peculiar chemical and 

 mechanical conditions, and of marked and consistent lithological cha- 

 racters, over large areas, affords, I conceive, independently of paleeon- 

 tological evidence, tolerably presumptive proofs of a sediment derived 



gradually evolve its component part of carbonic acid, which, being taken up by 

 the water percolating through these strata, would immediately act upon any car- 

 bonate of lime with which it came into contact. And as, from the general sili- 

 ceous character of the strata, the ^^vaters would pass through them without be- 

 coming saturated with other substances, and as the solution of cai'bonic acid 

 could not act upon the siliceous and argillaceous rocks, it would follow that its 

 action as a solvent would be almost entirely confined either to any disseminated 

 traces of carbonate of lime, or to the substance of the shells. In both cases the 

 bicarbonate of lime would be formed, which being readily soluble in water, would 

 by the gradual percolation thereof be entirely removed. 



In the cases especially where the carbonate of iron had segregated in the inte- 

 rior or around the exterior of the shell, the liberated carbonic acid would, as the 

 . carbonate passed into the hydrated peroxide, at once unite with the carbonate of 

 lime of the shell, converting it into a bicarbonate soluble in and removable by water. 



It would result tliat a solid cast of the hydrated peroxide of iron, in the one 

 instance of the interior of the shell, and in the other instance of the exterior, would 

 remain ; in both cases the space left by the removed shell would usually be left 

 unoccupied, and such is the condition of most of the fossils of the Bagshot sands. 



Where the shells were imbedded in the loose sands free from clay or ferruginous 

 concretions, w^hich could afford them any solidity of matrix, they might be thus 

 removed without leaving a ti'ace of their original presence. Witli regard to the 

 carbonate of iron, it is a mineral common in many I'ocks, especially in those below 

 the chalk, whence the materials forming this deposit were doubtlessly derived. 



This reasoning may possibly admit of extension to some other formations in 

 which siliceous sands and soft porous sandstones predominate. It is perfectly 

 well known that rain and spring- water will dissolve carbonate of lime, but I doubt 

 whether they are agents sufficiently powerful to produce the extensive effects v/e 

 have alluded to. 



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