54 
NORTH GALLERY. 
[upper 
Div, IV. are ortliorbombic in their forms and include Tephroite, di-manganous 
silicate ; Fayalite, di-ferrous silicate ; with Olivine and Hyalosiderite, 
which are the magnesiam and magnesio -ferrous silicates of the series. 
The Chrysolite is the name of the pale yellow gem into which the 
larger and clearer specimens of Olivine are occasionally cut ; while the 
Peridot is a pistachio-green variety, of which fine crystals and cut spe- 
cimens are exhibited in Case 36. Gadolinite is di-yttrious silicate 
(containing also cerium, &c.) ; and Humite, a mineral containing 
Fluorine, belongs also to the more basic silicates. 
Cases 36 Among the mono-silicates are arranged the large series of im- 
(ii.)to38. portant minerals which form the two parallel groups of the Augitefj- 
and the Hornblendes. In juxtaposition with these is seen WoUas- 
Case38 (ii.) tonite, the calcium monosilicate, and the anorthic minerals Rhodonite 
and Babingtonite, homotypic in composition, but crystallographically 
differing from the other members of the series. The Augitic and 
Hornblendic groups present two distinct crystallographic types. In 
Case36 (ii.) Enstatite, the magnesian, as in Bronzite a magnesio-ferrous mono- 
silicate crystallises in the orthorhombic system, though with certain of 
the angles of an Augite ; while in Diopside, and the other Augites^ 
clinorhombic in crystallisation, part of the Magnesium is displaced by 
Case 38 (i.) Calcium, and also by Iron, Manganese, or Zinc. So Anthophyliite, a 
magnesio-ferrous monosilicate, corresponding with Bronzite, presents- 
orthorhombic forms with angles belonging to the type of the Horn- 
blendes, as exemplified in Tremolite and the other members of the 
Case 38 (ii.) group, which, however, crystallise, in the oblique system. Certain 
varieties of Jade or Nephrite are assigned to these groups, as are also- 
different kinds of Asbestos. 
Cases 39 The hydrated section of this series contains the Serpentines and the 
and 40. talcose minerals. It comprises, also, Dioptase and Chrysocolla, cupric 
Case40(ii.) silicate hydrates; Hemimorphite, the zinc silicate hydrate; and Apo- 
phyllite, a hydrated calcio-potassium silicate, extraordinarily fine speci- 
mens of which are seen in Case 3.9, and in the glazed fronts of Cases- 
11 and 12. 
The second series in the class of the silicates consists of those of the 
epideutoxides (sesqui-oxides). Foremost among them is the Topaz, an 
aluminium silicate, in which part of the silicate is replaced by an 
CsL^ 41 (i.) analogous fluosilicate. The specimens of this mineral from the 
Ourulga river in Siberia, collected by Col. de Kokscharow, are singu- 
larly fine. They are of a delicate sherry-colour, but are preserved in 
the dark, as light speedily bleaches them. 
The third series of the silicates is constituted of those in which the 
monoxides and epideutoxides are associated in the same mineral. 
Tlie various groups known by the general names of the Garnets, 
Scapolites, Idocrase, Epidote, the Felspars, the Micas, and Dichroite 
( with a variety of minerals resulting from its alteration,) find their 
places in this scries, into the hydrated section of which fall the beauti- 
ful and extensive varieties of Zeolites and Chlorites. 
Ciuic 42. Among these the Garnets form a group of m'4,erals belonging to' the 
cubic system in which the chemical type (li^ R Si,) remains constant. 
