4
intense
heat is being slowly reduced by the slow wear of surfaces, and by quick
upward movements of the earth's crust, such as occur at times as a
consequence of volcanic adjustments.
By
some means or other the sedimentary rock masses of our State have been
permeated with minerals containing sulphur and arsenic. These are known
as sulphides. Associated with these are gold and silver in metallic
form, and perhaps in sulphide form also. The ordinary sulphide here is,
for the most part, iron pyrites, with a comparatively small percentage
of sulphides of lead (galena), and zinc (black-jack), and there is a
little copper, manganese, and bismuth. The greater weight of all
minerals compared with the weight of the rock material takes them as
low as the ascending heat will allow, and in this region, of course,
the heat is so intense that it stays their downward movement. Here all
rocks become very much permeated with minerals. As this mineral-soaked
region is brought slowly up into cooler temperatures, gravity, acting
on its minerals in association with water, takes them slowly down
again. Thus a continuous movement up and down is in progress in the
earth's crust, too slow for human experience to notice, but ever active
for all that.
It
is to this everlasting variation of temperature and movement of watery
liquids in the skin of the earth that we trace the origin of our lodes.
The wear of ages has brought " country " permeated with minerals up to
and near the surface, and gravity, by the aid of water, is gradually
taking them all down again. It would take them away to depths beyond
our reach in comparatively quick time, even for us, had the fractured
areas admitted of vertical fractures having unimpeded runs to the
depths where the heat resistance spreads them in the rock masses. This
is not the case, however. It has been so arranged that a more or less
twist strain has been applied in producing the fracturing of all rock
masses, and this fracturing, such as is produced in a twisted pine
board, is in evidence in most of our gold-fields.
In
the downward movement of all liquids, they naturally take the lines of
least resistance to the "pull" of gravity on them, and this is in the
fractured areas. The water makes its way down between the rock layers,
that is, between the layers of alternate slate and sandstone, and
between their leaves, and down the numerous shrinkage cracks in the
lavers, known to miners as " heads" and " floors." From these smaller
conduits it passes into the main systems of fracture, and in its
passage down it becomes loaded with minerals, including silica in
solution, the whole being taken from the rocks it passes through. As it
descends it meets with more or less resistance to its downward passage,
according to the inclination from the horizontal of its conduit for the
time being—Fig. 2 (a),
