148
SELENITE.
charged with sulphate of lime far more heavily instead
of much less so, than the London-Clay waters, seeing the
ample natural supply of the calcareous element. The boulder-
clay land is improved by lime-dressing, where the surface-soil has
been impoverished by the removal of its calcareous matter, but if
any sulphurous rainwater can percolate into the mass of the
boulder-clay (which Mr. Dymond holds to be permeable), there
should be far more abundant crystals of selenite formed there
than in the almost limeless London Clay; but none such are
to be found.
That the London Clay requires lime-dressing is prima facie
evidence of the normal acidity of the clay when the finely-
divided pyrites (to which the blue colour is due) has been
oxidised by atmospheric action into iron sulphate and free
sulphuric acid, which in the absence or deficiency of lime or
alkali has to remain unneutralised.
I would therefore answer Mr. Dymond's closing query in the
full, negative; it is not conceivable that the selenite in the
London or any older clay derives either its lime or its sulphur
from external sources; it owes a part only of its oxygen to the
air, and its water of crystallisation to the rain ; the important
constituents are native to the deposit. If I mistake not, selenite
nowhere occurs in superficial deposits, nor in unweathered and
absolutely impervious clays. The action of water charged with
oxygen is essential to the disruption of the pyrites, but it seems
necessary for the production of crystals that the supply of the
ingredients be slow, and under conditions which as yet we have
not been able to determine.
A totally different problem is presented by the sulphate of
baryta, constituting the crystalline lining of some Septaria. In
what condition was this almost insoluble mineral or its con-
stituents, at the time of the contraction of the nodule, and
the infilling of the radiating and concentric fissures resulting ?
The recently-detected occurrence, in a Lias septarium, of zinc-
blende is a kindred puzzle.
NOTE BY MR. DYMOND.
While I must defer to Mr. Dalton's opinion that selenite
nowhere occurs in superficial deposits of the London Clay, I do
not think the arguments he adduces against my suggestion
"that a part of the sulphate of lime that hardens the water