taking place in the heat stored in the earth's crust, whose
effects are appreciably communicated to the outside of the thin rind of
solid earth upon which we live.

Owing to the great igneous and volcanic activity at the close of the
deposition of the carboniferous system of strata, the coal-measures
exhibit what are known as _faults_ in abundance. The mountain limestone,
where it outcrops at the surface, is observed to be much jointed, so much
so that the work of quarrying the limestone is greatly assisted by the
jointed structure of the rock. Faults differ from joints in that, whilst
the strata in the latter are still in relative position on each side of
the joint, they have in the former slipped out of place. In such a case
the continuation of a stratum on the opposite side of a fault will be
found to be depressed, perhaps a thousand feet or more. It will be seen
at once how that, in sinking a new shaft into a coal-seam, the
possibility of an unknown fault has to be brought into consideration,
since the position of the seam may prove to have been depressed to such
an extent as to cause it to be beyond workable depth. Many seams, on the
other hand, which would have remained altogether out of reach of mining
operations, have been brought within workable depth by a series of
_step-faults_, this being a term applied to a series of parallel faults,
in none of which the amount of down-throw is great.

The amount of the down-throw, or the slipping-down of the beds, is
measured, vertically, from the point of disappearance of a layer to an
imaginary continuation of the same layer from where it again appears
beyond the fault. The plane of a fault is usually more or less inclined,
the amount of the inclination being known as the _hade_ of the fault, and
it is a remarkable characteristic of faults that, as a general rule, they
hade to the down-throw. This will be more clearly understood when it is
explained that, by its action, a seam of coal, which is subject to
numerous faults, can never be pierced more than once by one and the same
boring. In mountainous districts, however, there are occasions when the
hade is to the up-throw, and this kind of fault is known as an _inverted
fault_.

Lines of faults extend sometimes for hundreds of miles. The great Pennine
Fault of England is 130 miles long, and others extend for much greater
distances. The surfaces on both sides of a fault are often smooth and
highly polished by the moveme