GEMS AND PRECIOUS STONES. 671
JADE.
ALASKA.
P.
S. Smith * has given a few notes on the occurrence of nephrite in
Alaska. Bowlders of a hard, green, slightly translucent rock are
plentiful in nearly all of the streams of the Shungnak region north of
the Kobuk. These are
commonly called jade, but the majority are probably serpentine and
green quartzite and some may be nephrite. None of the nephrite seen was
of gem quality, since it contained many imperfections in the way of
cleavage and inclusions, of which magnetite is so abundant as to give
the mineral a spotted appearance. Several unsuccessful attempts have
been made to work the jade in the Jade Mountains, west of Ambler Eiver;
but the inferior quality of the mineral, combined with its inaccessible
location, will probably prove too great obstacles for its exploitation
for some time to come.
JASPER.
ARKANSAS.
Mr.
Francis Holstein reports an occurrence of jasper on sec. 23, Hot
Springs County, Ark., near Morrison Springs. This jasper is stated to
be beautifully colored and susceptible of receiving a high polish, but
the deposit has so far received no development.
CALIFORNIA.
The
variously marked and colored jaspers of the San Francisco region,
especially the "kinradite" variety, are meeting with increasing
appreciation in that city and among tourists. Some of the best of these jaspers
have been found on the beaches along the southern part of Marin
peninsula, between Point Bonita and Lime Point. Most of the :outhern
end of Marin peninsula terminates abruptly in cliffs at the water's
edge, but narrow beaches have formed in a few places below the cliffs
and on these the jasper can best be found. Some of the beaches are
exposed only at low tide and are difficult to reach.
The
rocks along this part of the peninsula belong to the Franciscan group
and consist of sandstone and radiolarian chert with intrusive basalt and diabase. The formations strike generally north or north-west chiefly with high dips. The radiolarian chert is a rather thin bedded jaspery
rock. The diabase and basalt may have a common origin presenting only
variations in texture. Near the water's edge they outcrop in
fresh dark greenish-black cliffs which show a large amount of jointing.
Fifteen feet above the water and higher up on the hills they have
reddish-brown weathered surfaces. These rocks contain jaspery
inclusions, some of which may be altered masses of radiolarian chert.
Veins and deposits of quartz have formed in joints and fissures in the
diabase and basalt. Some of this quartz grades into jaspery
material and other is greenish through chloritic or actinolite-like
inclusions. It should be possible to find good jasper hi the rock, but
the greater part is obtained from the beaches where the jasper
inclusions have fallen from the cliffs. The force of the waves has
ground off much of the adhering matrix and rounded
i The Noatak-Kobuk region, Alaska: U. S. Geol. Survey Bull. 536, pp. 154-155,1913.
