RY W. F PETTEINX 65 



In the Bischoff specimen the two minerals, pycnitc; and 

 cassiterite, form a coating in the cavities, and it is 

 but rarely the whole space is filled with the substi- 

 tution. The intensely black cassiterite, with the 

 milk-white base, forms a striking contr^t of con- 

 siderable interest. 



The occurrence forms a remarkable illustration of the 

 probable action on the original rock by chemical 

 changes, by which the topazisation occurred, and the 

 tin crystals were deposited in the cavities of dis- 

 solved orthoclase. 



At the Stanley River numerous remarkably fine 

 pseudomorphs of compact black cassiterite after 

 orthoclase have recently been discovered. This 

 replacement shews the clearly distinct crystal 

 outline of the felspar imbedded in an almost 

 milk-white porphyritic rock, mainly composed of 

 quartz and felted masses and radiating bunches of 

 short and opaque crystals of tourmaline. This last- 

 mentioned mineral is of an intensely dark-green 

 colour, and belongs to the variety which has been 

 named zeuxite, and which hitherto was supposed to 

 be almost restricted in this island to Mount Bis- 

 choff. Pseudomorphs of this mineral also after 

 felspar are somewhat abundant at the same locality. 



Mr. R. F. Waller has shewn me a small specimen of 

 topazised porphyry from Mount Heemskirk — a 

 quite new locality for this mineral. 



8 Chalcophanite {Hydrated manganese and zinc 'prot- 



oxide.) 

 In aggregates of well-fomied tabular crystals or a 

 bluish-black, and highly polished. Locality: Magnet 

 Silver Mine.- Magnet. 



9 Chrysoberyl {Alitminiate of heryUiutn,) variety 



Alexandrite. 

 A remarkably fine example of this highly-prized gem 

 stone was obtained by Mr. J. A. Thompson alt the 

 Weld River in stanniferous drift, during the ordi- 

 nary streaming process of dressing tin ore. It is of 

 a somewhat pale but attractive green colour, red by 

 transmitted light, as is characteristic of the variety. 

 It was by the kindness of Mr. Thompson tested with 

 the dichroiscope by Mr. W. H. Twelvetrees, Govern- 

 ment Geologist, with the result that its identification 



