N ON -METALLIC MINERALS. 
109 
The metal strontium, closely allied to barium , occurs in the 
form of sulphate and carbonate. The sulphate called Ceiestine, 
from the pale blue colour which it occasionally presents, occurs 
in fine crystals at Girgenti, in Sicily, associated with native 
sulphur. In the New Red Marl of Gloucestershire and Somerset- 
shire Ceiestine is now rather largely worked, and thousands of 
tons are annually shipped from Bristol. 
The carbonate of strontium known as Strontianite is found 
in the lead mines of Strontian in Argyleshire, a locality which 
originally gave its name to the mineral. 
The salts of strontium are remarkable for the red colour 
which they impart to flame, those of barium giving a green tint. 
Case H. — Gypsum, &c. 
In connexion with the Case in the Lower Hall, illustrating the 
applications of plaster of Paris, this mineral — a hydrated 
sulphate of calcium — has already been described (p. 40). The 
crystalline character of Gypsum , or, as the transparent varieties 
are called, Selenite, is well shown by the perfect crystals which 
not unfrequently occur in deposits of clay and marl ; a charac- 
teristic twin-form being seen in the fine arrow-headed crystal 
from the celebrated quarries of Montmartre, near Paris. Some 
beautiful specimens are shown from the New Red Marl of 
Newark in Nottinghamshire. Enormous crystals have recently 
been found in Utah. The beads and other objects of fibrous 
gypsum exhibit in a marked degree the pleasing lustre which 
has led to the name of satin spar ; whilst the carving illus- 
trates the application of the variety called alabaster s described 
at p. 28. The waterless sulphate of calcium, Anhydrite, has 
been already noticed (p. 40). 
Cases I. and J. — Calcite. 
Some idea of the great variety of forms assumed by this 
widely-distributed mineral will be gained from the numerous 
crystallised specimens grouped together in this section. On 
fracture, the crystals of calcite split with the utmost ease into 
regular six-sided solids, called rhomb ohedrons ; and it was 
indeed in this species that the property of cleavage in minerals 
was first observed. Calcareous spar, or calcite, is a carbonate of 
calcium, or as it is more frequently called, a carbonate of lime, 
often containing impurities upon which depend the colours 
assumed by the mineral. 
The highly transparent varieties are termed Iceland spar, 
the finest specimens being obtained in Iceland. This crystal is 
remaikable for its double refraction, the phenomena of which 
are well shown in the specimens exhibited. The power of 
refracting fight doubly is, however, enjoyed in a greater or less 
degree by all crystalline minerals, except those belonging to the 
cubic system. Some remarkably fine specimens of calcite are 
