proportion
increasing to some extent with the amount of silver, (a) The second is
of greater practical importance, being in some districts exceedingly
rich, and, next to the native metal, is the most prolilic source of
gold. Magnetic pyrites, copper pyrites, zinc blende, and arsenical
pyrites are other and less important examples,—the last constituting
the gold ore formerly worked in Silesia. A native gold amalgam is found
as a rarity in California, and bismuth from South America is sometimes
rich in gold. Native arsenic and antimony are also very frequently
found to contain gold and silver.
The
association and distribution of gold may be considered under two
different heads, namely, as it occurs in mineral veins, and in alluvial
or other superficial deposits which are derived from the waste of the
former. As regards the first, it is chiefly found in quartz veins or
reefs traversing slaty or crystalline rocks, usually talcose or
chloritic schists, either alone, or in association with iron, copper,
magnetic and arsenical pyrites, galena, specular iron ore, and silver
ores, and more rarely with sulphide of molybdenum, tungstate of
calcium, bismuth, and tellurium minerals: Another more exceptional
association, that with bismuth in calcite from Queensland, was
described by the late Mr. Daintree. In Hungary, the Urals, and Northern
Peru, silicates and carbonates of manganese are not uncommonly found in
the gold and silver bearing veins. In the second or alluvial class of
deposits, the associated minerals are chiefly those of great density
and hardness, such as platinum, osrovridum, and other metals of the
platinum group, tinstone, chromic, magnetic, and brown iron ores,
diamond, ruby and sapphire, zircon, topaz, garnet, Xc, which represent
the more durable original constituents of the rocks whose
disintegration has furnished the detritus. Native lead and zinc have
also been reported among such minerals, but their authenticity is
somewhat doubtful.
The distribution of gold-bearing
deposits is world-wide; although the relative importance of different
localities is very different, their geological range is also very
extensive. In Europe the principal groups of veins are in slaty or
crystalline schists, whose age, when it can be determined, is usually
Palosozoic, Silurian, Devonian, or Carboniferous, and less commonly in
volcanic formations of Tertiary age. The alluvial deposits, being more
extensive, are less intimately connected with any particular series of
rocks. Few of either are however, of much importance as compared with
the more productive deposits of America and Australia. In the lijited
Kingdom goldbearing quartz veins were worked during the Roman
occupation at Ogofau, near Llanpumpsant, in Carmarthenshire, and in
the year 1863 as much as 5'3°° oz- was produced
from similar veins in Lower Silurian slates at Vigra and Clogau mines,
near Dolgelly. In 1875 the mine was re-opened, and in 1878 it produced
720 oz. Tetradymite, native bismuth, and several other characteristic
associates of gold were also found in small quantity. In Cornwall small
pieces of native gold have at intervals been found in alluvial or
stream tin works; and similar but more important finds have been made
in the granite district of Wicklow, and more recently at Helmsdale, in
Sutherlandshire. The largest nugget of British origin weighs under 3 oz.
On
the continent of Europe the great rivers originating in the
crystalline rocks of the Alpine region, such as the Rhine and Danube,
are slightly auriferous in their alluvial deposits in several places;
but the proportion of gold is extraordinarily minute, so that the
working is only carried on by gipsies, or by the local peasantry at
irregular intervals, the return for the labour expended being very
small. The same remark applies to the Rhone and its affluents, and the
rivers of the central granitic mass of France. In the Austrian Alps the
g M quartz mines at the liathausberg, near Gastein, at a height
of about 9,000 feet above ths sea-level, and at Zell, in Tyrol, are of
interest historically as having developed the system of amalgamation in
mills, although they are economically of small import-
