the
winds, so that soil accumulates only in more favorable places while the
rock is left bare. The delicate shells of the lithophysEe are first
attacked and hollowed out by erosion. The small cavities thus formed
are enlarged and by uniting with others form miniature caverns, some of
them several feet across. Thus the rock presents a rough porous surface
suggestive of a huge dry sponge.
Three
types of topaz crystals are recognized from this locality— fine
transparent, rough opaque, and smooth opaque varieties. The opaque
crystals make interesting cabinet specimens. The transparent crystals
occur principally in lithophysse cavities, and less often in irregular
cavities with no trace of lithophysaB structure. The topaz crystals are
more abundant in the lithophysaj where the latter are
characteristically developed. The clear crystals grow upon the walls of
the cavities, being attached at one or both ends or along part of or on
a whole side. Clusters of topaz crystals occur in some of the cavities.
The crystals are also scattered over the surface, where they have been
left by the disintegration and erosion of their matrix. The crystals
vary from a beautiful wine color with brown tint to absolutely
colorless. The natural color of the crystals in the rock unexposed is
the wine color, and this fades on exposure to the light. After exposure
for fifty to seventy hours to sunlight, even the deeper-colored
crystals become practically colorless. The wine color of the crystals
fresh from the rock is quickly destroyed by heating. All the crystals
found exposed to the atmosphere are perfectly colorless, though it
sometimes happens that a cluster of crystals is partly embedded in the
surface, in which case the buried portions have retained their color,
while those exposed to the light are perfectly colorless. The
brilliancy of these transparent topaz crystals is exceptionally high
and does not seem to be affected by exposure to weathering. The
majority of the crystals are very small and but a small percentage are
over one-fourth or one-eighth of an inch long.
The
rough opaque topaz crystals occur scattered through the solid rhyolite,
and occasionally project into cavities where the free portion is
transparent. These crystals are larger than the transparent ones and
range from half an inch up to 2h inches in length. They
generally have rough prism faces and ragged ends. The interior is
crowded full with minute quartz grains and crystals which average about
0.05 millimeter in diameter. One crystal examined showed that the
quartz grains compose about one-sixth of the bulk of the crystal.
The
smooth opaque topaz crystals are similar to the rough opaque, except
that the faces are smooth and better developed. They were found at two
places only, and were embedded in fragments of rhyolite tuff that had
been caught up in the rhyolite flow. An analysis of one of these smooth
opaque crystals, based on the excess of silica, indicated that 18.78
per cent of the material was quartz.
Both
the transparent and the opaque topaz crystals were probably formed by
the same processes—that is, by vapors or solutions contemporaneous or
nearly so with the final consolidation of the rock. The crystals in the
cavities grew practically unhindered, while those in the rock formed
where the feldspar had been removed. In the latter case the topaz
included the resulting silica as quartz grains and crystals.
