of Mineral Species. 93 



of the oxygen in the sulphate. The form of the hexahedral lead- 

 glance, however, is not always recognizable in the products of its 

 decomposition, though there can be no doubt, that, in many cases, 

 the numerous crystalline species of the genus lead-bary te are form- 

 ed in this way in the veins. Those who might be still inclined to 

 doubt, should visit the repositories of these species at Lead-hills, 

 a place conspicuous in the annals of the mineral collector for the 

 beauty of the specimens with which his cabinet is adorned. They 

 occur there in a vein in greywacke, filled with a clayey mass, in 

 which nodules of the minerals containing the lead are imbedded. 

 On their outside, they are almost uniformly covered with crystals 

 of the carbonate, more rarely of the phosphate, of lead. In the 

 drusy cavities which they include, are deposited the rarer species 

 of the sulphato-carbonate, the sulphato-tri-carbonate, the cupreous 

 sulphate, and the cupreous sulphato-carbonate, and likewise the 

 phosphates and sulphates of lead. These cavities also are fre- 

 quently lined with fine crystals of the carbonate itself. A piece 

 of the sulphuret, with bright cleavage planes, is often discovered, 

 engaged among all these species, whose formation so much de- 

 pends upon its previous existence- In such cases, we find the 

 sulphuret corroded and rounded, presenting a surface nearly si- 

 milar to that of hexahedral rock-salt, or gypsum that have been 

 exposed to the dripping of water. The space between it and 

 the external coating is often filled with water, when the nodules 

 are found in the mine. Mr BAIRD, then surgeon at Lead-hills, 

 gave a pretty complete account of the changes by which the oxi- 

 dized species are formed from the sulphuret *. 



Miners pretty generally have an opinion, that the contents of 

 metallic veins are not always the same, and that they are often 

 working such as are not yet ripe, or would have been more pro- 

 ductive, if attacked at a later period. This opinion is founded 



* Memoirs of the Wernerian Natural History Society, vol. iv. p. 508. 



