A0 
ECONOMIC GEOLOGY 
by a marked unconformity, is the Ventersdorp System with 
a vast series of basic lavas; during their eruption the Rand 
was invaded by many dykes. The lavas were followed, after 
another unconformity, by the dolomites of the Pretoria 
System, which occupy the central part of the Rand syncline. 
The Banket (Fig. 20) contains much pyrites, which often 
forms 3 per cent. of the 
rock, and is mostly in 
small crystals and 
grains, or thin seams 
or streaks. Arsenical 
pyrites is absent. The 
mineral which is most 
conspicuous in micro- 
scopic sections after 
the pyrites and quartz, 
is chloritoid (Fig. 21), 
which has developed in 
situ and is usually a 
product of high pres- 
sure. A little mica is 
present and due to the 
decomposition of fels- 
pathic matter. The 
rarer constituents in- 
clude chalcopyrite, 
blende and galena, 
grains of rutile, zircon, 
corundum and tourma- 
line, and still more 
rarely diamonds, plati: 
num, and osmiridium. 
The occurrence of the 
gold in conglomerate at 
first suggested its placer origin ; but when it was found that 
the gold was always in the cement and not in the pebbles, this 
view was generally abandoned and the gold attributed to 
infiltration, as in ordinary lodes. The restriction of the 
solutions to the conglomerate was attributed to its being 
more permeable than the sandstone. It was also pointed 
out that the gold particles are minute, and that they and 
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