48 
NORTH GALLERY. 
[UPPER 
fracture -which distinguishes them from the ordinary Quartz, with its 
smooth conchoidal fracture. They are further distinguished by pe- 
culiar optical properties. 
Cases 22 to A series of minerals succeeds, formed by mixtures of the crystalline 
25, with the opaline silica, and of these with iron oxides and argillaceous 
and other impurities. They include the various kinds of Jasper and 
of Calcedony, Prase, Bloodstone, and Heliotrope, Hornstone, Carnelian, 
Sard, Plasma, while the various banded, ribbed, eyed, spotted, clouded, 
aud other fantastically figured and coloured stones of the Agate kind, 
including Onyx and Sardonyx, in every gradation of translucency, 
illustrate the modes in which these mixed minerals occur, and often 
evidence the successive action of the processes that formed them. Of 
the pseudomorphism of minerals, a good example is furnished in 
Case 24 (i.) Haytorite, a mineral composed of a Chalcedonic Hornstone, but pre- 
senting the forms of Datholite (see Case 51). 
Section ii. We next enter on the section of Oxygen Salts, the first class under 
Cases 27 to which is occupied by the Carbonates. The isomorphous character of the 
35 - several salts of the metals Calcium, Barium, Strontium, Lead, and Mag- 
nesium, and of the corresponding ferrous and manganous salts with 
them, finds illustrations in the long array of the anhydrous carbonates 
which are here exhibited, crystallized severally in forms which are equiva 
lent; or united in various proportions of admixture in the same crystal. 
These carbonates are divided by their crystalline forms into two 
large series or groups. The first comprises those crystallizing in 
prismatic forms on the type of Aragonite, the prismatic calcic carbo- 
nate. Among these are, besides Aragonite, Witherite, the barium car- 
bonate, Strontianite, the strontium carbonate, andCerussite,the plumbic 
carbonate. The specimens of this last mineral and those of Witherite 
are especially noticeable. 
The second series comprises those minerals of this chemical type 
that crystallize in rhombohedral forms isomorphously with Calcite, the 
rhombohedral calcic carbonate. These include the carbonate of 
Magnesium, Magnesite ; of Zinc, Calamine ; and the ferrous and 
manganous salts termed Chalybite and Diallogite respectively. They 
include also the mixtures of these in a very considerable variety, 
such as Dolomite, Ankerite, Brown Spar, &c. Baryto-calcite crystal- 
lizes in forms of the oblique system, and establishes the trimorphism 
of these minerals by exhibiting the barytic and calcic carbonates crys- 
tallized 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, are a 
very fine series, as well for their varied forms as for the conspicuous 
Cs 27 illustrations certain of them afford of the highly double-refracting pro- 
- fJ - perty of the crystal. Some singular pseudomorphs 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-iron- 
stone, with which the blast furnaces of Wales and Scotland are largely 
fed, spathose-iron, or Chalybite, enters as an ingredient in a high 
percentage. 
