FLOOR.] 
MINERALS. 
37 
of the Sapphire, notwithstanding the isomorphism of the two earths 
that compose it (viz., the rare oxide Glucina, and Alumina). Cussiterite, 
emphatically the ore of Tin (the binoxide of that metal), occupies the 
greater part of a Table Case (18), while in the rest of that Case is dis- 
played its isomorph, Eutile, the binoxide of Titanium ; the two other 
distinct crystalline types of the trimorphous binoxide of Titanium are 
illustrated by fine specimens of BrooMte, and by Anatase. 
Silica (teroxide of Silicium), ^Yhich acts as an acid in combining to 
produce many minerals (the Silicates), and which thus helps to form so 
considerable an ingredient of our globe's crust, is exhibited in its nume- 
rous varieties in four Cases. Quartz crystal is its purest form; the 
tinted specimens of which almost vie with jewels of denser substance, 
and higher refrangibility, iu beauty of colour. The Amethyst consists of 
two singularly interpenetrating layers of Quartz possessing subtle opti- 
cal characters of alternating opposite kinds, and the rijipled character of 
its "fracture " exhibits this structure as contrasted with the smooth con- 
choidal " fracture " of Quartz. The beautiful ribbed, banded, eyed, 
spotted, clouded, and other varieties of colouring, as also the gradations 
of transparency and translucency through which Quartz and its mix- 
tures with Opaline Silica pass into the opaque and impure Jasper, 
are represented in Calceclony and Carneliaii, Prase, Plasma, Helio- 
trope, &c. Very fine specimens of the pseudomorphous mineral 
Haytorite exhibit the crj'stalline forms of Datholite (see Case 40) in a 
material which is nearly pure Silica, and well illustrate this p)seudo- 
morplmm of Minerals. 
The OPxiL consists of Silica in a state chemically distinct from the 
crystalline variety, and is here exhibited in various forms of greater or 
less purity, including the Mexican Fire-Opal, and the beautiful Hun- 
garian gem, the " Noble Opal," conspicuous for its play of colours. 
Following the order of the Cases, those numbered from 25 to 39 inclu- 
sive, exhibit the series of The Silicates. The several species of these 
Minerals are displayed nearly in the order of the complexity of their 
chemical constitution. They include some of the most important 
Minerals that enter into the composition of igneous rocks. The Fel- 
spars, the Micas, the varieties of Hornllende, Aur/ite, Talc and Serpen- 
tine are conspicuous among these, while the Minerals more or less 
accidental to such rocks, are illustrated by Olivine (a coarser form of 
the gems called the Chrysolite and Peridot), Garnet, Topaz, Diallage, 
E'pidote, &c., &c. The Zeolites, an interesting group, containing water, 
but otherwise related in chemical type to the Feldspathic series, are 
exhibited in Cases 87, 38 and 39. The Emerald and the Aquamarine, 
varieties of Beryl ; the Zircon (a Silicate of the rare earth Zirconia), 
vrith its gem-forms, the Hyacinth and theJargoon; and the Mineral 
lolite or Dichroite, the Sapphire deau of jewellery, exhibiting a 
singular duplicity of colour (or dichroism) when its crystals are looked 
through in difi'erent directions, will not fail to attract notice in the 
Cases of this part of the collection. Case 40 contains the Boro-Si- 
LICATE3 AND BoRATEs, the fomicr including the Tourmaline, a mineral 
of great interest on many accounts to the student of physics, but 
