AUSTRALIAN MINING-FIELDS 121 



footwall side yields 2-35 per cent, of copper, 2 oz. of silver, 

 and '0725 oz. of gold per ton. The poorer pyrites, however, 

 pays to extract as fuel for the smelting of the rich acid ores 

 of the North Mount Lyell Mine, and for the manufacture of 

 sulphuric acid, used in the preparation of superphosphate, of 

 which the Mount Lyell Company has a factory at Melbourne. 



The ore mass is entirely surrounded by schists, but it is 

 close to the junction between the schists and the conglomerates, 

 which form the footwall side of the ore mass, though separated 

 from it by a thin layer of schist. The ore mass was therefore 

 at first regarded as part of a long lode, which was expected 

 to continue across the field, like a gold-quartz fissure lode. 



Dr. Peters, however, regarded the schists and conglomerates 

 as all part of one geological series, and explained the ore mass 

 as a lenticular-bedded precipitate, deposited on the floor of a 

 Silurian swamp. This theory necessarily fell, when it became 

 apparent, that the conglomerates and the schists belong to 

 different geological systems. 



The occurrence of the chief ores along the junction of the 

 schists and conglomerates then led to their description as 

 contact ores. But though the schists are of igneous origin, their 

 relations to the conglomerates are due to faulting and not to 

 intrusion. Contact ores are those which occur beside igneous 

 contacts. 



The clue to the distribution and origin of the ores is given 

 by the fault system of the Mount Lyell field (see fig. 1). The 

 Great Mount Lyell fault runs across the field from north to 

 south ; it is a powerful fault, in places reversed, so that the rocks 

 are locally inverted, while slices of the conglomerate have been 

 thrust in to the softer schists. Parallel faults have let blocks 

 of conglomerate down into the schists, as. in the outlier that 

 forms the North Lyell crags. The north and south faults are 

 crossed by about fifteen transverse faults, by some of which the 

 conglomerates that form Mount Lyell have been thrust westward 

 as a great buttress, overhanging the Queen River Valley ; the 

 track to the Sedgwick Valley climbs over this buttress almost 

 level with the summit of the Mount Lyell ridge. The transverse 

 faults are associated in places with thrust planes, which have 

 carried masses of the conglomerate over the schists. 



The chief ore masses occur in the acute angles between 

 the transverse faults, the great overthrust fault, and the thrust 



