126 SCIENCE PROGRESS 



3. The Mount Bischoff Tin-Mine l 



Mount Bischoff is another Tasmanian mine of unusual 

 geological interest. For the opportunity of examining it I am 

 indebted to the courtesy of the manager, Mr. T. Kayser, and to 

 the hospitality and guidance of Mr. H. Herman, the assistant 

 manager. 



Mount Bischoff is a hill, 2,600 ft. above sea-level, which rises 

 above the basalt plateau of North-western Tasmania, beside the 

 head-waters of the Waratah River, a tributary of the Arthur 

 River. The adjacent country consists of folded Silurian rocks, 

 pierced by quartz porphyry dykes. The summit of Mount 

 Bischoff is formed by part of one of these dykes, the course 

 of which is like a horseshoe, and surrounds the mine on the 

 west, south, and east. Branches from the dyke radiate into 

 the country outside it, while some of the dykes outside the 

 mine dip inward as if they all met below it. 



The tin ores are of three main types: (1) A quartz vein 

 charged with tin forms the Queen lode, on the north-eastern 

 side of the mine; this lode follows along a porphyry dyke, 

 which it overlies near the outcrop ; when it was followed down- 

 ward, it was found to cut across the dyke and then continue 

 beside, but underneath it. (2) Cassiterite also occurs in masses 

 and veins in contact with the porphyry dykes, in pockets and 

 veins in the Silurian slates, and as impregnations in the porphyry. 

 (3) The main wealth of the mine comes from a large mass of 

 iron-stained material, known as the Brown Face, which includes 

 some coarse bedded sand, yielding up to 10 and even 15 per cent, 

 of cassiterite. This Brown Face is bounded to the east by a 

 sloping face of the Silurian slates, the junction being a strong 

 fault plane. To the west it is bounded by altered Silurian rocks, 

 separated from unaltered contorted slates by the western arm of 

 the quartz porphyry dyke. 



The special problem in the geology of the Mount Bischoff 

 Mine is in the nature of the Brown Face. Its coarsely bedded, 

 loose material at first inevitably suggests that this brown, iron- 

 stained mass is a sedimentary deposit, filling up a crater-shaped 

 basin within the dykes. But this simple hypothesis unfor- 

 tunately does not seem adequately to explain the facts. Part 



1 Mr. Kayser has given a general account of the geology of the mine in " Mount 

 Bischoff," Rep. Austral. Assoc. Adv. Sci. vol. iv. 1892, pp. 342-58. 



