324
ON THE NATURAL HISTORY
"moon-stone." But the mineral is to be carefully distinguished
from the "moonstone" of the jeweller, which is an opalescent
adularia, and therefore much harder than gypsum, Gypsum, or
selenite, is indeed so soft as to be readily scratched, even by the
finger-nail, and hence when fibrous gypsum, or satin spar, is cut
and polished as an ornamental stone, for sake of its sheen, the
material suffers from the slightest abrasion.
Selenite crystallizes in what is called the monoclinic, or
singly-oblique, or monosymmetric system, and the crystals
present perfect cleavage parallel to a certain form which is
termed the clinopinacoid. In the most common type of crystal,
the plates which are cleaved parallel to this form are diamond-
shaped. It has been suggested that the small lozenge-shaped
plates of glass so common in old-fashioned casements represent a
survival of the cleavage-plates of selenite used in days before
glass was common. Even after glass came into general use for
windows, selenite continued to be used in certain religious
houses. Such plates, by reason of their softness, soon lost
transparency and behaved rather like ground glass—a decided
advantage in places which required a "dim religious light."
The mineral is still called in Germany Marienglas or Fraueneis.
Probably the largest known crystals of selenite are those
from the cave, or goede, in Utah, whence Dr. J. E. Talmage
obtained the specimens which he has so generously distributed
in this country. Much nearer home there is the well-known
Marienglashohle of Friedrichroda, which presents an example
of a grotto with selenite forming the sparry garniture of roof and
wall. This cave is situated near the great ducal castle of
Reinhardsbrunn, not far from Gotha, surrounded by the pine-clad
hills of the Thuringer Wald. A cottage near the entrance offers
the miners' greeting, "Gluck Auf !" but let no visitor be tempted
by the beautiful crystals to use his hammer, for over the doorway
hangs the not unnecessary notice, "Das Abbrechen von
Marienglas ist streng verboten !"
Magnificent specimens from this locality may be seen, how-
ever, in the British Museum, and smaller examples in the Museum
of Practical Geology—the specimens in both cases having been
presented by the Prince Consort. It is curious to note how some
of these fine crystals are bent and contorted as though they had
been at one time plastic as sticks of warm sealing-wax.