326 ON THE NATURAL HISTORY 



sometimes in beautiful crystals, is never abundant enough to be 

 of economic value. Similar crystals occur in many other clays 

 like the Gault and the Kimeridge clay. It is the latter 

 formation which yields the fine crystals for which the Headington 

 pits, near Oxford, are famous. With reference to the formation 

 of these crystals. Professor Phillips regarded them as " still in 

 progress of growth, "^^ a remark which may also apply to the 

 crystals in our London clay. Minerals, let us remember, have 

 their periods of growth and decay, and in fact pass through a 

 kind of life-history. 



Nearly forty years ago the late Professor P. M. Dimcan 

 called attention to the decay and disappearance of crystals of 

 selenite in the London clay of the Tendring Hundred in Essex. 

 The clay enclosed definitely-shaped cavities, which had evidently 

 been occupied at one time by stellate crystals of selenite. 

 Gypsum is a fairly soluble substance, but it was held that the 

 crystals of selenite in this case had been removed not by simple 

 solution, but by chemical decomposition. Selenite in the 

 neighbourhood of decaying organic matter might readily suffer 

 deoxidation, and the sulphate would thus become reduced to the 

 state of sulphide. Calcium sulphide is a very unstable body, 

 and may easily be removed. In the presence of water, the 

 sulphide evolves sulphuretted hydrogen which may, by oxidation,, 

 yield sulphuric acid. Then this acid acting on carbonate of 

 lime will regenerate gypsum, and so the cycle of changes goes 

 on — the death of one crystal being followed by the birth of 

 another. 



Moreover, when calcium sulphide, formed from gypsum by 

 decaying organisms, comes into relation with a solution of acid 

 carbonate of iron, such as must often be present in waters 

 circulating through the upper part of the earth's crust, a reaction 

 is set up, whereby iron sulphide and calcium carbonate are 

 produced. Curiously enough then it appears that pyrites may 

 be formed from the decay of gypsum.^^ 



That pyrites is at present in course of formation, under 

 certain conditions, is a well-known fact, and examples of 

 recent pyrites have frequently been cited. Even Borlase, the 



18 Geology of Oxford, 1871, p. 325. 



19 See Prof. A. Lacroix's fine monograph : Le Gypse de Paris et les Mincraux qur 

 raccompagnent." Nouvelles Archives du Musi urn d'histoire naturelle, 3 me. serie, t. xi^ 

 Paris, 1897, pp. 201 to 294. 



