Pakk. — Thermal Activity and Metalliferous Veins. 25 



feldspars and pyroxenes sometimes showed signs of alteration. 

 The samples were selected from the least-altered rocks obtain- 

 able, and in no case did they contain visible pyrites. 



The evidence is by no means conclusive that the gold and 

 silver are primary constituents. Whatever the source of the 

 gold may be, I am inclined to agree with Percy Morgan" that 

 the quantity of gold and silver in the veins is too great to be 

 accounted for by the traces existing in the andesite. 



Dr. J. E. Don, I in the preparation of his excellent thesis 

 on " The Genesis of certain Auriferous Lodes," made an in- 

 teresting examination for the presence of gold in the andesites 

 and propylites of the Thames Goldfield. He states that his 

 tests were made upon the concentrates obtained from large 

 samples, by the method of crucible fire assay. His results, 

 in the case of the unaltered andesites, were negative, from 

 which he concluded that these rocks contained no gold. The 

 question that will naturally suggest itself to the mind of the 

 metallurgical chemist, accustomed to the estimation of in- 

 finitesimal quantities of gold in cyanide solutions and residues, 

 will be, is the method of crucible or pot assay capable of 

 sufficient refinement to indicate the presence of gold in the 

 proportion of a grain or two to the ton of rock? 



My early tests of the Hauraki andesites in 1894 were made 

 by the crucible-assay method. The results, however, were 

 often discordant and unsatisfactory, chiefly on account of 

 the many sources of possible error inherent to the method — 

 errors that it was found impossible to entirely eliminate. 

 Believing that trustworthy results could not be obtained by 

 the pot assay, I adopted a method of leaching the pulverised 

 rock with dilute solutions of potassium-cyanide. By this pro- 

 cess larger samples could be tested than by fire assay, and 

 the possible sources of error were reduced to a minimum. 

 The crucible assay is clumsy, laborious, and, in my experi- 

 ence, incapable of the refinement required for the estimation 

 of minute traces of gold even in the hands of the most skilful 

 manipulator. 



Luther Wagoner,]: of San Francisco, who in 1902 made 

 a number of tests for gold and silver in sea-sediments, sand- 

 stones, syenite, granite, basalt, diabase, &c, by the cyanide 

 method used by me in 1894 and 1896, arrived independently 

 at the same conclusion. Discussing the assay of rocks, he 



* Percy Morgan, " Notes on the Geology, Quartz Reefs, and Minerals 

 of Waihi Goldfield," Trans. Aust. Inst. Min. Eng., vol. viii, 1902, p. 164. 



t J. R. Don, "The Genesis of certain Auriferous Lodes," Traus. 

 Am. Inst. Mm. Eng., vol. xxvii, 1898, p. 564. 



\ Luther Wagoner, " The Detection and Estimation of Small 

 Quantities of Gold and Silver," Trans, Am. Inst. Min. Eug., vol. xxxi, 

 1902, p. 198. 



