386 Forbes — Appearance of Gold in the Earth's Crust. 



boiirhood, or have no reason to ' suppose tlie existence of Dioritic 

 rocks which may not come quite to the surface, there we find 

 no gold. 



The specimens from California which I have alluded to, fully 

 confirm these views — and specimens of Diorite are mineralogically 

 and chemically the same as those of similar age from Chile, Peru, 

 and Bolivia — ^whilst the microscopical examination of their sections 

 gives strikingly identical results. 



With regard to the time of introduction of gold, or rather auri- 

 ferous eruptive rocks, into the crust of the earth, a continued study 

 of the subject, and the collection of more data from other parts of the 

 world, not only confirms me in the views expressed in a communica- 

 tion to the British Association, in 1865 (a short abstract of which 

 is given in the GtEological Magazine, for January, 1866, p. 23) 

 but makes me further believe the views therein expounded to be of 

 universal application, and — ■ 



§ bearing in mind that any bed of sedimentary origin may 

 contain fragmentary debris of any auriferous eruptive rock or 

 vein substance which was of previous geological age — 



I am of the opinion that gold is not in itself characteristic of any 

 sedimentary stratum or formation, and when found in such beds, 

 introduced otherwise than stated in the proviso §, that its presence 

 there is due to subsequent intrusive causes. 



My researches have led me to conclude that, universally, gold has 

 been introduced into the crust of the earth at two^ very distinct 

 geological epochs, and that in both these cases it has been carried up 

 in direct consequence of, and in conjunction with, the outbursts of 

 distinct and characteristic plutonic rocks. 



These two epochs of auriferous impregnation I designate re- 

 spectively as — -1. The older or auriferous granite outburst. 2. The 

 younger or auriferous diorite outburst. 



(1). The older, or auiiferous, granite intrusion appears to have 

 occurred at some time between the Silurian and Carboniferous period ; 

 certainly not older than the Upper Silurian, or younger than the 

 Carboniferous strata ; probably not younger than the deposition of 

 the first members of the latter formation. 



Gold formations, belonging to this period, present themselves in 

 Australia,* Bohemia, BoKvia,'-'' Brazil, Buenos Ayres, Chile,'-' Corn- 

 wall, Ecuador, Hungary, Mexico,'-'-' New Granada, Norway, Peru,* 

 Sweden, Ural,* Wicklow. 



To this period and cause I also attribute most of such deposits of 

 gold as are found intruded as quartz nodules and veins in many 

 places, as if interstratified in the Cambrian and Silurian (and pro- 

 bably also Laurentian and Devonian) systems, which I believe to 



1 Although subsequent researches may render it necessary to modify these conclu- 

 sions, at present I am not inclined to admit that gold has appeared at other than 

 these two epochs. 



* These so marked, as well, I believe, as California and many others, have 

 ' gold deposits of both ages. 



