The mode of working mines is two-fold; by _open excavations_, and
_subterranean_.
Workings in the open air present few difficulties, and occasion little
expense, unless when pushed to a great depth. They are always preferred
for working deposits little distant from the surface; where, in fact,
other methods cannot be resorted to, if the substance to be raised be
covered with incoherent matters. The only rules to be observed are, to
arrange the workings in terraces, so as to facilitate the cutting down
of the earth; to transport the ores and the rubbish to their destination
at the least possible expense; and to guard against the crumbling down
of the sides. With the latter view, they ought to have a suitable slope,
or to be propped by timbers whenever they are not quite solid.
_Open workings_, are employed for valuable clays, sands, as also for the
alluvial soils of diamonds, gold, and oxide of tin, bog iron ores, &c.,
limestones, gypsums, building stones, roofing slates, masses of rock
salt in some situations, and certain deposits of ores, particularly the
specular iron of the island of Elba; the masses of stanniferous granite
of _Geyer_, _Altenberg_, and _Seyffen_, in the Erzgebirge, a chain of
mountains between Saxony and Bohemia; the thick veins or masses of black
oxide of iron of Nordmarch, Dannemora, &c., in Sweden; the mass of
cupreous pyrites of Ræraas, near Drontheim, in Norway; several mines of
iron, copper, and gold in the Ural mountains, &c.
_Subterranean workings_ may be conveniently divided into five classes,
viz.:--
1. Veins, or beds, much inclined to the horizon, having a thickness of
at least two yards.
2. Beds of slight inclination, or nearly horizontal, the power or
thickness of which does not exceed two yards.