of Mineral Species. 95 



axotomous lead-bary te ; but, beside white crystals of the di-pris- 

 matic, 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 phosphate have replaced 

 the sulphato-tri-carbonate of lead. 



A very interesting change of the sulphuret of lead into a gra- 

 nular mixture of carbonate and phosphate, was mentioned to me 

 by M. VON WEISSENBACH of Freyberg, who had first observed 

 it, and who likewise shewed me the specimens he had collected 

 on the spot, at the mine called Unverhofft Gliick an der Achte, 

 near Schwarzenberg in Saxony. The original forms of the lead- 

 glance, regular octahedrons, were still distinctly visible ; but they 

 consisted of a tissue of white and green crystals of the di-pris- 

 matic 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 examples 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, 

 originating from, and containing particles of, lead-glance, from 

 the Michael mine in the territory of Geroldsegg in Swabia *. 



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 si- 

 tuated in metalliferous beds. It has been very generally ob- 

 served, 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 gene- 



* LEONHARD'S Handbuch der OryJctognosie, 2d edit. p. 293. 



