OF PYRITES AND GYPSUM.
325
This selenite of Friedrichroda is in Permian rocks, and it
is notable that the greatest deposits of gypsum in our own country
also occur in the New Red Sandstone formation. At Newark in
Nottinghamshire, at Fauld in Staffordshire, and at Chellaston
in Derbyshire, gypsum is, or has been, very extensively worked
in the Keuper marl, whilst in the north of England the mineral
occurs, as Mr. J. G. Goodchild has pointed out, at a lower
horizon.16 The gypsum of our New Red rocks is mostly in the
granular form called alabaster, and, with its veins and cloudings
of iron oxide, is valued as an ornamental stone, especially for
internal ecclesiastical decoration. As it is extremely soft it is
carved into elaborate forms much more readily than true
marble.
These great masses of alabaster, occur as balls, or
lenticular cakes, or irregular beds, in the marl,17 and often enclose
cavites, the walls of which are studded with crystals of selenite,
not unlike those disseminated through many clays. In the
Triassic and Permian rocks, however, the gypsum seems to
have been precipitated from the salt water of lakes, which
received the inland drainage of a region probably of desert-like
features ; or it may perhaps have been deposited in a land-locked
sheet of water, representing an arm cut off from the sea.
Large deposits of gypsum occur not only in the Midlands
and the North of England, but also in the south-east, notably in
the Purbeck beds of Sussex. Here it was discovered by the
famous Sub-Wealden boring at Netherfield, near Battle,
commenced in 1872 to commemorate the visit of the British
Association to Brighton. At the present day this gypsum is
largely worked, chiefly for conversion into "Plaster of Paris"
—a use to which much of our gypsum elsewhere is applied.
By careful calcination a large proportion of the water of this
hydrous sulphate is expelled, and the partially dehydrated
gypsum becomes a valuable material, of wide application in the
industrial arts. If all the water be expelled the mineral has the
composition of anhydrite, and becomes spoilt as a cement, being
as the workmen say, "over-burnt."
The gypsum of the London clay, although occurring
16 An admirable account of gypsum will be found in Mr. Goodchild's paper entitled
"Some Observations upon the Natural History of Gypsum." Proceedings Geologists'
Assoc, Vol. X., p. 425. For the selenite of Utah see Rep. Museums Assoc., 1897, p. 47.
17 " The Gypsum Deposits of Nottinghamshire and Derbyshire." By A. T. Metcalfe,
Trans. Fed. Inst. Min., Vol. XII. (1896), p. 107.