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ECONOMIC GEOLOGY

Mercury is sold in iron flasks containing 75 Ib. each. Its
range in price from 1850 to 1918 was between £5 16s. and
£26 a flask. The pre-War maximum was £21 10s. per
flask-in 1874. The price in 1926 ranged about £14 a flask.
The average output in recent years has been from 2000 to
4000 tons mainly from the United States, Italy, which now
includes the mines of Idria, Spain, and China.
ALMADEN Mines—Historically the most important mercury
 mines are at Almaden (Arabic, the mine) del Azogue
(meaning doubtful) in central Spain. They were worked by the
Carthagenians, and supplied vermilion to ancient Rome and
later to the Moors; from 1525 to 1645 they were leased to
the famous Bavarian bankers, the family of Fuggar. They
are now worked by the Spanish Government. The mine
records give the annual output since 1499.
The ore occurs in a steeply tilted Ordovician quartzite,
in a series of faulted and slicken-sided rocks which range
from Lower Ordovician to Devonian. An intrusive porphyry
sill above the quartzite sends dykes through it. A gray crush
conglomerate, the frailesca, underlies part of the ore. There
are three adjacent lodes. The most important, the S. Pedro
and S. Diego, is 25 feet thick and contains thick veins with
30 per cent. of cinnabar and some masses with 75 per cent.
Two subparallel thinner lodes, the S. Francisca and the S,
Nicolas, each yield 3 per cent. ore. The cinnabar occurs in
veins along fissures and joints, and in masses due to replacement
 of quartzite.
The cinnabar is usually the only conspicuous sulphide ; but
pyrites occur in veins and grains, and forms a shell around
carbonaceous nodules. The chief veinstone is quartz. The
adjacent slates contain many graptolites and are richly
carbonaceous, and are traversed by curved slicken-sided
graphitic surfaces.
The cinnabar has been introduced in solution, probably
as a double sulphate of mercury and sodium, and was deposited
 by the reducing action of the carbonaceous matter
and by cooling near the surface. Deposition was mainly in
the quartzite as its fracture produced innumerable fissures,
whereas the slate was compressed into an impermeable mass.
The slate contains a little cinnabar along the joints.
The main problem in connection with the Almaden ore is