THE GENESIS AND DISTRIBUTION OF GOLD. 617 



THE GENESIS AND DISTRIBUTION OF GOLD. 



BY PROF. J. S. NEWBERRY. 



Most of the quartz veins which carry gold belong to the class of what are 

 called segregated veins. These occur only in metamorphic rocks, are lenticular 

 sheets, Hmited in depth and lateral extension, and generally showing little of the 

 banded structure so characteristic of fissure veins. They consist mainly of quartz, 

 in which the gold is sometimes free, but more commonly contained in iron pyrites, 

 with which yellow copper is often associated. Sometimes the gold is not strictly 

 confined to the quartz veins, but extends more or less into the inclosing rocks, 

 which are oftener than otherwise magnesian slates. 



The gold in segregated veins would seem to be indigenous to the formation 

 in which it occurs, and not, as in fissure-veins, to have been derived from some 

 foreign source. It is usually supposed that, before they were metamorphosed, 

 the rocks which inclose the segregated veins contained gold generally, though 

 sparsely, disseminated through them, and that, in the process of the segregation 

 of the siliceous matter to form sheets of quartz, the gold was somehow gathered 

 and concentrated by it. 



Sir Roderick Murchison, guided by his study of the gold deposits of the Ural 

 Mountains, supposed that auriferous quartz veins were confined to Paleozoic 

 rocks, but that the gold impregnation had taken place at a comparatively recent 

 date. It was demonstrated, however, by Professor Whitney, in the prosecution 

 of the geological survey of California, that the metamorphic slates which carry 

 gold in the Sierra Nevada are of Triassic and Jurassic age ; and in the light of 

 later observations, we may say that metamorphic rocks of all ages contain aurifer- 

 ous veins. Nearly all the great mountain chains of the world contain more or 

 less of such veins, and as these mountain chains have been the great condensers 

 of moisture, and erosion has been constantly wearing down their slopes, placer 

 deposits have been formed which have supplied most of the gold yielded by the 

 earth to man. As it can be procured from them by the simplest methods, the 

 work of its extraction was begun by prehistoric races, and the Altai, the Himal- 

 ayas, the Ural Mountains, the Australian Alps, the Sierra Nevada, and the 

 Rocky Mountains, have in turn contributed their millions to the treasuries of the 

 world. These mountain chains are of very different ages, and we have abund- 

 ant evidence that gold has existed in some of them from the earliest geological 

 times. The oldest mountains of which we have any knowledge — the Laurentian, 

 of Canada, now nearly removed by erosion — contained auriferous quartz veins 

 that have supphed gold to all the successive formations which have been derived 

 from their ruins. The gold impregnation of the Laurentian rocks dates back 

 certainly to the period of their metamorphism ; and this was pre-Silurian, for the 

 undisturbed lower Silurian strata overlap and partially cover these gold-bearing 

 rocks. 



