THE 



GEOLOGICAL MAGAZINE. 



No. XXVII.— SEPTEMBER, 181 



Oie-ICa-IItTJLXj JLiaTIOLES. 



I. — On the Geological Epochs at which Gold has made its 

 Appearance in the Crust of the Earth. 



By David Forbes, F.E.S., F.G.S., etc. 



rthe Geological Magazine, 1865, Vol. II., p. 308, reference is 

 made to the recent discoveries of Whitney as to the occurrence of 

 Gold in the rocks of California even of an age as recent as Creta- 

 ceous. These discoveries fully confirm my previous results in South 

 America,^ yet the examination of a series of specimens recently sent 

 me from California makes me communicate these few observations 

 upon the relations of the gold-bearing rocks in question and some 

 remarks on the views which I have expressed^ with reference to the 

 geological periods at which gold has made its appearance in the 

 crust of our globe. 



When Mr. Whitney speaks of auriferous Triassic and Jurassic 

 strata, the impression left upon the mind of the reader seems to me 

 to be, that the strata pertaining to these formations contain sedi- 

 mentary beds having gold disseminated in them. 



Not having been able to obtain as yet the original reports of 

 Mr. Whitney, I cannot judge conclusively whether this is his exact 

 meaning or not ; but the examination of the rocks and other speci- 

 mens sent me from California makes me believe that such impression 

 is decidedly not the reality of the case, but that the mineral deposits 

 of California are precisely identical with those which I have met 

 with in Chile, Peru, and Bolivia, and in part described. 



In South America I do not state that the Upper Oolitic strata, etc. , 

 are auriferous ; but I report that the presence of gold in such strata 

 is due to the eruption of Dioritic rocks of still younger age, which 

 carry up the gold into the neighbouring rock in the form of veins, or 

 metallic impregnation, extending a greater or less distance into the 

 sedimentary strata, which are more or less altered by contact 

 with the eruptive rock. Where we have no Dioiite in the neigh- 



1 Geol. Soc, Nov., 1860. Quart. Joiirn. Geol. Soc, vol. xvii. 



2 Vide abstract, Geological Magazine, January, 1866, p. 23. 



VOL. III. NO. xxvii. 25 



