The stratified rocks have been divided into great groups according to their geological age: these groups are recognised by the fossils which they contain. To the coal-miner that group known as the Carboniferous is the most important. Although the presence of coal has given the name to the formation, coal itself forms but a small proportion of the beds constituting the Carboniferous gro up. These beds, while they differ from each other in the various coalfields, have certain characteristics which are represented throughout the chief coalfields and are referred to by Professor Hull as follows

UPPER CARBONIFEROUS.

Upper Coal-Measures. — Reddish and purple sandstones, red and grey clays and shales, thin bands of coal, ironstone, and limestone with spirorbis and fish remains.

Middle Coal-Measures. — Yellow and grey sandstones, blue and black clays and shales, bands of coal and ironstone, fossil plants, bivalves, and fish, with occasional beds of marine origin.

MIDDLE CARBONIFEROUS.

Lower Coal-Measures or gannister series with thin coal seams and marine fossils.

Millstone Grit with thin coal seams.

Yoredale beds, or upper limestone shale with marine fossils.

LOWER CARBONIFEROUS.

Mountain limestone with marine fossils.

Calciferous Sandstone Series or lower limestone shales with marine fossils.

In Wales and the South of England, where the rocks are of carboniferous age, the lower portion is formed by beds of limestone from 1000 to 2000 feet thick, with a few bands of calcareous shales near the bottom. In the central area of Staffordshire and the borders of South Lancashire and Yorkshire, the limestone thickens out to from 2000 to 4000 feet; while in the northern portion through North Yorkshire, Durham, Westmoreland, and Northumberland, sheets of mechanically formed sediments come in and split up the limestone series into beds of grits, sandstones, and shales, which contain persistent bands of limestone and a few thin seams of coal. Crossing the border into Scotland, a further series of changes occur; the lower part of the series now changes to beds of calciferous sandstone, while coals and limestones constitute the upper beds. Thus, speaking generally, we may say that no coal is found beneath the millstone grit series in South or Central England, and little of any consequence in the north, while in Scotland the finest seams of coal on the eastern side occur in the limestone series underlying the millstone grit.

Bedding and Folding. — The stratified rocks cover an immense area of country and are of great thickness. In some districts they are tilted on end and thrown in great folds, forming hills and mountains and deep valleys. Thus an opportunity is afforded for their examination which enables their relative positions to be ascertained. The order of deposition and the fossil contents furnish information that allows their relative age to be determined.

Strata seldom lie horizontally, but are usually more or less inclined. The inclination to the horizontal is called the dip of the beds, and a line at right angles to the dip is called the line of strike or level course (Fig: 2).

Stratified rocks sometimes dip uniformly in one direction, in other cases they may dip away from or towards an axis, thus forming folds. If the beds dip in towards a common point a syncline is formed, whereas if they dip away from such a point an anticline is formed. The general appearance of these anticlinal and synclinal folds is shown in Figs. 3 and 4. Sometimes the beds lie on the top of each other with great regularity; in such ease the beds are said to be conformable. In other cases the beds may lie horizontally, or at a slight angle, upon the upturned edges of more highly inclined beds; in this ease the beds are said to be unconformable. Conformable and unconformable strata are shown in Figs. 5 and 6. The occurrence of an unconformity shows that a break in the continuity of the stratification has taken place and that a greater or lesser period of time must have elapsed between the deposition of the materials forming the two sets of beds.

False Bedding. — The various layers of a stratified deposit usually lie parallel to each other, their surfaces marking the bottom of the water at the period of deposition. Occasionally the layers may be found to lie at various angles to each other and to show distinct diagonal lamination. This characteristic appearance is shown in Fig. 7, and is known as false or current bedding. This has been caused during the deposition of the sediment by currents sweeping first in one direction, then in another, the layers of sand lapping over those already deposited and thinning out in the direction of the currents. In fixing the real direction of dip in such rocks, attention must be directed to the main lines of bedding, and observations should not be too closely confined to one point.

Joints. — All sedimentary rocks contain joints. These consist of a series of divisional planes running at right angles to each other which divide the rock up into rectangular blocks. In some cases the joints form well-defined fissures, in other cases, especially in close-grained rocks, they are almost imperceptible until the rock is broken, when being planes of weakness the rock parts or splits along the line of jointing. Joint structure is mainly due to contraction of the material from which the rock is formed (luring drying. This contraction set up stresses in the material, which were ultimately relieved by the material parting at intervals. This system of cracks so induced form what we now call joints.

Faulting. — Besides bending to which beds may have been subjected, they are sometimes broken through, and the broken ends raised or depressed relatively to each other. Such an occurrence is described as faulting. Faults, then, are dislocations of the strata caused by the movements of the crust of the earth. These dislocations are frequently of considerable magnitude, and when they pass through coal seams they interfere with the operations of mining, and cause considerable expense before the workings can be extended beyond the faulted part. Several classes of faults are met with, but those of greatest importance to the coal-miner are : (1) Normal faults, (2) Reversed faults, (3) Trough faults, (4) Step faults, and (5) Wash-out faults. All of these faults have been produced at a period subsequent to the formation of the coal seams in which they may occur. Other irregularities have been formed at the same time as the coal seam and are often called faults by the miner, since they interrupt the working of the seam. They are not, however, true faults. These are: (1) Splitting, (2) Rolls, (3) Swellies, (4) Overlaps, and (5) Thinning-out. The foregoing terms are those employed by geologists, but the miners in different districts use such terms as 'slip,' 'slide,' 'throw,' 'check,' 'trouble,' 'dyke,' and other names in describing them. The 'throw' of a fault is the amount of displacement of the two portions of the faulted bed measured in a vertical direction. The fault is said to be an upthrow or a downthrow according to the side from which it is approached.

The 'hade' of a fault is the angle the fault makes with the vertical. The width of a fault is the horizontal distance between the two broken ends of the bed; its dimensions depend upon the throw of the fault and the hade. in normal faults the width represents the extent of barren ground, and in no way applies to the actual width of the fissure itself.

The distance along the line of fracture through which the dislocated part of the bed has moved is known as the 'slip.' In practice the actual amount of this oblique movement is seldom referred to.

Faults rarely affect the level of the surface; which points towards the movements producing them having been gradual rather than abrupt.

The actual appearance of the fracture varies in character with the nature of the rocks through which it passes. In some cases these fractures are mere planes of division, in others the opposite surfaces are smoothed and polished, while occasionally the fractured surfaces are some distance apart, the space between being filled with brecciated matter. Sometimes the beds terminate abruptly with little or no distortion; at others they are bent and distorted along the plane of the fault, and are cut up by numerous small and irregular fractures, suggesting a jarring and grinding action during the period of movement. Where such distortion is present, it usually corresponds to the direction of the faults, but cases where the opposite conditions exist are known.

Normal Faults. — A diagram to show this form of fault is given in Fig. 8. This is the type of faulting usually found in coal seams. The most important point for the coalminer to observe is the direction of the fault, as this shows the direction in which the faulted seam will be recovered. The practical rule may be stated as follows: If the fault makes an angle less than a right angle with the bottom of the bed of coal, the coal must be looked for at a lower level on the other side, as the fault is a downthrow. If, on the contrary, the angle is greater than a right angle, the seam will be recovered at a higher level, as the fault is an upthrow.

It may be noticed that the distance the coal is heaved is not indicated by the nature or direction of the fault. An examination of the strata on the other side may give definite information as to the displacement, provided the observer knows the sequence of the rocks.