40 THE MINERALS OF TASMANIA. 



67. Cerussite (Carbonate of Lead). 



The native lead carbonate is almost an invariable 

 cLCCompaniment, in more or less quantity and perfection, 

 of the sulphide in lead mines. It is subject to ready 

 formation by the action of carbonic acid derived from a 

 gangue or adjacent rock containing soluble calcium-car- 

 bonatte such as ordinary calcite, dolomite or ankerite, and 

 is thus a secondary mineral, where its formation is due 

 to epigenic action mainly in the upper or more superficial 

 portions of the lode. It crystallises in the orthorhombic 

 system, and but few mineral species afford such extreme 

 variation of crystal habit or diversity of arrangement of 

 the individual groups or bunches. It is at times in the 

 form of a white amorphous mass, or thin coating on its 

 parent galena, or a thin layer of sulphate may rest between 

 the original sulphide and the external carbonate. Within 

 the zone of oxidation it is frequently in enormous quan- 

 tity, and cavities in the surface outcrop or galena may 

 contain fairly developed crystals; these are often met 

 with in exquisite perfection. When attached to a gos- 

 sany base it attests the superficial origin of the group. 

 It is frequently in the form of long slender crystals, often 

 acicular and of extreme delicacy. They may be almost 

 water-clear or snow-white, in which latter case when 

 implanted, as is not rarely the case, on almost black ferro- 

 manganese gossan, they afford specimens of remarkable 

 refinement. Again, the arrangement of the long acicular 

 crystals may be in a crudely stellate pattern of equal 

 attractiveness. Such masses occur in both these states 

 at the Comet Mine, Dundas. The large vughs which have 

 occasionally been met with at the Hercules Mine, Mt. 

 Read, have been found on many occasions completely 

 lined with a comparatively thick coating of exquisitely 

 beautiful crystals of this mineral, oftentimes much stained 

 externally with cupric carbonate and iron oxide, the pure 

 white of the cerussite strongly contrasting with the green 

 and blue of the copper and yellow of the iron. At the 

 Magnet Mine many perfectly-formed groups of crystals 

 have been frequently obtained. They are often seated 

 on the parent mineral galena or nestling in the cavities. 

 In the gossan zone they are very frequent and of perfect 

 form, showing single crystals as well as mackles and tril- 

 lings. In certain positions they are of a bright yellow 

 colour, owing to chromic acid influence, but the clear to 

 glassy condition prevails. Fairly large quantities have 

 been mined at the Silver Queen, Sylvester, Austral, and 



