1865,] DUNCAN IMPRESSIONS OP SELENITE. 17 



selenite and amidst gypsum is remarkable, as is also the frequent 

 neighbourhood of plant-beds or lignites. 



It is by no means improbable that the deoxidation of the vegetable 

 remains before their complete transformation into lignite, may have 

 been produced by the gases resulting from decomposing contiguous 

 organic substances in contact with water containing usual amounts 

 of sulphate of lime in solution. The sulphuretted hydrogen, deriving 

 oxygen from the vegetable matters undergoing their transformation 

 into more or less pure hydrocarbons, yielded the sulphuric acid re- 

 quisite for the formation of the gypsum. 



How constantly peat, coal, and several forms of gypsum are asso- 

 ciated is well known, and the theory should hold good in all 

 instances. 



It would appear then most consistent to assert that the selenite of 

 the London and Woolwich clays was formed in amorphous gypsum, 

 the result of the decomposition of carbonate of lime by sulphuric acid 

 in minute quantities, probably in percolating water. 



The solution of the gypsum around the crystals of selenite and 

 the closing in of the clay are inferred to have then taken place. 



7. How selenite may have been removed from the deposits. — The se- 

 lenite must have been removed from the spaces in one of the fol- 

 lowing manners : — 



1 . By being washed out mechanically. 



2. By being simply dissolved by percolating waters. 



3. By being decomposed. 



1. The spaces were all closed, and the clay was not laminated until 

 it became dry. The impressions of the planes and angles were per- 

 fect. These facts militate against the removal mechanically. 



2. The improbability of the solution of the selenite may be gathered 

 from the following considerations. 



In the same London Clay, in more exposed situations, where con- 

 stant moisture and sea-spray act in all seasons, selenite is still found 

 in great abundance. If selenite be not dissolved on the Walton cliffs, 

 why should it be dissolved inland, where the only moisture is that 

 of very slowly percolating meteoric waters? Small particles of 

 selem'te remain undissolved close to the spaces, and there is an abun- 

 dance of soluble matter around them. The hardness and perfection 

 of the planes and angles of the impressions could not have been ex- 

 pected after so prolonged a percolation as that required to dissolve 

 out the selenite. 



3. The production of a salt which would entail a gradual destruc- 

 tion of the lamellar structure of the selenite, and would bring the 

 mineral down to the level of solubility witnessed in gypsum, might 

 account for the empty spaces. The theory which appears to be most 

 reasonable is partly that by which Bischof accounts for the decom- 

 position of beds of gypsum, and the consequent formation of other 

 beds of gjrpsum out of strata of carbonate of lime. Bischof may be 

 thus quoted* : — " If gypsum is impregnated with organic substances 

 (bitumen) and comes in contact with water, it will be gradually de~ 



* Op. cit. vol. i. p. 419. 

 VOL. XXII. PART I. C 



