16 PROCEEDHiTGS OF THE GEOLOGICAL SOCIETY. [NoV. 22 ^ 



jority of instances where it is ultimately deposited by evaporation. 

 It is very evident that the selenite of the London Clay could not 

 have been the result of surface-evaporation, for it is found through- 

 out the thickness of the deposit, which, moreover, is of deep-sea 

 origin. 



In the Woolwich beds the spaces were found, not in layers, but 

 scattered and at various depths. It is possible that the facilities 

 for evaporation were occasionally considerable in this deposit, but 

 there are many reasons to be adduced against this idea. 



There are no evidences of either the Woolwich or London Clay beds 

 having been percolated by an intensely gypseous water. There is a 

 difficulty in this part of the inquiry in consequence of the difference 

 between selenite and the other forms of crystalline gypsum. 



All the specimens which have resulted from evaporation which I 

 have seen, cannot be called selenite, they are more fibrous, less 

 laminar, and not so regularly crystalline. The distinction between 

 the two forms is acknowledged by some, but ignored by other mine- 

 ralogists, and Bischof must be classed amongst those who consider 

 all the forms, except Anhydrite, to be identical in the chemico-geo- 

 logic sense. But although burnt selenite acts chemically like burnt 

 gypsum, still in its crystalline form it is infinitely more durable, less 

 soluble, and less influenced by heat, cold, and moisture. The struc- 

 ture of selenite infers long periods of time for the perfection of its 

 large laminate crystals ; and if this be the case, it is, especially when 

 the extra amount of water of composition is considered, a strong- 

 argument against its origin by simple and immediate deposit. 



As there is some doubt whether selenite is ever the result of simple 

 deposition, the examples now and then presented of selenite crystals 

 developing in masses of amorphous gypsum, become very important. 

 In fact the only definite knowledge of the origin of the mineral is 

 derived from them, and all other explanations are hypothetical. 



. Gypsum may have been deposited in the clays from either the per- 

 colating river- or sea-water ; but still this is less probable than its 

 secondary and chemical* origin by the changes induced in carbonate 

 of lime by decomposing organic matter. Although carbonate of 

 lime does not now usually exist p^r se in the clays, still masses of 

 it are found here and there, although they have been exposed to 

 great water- washing. During the earlier years of the London and 

 "Woolwich beds, large amounts of carbonate of lime must have been 

 formed from the detritus of shells, and in many places they must have 

 been exposed to the influence of sulphuretted hydrogen, and its asso- 

 ciated gases — the result of the decomposition of organisms. 



If it be admitted that water containing sulphate of lime will pro- 

 duce an evolution of sulphuretted hydrogen, carbonic oxide, and 

 carbonic acid gases from dead organic matter, it simply remains to 

 account for a sufficiency of oxygen to develope small quantities of sul- 

 phuric acid, before gypsum can be produced from the carbonate of 

 lime. The ^constant presence of organisms between the layers of 



* See " Remarks on the Production of Crystals," by John Morris, Mag. Nat. 

 Hist., November 1837. 



