256 BULLETIN OF THE IMPERIAL INSTITUTE
Rhyolites
and intrusive diorites also occur. Slates predominate in the eastern
portion of the district. In some places the slates dip vertically and
are disturbed by diorite intrusions.
There
is a conspicuous break in succession between these older rocks and the
next younger formation—the Drummond beds. The Drummond beds consist
chiefly of shales, sandstones, and conglomerates. They are of doubtful
age, but possibly Permo-carboniferous.
No Mesozoic beds are found in situ;
but there occur in the surface alluvial deposits certain boulders of
rock that may have been derived from younger beds which formerly
existed in this area, but which have been denuded away. These boulders
are known locally as " billy "; they consist of hard flinty quartzite,
and are of frequent occurrence in the alluvial deposits of Central
Queensland. Their exact origin is doubtful. They have been regarded as
remnants of a Mesozoic formation, but it has also been suggested that
they have been formed by the cementation of sand underlying the basalt
(see below).
The
youngest formations of the district are alluvial deposits and flows of
basalt. The alluvium partly underlies and partly covers the basalt.
Peaks of basalt occur in many places, but no extensive sheets of it are
known at present. It is suspected, however, that basalts formerly
covered a large area in the district, and that they have been largely
removed by denudation. Some of the basaltic peaks reach a considerable
altitude, and heights of 2,000 ft. or more above sea-level are recorded.
Origin of the Sapphire
The
study of the basalt has thrown some light on the problem of the origin
of the sapphire. At Mount Hoy, spinel of the pleonaste variety was
found to occur abundantly in the basalt, and the crystals had the
appearance of being corroded. A specimen of pale-blue sapphire was
found on the summit of Mount Hoy, at a height of 500 ft. above the
highest of the sapphire-bearing alluvial deposits, and it is considered
probable that the sapphire, like the
