96 Mr HAIDINGER on the Parasitic Formatiort 



ral from the oxidation of the original substance. I have seen 

 only one example of the contrary, which was shewn to me by 

 Professor HAUSMANN, in the museum at Goettingen. Impres- 

 sions, of a hexahedral form, produced by lead-glance, contained 

 a residue, of a very loose texture, of native sulphur. This spe- 

 cimen was found in Siberia. 



The mineral usually designated by the name of Blue Lead, 

 is in some respects the converse of the changes considered above. 

 Its forms are those of the rhombohedral lead-baryte, namely, re- 

 gular six-sided prisms. The compound of phosphate of lead and 

 chloride of lead, of which their substance originally consisted, 

 has given way to the sulphuret, which usually appears in granu- 

 lar compositions, filling the crystals. The first varieties that 

 were noticed by mineralogists, were those from Tschopau in 

 Saxony. I remember having seen specimens of it, entirely con- 

 sisting of compact galena, but I have not had an opportunity of 

 comparing any again, after having examined some of the other 

 varieties of the same substance. At Huelgoet in Brittany r six- 

 sided and twelve-sided prisms are found, often upwards of an 

 inch in length, and nearly half an inch in thickness, which con- 

 sist of a coarse-grained compound variety of lead-glance, the 

 component individuals being so large that it is very easy to 

 ascertain their hexahedral cleavage. Sometimes these indivi- 

 duals have one of their hexahedral faces of crystallization co- 

 incident with the original surface of the hexagonal prism. The 

 stratum of lead-glance contiguous to the surface of the origi- 

 nal crystal, is usually separated from the body of it by an empty 

 space, so that it may be very easily broken off. Sometimes only 

 this stratum is in the state of lead-glance, while remains of the 

 original species are still visible in the interior, or part of the crys- 

 tal only has begun to have a portion contiguous to the surface 

 converted into lead-glance, while the rest presents the ada- 

 mantine lustre and brown colour of the rhombohedral lead-ba- 



