Sills or Sheets. — While dykes penetrate the enclosing stratified rocks more or less at right angles to the stratification, a sill or sheet consists of a mass of igneous rock which has made its way along the bedding plane between two successive beds, forcing them apart, and finally solidifying in this position. These sills are found in many of the coalfields, and exercise considerable influence on the underlying or overlying coals. When in close proximity to a coal-seam, the coal is usually charred and burnt to an extent sufficient to lower its market value, or even to render it unsaleable. In other cases, if not too near, the coal may be hardened and converted into an anthracite, thus increasing its commercial value. On the whole, however, the presence of much 'floating whin,' as these sills are often called, has a decided tendency to render the successful working of coal-seams in their vicinity a matter of uncertainty.

Where igneous intrusions are found in coalfields they may be associated with faults, the beds being dislocated along the line of the dyke: oftener they have merely burst through the beds causing little or no displacement. In such cases the coal-seam is found lying at the same horizon on the other side, their only effect being to produce burning and charring already referred to, and to break the continuity of the seam.

Organic remains of the Carboniferous Period. — It will be noticed that there is a distinct resemblance in the character of many of the rocks described in the foregoing pages, and if a comparison of the rocks of two localities some distance apart were to be made their general appearance would not be a great aid in determining their relative age and position.

The rocks themselves fortunately provide us with another means of determination which enables very definite information to be obtained. This information is derived from the fossil remains of animals and plants which lived in the waters or on the land during the time the rocks enclosing them were being formed. Each group of beds contains certain fossils which differ from those occurring in the preceding and succeeding groups, and beds occupying the same relative position are thus known by their fossil contents no matter in what part of the world they are found. Speaking broadly, the types of life whose remains are enclosed in the rocks are represented by the least highly organised forms in the older deposits and by more complex forms in the newer. The rocks thus bear testimony to the gradual evolution of life from lower to higher and more complex forms, and, as already indicated, these forms are not scattered indiscriminately through the rocks but occupy definite positions in them. It is quite impossible to describe here all the fossils found in the Carboniferous System, but a few representative types will be dealt with. The fossils found in the carboniferous limestone are all of marine origin, that is, they represent types of animals that inhabited salt water. The millstone grit rarely contains fossils, and may be dismissed once for all. The coal measures contain the remains of land plants and a few freshwater or estuarine animals, thus helping to complete the evidence of our present coalfields being formerly swamps. Fossils thus give valuable evidence of former geographical changes, conditions of climate, and sequence of deposition, and enable us to arrive at a chronological arrangement of the stratified rocks.

SIGILLARIA — The remains of this plant are often found in coal, and impressions of the bark are fairly common in the associated shale. The plants grew to a large size, sometimes as much as 4 feet in diameter and from 40 to 50 feet in length. They are easily recognised by the symmetrically scarred impressions of the bark (see Fig. 18). They are closely allied to the humble 'club mosses' of the present day. Stems of this plant are occasionally found rising up out of the coal and penetrating the roof shales — the thin film of coal formed by the bark forms a line of weakness and large rounded masses are apt to fall without warning where these 'pot bottoms,' as they are called by the miner, occur.

STIGMARIA. — This fossil is commonly found in the clays underlying the coal-seams. At one time it was considered to belong to a distinct family, but is now known to be merely the roots of Sigillaria. The abundance of these rootlets in practically all underclays goes to show that a considerable portion of the overlying coal seam must have been formed from Siglllariae. The general appearance of this fossil is shown in Fig. 19.

LEPIDODENDRON. — This is another ancient representative of the present day 'club mosses' that is of common occurrence in the coal measures. Some of the plants belonging to this family were over 40 feet high and of considerable diameter. They had long, straight stems with branches near the top, the stems being covered with scars, caused by the separation of the leafstalks. The scars are usually well-marked, and it is this marking that gives the name to the genus. Seed cones of this plant are often found well preserved among the coal shales, and are known as Lepidostrobus. Fig. 20 shows the characteristic marking of this fossil plant.

CALAMITES. — This order of plants was largely represented during the coal period, and their jointed and striated stems are found abundantly in many of the coal measures shales. These plants were closely related to our modern 'horse-tails,' as the equisetum fluvialile of our marshes and ditches are popularly called. Specimens of this fossil showing a height of 8 feet and a diameter of 5 inches have been found. The general appearance of this fossil is illustrated in Fig. 21.

FERNS. — A very large proportion of the plant remains found in the coal-bearing strata consist of ferns. These graceful relics of a former luxuriant vegetation are found in the roof shales and also in the coal-seams themselves. The impressions, especially in the shales, are most striking, and many different species can be readily recognised. There is on the whole a general resemblance to present-day forms, but the coal-measures ferns are now quite extinct and their place in nature filled by new species. More than a mere passing glance should be given to such specimens of ferns as may fall into the hands of the young miner, because not only are they important as type specimens but they are generally so well preserved that even the seed spores on the back of the leaves can be recognised.

Neuropteris. — This fern bears a close resemblance to the Royal Fern (Osmunda regalis) of the present day. Its leaves are heart-shaped and show beautiful veins arching out from the base. The occurrence of this form is restricted to the lower and middle coal measures. Its general appearance is shown in Fig. 22.

Pecopteris. — This fern has long, narrow primules, branching alternately from each side of the stem. The leaflets are traversed by a mid-rib, from which a number of veins branch off. These veins run straight out almost perpendicular to the mid-rib; in some cases they divide, but in no case are they found intersecting each other. The occurrence of this species is confined to strata belonging to the upper coal measures. Its general appearance is shown in Fig. 23.

Sphenopteris. — In this type the leaves are of somewhat variable shape, but usually show some contraction at their base and are frequently lobed. The veins run from the base of the leaflets and radiate towards the apex. They are found in strata belonging to the middle or lower coal measures. The general appearance is shown in Fig. 24.

Plants as an index of horizon. — Many of the fossil plants of the coal measures are common to all the beds of the Upper Carboniferous Series, and their presence are only of service to the miner in a general sense. An experienced geologist can, however, often deduce important information from the abundance of some types and the rarity of others, but such knowledge can only be acquired after years of study and experience. On the whole the estuarine and freshwater types of animal life whose remains are found associated with coal-bearing strata give more definite information with regard to the sequence of the beds.

MOLLUSCA. — Animals belonging to this sub-kingdom have soft bodies with no backbone, and are usually protected by a calcareous shell. Fossil mollusca are found in large numbers in the coal measures, and are preserved by being enclosed in ironstone nodules or flattened between beds of shale. The fossils of the lower measures are marine and are usually found in the roof shales. Chief among them are Goniatites, and occasionally Productus (see Figs. 25 and 26). Freshwater and estuarine types are Anthracosia and Anthracomya, which are abundant throughout the middle and upper measures, and are widely distributed, being met with in the coal measures all over the world. In some districts they occur in such quantities as to form beds several inches thick. These beds are known as 'musselbands.' Figs. 27 and 28 show these types.

While the foregoing are important coal-measures fossils, too much dependence should not be placed on the occurrence of single specimens when trying to correlate seams or determine the sequence of beds, as stray specimens may be found both above and below the main horizon. This is what one would expect, as it is only reasonable to assume that particular specimens would have to develop from a small beginning, gradually reach their maximum, and then slowly die out.

Oilshale. — It has already been pointed out that the lower part of the carboniferous formation consists in Scotland of a group of rocks known as the Calciferous Sandstone Series. In the counties of Midlothian and Linlithgow, and along the southern coast of Fife the upper part of the Calciferous Sandstone Series contains several seams of oilshale interstratified with beds of sandstone, shale, fireclay, and estuarine limestones. These beds of oilshale are of considerable economic importance, and furnish the raw material for an important industry. When subjected to destructive distillation in retorts, they give crude oil, ammonia, and gas. The gas is used as fuel, the ammonia, held in solution by water, is precipitated by sulphuric acid as sulphate of ammonia, while the crude oil furnishes the following marketable products after further distillations and treatment.

(1) Shale Spirit or naphtha, used largely as a solvent and for motor spirit.

(2) Burning Oil, used for lamps and certain classes of oil engines.

(3) Gas, or Intermediate Oil, used for gas-making and enrichment, for cleaning purposes, and as fuel for the Royal Navy.

(4) Lubricating Oil, used for lubricating machinery.

(5) Solid Paraffin, used for making candles, and for other purposes.

Physical Characters of Oilshale. — Oilshale is a fine grained black or brown coloured shale easily distinguished from ordinary carbonaceous shale by its toughness and resistance to weathering. It is easily cut by a sharp knife, the shaving curling up in front of the blade; two varieties are recognised, 'plain,' being flat and smooth and dull black, while 'curly' is twisted and contorted and shows a fine glossy polish on the contorted parts. It is finely laminated, although the laminations are not visible in the raw shale, but are easily recognised in the spent material after distillation. The specific gravity of oilshale is less than that of ordinary carboniferous shale.

Fossils. — These are not found in large numbers but two distinct fossiliferous bands are now recognised, one near the top of the shale-bearing strata, and the other near the bottom. Both bands contain Orthoceras and Goniatites which are marine types.

It is generally believed that oilshale is the remains of vegetable matter, containing numerous seed spores, and a certain proportion of animal matter, which has been deposited along with clay as drift material at the bottom of lagoons, and has afterwards been subjected to bacterial action.