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PRECIOUS STONES.
841
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.