AUSTRALIAN MINING-FIELDS 127 



of the gossan is certainly porphyry in the last stage of decom- 

 position. The grains in the tin-bearing sands are angular, and 

 do not appear water-worn, and they may be derived from rocks 

 containing quartz grains, which have broken down into rotten- 

 stone by the removal of the cement simultaneously with the 

 introduction of the tin. The whole Brown Face seems to me 

 to be a gossan. It was probably formed by the oxidation of a 

 mineralised mass of quartzites and slates, with some intrusive 

 porphyry dykes ; most of this mass had been impregnated with 

 pyrites and the rest of it with cassiterite. The pyrites has been 

 converted into limonite, the bases removed (probably as soluble 

 sulphates), and the settling of the decaying rock mass has in places 

 produced a false bedding. The intensity of the chemical changes 

 that have taken place in the rocks of this mine is shown by the 

 alteration of some of the porphyry into radial clusters of topaz, 

 and the partial alteration of the rock into a topaz greissen. 



The economic problem of the mine is, What will happen 

 when all the Brown Face material has been removed ? It will 

 be exhausted some day, and the mine will then depend on the 

 veins and masses of cassiterite developed along the course of 

 the quartz porphyry dykes. If the view be correct that the 

 Brown Face is a decomposed gossan, there seems no reason why 

 shoots and veins of rich tin ore should not go down along the 

 course of the quartz porphyry dykes. We may be, at any rate, 

 confident that if the shoots are there, they will be found, and 

 worked with the economic skill, which has given Mount Bischoff 

 such a long career of financial prosperity. 



4. The Australian Saddle Reefs and their Distribution 



(a) Bendigo 



The mining-field of Bendigo illustrates the valuable results 

 attained by combination of skilled, economic management, and 

 sound knowledge of the geological structure of a mining-field. 

 Years ago Bendigo was described as exhausted ; but the 

 mining industry is still flourishing there. It shows the deepest 

 gold-mining in the world, and low working costs, which are 

 regarded with wonder by some other fields. 



Bendigo is a mining town on the northern slope of the 

 Victorian Highlands, near the edge of the Murray Plains, 100 

 miles north of Melbourne. Its mining dates from 185 1. It 

 was at first famous for the wealth of its alluvial gravels, and 



