Forbes — Igneous Rocks of Staffordshire. 23 



gration of granitic rocks of an age later than mucli, if not of all, of 

 the Silurian strata, but probably not later than the Devonian period. 

 The largest gold washings of South America, and probably of the 

 whole world, he looked upon as derived from this source as well as 

 the auriferous quartz veins, as they could be traced to the proximity 

 of the granite, and which he believed to have originated in or been 

 injected from the granite into the neighbouring strata, carrying the 

 gold, which was a normal constituent of the granite itself, along with 

 it. This granite, wherever met with, is invariably auriferoixs in itself, 

 and although it would not pay to grind down granite mountains, 

 and work out the gold in them, yet in many parts of South America, 

 in Brazil, near Valparaiso, etc., the granite apparently solid, was 

 frequently decomposed in situ to depths of even over 200 feet, as 

 shown frequently in railway cuttings, and then it sometimes repaid 

 the labour of washing the whole mass for the sake of the gold in it. 

 To this class also belong many metallic veins injected also from the 

 granite into the neighbouring Silurian strata, which contain gold, 

 and are remarkable for the presence of other minerals, very charac- 

 teristic, as oxide and sulphides of Tin, Tin pyrites, Copper pyrites, 

 compounds of Bismuth Tellurium, Selenium, etc., many of which are 

 seldom or never met with in later rocks. 



The second appearance of gold is however totally distinct from 

 the above in mineral character, as well as in geological age, and 

 results from the eruption of Dioritic (Greenstone) rocks, composed 

 of hornblende and feldspar (without quartz), which break through 

 strata, even as late as those containing Oolitic fossils, and conse- 

 quently must be regarded as younger than the OoKtic period, but as 

 far as researches have yet shown are probably not posterior to the 

 deposition of the Cretaceous strata. In this case, instead of quartz 

 veins carrying the gold from the granite into the neighbouring 

 strata, veins of metallic sulphides and arsenides act in the same 

 manner, and the gold is found imbedded in its metallic state in the 

 compounds of sulphur and arsenic with iron, copper, etc., and from 

 some unknown cause the more superficial parts of these veins appear 

 as a rule to be much richer in gold, which by the miners is generally 

 supposed to decrease in depth. The minerals commonly found in 

 these veins are not the same as in the metallic veins mentioned as 

 occurring with the granitic rocks under the first head, and as far as 

 observations have gone, the metals. Tin, Tellurium, Tungsten, 

 Titanium, Selenium, etc., are never found in the auriferous veins of 

 later dates. Nothing could be more conclusive than the totally 

 distinct age of these two sets of auriferous eruptive rocks which the 

 author believes to represent the only ages at which gold has been 

 introduced into the upper crust of the globe, and thinks it probalile 

 that this generalization may be carried into other parts of the world 

 if it be not altogether universal. 



II. — On the Igneous Eocks op Staefoedshiee. By D. Forbes, F.E.S., F.G-.S. 

 "E. DAVID FOKBES read a paper, in which he stated that, 

 at the request of the committee of Section C, he undertook 



