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190 The Sapphire.
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is
a dark, friable slate, broken through by eruptive dykes, in some places
hornblendic and dioritic, in other places quartzose. At one point near
the river, the dykes appear to be trachytic and porphyritic, with
amygdules and dark mica. At other points the dykes are more like grey
lava. Prof. H. A. Miers, has described one of the dykes as a
mica-augite-andesite. The dykes contain Sapphires, Garnets, and other
minerals, in well-defined crystals and in rounded masses. It is evident
that the denudation of these dykes has set free the Sapphires, and
other stones, now found loose in the gravels with the gold.
Sapphires
have recently been discovered at Yogo Gulch, on the Judith River, and
elsewhere near Utica, in Montana. Some of the Yugo Gulch stones are of
deep colours, including cornflower and peacock blue. Their mode of
occurrence has been described by Mr. Kunz, whilst the character of the
crystals has been studied by Mr. Pratt. The Sapphires have been traced
to certain igneous dykes composed of a rock recognized by Mr. Pirsson
as a dark basic larnprophyre, and he believes that the Sapphires were
actually formed in this rock as a true matrix.
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AUSTRALIAN SAPPHIRES.
The
Sapphire is found in many parts of Australia, but the stones are
usually of too dark a colour to be of value for jewellery. In the
wide-spread auriferous drifts of the goldfields of Victoria, the
Sapphire is by no means an uncommon mineral. Probably it has here been
derived from the basaltic rocks which, by their disintegration have
yielded most of the constituents of the gold-bearing gravels.
The Sapphire is also widely distributed in New South Wales, especially in the New England district, where it
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