


262 STRUCTURAL AND FIELD GEOLOGY 
that gold is confined to particular beds of conglomerate— 
other layers of the same kind of rock containing little or none. 
Possibly this may be explained by the presence of reducing 
agents in the one case and their absence in the “offem 
According to Messrs Hatch and Corstorphine it is difficult 
to say what this reducing agent was. They point to the 
frequent association of gold with pyrite as suggesting that 
the latter had something to do with the precipitation ; and 
they suspect that the carbonaceous matter—plentifully present 
in some richly auriferous portions of a conglomerate—may 
have played a greater rdle as a reducing agent than is 
commonly supposed. 

FIG. 106.—REVERSED FAULT IN THE GOLD-BEARING ROCKS AT JOHANNES- 
BURG. (After Schmeisser.) 
$s, s, sandstones, etc.; c,c, c, beds of gold-bearing conglomerate (so-called ‘‘reefs”); d, dyke 
lying in fault. 
Yet another example of disseminations may be given. 
At Keeweenaw Point, Lake Superior, occur certain much 
decomposed igneous rocks (melaphyres) with interbedded 
conglomerates. Native copper is found both in the con- 
glomerates and the igneous rocks, which are old lavas, the 
pebbles of the former being often encrusted with it, while 
the amygdaloidal cavities of the latter are frequently lined 
and occasionally completely filled with the same metal. 
Copper occurs also in the joints of the rocks and the fault- 
fissures traversing the strata, so that at Keeweenaw Point 
BeOS th So Bt SRN SARE Sc el a ARES Teg. 
SOS lg 
Mie Gh Sigg ae 
> nae eee 
