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896
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MINERAL RESOURCES, 1916----PART II.
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The
value of the domestic production of precious stones shows a similar
decrease for the years 1914 and 1915 as compared to the five-year
period 1909-1913, but the production in 1916 has not recovered to the
average value for 1909-1913, being in fact but little more than half as
much. If the average values of the domestic production and of imports
for the years 1909 to 1913, inclusive, are taken as the standard, the
production in 1914 was valued at 34 per cent and the imports (exclusive
of pearls) at 44 per cent; in 1915 the production was 47 per cent and
the imports 56 per cent;; in 1916 the production was 60 per cent, the
imports 99.7 per cent—almost a complete recovery.
Diamonds and other precious stones imported and entered -for consumption in the United States, 1907-1916
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o Including agates. Agates in 1906, $20,130; in 1907, $22,644; in 1915, $31,657; in 1916, $18,681.
EXHIBIT OF TOURMALINE AND OTHER GEM MINERALS. IN THE PEGMATITES OF SOUTHERN CALIFORNIA.
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The
gem-bearing pegmatite region of southern California has been somewhat
intensively studied by the writer under the joint auspices of the
United States National Museum and the United States Geological Survey,
and an exhibit representing the region has been deposited in the
National Museum, Washington, D. C.
The
gems are contained in one case which has three horizontal shelves and
two sloping shelves. The exhibit consists of specimens of the country
rocks (granite and gabbro) and of their associated pegmatite dikes, in
which are found the gem tourmalines and other gem minerals. The
collection is shown on the first floor, main hall, east end, of the new
National Museum Building, Washington, D. C.
The
nearly flat-lying pegmatite dikes, from which most of the specimens
shown were obtained, crop out on the hills north and east of Pala, San
Diego County, Cal., and are of the compound, unsym-metrical type whose
different parts are thought to be due to differentiation processes
rather than to multiple injections of material into reopened fissures.
The upper portion of the dike is locally known as the " top rock " and
is a mixture of a coarse, granular aggregate of quartz and feldspar
and of a graphic pegmatite. No gem stones are found in this " top
rock." The lower portion of the dikes, locally called the "bottom
rock," is a much finer grained granular
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