ORES OF GOLD 61
he pyrites are angular and not water-worm, and that the
rounded pyrites might be concretionary. Tt was also claimed
that as the Rand gold contains from 100 to 120 parts per
1000 of silver, it cannot be alluvial. "On these grounds the
gold was attributed to Ventersdorp age, when the Rand beds
were intersected by dykes, against which a patch of rich
ore may end abruptly. (The latest full statement of this
Sho is by C. B. Horwood, Gold Deposits of Rand,
917.
These arguments are however inconclusive. Gold in
blacer deposits occurs in the cement and not in the pebbles,
which represent the hard barren “ huck-quartz.” Some
placer gold, as in Queensland, contains 50 per cent. of silver
and is of much lower grade than that of the Rand. Theangu-
larity of the gold is due partly to its having been squeezed
between the grains of sand, and partly to the gold having been
P16. 22,—A PyriTic PEBBLE FROM THE BANKET,
A pyritic pebble from the Banket with wind-shaped
surfaces, pseudomorphic after 2 pebble of iron oxide.
The pebble ic half-an-inch in width.
dissolved and redeposited. The sudden ending of rich
Patches against a dyke is due to its rise along a fault; the
abrupt termination of the patch is due to the fault.
The placer theory of the Rand was faced by two difficulties
—the rarity of pyrites in ordinary placers, and the rich
concentration of such minute particles of gold. Pyrites
occurs in placers containing abundant vegetable matter, but
20t in quartz sands where it would be destroyed by oxidation.
The Rang pyrites however often occurs in streaks and patches
like black iron sand on a sea beach; the Rand pyrites was
Probably deposited as iron oxide. Some of the larger pebbles
of pyrites (Fig. 22) have sand-worn faces, and must have been
originally pebbles of either ironstone or quartz which has been
‘placed by pyrites.
X Gold in particles as minute as those in the Rand occurs in
sands ang silt, but rarely amounts to more than a penny-
weight or so per ton: whereas some Banket contains Over
