432 R. DAINTREE ON CERTAIN MODES OF 



Upper Silurian strata on each side of it, become richly auriferous 

 throughout the width of the dyke (about 90 feet). 



The practical value of these facts is great, as it narrows the search 

 of the prospector for gold in the Devonian and Upper Silurian areas 

 of the Australian Alps to the portions penetrated by certain 

 intrusive plutonic rocks, or at all events primarily and especially to 

 such areas. I have therefore been induced to lay before the 

 Society some further notes on the subject. 



With regard to the practical value of the rock containing the 

 sporadically diffused pyrites, I previously expressed an opinion that 

 ' ' the noble metal would not often be found in sufficient concentra- 

 tion in this form to become a source of supply in any material 

 degree." 



I instanced the case of the "Tunnel" and "Greens" so-called 

 reefs, at the Upper Cape, Queensland (which were simply decom- 

 posed pyritous trachytes or felsites), as the only evidence we then 

 had of such a trial being made ; and in neither case had the result 

 induced the owners to continue their work beyond a few trial 

 crushings, which yielded about 12 dwts. per ton. 



Since this was written, Mr. Ulrich has described a parallel case in 

 the so-called Peninsular Eeef at Portobello *. 



According to Mr. Ulrich this so-called reef has been opened in 

 four places, lying nearly in a line down the steep slope of a mountain 

 there, composed of greyish-white trachyte. The highest opening is 

 a short prospecting drive in which quartz of a rather concretionary 

 character was found in an. irregular bunch, amounting in quantity 

 to about one ton ; in this no gold was seen. The second opening, 

 about 100 feet lower down, is a shallow excavation in trachyte, 

 which is here full of irregular siliceous segregations like quartzite, 

 containing an abundance of very fine grains of pyrites. A trial 

 crushing of 2 cwt. yielded 18 grains, and one of half a ton J ounce 

 of gold ; and the prospectors stated " that they could wash a pretty 

 fair prospect of gold out of every tin dish full of the stuff excavated." 

 The third opening, about 60 feet lower down, is a small open cutting 

 in a very close-grained white trachyte, a trial crushing of one ton of 

 which yielded 3 dwts. of gold. The fourth opening, at the foot of 

 the mountain, is a shaft 40 feet deep, with a small drive at the 

 bottom. The first 25 feet passed through loose ground, and the 

 remainder through a decomposed cap into a hard coarsely crystalline 

 rock, composed of hornblende, triclinic felspar and some quartz, 

 and more or less densely charged with fine grains of pyrites. Two 

 trial crushings of the hard rock, of one ton and half a ton, yielded 

 respectively 8 dwts. and 11 dwts. of gold, whilst a ton of the 

 softer decomposed rock, which might have been expected to be 

 richer, only gave 6 dwts. As there is no evidence that the above- 

 mentioned openings have been made in the line of a peculiarly 

 auriferous zone or streak, and it is very improbable that the 

 auriferous portions of the rock were hit upon by accident, Mr. 



* s Geology of Otago,' Hutton and Ulrich, p. 165. 



