346 Proceedings of tlic Royal Society of Victoria. 



similar class of rocks altered to kaolin at the surface, but little 

 altered in depth, which cut through G, F and E as dykes (H). 

 ^ These dykes are clearly of later date than the massive igneous 

 rocks (G), but they are older than, and do not intersect A, B, 

 C, or D. TTiey formed a conspicuous feature on the surface 

 of the mine, ranging from 25 feet in thickness down to mere 

 threads. 



GOLD-BKAKIXG OrES. 



Enclosed within boundaries formed of igneous rocks is a great 

 body of quartz of irregular form, and covering many acres at 

 the surface. In depth the quartz is thickly impregnated with 

 sulphides. At the surface, and for a considerable depth below, 

 the sulphides have been decomposed and removed, leaving 

 the siliceous skeleton. This surface rock of siliceous composi- 

 tion and cellular structure was described by Dr. Jack as sinter, 

 but further workings have fully established the fact that the 

 ca.vernous siliceous rock at the surface is continuous with the 

 upper portion of the quartz and sulphide ore met with in the 

 deeper levels of the mine. Even at the surface odd nodules of ore 

 were met with in the siliceous rock containing unaltered pyyites. 



This siliceous sulphide body is undoubtedly the original source 

 from which all the Mt. Morgan gold has been derived. 



At the lowest depth attained in the mine, some 850 feet from 

 the summit of the Mount, the ore is a dark-grey, finely- 

 saccharoidal quartz, thickly studded with iron and copper 

 pyrites, the former gi-eatly predominating. This ore carries 

 from 2-J to 3| per cent, copper and from H dwt. to 8 dwt. of 

 gold per ton. It is noteworthy that some of the iron pyrites is 

 in dodecahedral crystals. In the Pilgrims' Rest Goldtields, S. 

 Africa it was observed that the iron pyrites of pentagonal dode- 

 cahedral form was rich in gold, wliile the cubical crystals were 

 barren. 



A prominent feature of this mine is the increase of copper 

 sulphides in depth. Stains of copper were noticeable even at 

 the surface, showing that copper sulphides existed there for- 

 merly, and the explanation probably is that the copper sulphides, 

 beinw more readilv oxidised and removed in solution, have dis- 



