eo a eae Lee a ee eee 
ea a a eS ees et mee 
O. A. Derby—Occurrence of Gold in Brazil. 443 
of oe gathered and rested for some time, or until it evap- 
orated, i 
During the deposition of the iron the upper portion of the 
liquid, becoming lighter than the lower, 
lowest point where the accumulation was greatest and, carry- 
ing with it gold and a relatively small proportion of iron, pro- 
cee the thinner film of iron with gold below the angle of 
‘the V. : 
2. GoLp IN GNEISS. 
Gold, almost universally worked from veins or from debris 
derived from them, is supposed to have come in some way 
from the adjoining rocks (Dana’s Mineralogy, 5th ed., p. 6), 
but thus far no case of its occurrence in workable quantities 
throughout the mass of such rocks seems to have been recorded. 
useum specimens of country rock charged with gold are 
comparatively common but in such cases the deposition of the 
precious metal in the rock is clearly connected with the filling 
of the adjacent vein. The gold-bearing itabirites of Brazi 
come nearer to a case of the distribution of gold throughout a 
rock independent of well-defined veins, but even here the 
irregular pockets of friable iron ore (jacutinga) and lithomarge, 
in which the gold is found so abundantly, partake somewhat 
of the nature of veins. The district of Campanha and Sao 
Gongalo in southern Minas Geraes, however, afford an example 
of extensive mining operations in decomposed gneiss in which 
the almost complete absence of veins and of the other usual 
concomitants of gold is remarkable. 
This district hes some fifty or sixty miles to the southwest 
of Sao Joao. D’el Rei which may be taken as about the 
southern limit of the rich belt of auriferous slates, quartzites 
and itabirites which constitutes the best known and most 
