NON-METALLIC MINERALS. 
113 
Case </>. — Chalcedony, Agates, &c. 
In this Case the flint-series is continued by specimens of 
silicified sponges and other organic remains preserved in flint. 
There are also here some curious examples of beekite, or orbicular 
silica. Then fellows a beautiful collection of the different 
varieties of chalcedony , a translucent siliceous mineral, of a wax- 
like appearance, occurring often in stalactitic forms, and 
exhibiting a crypto-crystalline structure. Many of our most 
beautiful siliceous minerals are simply coloured varieties of 
chalcedony ; oxide of nickel, for example, producing the apple - 
green tint of the chrysoprase , while peroxide of iron gives rise 
to the bright red tint of carnelian, and the deep reddish brown 
colour of sard. 
Agates consist essentially of different kinds of quartz — 
chalcedonic, jaspery, and crystalline — disposed in successive zones, 
due to sequence of deposition from siliceous solutions. Whilst 
a few typical specimens are placed here, a large collection is 
exhibited in a neighbouring Table-Case No. 30, and described at 
p. 54. Regular alternations of light and dark coloured chalcedony 
are presented in the onyx, and on this depends its value for 
cameo work : when the layers consist of sard juxtaposed with 
strata of white chalcedony, the stone is called a sardonyx. A 
large number of cameos will be found here. Heliotrope , or 
blood-stone, is a chalcedony of a deep green colour, interspersed 
with blood-red spots of jasper, found in India, Iceland, the 
Island of Rum, Scotland, &c. 
Case Q. — Opal ; Alumina ; &c. 
In the moss agate and mocha stone placed in this Case, there 
are certain dendritic or moss-like delineations of an opaque 
brownish yellow or green colour, mostly due to oxide of 
manganese or of iron. These are followed by a large series 
illustrating the various kinds of Opal. In opal the silica exists 
in an amorphous, soluble, and usually hydrated condition, having 
solidified probably from a gelatinous or colloidal state. 
The precious or noble opal occurs in porphyry at Czerwenitza, 
near Kashau, in Hungary, and, under similar conditions, in 
Honduras. The fine specimens from Queensland are obtained 
from the walls of fissures in nodules of ironstone. The fire 
opal is brought exclusively from Mexico, while the common opal 
is abundant as a secondar} r product in volcanic rocks in many 
localities. Hyalite, or Muller’s glass, is a colourless transparent 
opal; while menilite is an opaque brown opaline concretion 
occurring in the tertiary strata of Menil-montant, near Paris. 
The remainder of this Case is occupied by a fine series of 
specimens of native alumina. The coarser forms of alumina, 
or oxide of aluminium, having been already described under the 
names of emery (p. 39) and corundum (p. 90), it remains to notice, 
in this place, only these fine transparent varieties which, from 
