15 



Aet. III. — On the Geological distribution of Gold, with special 

 reference to some Auriferous Rocks in South America. 

 By P. Nisser, Esq., many years practically engaged in 

 the Gold Mines of that country. 



[Bead before the Institute, 30th March, 1859.] 

 To the question — "Where is gold to be found?" great autho- 

 rities have given the following answer: — " Gold is found in 

 the rocks of igneous origin, in the Azoic and Palaeozoic meta- 

 morphic rocks, also among the drifts of diluvial and alluvial 

 deposits,* as the results from disintegration of the auriferous 

 compounds in the rocks." 



By this general definition of the geological position of gold, 

 we will here take a view of this metal, under two heads, viz., 

 Gold in the Rock and Fluvial Gold. 



GOLD IN THE ROCK. 



Gold is found in the fissures of the rocks filled with mine- 

 rals and metallic compounds, forming veins. The rocks in 

 which such auriferous veins are found may be classed under 

 four groups — the volcanic, trappean, granitic, and most of 

 the metamorphic rocks. 



Among the rocks producing auriferous materials some 

 rare exceptions exist where gold occupies the whole mass 

 of the rock. As some of these may not yet be known, 

 a view of the same is here offered: — Silurian epoch — a rock 

 of a silico- argillaceous composition, having the appearance of 

 schist, containing particles of quartz and iron-oxide, inti- 

 mately mixed and bound together with clay, being hard and 

 compact, of an uneven fracture ; colour green or dark grey. 

 Small fissures divide the almost vertical stratification into 

 small, irregular, angular compartments, the fissures of which 

 are occupied with a coating of minute quartz crystals, 

 i. e., both sides of these fissures are studded with the crystals, 

 the fissures being ^ to J of an inch wide. The seams so coated 



* The geological periods for diluvial and alluvial gold will be given under 

 fluvial gold. The term fluvial expresses atiriferous detritus of the earliest 

 and latest geological dates. 



