FLOOR.] 
MINERALS. 
55 
that crystallise in rhombolieclral forms isomorplious with those of Div. IV. 
Calcite, the rhombohedral calcium carbonate. These include the 
magnesium carbonate, Magnesite; zinc carbonate, Calamine; and 
the iron and manganese salts termed Chalybite and Rhodochroisite 
respectively. They include also the mixtures of these in a very con- 
siderable variety, such as Dolomite, Ankerite, Brown Spar, &c. 
Baryto-calcite crystallises in forms of the clinorhombic system, and 
establishes the trimorphism of these minerals by exhibiting the 
barium and calcium carbonates crystallised in a third set of distinct 
crystalline forms. The crystals of Calcite in Cases 29 to 31, and in 
the fronts of Cases 27 to 29 ,form, with one very large crystal in a sepa- 
rate case in Room III., a very fine series, as well for their varied forms 
as for the conspicuous illustrations certain of them afford of the highly 
double-refracting property of the crystal. Some singular pseudo- 
morphs from Devon, in the Chalybite Case, are well worthy of notice. 
The Limestone and Dolomite rocks are formed of Minerals from this 
series, in various massive, granular, or crystalline aggregations, the latter 
of which frequently form Marbles; while into the Clay-ironstone, with 
which the blast furnaces of Wales and Scotland have been largely 
fed, spathose-iron, or Chalybite, enters as an ingredient in a high 
percentage. 
Among the hydrated carbonates, and carbonates combined with Case34 ui. ) 
hydrates, or with compounds belonging to the previous divisions, 
attention may be called to the green and blue copper ores, Malachite 
and Chessylite, of which latter a very fine series of crystals is exhibited. 
Case 35 contains also fine specimens of Phosgenite, a combination Gase35(^ii.) 
of the chloride and carbonate of lead; and of Piirisite, an analogous 
compound from the Emerald Mines of Santa Fe di Bogota, containing 
the fluoride combined with the carbonate of calcium and of the rare 
metals of the cerium group. 
The Silicates, occupying no less than fifteen Gases, form the next 
class in this section. The minerals comprised in this large, varied, 
and important class are arranged in series distinguished by the type of 
oxide that characterises the bases in the silicate. Thus the silicates cor- 
responding to monoxide-bases (ferrous oxide, magnesia, &c.) are arrayed 
in one series; those the bases of which are sesqui-oxides are in another; 
and such as contain bases of both kinds fall into a third. The respec- 
tive hydrates are comprised under the series to which the minerals 
of corresponding anhydrous types belong. 
The first of these series is composed of such silicates as are formed CaseoO u.) 
by the combination of silica with monoxides only, or in which 
sesqui-oxides are met with only as accidental or intrusive ingredients. 
The anhydrous section of this series contains, among others, the 
following minerals. Phenakite, the di-glucinum silicate, and Wille- 
mite, a zinc-silicate corresponding and isomorphous with it, represent 
a rhombohedral series of dibasic silicates. The specimens of Phenakite 
from the emerald mines of the Urals are extremely fine. Of the same 
chemical type are the minerals comprised in the Olivine group, which 
are orthorhombic in their forms, and include Tephroite, di-manganous 
