1865.] DTJIfCAN IMPRESSIONS OF SELENITE. 15 



meets the difficulty of accounting for the slight evidences of decom- 

 position in the beds which are so crowded with shells. 



It must be remembered, in considering the subsequent geologic 

 chemistry of the beds, that as the depression of the estuarine deposit 

 proceeded, it was covered with the marine London Clay, and that 

 salt water percolated through its beds until all was elevated, and 

 subaerial denudation commenced. 



The London clay in the neighbourhood of the impressions is un- 

 fossiliferous, and contains no carbonate of lime, . but doubtless the 

 organisms common to all sea-bottoms were once present, and deter- 

 mined in a decided manner the mineral condition of the deposit. 

 The small particles of selenite near the impressions were probably 

 carbonate of lime at one period. Kiver- and rain-waters had nothing 

 to do with these clays at first, for they were washed and percolated 

 by sea-water, until the elevation of the lower Eocene beds subjected 

 them to the action of fluviatile and meteoric waters. 



The slow rate of drainage through the London Clay is tolerably 

 evident. It wiU. be noticed that in one instance the evidences of the 

 deposition and removal of selenite were found in beds which were 

 shallow- and fresh- or brackish -water deposits, and that a prolonged 

 exposure to percolation by sea-water was followed by a rapid perfect 

 drainage by fresh water ; whilst in the other instance the proofs were 

 discovered in marine deposits which were not formed near the sur- 

 face, and which have been finally slowly drained by fresh water. 

 Moreover the selenite is common in the marine but rare in the es- 

 tuarine deposit*, and in both it is scattered and is not in horizontal 

 layers. 



6. Origin of Selenite. — There are many instances of the formation 

 of crystaUine gypsum during the recent period, consequent on the 

 evaporation of waters holding sulphate of lime and other salts, such 

 as sulphate of soda and chloride of sodium, in solution. Darwin 

 found large crystals in the banks of a South American salina which 

 deposited large amounts of sulphate of soda; Bischoff and others 

 notice the formation of laminar crystals on the faggots placed for 

 retarding the percolation of certain sahne waters, and there are some 

 fine specimens of recent crystalline gypsum impregnated with sand 

 in the museum of the Society. The deposition of small crystals and 

 of the amorphous mineral occurs very constantly J. 



Now all these deposits occur under somewhat identical conditions, 

 and often have a relation to the comparative solubilities of the chloride 

 of sodium and sulphate of soda which are more or less present in the 

 waters. The excess of sulphate of lime is rarely observed, and it is 

 pernicious to molluscous life ; and as it is not advisable to travel out 

 of the usual course of things in attempting to account for difficult 

 natural problems, it may be conceded that no more than an average 

 amount of the salt is, in the first instance, present in the great ma- 



^ Mr, Prestwich notices the absence of selenite in the mottled clays of the 

 Woolwich beds (op.'cit.). 



t Chem. and Phys. Geol. Cavendish Society, vol. i. p. 426. 

 I Ibid. vol. i. p. 154. 



