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6 ECONOMIC GEOGRAPHY

movement and volcanic activity it may occur in more recent forma-
tions. When gold occurs in this way the ore in which it is con-
tained Ms to be crushed in stamp mills before the gold itself can
be extracted. On the other hand, much gold is still obtained from
alluvial deposits or "placers/' which are due to rivers bringing
down, and depositing in sandbanks and elsewhere, gold which has
been liberated by weathering from veins and lodes along their
courses. The Rand goldfield in the Transvaal is perhaps a much
altered placer deposit of early geological times.

SILVER, COPPER, LEAD, AND ZINC. Part of the world's silver is
obtained from the same ores as gold, but much of it conies from
ores which contain copper or lead, cobalt or nickel. Copper ores
are found in various formations ranging in age from pie-Cambrian
to Tertiary, but the most important are those which occur in
igneous and metamorphic rocks. Gold and silver are almost always
present in copper ores, and are recovered during the electrolytic
refining of the copper. As it is a good conductor of electricity,
copper is much used in electrical engineering; it is also the main
constituent in bronze and brass. Lead commonly occurs with
silver and zinc, the principal ore from which it is obtained being
galena. When it is found in sedimentary beds independent of
igneous intrusion, it is usually associated with zinc, but is free from
silver and copper; when it is associated with igneous intrusions,
the latter minerals may also be present. , - , , ,,

PLATINUM is originally derived from basic igneous rocks which
have been raised to the surface by mountain-forming uplifts, but
the greater part of the world's production comes from placer de-
posits—more especially those in the Ural mountains. Platinum,
which is now much more valuable than gold, is largely used in the
chemical and electrical industries, in jewel-setting, and in dentistry.

ALUMINIUM is widely distributed in nature, but for commercial
purposes is mainly obtained from bauxite, a mixed mineral believed
to be the result of weathering and chemical action. As an abun-
dance of electrical energy is required for the reduction of the ore,
the smelters are frequently situated where hydro-electric power can
be cheaply generated. Important deposits of bauxite occur in"'
Arkansas (ILS.A.), French Morocco, and British and Dutch Guiana.

TOT has its chief source in the oxide cassiterite which is found in
veins closely related to granitic rocks. In actual, fact, however,



	6 ECONOMIC GEOGRAPHY movement and volcanic activity it may occur in more recent forma- tions. When gold occurs in this way the ore in which it is con- tained Ms to be crushed in stamp mills before the gold itself can be extracted. On the other hand, much gold is still obtained from alluvial deposits or "placers/' which are due to rivers bringing down, and depositing in sandbanks and elsewhere, gold which has been liberated by weathering from veins and lodes along their courses. The Rand goldfield in the Transvaal is perhaps a much altered placer deposit of early geological times. SILVER, COPPER, LEAD, AND ZINC. Part of the world's silver is obtained from the same ores as gold, but much of it conies from ores which contain copper or lead, cobalt or nickel. Copper ores are found in various formations ranging in age from pie-Cambrian to Tertiary, but the most important are those which occur in igneous and metamorphic rocks. Gold and silver are almost always present in copper ores, and are recovered during the electrolytic refining of the copper. As it is a good conductor of electricity, copper is much used in electrical engineering; it is also the main constituent in bronze and brass. Lead commonly occurs with silver and zinc, the principal ore from which it is obtained being galena. When it is found in sedimentary beds independent of igneous intrusion, it is usually associated with zinc, but is free from silver and copper; when it is associated with igneous intrusions, the latter minerals may also be present. , - , , ,, PLATINUM is originally derived from basic igneous rocks which have been raised to the surface by mountain-forming uplifts, but the greater part of the world's production comes from placer de- posits—more especially those in the Ural mountains. Platinum, which is now much more valuable than gold, is largely used in the chemical and electrical industries, in jewel-setting, and in dentistry. ALUMINIUM is widely distributed in nature, but for commercial purposes is mainly obtained from bauxite, a mixed mineral believed to be the result of weathering and chemical action. As an abun- dance of electrical energy is required for the reduction of the ore, the smelters are frequently situated where hydro-electric power can be cheaply generated. Important deposits of bauxite occur in"' Arkansas (ILS.A.), French Morocco, and British and Dutch Guiana. TOT has its chief source in the oxide cassiterite which is found in veins closely related to granitic rocks. In actual, fact, however,