396 PROCEEDINGS OF THE GEOLOGICAL SOCIETY. [May 26j 



believe that their absence partly arises, not so iiiucli from original 

 causes so entirely unfavourable to the existence of testacea, as from 

 the subsequent operation of chemical agencies ; for under favourable 

 circumstances there are, in the central and upper divisions, traces of a 

 fauna far from restricted. 



A slight quantity of clay and peroxide of iron has sufficed at Chob- 

 ham Place enough to consolidate the usually incohesive green sands, 

 so as to form a semi-solid miass full of the casts and impressions 

 of shells. At Knovfles Hill and near Long Cross the green sands 

 contain considerable numbers of the solid sandstone casts of the Ve- 

 nericardia planicosta. At Goldsworthy the iron pyrites, wliich 

 occurs in the green sands, has preserved many casts of the entire shell, 

 whilst at Shapley, where in one part the green sands pass into a sandy 

 grey clay, midecomposed, but very friable shells, are in places com- 

 mon. Again, in those parts of the upper sands where concretions of 

 iron sandstone occur in rather more abundance than usual, traces of 

 shells, although indistinct, are, in places, not rare. But in the great 

 mass of the loiuer and wpjier Bagshot sands, where there is a want of 

 any substance to give compactness to the loose and incohesive sands, 

 there is almost or rather a complete absence of organic remains. 



Yet, if preserved in remnants, why should they not have existed in 

 masses ? why not in the sands as well as in occasional concretions ? 



It is evident from the exceptional, yet uniform conditions under 

 which the fossils are preserved, that there must have been some general 

 cause in operation to produce so common a result*. 



* Supposing there had been a sea-bottora of mueli regularity of depth and of 

 similar mineral character (muddy green sand), is it probable that the mollusks 

 would have, under such' circumstances, fixed their habitat's only in many very 

 limited and distant parts of this area ? ov else supposing them to have existed 

 throughout the whole area, could they have been fossilized in certain localities 

 only, by the oxide of iron, which must have affected them all equally, as it would 

 have been generally diffused over the spaces in which it is now segregated only in 

 occasional masses ? 



Conditions such as these would, on the contrary, appear to be favourable to an 

 average uniform diffusion of life ; for the causes in operation would not be local, 

 such as might give rise to local conditions favoiivable to local existences, but 

 general. Further, it is improbable that animal life should have been developed 

 in exact measure with the action of a subsequent chemical agency. If an origi- 

 nally limited and local distribution did not prevail, then there must have been a 

 more widely diffused fauna than now appears, of which all but the traces now 

 remaining must have been destroyed by some general cause. 



It has been stated that, excepting where the beds were impervious, or nearly so, 

 to the action of water, the original substance of the shell nowhere remains. In 

 all the pervious strata only the impressions and casts, and that rarely, at present 

 exist, and even in this state there are, as before mentioned, favourable mineral 

 conditions which do not generally prevail. 



We know" that the hydrated peroxide of iron is of common occurrence in the 

 V2)2)er and middle Bagshot sands, both generally disseminated in an earthy state, 

 and in concretions and layers. Now this mineral is readily derived from the car- 

 bonate of the protoxide of iron, which by exposure slowly parts with its carbonic 

 acid, and uniting, Avithout change of form, with a further portion of oxygen 

 and a proportion of water, passes into the hydrated peroxide. The consequence 

 of this change would be, that if a carbonate of iron v»-ere originally disseminated 

 in the mass of the sedimentary deposit, and segregated in and upon any ex- 

 traneous organic bodies which may have offered, it would by decomposition 



