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The theory of reducing metallic ores, of whatever constitution, is to bring them to the state of oxides; and then, by the addition of charcoal, and with the aid of heat, to expel the oxygen in the form of carbonic acid; after which the pure metal is left. In practice, the reduction of copper-ores is slightly different.
Here, then, is a connexion sufficiently close established between the phonolites and the green slates, between the pyroxenic amygdaloids and the serpentines containing copper-ores, between volcanic substances and others that are included under the vague name of transition-traps. All these masses are destitute of quartz like the real trap-porphyries, or volcanic trachytes.
It became apparent at length, however, both from the large quantity of coal necessary for the reduction of copper-ores, and the great expense of the transport, that instead of carrying coal to Cornwall to smelt the ores the greater quantity to the less an opposite course must be pursued, and the ores carried to the coal districts, and there smelted.
The mines from which this copper is extracted, are in a lateral valley, opening into that of Aroa; and which is less hot, and less unhealthy, than the ravines nearer the sea. In the latter the Indians have their gold-washings, and the soil conceals rich copper-ores, which no one has yet attempted to extract.
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