112 
THE PRINCIPAL FLOOR. 
Case 0. — Quartz — continued. 
Amethyst is another form of crystallised quartz, usually 
presenting a purple colour, due as some suppose, but perhaps 
incorrectly, to the presence of oxide of manganese. Amethysts 
are found in India, Ceylon, Persia, Brazil, Siberia, and various 
parts of Western America. The pink colour of rose quartz is 
probably referable to a slight admixture of oxide of manganese. 
The eye will be arrested by the gold spangled appearance of 
the Aventurine, which seems to be generally due to the presence 
of minute scales of mica. The aventurine glass will be alluded 
to at p. 131. In the cat’s eye, from Ceylon, the presence of 
parallel fibres of asbestos gives rise to a transverse luminous 
band, well seen when the stone is cut en cabochon, or with a 
convex surface. The brown South African cat’s eye, or Tiger- 
eye stone, is composed mainly of silica and ferric oxide, and 
owes its fibrous structure to the presence of altered crocidolite. 
In the ferruginous quartz , or Eisenkiesel, the mineral is deeply 
coloured, aud usually rendered opaque, by the presence of 
hydrous peroxide of iron. The specimens of capped quartz 
curiously show how a deposit of quartz has been thrown down 
upon a crystal of the same substance, with sufficient interruption 
to prevent perfect cohesion between the crystal and its cap ; whilst 
the pseudomorphs of quartz suggest changes by which various 
minerals have impressed their specific forms upon the quartz, or 
have suffered removal, whilst the quartz has taken their place. 
Case P. — Jasper, Flint, &c. 
Passing from the crystallised varieties of quartz, we find here 
a group of siliceous minerals formed by the jaspery varieties, 
which are compact, opaque and more or less impure. The 
jaspers are usually coloured red or brown by association with 
oxide of iron. When the colours are in stripes the mineral 
is called riband jasper, of which some Siberian specimens are 
here exhibited. In the Egyptian jasper, which occurs usually 
in the form of rolled pebbles, the brown colours are disposed 
in concentric zones. The curious flexibility of the flexible 
. sandstone , conspicuous in this Case, has been referred i o the 
peculiar structure of the rock, the grains of sand being so loosely 
united as to enjoy a certain freedom of motion. Several impure 
forms of silica are presented in the shape of hornstone, chert, 
.and flint : in flint the silica appears to be partly in the soluble 
and partly in the insoluble state. Flints occur chiefly in the 
upper chalk, in the form either of nodules or of regular bands : 
some characteristic examples will be found among the rock- 
specimens in the Upper Gallery. Some of the applications of 
flint receive illustration here, and a collection of flint implements 
is arranged in Table Cases Nos. 22 and 23 on the opposite side 
of the room (p. 51). 
