116 
THE PRINCIPAL FLOOR. 
known is the potash- felspar, Orthoclase. The transparent ortho- 
clase called Adularia is used for purposes of ornament under 
the name of Moon- stone. The opalescent orthoclase from the 
syenite of Fredriksvaern, in Norway, was formerly regarded as 
microcline. Amazon stone is a variety of microcline, noticeable 
for its beautiful apple-green colour. Obsidian or volcanic glass , 
is a vitreous form of felspathic lava employed by many savage 
races for making cutting instruments ; it was largely used in 
this way by the ancient Mexicans. The plagioclastic section of 
the felspar group comprises such species as crystallise in the 
triclinic or doubly-oblique system. Albite and Oligoclase are 
closely-related felspars, the former containing soda and the latter 
both soda and lime. The Norwegian Sun-stone or Aventurine- 
felspar is, in part, a variety of oligoclase, notable for enclosing 
certain minute crystals from which light is brilliantly reflected. 
Attention need scarcely be directed to the beautiful play of 
colour exhibited by the polished slabs and cameos of the soda- 
lime felspar called Labradomte. The mineral was first sent to 
England by the Moravian missionaries in Labrador. 
Case 0 . — Various Anhydrous Silicates. 
In this Case the large series of anhydrous silicates is continued 
with the well-known lapis lazuli. This is a composite mineral 
substance, of which the essential part has been separated in a 
pure state, and is called by Brogger lasurite. The lasurite 
seems to be a silicate of aluminium and sodium, with sodium 
sulphide and sulphate ; in the lapis itself calcium is always 
present. Lapis lazuli occurs generally in crystalline limestone, 
in Persia, Bokhara, China, Siberia, &c. The richer varieties of 
lapis lazuli are employed in the manufacture of ornamental 
artic'es ; and when subjected to careful powdering and washing, 
to free it from all foreign admixtures, the mineral forms the 
ultramarine of the artist. 
An artificial ultramarine is prepared by carefully heating a 
mixture of clay, carbonate of soda, and sulphur. Examples 
of the genuine and of the artificial ultramarine s'and side by 
side. 
Several beautiful crystallised minerals such as Scapolite, 
datolite, and epidote are fairly represente 1 in this Case, but as 
they have no general interest, any notice would be out of place 
here. The visitor will, however, be attracted by the pink slabs of 
the Norwegian Tliulite and the Siberian Rhodonite, as also by the 
emerald-like Hiddenite. Some specimens of jadeite from China, 
Burma, and Mexico, are illustrations of a material often con- 
fused with the jade exhibited in the next Case. 
Case U. — Augite; Hornblende; &c. 
Here the visitor will find a very instructive group of silicates 
embracing the different varieties of the closely- allied rock' 
