A492 
ECONOMIC GEOLOGY 
1000 oz. to the ton. The concentration of this gold is prob- 
ably due to each seam of Banket having been formed during 
a long pause in deposition, so that the layer was subjected 
to prolonged wave action, during which the gold was collected 
in the hollows between the pebbles, as in the riffle of an ore- 
concentrating plant.! The beds of ‘‘false banket,” or 
“ bastard reef,” in which the pebbles are angular, accumulated 
quickly, and gave no opportunity for such concentration of 
the gold ; and they may contain only a few grains or I or 
2 dwt. per ton of gold, although in all respects, other than 
those due to long-continued beach action, they are identical 
with a rich underlying leader. The concentration of gold 
by water action is also shown by the so-called “shoots” 
inthe East Rand. They consist of gravelly stream beds across 
the beaches; and the flow of water down these pebbly 
channels concentrated gold in them. 
The minerals found in the Banket include diamonds, 
platinum, tourmaline, corundum, osmiridium, and zircon, 
and they are characteristic of alluvial deposits. The typical 
hydrothermal minerals are absent. 
If the ore had been formed by infiltration, it is improbable 
that the gold solutions would not have occasionally passed into 
the bastard reef, which in many places must have been at 
least as permeable as the Main Reef Leader. 
The Banket has been worked more extensively than any 
other gold ore, and the sections have been most carefully 
sampled and studied ; yet no case has been described of any 
infiltration channels, like the verticals of Dakota, by which 
the gold could have been introduced. 
That the gold was introduced during the deposition of 
the conglomerate, and not during the injection of the dykes 
at the long subsequent Ventersdorp period, has been shown 
where contemporaneous erosion has left a patch of the gold- 
bearing Banket surrounded by quartzite; the first case was 
recorded from the May Consolidated Mine (Gregory, Tr. 
I.M.M., xvii, 1908, p. 21; confirmed by E. T. Mellor, 
F. Chem. Met. Soc. S. Afr., 1916, xvi, p. 158). 
The placer theory of the Rand was adopted amongst 
*E. T. Mellor however regards the Banket as formed by sudden floods, 
a view rejected by R. B. Young, Journ, Chem. Met. Soc. S. Afr., 1916, 
xvi, p. 230; and by du Toit, Geol, S. Africa, 1926, p. 66.