MINERALOGY. 



87 



Quartz crystal in Case He. Its tinted specimens may vie in point of 

 colour with jewels of denser substance and higher refrangibility. 

 Among these are the lilac-hued specimens of the Amethyst, the 

 Brazilian specimens of which, as well as of the yellow kind, show 

 the "rippled" fracture which distinguishes them from the ordinary 

 Quartz, with its smooth conchoidal fracture. They are further dis 

 tinguished by their optical properties. 



A series of minerals succeeds, formed by mixtures of the crystalline 

 with the opaline silica, and of these with iron oxides and argillaceous 

 and other impurities. They include the various kinds of Jasper and of 

 Chalcedony, Prase, Bloodstone and Heliotrope, Hornstone, Carnelian, CaS€J5 

 Sard, Plasma, while the various banded, ribbed, eyed, spotted, clouded, 15 & 16> 

 and 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. 

 We next enter on the section of Oxygen-Salts, the first class under 

 u- which is occupied by the Carbonates. The isomorphous character of 

 the several salts of the metals Calcium, Barium, Strontium, Lead, and 

 Magnesium, and of the corresponding iron and manganese salts with 

 nates, them, finds illustrations in the long array of the anhydrous carbonates 

 which are here exhibited, crystallised severally in forms which are 

 equivalent, or united in various proportions of admixture in the same 

 crystal. 



These carbonates are divided by their crystalline forms into two Cases 

 large series or groups. The first comprises those crystallising in 1 ^ & 

 forms on the type of Aragonite, the orthorhombic calcium carbonate. 

 Among these are, besides Aragonite, Witherite the barium carbonate* 

 Strontianite the strontium carbonate, and Cerussite the lead carbonate. 

 The specimens of this last mineral and those of Witherite are espe- 

 cially noticeable. 



. The second series comprises those minerals of this chemical type 

 that crystallise in rhombohedral forms isomorphous with those of 

 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 oblique system, and estab- 

 lishes the trimorphism of these minerals by exhibiting the barium 

 and calcium carbonates crystallised in a third set of distinct crystal- 

 line forms. The crystals of Calcite here exhibited form, with two 

 very large crystals in separate cases, 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. 



The Limestone and Dolomite rocks are formed of minerals from this Cases 

 series, in various massive, granular, or crystalline aggregations, the latter 19 & 20. 

 of which frequently form Marbles; while into the Clay-ironstone, with 

 which the blast furnaces of Wales and Scotland have been largely 



