286 Mr Haidinger on the Parasitic Formation of Minerals, 



lead. The third specimen, like the preceding one, has the 

 form of the axotomous lead-baryte ; but, beside white crystals 

 of the di-prismatic, also yellow ones of the rhombohedral lead- 

 baryte are found to occupy the space originally taken up by 

 the axotomous lead-baryte. Here the carbonate and the phos- 

 phate have replaced the sulphato-tri-carbonate of lead. 



A very interesting change of the Sulphuret of lead into a 

 granular mixture of carbonate and phosphate, was mentioned to 

 me by M. Von Weissenbach of Freyberg, who had first ob- 

 served it, and who likewise showed me the specimens he had 

 collected on the spot, at the mine called Unvej'hofft Gliick an 

 der Achte, near Schwarzenberg in Saxony. The original 

 forms of the lead-glance, regular octahedrons, were still dis- 

 tinctly visible ; but they consisted of a tissue of white and 

 green crystals of the di-prismatic and rhombohedral lead- 

 baryte. There was a black friable residue left, which was 

 considered as friable lead-glance. Such a substance is often left 

 on the surface of decomposing lead-glance, where, even in the 

 portions that yield to the pressure of the nail, and soil the 

 fingers, some traces of cleavage continue. Very good ex- 

 amples of it occur at Mies in Bohemia, along with the well- 

 known large crystals of carbonate of lead. Selb also observed 

 black di-prismatic lead-baryte in the shape of cubes, originat- 

 ing from, and containing particles of, lead-glance, from the 

 Michael mine in the territory of Geroldsegg in Swabia. (Leon- 

 hard's Hand. Oryktog. M edit. p. 293.) 



The changes described above are not of a rare occurrence 

 in the various mining districts, not only in such where the 

 works are carrying on in actual veins, but also in those which 

 are situated in metalliferous beds. It has been very generally 

 observed, that such mineral repositories yield crystals chiefly 

 in their upper levels, and that they are found more compact 

 when the works are carried to a greater depth. They follow, 

 in general, from the oxidation of the original substance. I 

 have seen only one example of the contrary, which was shown 

 to me by Prof. Hausmann, in the museum at Gcettingen. Im- 

 pressions, of a hexahedral form, produced by lead-glance, con- 

 tained a residue, of a very loose texture, of native sulphur. 

 This specimen was found in Siberia. 



