16 S. f. EMMONS 
estimates $15.00 per ton as the average tenor of the ore mined 
throughout the whole district. Hatch puts it at $10.00, or half 
an ounce. 
The central district of the Rand, on the Johannesburg side of 
the basin where the greatest concentration of mines exists at — 
present, is 1144 mileslong. The workings of these mines extend 
now to a depth of 800-900 feet in the case of those which started 
from the outcrop, while deep-level mines, or those which do not 
own the outcrop, are down as far as 2000 feet, in each case reck- 
oned on the dip. Drill holes sunk at various points in this 
extent, to vertical depths of between 2000 and 3000 feet, have 
proved the gold bearing conglomerates to distances of up to 8000 
feet from the outcrop, and found, for the points thus tested, about 
the same average tenor in gold as higher up, with similar varia- 
tions from point to point. 
The conglomerate beds and the sandstone which enclose them 
have a reddish tinge near the surface which is due to the oxida- 
tion of the finely divided iron, but in depth have the greenish 
or bluish-gray color common to rocks containing sulphides of 
iron. The conglomerate beds vary in thickness from a few inches 
up to six or more feet, and the pebbles of which they are com- 
posed from the size of a pea to that of a hen’s egg or even larger. 
These pebbles are generally smooth and well rounded, some- 
times flattened and not infrequently cracked and fissured; angu- 
lar pebbles are also found. They are mostly composed of white 
or smoky quartz; quartzite is also mentioned as a constituent. 
Gibson speaks of pebbles of a yellowish talcose material, like 
hardened clay, which may be altered igneous rocks. The cement 
consists mostly of small quartz fragments, and abundant but irreg- 
ularly distributed pyrite grains. Under the microscope it is seen 
to contain, beside quartz and gold, pyrite, magnetite, zircon. 
rutile, muscovite and chlorite, the last two minerals and some of 
the quartz being of secondary origin. The gold, which occurs 
almost exclusively in the cement, is generally invisible to the 
naked eye. When it does occur in the pebbles it is found in the 
delicate cracks or fissures associated with quartz which appears 
