On some Spaces, formerly occupied by Selenite, in the Lower Eocene Clays of the London Basin; with remarks on the Origin and Disappearance of the Mineral

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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 Woolwich clay is remarkable, for they cannot be all derived from the percolating surface-water, but from the original constituents of the bed. It is a proof of the great lapse of time required, even with the assistance of alterations in the level and dip which determine increased drainage, before such beds can be reduced to the mineralogical simplicity illustrated by clay-slate. The formation of gypsum, and of its more durable replacing crystalline form selenite, suggests the decomposition and destruction of organic 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 of selenite with that of the organisms to whose former existence it owed its origin. It is clear that if organisms deposited in sediment contribute at the commencement to the formation of such minerals as selenite, which are removeable in their turn, after a while the sediment may become free from all trace of former organic matters. In other words, there is no reason why the purest clay-slate may not have