120 SCIENCE PROGRESS 



places they overlie the conglomerates ; but this superposition 

 is due to a vast overthrust fault, which traverses the whole 

 district, and is one of the most important features in the geology 

 of Western Tasmania. 



The ores occur in two conditions — in fahlbands, or long 

 bands of mineralised schist, and as thick, short masses. The 

 fahlbands are confined to the Mount Lyell schists, but some 

 bands of pyritiferous quartzite in the Devonian rocks are of 

 similar origin. The fahlbands are best developed in the neigh- 

 bourhood of great transverse faults, which cut across the schists. 

 Their ores are pyrite, chalcopyrite, and fahlore, containing some 

 gold and silver. The minerals usually occur in bunches, in 

 bands of crushed schists, and the bunches may be arranged in 

 shoots. The wide distribution of these fahlbands led to the 

 whole of the schist country being taken up by mining com- 

 panies, but they have never paid to work independently. They 

 have, however, been profitably worked as metal-bearing fluxes ; 

 for the extremely basic, pyritic ores of the Mount Lyell Mine 

 require a siliceous flux ; the acid ores of the larger fahlbands 

 serve this purpose, and contribute their own copper at the same 

 time. Hence the South Tharsis, Royal Tharsis, and Lyell 

 Tharsis mines have been acquired and successfully worked by 

 the Mount Lyell Company. 



The ore masses are economically of far greater importance 

 than the fahlbands. The largest is that of the Mount Lyell 

 Mine, often known as the " Parent Mine," at Gormanstown. It 

 consists of a somewhat boat-shaped seam of pyrites, containing 

 quartz and barite, with chalcopyrite, some gold and silver, and 

 insignificant amounts of galena and blende. The ore mass 

 trends north-west by west, and at the surface is 800 ft. long and 

 200 ft. wide. It is worked by an open cut, about 300 ft. deep, 

 quarried to the extent of 1,000 tons a day. The ore mass 

 underlies to the west. It tapers below, and is abruptly cut 

 off by a thrust plane. It is twisted, the major axis varying 

 from north-west by west at the surface, to north-west in the 

 fifth level, and to almost due west at the eighth or lowest level. 

 The amount of pyrites above the fifth level was originally 

 4,200,000 tons, the whole of which, however, would not pay 

 to extract as ore ; for on the western or hanging-wall side of 

 the deposit, the average value is only "64 per cent, of copper, 

 •06 oz. of gold, and i'6 oz. of silver to the ton. The ore on the 



