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68 GEMS AND PRECIOUS STONES IN THE
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beautiful topazes occurring in the lithophyses of rhyolite, which is
the first noted occurrence of this gem in an eruptive rock.' This rock
was, however, first identified by Whitman Cross, and its exact locality
is directly opposite Nathrop, Col., on a ridge a quarter of a mile in
length and about 200 feet in height. Here the topaz is found in more or
less rounded cavities, partially filled by its curved walls, which by
concentric arrangement and an overlapping often produce a roselike
form. These cavities are often lined with minute, glassy quartz
crystals, and on them are found the topazes, which are prismatic in
form, and, being attached to
TOPAZ
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the sides of the cavities in all positions, are often found doubly terminated. The crystals are from i to
(rarely) i inch in length and i to i inch across the prism. In color
they are generally transparent and flawless, and are either colorless,
pale-blue, or distinctly sherry-colored. A similar occurrence is noted
by Mr. Cross, in the nevadite of Chalk Mountain, but the crystals are
somewhat smaller. Chalk Mountain is situated at the juncture of Lake
Eagle and Summit Counties in Colorado.1 Many fine large
topaz crystals have been found at Crystal Park, near Pike's Peak, El
Paso County, Col. Three crystals from this lo-
1 Report on the Development of the Resources of Colorado, p. 36, 1881-1882. ' Am. J. Sci. III., Vol. 27, p. 94, Feb., 1884.
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