ANNIVERSARY ADDRESS OF THE PRESIDENT. Ixvii 
cites showing a much more considerable decomposition of the ori- 
ginal vegetable matter, the carbon occurring in greater proportion to 
the oxygen and hydrogen. 
Adverting to the scarcity of organic remains in the Bagshot sands, 
Mr. Prestwich supposes that this scarcity or absence of them arises, 
not so much from original causes unfavourable to the existence of 
testacea and other animals, or to the entombment of the remains of 
life, as to the subsequent operation of chemical agencies. He ob- 
serves that where conditions for preservation were favourable, as in 
the central and upper divisions of these sands, traces of a fauna are 
far from restricted, and among other instances points out, that at 
Chobham Place, where a slight quantity of clay and peroxide of iron 
has consolidated the usually incoherent green sands, there is a semi- 
solid mass full of the casts and impressions of shells. 
Mr. Prestwich, after remarking that in certain beds only the im- 
pressions and casts (and that rarely) at present exist, and that in this 
case there are favourable mineral conditions which do not generally 
prevail, proceeds to show that the hydrated peroxide of iron occurs 
commonly in the upper and middle Bagshot sands. He infers, that 
if a carbonate of iron were disseminated through the mass of these 
beds, and segregated in different places, it would, by decomposition, 
gradually evolve its carbonic acid, which being taken up by water 
percolating through the mass, would thus render this water capable 
of dissolving the carbonate of lime which might come in its way, thus 
acting upon the shells when it reached them in the pervious beds. 
It could scarcely but happen, where there may be sufficient free car- 
bonic acid, no matter how derived, in waters percolating through 
rocks, that shells might be readily removed from beds, so raised 
into the atmosphere, that the waters containing the bicarbonate of 
lime in solution can readily drain off as springs, and the matter of 
the shells be thus removed to lakes and seas, perhaps to be again 
employed by molluses for their harder parts. 
The preservation of the matter of the shells themselves, in clays or 
other substances, usually called impervious, always however to be re- 
garded as a comparative term, is well known to geologists, as also 
their absence in sandstones and other rocks. Even in some lime- 
stones, the carbonate of lime constituting the fossils is not that of the 
original shell, but a crystalline body replacing the actual shell, and 
therefore entering a cavity produced by the solution and disappear- 
ance of the original body. very cast of a shell we find is an ex- 
ample of the solution and disappearance of the original. And not 
only do we find organic remains thus acted upon, but occasionally 
the carbonate of lime of rocks, in part formed of that substance, has 
disappeared partially or wholly. This may frequently be observed 
in some districts composed of the older rocks, and the origina! mixed 
rock, partly a mechanical accumulation of mud and silt, and partly 
a chemical product, from the admixture of carbonate of lime, presents 
only a sandy or silty appearance for many feet, where it has been ex- 
posed to atmospheric influences. 
The manner in which the harder portions of animal remains have 
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