18 PROCEEDINGS OF THE GEOLOGICAL SOCIETY. [NoV. 22, 



composed into sulphuret of calcmm, while carbonic acid will at the 

 same time be formed. Should the carbonic acid pass from deeper to 

 higher strata, which are likewise undergoing decomposition, it gives 

 rise, in the presence of water, to a disengagement of sulphuretted 

 hydrogen, while the gypsum is converted into carbonate of lime. If 

 these exhalations of sulphuretted hydrogen become converted by 

 attraction of atmospheric oxygen into sulphuric acid, and this comes 

 in contact with the younger strata of limestone, gypsum will again 

 be formed." 



It may be noticed in support of the adaptation of this theory to 

 the cases under consideration, that the organic substances, some of 

 them even bituminous, were at hand, and that the clays were doubtless 

 always wet. 



The former soluble condition of portions of the plant-beds may be 

 estimated from the blackening of the clay and loam immediately 

 around them ; and it is probable that this solubility increased after 

 the deoxidation of the vegetable matter had been completed. 



It is remarkable that the only trace of a sulphide in the portion of 

 the Woolwich beds under consideration, was found close to the sele- 

 nite impressions ; and it is equally interesting in reference to the 

 theory I have adopted, that particles of selenite should have been 

 found close to the large moulds in the London Clay. 



If sulphide of calcium was formed, as already noticed, and the 

 interlaminar spaces of the selenite were gradually the seat of its for- 

 mation, thus offering a large surface for its development and for the 

 action of percolating water, the gradual disappearance of the crystals 

 was but a matter of time. If these theories of the origin and removal 

 of selenite are correct, the decomposition of the plant-remains in one 

 case, and of other organisms in the other, were of equal importance ; 

 and it is from the abundance of the evidences of such decomposition 

 in the London Clay that the operation of solutions of organic matter 

 is inferred. 



8. Conclusion. — The preservation of the impressions of selenite is 

 doubtless exceptional, and it is very probable that this mineral, like 

 many others, is formed, decomposed, and removed without any trace 

 being left of the complicated chemical operations which determined 

 its existence and decay. The existence of soluble salts in the Wool- 

 wich clay is remarkable, for they cannot be all derived from the per- 

 colating surface-water, but from the original constituents of the bed. 

 It is a proof of the great lapse of time required, even with the assist- 

 ance of alterations in the level and dip which determine increased 

 drainage, before such beds can be reduced to the mineralogical sim- 

 plicity illustrated by clay-slate. 



The formation of gypsum, and of its more durable replacing crystal- 

 line form selenite, suggests the decomposition and destruction of or- 

 ganic remains, and the disappearance of these minerals is equivalent to 

 the destruction of the evidence of the former existence of organisms. 



How organisms are preserved and destroyed in sedimentary strata, 

 are questions constantly before the geologist ; and these observations 

 have been made with the intention of correlating the disappearance 



