202 



limestone. Slight variations in tint occur, and some specimens 

 are of a greyish colour, while others are almost of a rose red tint. 

 Analyses of Alstonite from Bromley Hill gave^ 



Barium Carbonate ... ... ... 62 •i6 



Calcium Carbonate ... ... ... 30'29 



Strontium Carbonate ... ... ... 6-64 



Like Aragonite and Calcite, Pyrites and Marcasite, and some 

 other minerals not so well known, the chemical compound now 

 under consideration presents us with a good example of dimorphism 

 — the compound crystallizing, in the case of Alstonite, in forms 

 belonging to the Hexagonal System, while in the other form of the 

 same chemical compound, Barytocalcite, the crystals belong to 

 the System known as the Monoclinic. Like Alstonite, Baryto- 

 calcite is rarely, or never, found in any locality far from Alston, so 

 that it has as good a claim to be regarded as a mineral peculiar 

 to Cumberland as Alstonite has. It occurs in association with 

 Galena, Barytes, Fluor, etc., in veins traversing the Yoredale 

 Rocks — generally in connection with the beds of limestone of that 

 series — and is found both massive, and crystallized, either lining 

 cavities in the rider or in the rock adjoining the veins, or else 

 investing fragments of fault-breccia. The crystallized forms occur 

 as divergent groups of slender, prismatic crystals, shooting out in 

 various directions from their basal points of attachment. The 

 individual crystals sometimes attain a length of nearly two inches ; 

 but a more common size is about one-fourth of that. The terminal 

 faces are oblique to the axis, so that the general form of each 

 individual crystal reminds one somewhat of the graver used in 

 engraving on wood. The mineral is usually translucent, and has 

 a lustre between that of glass and of resin. It varies in tint from 

 nearly colourless to various shades of yellowish and grey, 



Another division of the same group is formed by the Hydrated 

 Carbonates. 



One of these a rare mineral, well-represented in the British 

 Museum by Cumberland specimens, is Hydrozincite, or Zinc 

 Bloom. This occurs in the veins of the Alston district chiefly in 



