318 
The American Geologist. 
May, 1896 
Bird reef, Kimberley reef, Elsburg reef and Black reef. 
The South reef and the Main reef leader ar° the richest, the 
most continuous and the most largely developed. Their thick¬ 
ness is not great, varying from a few centimeters to one 
meter. Their gold content varies from 10 to 45 grammes per 
ton of ore crushed. 
The Main reef, which is larger (one to eight meters) is of 
lower grade (10 to 15 grammes.) It is now being exploited. 
It does not fall within the province of this paper to follow 
the reefs across the Rand and to examine their variations. 
This task has been undertaken by de Launay, and the reader 
is referred to his article in the Annoles des Mines and to his 
work now being published by Baudry. 
In short the ore appears to be a conglomerate or sandstone 
impregnated with auriferous pyrite and silicilied. It has 
sometimes undergone metamorphism as at Ferrieira, Buffels- 
dorn and elsewhere. The gold is never found in the quartz 
pebbles; but only in the cement which holds them. The prin¬ 
cipal ore is conglomerate, whose pebbles vary in size from a 
few millimeters to ten centimeters. 
Mineral waters have followed the porous strata,—the sands, 
sandstones and conglomerates of this Devonian formation,— 
and have produced the reefs which, as we have said, contain 
the auriferous pyrite. 
The origin of the Black reef is very interesting. In its vi¬ 
cinity there has been an intrusion of diabase, which entered 
between the quartzyte and limestone, following a general east- 
west course to Klipriversberg. Belts of quartzyte sometimes 
occur at its contact with the dolomyte. 
The mineral waters, following the line of contact, have 
rounded the pyrite as it was formed and have produced strat¬ 
ified deposits containing fragments of diabase. The flow of 
thermal water must have been sufficiently strong to carry 
along fragments and pebbles of the disintegrated conglomer¬ 
ate and to erode thalwegs or water courses on the surface of 
the diabase whose intrusion must have immediately preceded 
the advent of the mineral waters. This diabase is of course 
of an earlier period than that in the dykes which have dis¬ 
turbed the formation. 
