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PRECIOUS STONES.
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Professor
Bleeck concludes that the jadeite was formed by the metamorphism of an
albite-nepheline rock, both of these minerals being found with the
jadeite in places. The change would be represented chemically by
NaAlSi04 (nepheline) + NaAISi308 (albite) = 2NaAlSi20„
(jadeite). Under certain conditions albite-nephelite rock might form,
while under conditions of high pressure, during consolidation or
after, jadeite with a much lower molecular volume would be produced.
The color of pure jadeite is stainless white and specimens closely
resemble marble in appearance. The rich emerald-green colored jadeite
is the most highly prized and is not abundant compared with the white
or dull-green varieties. Some jadeite has a pale amethystine color. The
emerald-green color of the best variety of jadeite is duo to chromium;
the dull-green color of other varieties is due to iron; and the
amethystine color is supposed to be caused by the presence of manganese.
JASPER, PETRIFIED WOOD, ETC.
ARIZONA.
Probably
the largest jasper deposits in the world are those of the petrified
forests of Arizona. It is not alone the sight of so many petrified
trees that causes wonder to the visitor, but the large variation of
brilliant colors displayed by these trees. The deposits have been
described by L. F. Ward,a and their beauties portrayed by
many writers. Geologically the petrified forests may be briefly
described as occurring in formations of Triassic age. The trees were
not petrified in place except in a few instances, but were washed down
from high levels and scattered over large areas or accumulated in
comparatively confined areas. They were deposited in and covered by a
conglomeratic sandstone stratum overlying purplish and gray marl beds.
This stratum now forms the capping over large areas of mesa country. In
their present positions the petrified trees are visible where the
sandstone has been cut into by the erosion of valleys and gulches and
washes. Some of the trees remain in the sandstone, while others,
principally fragmentary, have been dropped into the washes by the
erosion of their parent rock. In a few cases the petrified tree trunks
are preserved nearly whole, while as a rule they occur in fragments
large and small. The smaller material is abundant over large areas, and
in places the large blocks or sections of trees are numerous.
The
trees have been petrified by silica in its various forms with varying
quantities and kinds of impurities acting as pigments and furnishing
widely diverse colors. The general form of the tree trunks and limbs
and a few details of structure have been preserved during
petrification, though the minute detail of structure seen in some
petrified woods is wanting. The mineral matter composing the trees is
largely jasper with varying amounts of chalcedony and quartz. The
jaspers range from brilliant red, through orange to yellow in color.
The large number of shades of these colors, particularly the reds, is
striking. From the brownish and maroon reds there are all gradations as
terra cotta, cardinal, scarlet, cherry-red, etc., to orange,
a Geology
of the Little ColoradoValley, Ariz.: Am. Jour. Soi., 4th ser., vol.
12,1901, pp. 401-413. Petrified forests of Arizona: Ann. Kept.
Smithsonian Inst, for 1899,1901, pp. 289-307.
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