OF PYRITES AND GYPSUM.
323
origin—and thus it comes about that we may sometimes search
for fossils, and find only crystals of gypsum. The appearance of
the gypsum is in truth correlative with the disappearance of the
fossils. Less commonly the acid from the disintegration of
the pyrites attacks the silicate of alumina in clay, and so
produces an aluminous sulphate, like the mineral known as
Websterite or aluminite.
In an old journal entitled The Mining Review there appeared
a series of articles on "The Mineral Topography of Great
Britain," by Mr. A. W. Tooke, M.A., F.G.S., and in the
number for February, 1837, the minerals of Essex are enumerated.
Excluding rocks, which are not here separated, the Essex
minerals include iron-pyrites, copper-pyrites (which can have
been only occasionally, if ever, found) and selenite, or crystallized
gypsum. With regard to the selenite the following local infor-
mation is given :
" Upon the sea-shore, between Little Holland and Harwich, copper-pyrites and
selenite, there called Frinton glass, from being most plentiful opposite that
village."
Probably the good folk of Frinton never used this "glass"
for glazing their windows, but, ages before Mr. Tooke wrote, the
mineral was undoubtedly employed elsewhere for a similar pur-
pose. It is said that the Lapis Specularis, which most authorities
believe to have been gypsum, was introduced at Rome in the
time of Seneca, and that Tiberius got cucumbers at his table
almost every month in the year by using it for the protection of
the plants. Pliny, describing this glass-stone, cites the opinion
of certain authorities to the effect that it was (to borrow from
the old translator, Holland) "a liquid humour of the earth,
congealed to an ice after the manner of crystal." And to this
day workmen are in the habit of calling crystals of gypsum
"congealed water." Samuel Dale, in referring to the selenite of
Harwich, calls it "Mock Crystal," composed of the same
material as Talc. ''Certeine it is," said parson Harrison, "that
antiquitie used it before glasse was knowen, under the name of
Selenites."
Transparent gypsum is still called Selenite—a word supposed
to express the character of the lustre exhibited by the mineral on
its cleavage-planes. This lustre has been compared to the
soft brightness of moonlight, and hence the word selenite or