258 BULLETIN OF THE IMPERIAL INSTITUTE
reddish
clay. The gravel is in some places very clayey ; in other places it is
practically free from clay, and can be more readily sifted and worked.
The sapphire is not distributed uniformly through the gravel; in some
instances small patches only are found to be sapphire-bearing, and
these are surrounded by large quantities of barren gravel. In other
instances there is a nearer approach to uniformity of distribution, the
sapphire occurring more generally throughout large masses of gravel.
The colour of the gravel, where its rests on a foundation of the older
rocks, varies with the nature of the rock. It has been observed that
the gravel tends to a reddish colour where it rests on schists and
slates, and to yellowish on granite, whilst it is almost black where it
rests on basalt or other basic igneous rocks.
The
mineral composition of the gravel does not vary very much in different
parts of the district, the most noteworthy difference being that "
billy" is abundant in the gravels of Retreat Creek and other creeks
trending eastward, whereas it is absent in most of the deposits on
Tomahawk Creek and other creeks trending northward.
Minerals
other than sapphire found in the sapphire-bearing gravels include
spinel (spinel ruby and pleonaste varieties), garnet (pyrope variety),
zircon, quartz (rock crystal, amethyst, and cairngorm varieties),
chalcedony (carnelian variety), rutile, magnetite, ilmenite,
tourmaline, hornblende, topaz, and diamond. Diamonds, however, appear
to be very scarce. A colourless, flawless crystal of diamond weighing
1-1/4 carats was found some years ago at Policeman Creek; and two
straw-coloured diamonds weighing about 1 carat each are reported to
have been found in Retreat Creek.
Method of Working the Gravel
The
mining of the Anakie sapphire deposits has been carried on by holders
of small claims, and the methods adopted in obtaining and treating the
gravel have been rather simple and perhaps lacking in efficiency. The
methods of digging adopted are described as (i) "surfacing," or simply
removing and treating the soil; (2) "deep
