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THE OPAL.
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posited little by little, produces at last enormous piles.
In
these silicious masses are found branches of the birch-tree completely
silicified, and in the midst of a reddish clay a thin layer of
chalcedony, which, so long as it remains watery, is translucid, but
when dry becomes opaque and like white enamel.
In
these same deposits of the geysers small portions of silica are found,
which perfectly resemble the noble opal so long as they remain
hydrated; but they lose their vivid colours when they are dried.
M. Descloizeaux is disposed to conclude from this observation that opals and chalcedonies found in volcanic earths had their origin in phenomena analogous to those of the Iceland geysers.
THE OPAL.
The
opal is formed of silica like the stones of the first group; but it
differs from them by the constant presence in its composition of a
certain quantity of water, making 5 to 12 parts in 100 of its weight.
M. Damour has shown, moreover, that when sulphuric acid is applied to
the opal the stone turns black, leading to the conclusion that it
contains organic matter, probably bituminous. This the sulphuric acid
would seem to destroy by setting its carbon at liberty.
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