198 



one of the commonest minerals at this locality, Calamine should be 

 comparatively rare, while the Zinc SiUcate should be much more 

 common. 



One of the prettiest minerals occurring in Cumberland and 

 Westmorland is the pearly-looking, usually cream-coloured, mineral, 

 with curved rhombic faces of crystallization, so commonly found 

 as a sparry incrustation on the haematites from Cleator, the 

 Blende from Alston and from Keswick, and the crystals of Galena 

 from Greenside Mines. This is one form or the other of Dolo- 

 mite, which is essentially a double carbonate of Lime and 

 Magnesia. The best known specimens of this form from the 

 Alston district, come from Nent Head and Garrigill. But the 

 mineral in a less-showy form is of common occurrence as the 

 crystalline lining of the geodes, or drusy cavities, that characterise 

 the limestones of the Carboniferous series wherever their original 

 condition has been changed by the infiltrations of magnesian 

 solutions from the New Red. This is especially the case around 

 Shap, and all along the line of outcrop of the impure, arenaceous, 

 or earthy, limestones of that part of the Mountain Limestone so 

 well exposed there. The original carbonate of lime of the lime- 

 stone has been partly changed into magnesian limestone, as a 

 consequence of the infiltration referred to ; and the rock, in 

 undergoing the change in its comppsition, has changed also in its 

 dimensions, shrinking to a smaller bulk, so that a process the 

 reverse of what takes place in a cooling lava, has favoured the 

 development of a cellular or cavernous, or, as it is termed, 

 drusy, character throughout the more highly-altered portions of 

 the rock. In these drusy cavities the Dolomite has crystallized 

 out, sometimes in very pretty crystals, as may be seen by 

 the beautiful specimens presented to the Carlisle Museum by 

 Mr. Robert Crowder. Some Iron-peroxide, and, apparently also, 

 some Magnesian Carbonate, have been combined in varying pro- 

 portions with the other constituents of the Dolomite, and have 

 imparted to it sometimes the one, sometimes the other, of their 

 respective characteristic shades of colour. Some of these druses 

 are sufficiently capacious to hold half a pint of water, but more 



