180 THE MINERALS OF TASMANIA. 



tion, with all the habit of the regular tetrahedron which 

 the crystals usually present. The regular tetrahedron 

 is commonly levelled on the edges by planes of the three- 

 faced tetrahedron. 



Although tetrahedrite may be essentially a sulphan- 

 timonite of copper, the copper may be partially replaced 

 by zinc, iron, or silver, whilst the antimony may be 

 replaced by arsenic. When isomorphous silver is present, 

 the ore is known as freibergite, and when much arsenic 

 has replaced the antimony it is known as tennantite. Mer- 

 cury and bismuth may also be present in limited quantity. 

 As occurring in this island, fahl ore is justly noted for the 

 somewhat unusually large returns of silver it gives on 

 assay ; this remark applies to its occurrence at all the 

 mining localities. It is not only an important ore of silver 

 on account of the high value for that metal, but also from 

 the established fact that it has been proved to live to 

 great depths, as has been shown by the deep levels of the 

 more celebrated mines of Saxony. At the Western Mine, 

 Zeehan, it was obtained in beautifully bright, well-formed 

 crystals, extremely rich in silver, from the deepest work- 

 ings. A small quantity was obtained intermixed with 

 pyrite at the old Penguin Silver Mine ; at the Long Tun- 

 nel Mine, Heazlewood, it has been found intermixed with 

 sphalerite, galena, and jamesonite; at Mt. Lyell it occurred 

 in close association with cupriferous pyrites, and afforded 

 remarkably high returns for silver, and also for gold ; 

 at the Hercules and adjacent mines at Mt. Read it occurs 

 in limited quantity, often assaying at the rate of 2000 to 

 3000 oz. of silver per ton, and at the mine named nice 

 patches of crystals have been obtained ; at the South Cur- 

 tin and Davis and other mines at North-East Dundas it 

 is commonly associated with bladed bismuthinite. Crystals 

 are occasionally found, but they are by no means abun- 

 dant. In the Zeehan silver-lead mining field it very often 

 occurs sparingly, accompanying galena and pyrites, some- 

 times in the form of crystals implanted on brownish 

 siderite. The Oonah Mine has afforded some fairly good 

 crystal groups. It is the staple form of nearly all the 

 product of the North-East Dundas silver mines, very com- 

 monly occurring, as at the Ring Valley and Curtin-Davis 

 Mines, as an amorphous lode-mineral, with siderite as the 

 prevailing gangue. In the majority of cases it is remark- 

 able for its consistent purity and satisfactory silver con 

 tents. 



