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,"18 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. Duncan
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.19
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 Mineraux qui
l'accompagnent," Nouvelles Archives du Museum d'hisoire naturelle, 3 me. serie, t. xi.
Paris, 1897, pp. 201 to 294.