Reversed faults. — This type of fault is shown in Fig. 9, and, as its name implies, shows conditions of the opposite character to those found in faults of the normal type. While not entirely unknown in coal seams, their presence is of comparatively rare occurrence, although examples are to be found in the coalfields of Wales and Belgium. As will be seen from the diagram the fault in this case hades, or inclines, towards the upthrow side. The inclination of the fault plane in such cases is usually less steep than in the case of normal faults. Faults of this type may be looked for in districts where anticlinal or syncinal folding is common. Their occurrence produces doubling of the beds through which they pass along the width of the fault. The rule for finding the direction in which the seam will be recovered is the reverse of that given in the last case, but since this type of fault is uncommon in coal-bearing areas it is usual to assume all faults as normal unless there should exist evidence to the contrary.

Trough Faults. — This type of fault is shown diagrammatically in Fig. 10. It consists of a pair of normal faults hading towards each other, and enclosing a wedge-shaped mass of rock. In this case the parts with the broad bottoms have probably risen and allowed the part between to settle down. Such faults have, in all probability, originated from a single fissure splitting up into two before reaching the surface. In such eases one fault may be larger than the other, as is frequently the case, and the displacement below the line of intersection will, under such circumstances, equal the difference of the two displacements. Since both faults are normal, the rule for recovering the faulted bed is that given under the head of normal faults.

Step Faults. — This type of fault is shown diagrammatically in Fig. 11. It consists of a series of parallel and adjoining faults by means of which the beds are displaced by a succession of steps, the full displacement being reached by a number of small individual displacements. Such faults frequently result from the breaking up of larger ones. They are of the normal type, and are dealt with by the miner in accordance with the already stated rule. A common occurrence in connection with large faults is branching out of the fault towards one or both sides. These branches may run for considerable distances, or they may spread out into a fan-shaped series of small displacements and then disappear. This splitting up is thus frequently the sign of the termination of a fault. An examination of the maps of the Geological Survey showing any of the important coal-bearing areas, furnishes much instructive information of the way in which the earth's crust is broken up by dislocations, which in many cases divide the ground into an irregular network.

Wash-Out Faults. — This type of fault is shown in Fig. 12. It is due to the denudation of the seam at some later period than that of its formation. The denuded part is filled with sandstone, or in some cases with gravel and boulder clay. When filled with sandstone, the infilling material invariably shows signs of current bedding, which points to the coal in its early stages having been removed by a strong current of water. Where the infilling material consists of pebbles and clay, the pebbles are generally elliptical in shape, and scratched and striated in the direction of their longer axis, which indicates the removal of the coal by glacial action. When the first mentioned conditions are met with, it is quite usual to find the coal which has been washed out by the current deposited on the top of the existing seam at some point or points where the current has lost its velocity, and therefore its transporting power. The coal when thus dropped is usually much broken up and mixed with layers of sandstone, thus rendering the thickened part of little commercial value. These wash-out faults vary from a few feet to hundreds of feet in width, the wider ones probably representing old river valleys. Wash-out faults are not accompanied with dislocation of the beds, and are easily recognised by the signs of current bedding, or ice action, and the entire absence of any definite break in the rocks. In such cases the seam is recovered by driving through the obstruction at the same inclination as that of the obstructed seam.

Splitting. — This feature is of common occurrence in many coal-seams, and is due to irregular depression of the old land surface, and irregular deposition of the sand and mud during the intervals of depression. In some cases we find a seam consisting of two parts separated by a mere film of hardened mud or clay, forming what the miner terms a 'dirt parting.' This layer may grow in thickness in certain parts of the field, and may cause the seam to split into two distinct seams separated by a thickness so great as to make it necessary to work them as separate units. Examples of seams splitting in this way, and in some cases again reuniting, are to be found in nearly all our coalfields. Fig. 13 shows splitting.

Rolls. — This is an extremely common feature in nearly all coal-seams, and is due to the coal having been deposited on an irregular surface. While of common occurrence they are rarely of large size, and, as a rule, do not greatly interfere with the operations of mining. Their presence may, however, cause inconvenience especially in cases where the coal-seam is holed or under-cut by means of coal cutters. The general appearance of rolls is shown in Fig. 14.

Swellies. — These are abnormal thickenings of the coal seam, and are due to the same causes as rolls, although in some cases irregular pressure may have caused thinning of the bed at one part more than another. As in the case of rolls, the line of inclination of the roof beds usually remains fairly uniform, the thick parts being formed by depressions in the floor. The general appearance of an occurrence of this type is shown in Fig. 15. The effects on the working of a seam are exactly similar to those noticed under rolls.

Overlap. — The appearance of overlap is shown diagrammatically in Fig. 16. Overlap occurs where coal-seams have been laid down on a sloping surface, and may occur in a perfectly conformable series of beds where the upper ones extend over a wider area than the lower, either in one direction or on all sides. It is caused by the gradual sinking of the old surface, and the gradual extension of each bed over those immediately below. While the upper beds may in a case such as this be themselves conformable, they will rest unconformably on the old surface beneath.

Thinning-out. In certain eases coal-seams become thinner and finally disappear. This feature is due to deposition having taken place on a sloping floor, or to failure of the vegetation from which the seam was formed. This feature is not of frequent occurrence, and is oftener met with in the beds of shale and sandstones forming the coal measures than in the seams themselves.

Dykes. — During periods of volcanic activity fissures have been formed in many of the carboniferous beds; these fissures were subsequently filled with igneous rock in the molten state, which upon cooling solidified. Such fissures are known as dykes, the general appearance being shown in Fig. 17. The effect of these eruptive rocks upon coal seams where they penetrate them is in some cases most remarkable. In coalfields much cut up with igneous rocks the coal usually suffers to a greater extent than the other strata, being particularly liable to alteration by heat and because it presents easy lines of escape for the igneous matter being pressed up from below. The igneous rock is thus found following the upper or lower surface of the coal-seam, or even in some cases pushing its way along the centre of the seam.

These intrusive rocks produce alterations on seams of coal in their vicinity, the nature of the alterations varying with the size and nature of the eruptive sheet, and the thickness and composition of the coal-seam, and with other causes such as the temperature of the molten rock at the period of injection. In some instances the coal is completely fused, in others it changes to a hard, brittle anthracite, owing to the driving off of volatile constituents, while again it may be found passing into hard cinder or changing into a sooty mass. In all cases there is distinct evidence of the high temperature that must have existed during the ejection of the molten material, and the miner recognises this by speaking of these altered coals as 'burnt coal.'

Alteration is not confined to the coal and the overlying and underlying beds, but occurs in the invading rock. A hard black basalt or dolerite where in contact with coal or a highly carbonaceous shale, becomes white or yellow, and quite friable, its crystalline texture disappears, while at the same time it loses weight. In some cases this change is so complete that it is only the distinctively intrusive appearance of the mass that enables it to be recognised. Where the fissure is narrow the intrusive rock may be changed throughout. Where the fissure is wide only the outer parts of the intrusive rock are changed, a portion near the centre retaining its original semi-glassy structure. Dyke rocks so changed are spoken of by the miner as 'white rock' or 'white trap,' and in some cases the dyke is spoken of as a 'whitehorse.'