30 



which the reef traverses younger strata — the Copeland Gold- 

 field, for instance, is in Carboniferous rocks ; but, generally- 

 speaking, it is in strata of the Silurian, or older periods, that 

 they occur. Gold ores are frequently associated with dykes of 

 igneous rock (often diorite), but the connection between the 

 occurrences is not understood. 



Gold may be described as occurring in two ways — 



(a) As free gold, which may or may not be visible to 

 the eye. Such gold would generally be extracted by 

 passing the finely crushed ore {pulp) over plates of 

 copper that have been coated with quicksilver. The 

 quicksilver combines with the gold, forming amalgam, 

 and is subsequently driven of£ again by heating in a 

 retort, 

 (i) In combination with iron-pyrites and other minerals, in 

 which case the presence of gold can only be detected by 

 assaying the stone. Special machinery is then necessary 

 to concentrate the gold-bearing minerals from the 

 crushed ore, and special processes, such as chlorinatiou, 

 smelting, or cyanidation, have to be resorted to to 

 extract the gold. 



It must be remembered that in general the gold-bearing 

 character of an ore cannot be detected by the eye. The pro- 

 portion of gold, to make an ore payable may be very small, 

 under favourable conditions a few pennyweights of gold to a 

 ton of ore being sulficient. All gold as found in nature contains 

 more or less silver, so that the value of the hullion (i.e.. the gold 

 imd silver together) per ounce varies in different cases.* 



The general collection of New South Wales gold ores is 

 contained in eight cases, viz., 50-57. These should be studied 

 f fom right to left. The ores are arranged in districts, Northern, 

 AVestern, and Southern respectively ; each being subdivided into 



* In Australia it is usual to express the value of an ore by the weight (tro.y) of gold 

 contained in a ton (2240 lb.) of stone. In some other parts of the world the value is 

 expressed in coin of the realm. An American mining engineer would speak of an ore as 

 being worth so man}' dollars to the ton. 



