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 movement which has taken place in the strata.  They then show the phenomenon known as slicken-sides.  Many faults have become filled with crystalline minerals in the form of veins of ore, deposited by infiltrating waters percolating through the natural fissures.

In considering the formation and structure of the better-known coal-bearing beds of the carboniferous age, we must not lose sight of the fact that important beds of coal also occur in strata of much more recent date.  There are important coal-beds in India of Permian age.  There are coal-beds of Liassic age in South Hungary and in Texas, and of Jurassic age in Virginia, as well as at Brora in Sutherlandshire; there are coals of Cretaceous age in Moravia, and valuable Miocene Tertiary coals in Hungary and the Austrian Alps.