Stratified rocks of Mechanical Origin. — These have been referred to under the head of Stratified Rocks, and only a short description of a few of the commoner will be given.

CONOLOMERATE or PUDDING STONE is a rock made up of pebbles, which are cemented together with sand and some finer binding material, usually silica, or carbonate of lime. The pebbles are worn and rounded by river or sea action, and vary in size from that of a pea to large boulders. They may consist of almost any hard mineral or rock, though the sand about them is largely quartz. In some cases the rock is quite soft and the pebbles can be easily detached. In others it is so hard that the pebbles are broken across with the fracture of the rock. Where a rock contains angular fragments in place of rounded ones it is called a Breccia.

SANDSTONE is a rock composed of fine quartz grains, cemented together either by calcareous, siliceous, or ferruginous materials. When the sandstone is brittle and the surface grains easily rubbed off, it is spoken of as friable, or when the grains are more firmly fixed in the cementing matrix it is called compact.

Sandstones are often distinctly banded, and are sometimes stained red or brown with the oxide or the hydrated oxide of iron. In quite a number of sandstones mica flakes are found, and when very numerous the rock is called a MICACEOUS SANDSTONE. Sandstones form much thicker layers than conglomerates, the latter usually constituting a basal layer only of the sandstone formation.

SHALE is a consolidated mudstone forming a laminated deposit. In some cases the clayey material is intermixed in varying proportions with fine sand grains. Shale is a very abundant rock and is found almost everywhere. Some shales are full of fossil plants, others yield petroleum distillation, and are known as OILSHALES. Shales disintegrate readily under the action of rain and frost. Rocks belonging to this class are more or less impermeable to water and do not undergo solidification by the deposition of cementing material from underground water like sandstones. Their consolidation from the original mud is a result of pressure exerted by the mass of material deposited above, the lamination being caused by slight differences in the character and sizes of the fine particles forming the original deposit.

Rocks formed by Chemical Precipitation. — The rocks coming under this class do not as a rule show marked laminated structure, but are frequently distinguished by concentric concretionary, or radial fibrous structure. They sometimes occur in irregular masses but usually interbedded with other stratified rocks.

OOLITIC LIMESTONE is a rock made up of small spherical nodules resembling fish roe. When broken apart individual grains may reveal in the centre a core of silica about which the carbonate of lime has been deposited in concentric layers. When replaced, as it sometimes is, by oxide of iron oolitic iron ore is formed.

TRAVERTINE is a rock formed by the evaporation of water containing lime in solution, it is usually porous, and formed of more or less concentric layers, or it may show a peculiar fibrous formation. Further infiltration may fill up the pores and render it compact.

Sedimentary Rocks of Organic Origin. — Certain rocks are formed by the accumulation of the remains of plants and animals and hence are of organic origin.

LIMESTONE is generally a white or grey coloured rock composed of carbonate of lime with varying proportions of silica, clay, and other substances. The carbonate of lime is usually derived from the hard parts of marine organisms, and the siliceous and clayey impurities are from fine sediments, drifted in from the land, which have settled down with them. A limestone composed largely of organic remains may in certain cases show little trace of its origin where a freshly broken surface is exposed. On the other hand a weathered surface will usually show traces of the organic remains, as these usually withstand the action of the atmosphere better than the matrix in which they are embedded.

PEAT is a deposit of decaying vegetable matter in the presence of stagnant water. It is formed of mosses, marsh, heath, and moorland plants, but in many instances, especially in the hill peats of Scotland, pine, oak, birch and other trees have contributed largely to the accumulation. In a peat bog the surface is generally covered with growing plants, while the lower portion is formed from the decomposing matter. These bogs usually form the sites of old lakes which have been filled up. The depth of peat is in some cases only a few feet or even less, while in others a thickness of thirty feet or more is attained. In the deeper deposits the part nearest the bottom is usually very dark in colour and the plant structure is almost entirely destroyed.

Peat represents the first stage in the formation of coal from vegetable matter, and differs from coal by its larger proportion of contained water. Because of this water its value as a fuel is correspondingly small.

COAL consists of uncombined carbon along with various combinations of carbon and hydrogen, and is plainly of vegetable origin, as it contains the recognisable remains of plants.

George Stephenson the great engineer once referred to a block of coal as being so much 'bottled sunshine,' and better description could have been framed. Coal is a mass of compressed and mineralised vegetation, and consists of the altered remains of a large variety of plants, whose growth in ages long past was fostered by the powerful rays of the sun aided by a moist atmosphere. The plants were all of a soft nature without woody fibre, and because of this were easily decomposed. The rapid growth and decay gave rise to an atmosphere highly charged with carbonic acid gas, which pushed on the growth at a still more rapid rate until the plants, weakened by excessive development, fell down and were squeezed into one solid mass. New plants sprung up to take the place of those perishing by decay, and bit by bit the thickness of the mass increased until by the gradual sinking of the surface, the whole mass was submerged in water, forming a bed which later became coal. Sand and mud washed in from the higher parts of the land gradually covered this up and formed a new surface. After a time a new mass of vegetation began to grow and decay, and was then covered up in its turn, and thus a second bed was formed. In this way bed after bed of coal was formed with different thicknesses of mud and sand between, and these beds are now known as the carboniferous or coal-bearing beds.

The changes which take place in the conversion of vegetable matter into coal are partly chemical and partly structural. The plant remains contain Oxygen, Hydrogen, and Nitrogen, which are gradually expelled until only a small proportion of these substances is left along with the carbonaceous material of which the plants are principally composed. The vegetable structure at the same time gradually disappears, and coal is formed. The process of formation can be traced through its different stages from peat, which is simply decaying vegetation, to Lignite or Brown Coal, which contains more carbon and less moisture than peat. Then from Lignite to Cannel Coal, from Cannel to Bituminous Coal, and from Bituminous Coal to Anthracite, each of these varieties of coal containing a greater amount of Carbon and less volatile matter as we pass from one to the other in the order named.

Some coals were undoubtedly formed on dry land or swampy marshes, and were covered over by the gradual subsidence of the land surface. Evidence to prove this is obtained by the finding of trunks of plants in the position in which they grew, and by rootlets found in the beds immediately underlying many coal seams. One of these mineralised or fossil plants is shown in Fig. 1.

Other seams of coal were undoubtedly formed by the drifting of large masses of vegetation into lakes, estuaries, and shallow seas, by rivers and floods.

A careful examination of a thin section of coal under a microscope will reveal the presence of traces of plant tissue and seed spores, and frequently the presence of vegetable cells and fibres in a fine state of preservation. In addition to the evidence obtained by a minute examination with the microscope, a coal-seam often shows under ordinary examination imprints of ferns and other plants, which have been so well preserved as to furnish unmistakable evidence of its vegetable origin even to the casual observer.

Hugh Miller, the great Scottish geologist and man of letters, standing on a heap of recently dug coal gave the following graphic description of its formation. He said :–

'All round us are the relics of innumerable forms of plant life which flourished and waved luxuriantly in the warm breezes long before Egypt was dreamed of, or Nineveh ever knew Nimrod, or Rome knew Romulus, or to begin aright Adam knew Eve. Every tree and plant whose ruins are here compressed into these beds of coal was green and was wood centuries and centuries before Eden had her first rose and Eve had her first walk amid the beautiful lowers. The age of the Pyramids of Cizeh is nothing compared with this great pyramid of coal. Long, long before that pyramid rose above the sands these seams of coal were packed up close, arranged and ready for human discovery and future use.'

A coalfield may be described as an area or tract of country the underlying rocks of which contain one or more coal seams. It is a noteworthy fact that all coalfields take the form of a basin or elongated trough, the beds dipping from all sides towards a central axis. The thickness and quality of the coal-seams are not necessarily uniform throughout the field, and in many cases the strata are much faulted, and may also be pierced with dykes of igneous rock due to volcanic action of later date than the period of the formation of the coal-seams.

Strictly speaking coal should be classified as a metamorphic rock as it has undoubtedly undergone considerable changes since its formation. The nature of the changes has already been indicated, and the only reason for placing coal among the sedimentary rocks is that these changes are due to the same causes that led to the consolidation of all sedimentary rocks, namely pressure of superincumbent deposits, increase of temperature with increase of depth, and action of underground waters.

Rocks formed by Contact Metamorphism. — This type of metamorphism is produced when great masses of igneous rock are injected into the upper part of the earth's crust.

The surrounding strata are baked and hardened, and in some cases re-crystallisation of certain mineral substances may take place. The alteration is purely local and the effects may be studied at the junction of an igneous with a stratified rock, an extremely good example being a passage of a dyke rock through a coal seam.

PORCELLANITE is the name given to partly fused shale where it has been in contact with intrusive igneous rocks. It is usually red or grey, hard, brittle and fine-grained, much resembling the half-burned refuse of a pottery.

MARBLE (Crystalline limestone) is formed by the action of heat, or of heat and pressure combined, upon limestones. These rocks are of various colours depending upon the purity of the original limestone. Those found in the coal measures are usually black, due to the admixture of carbonaceous matter.

Rocks formed by Dynamic Metamorphism. — GNEISS is the chief rock belonging to this class nd has the same composition as granite, although the name Gneiss is now frequently applied to foliated rocks with a structure like granite, irrespective of the nature of the minerals they may contain. The crystals of the different minerals are arranged in roughly parallel planes or folia, the planes being usually bent and curved in the rock mass. These pressure effects are easily traced in most gneisses, and give evidence that the structure has been largely influenced, if not entirely caused, by the action of shear forces on the rock.

MICA SCHIST is composed mainly of quartz and Muscovite mica. It is a typical metamorphic rock and covers wide areas where the older rocks are exposed. The constituent minerals are arranged in layers or folia along which the rock is easily split. It is easily distinguished from gneiss by its finer texture and more wavy appearance.

SLATE is a metamorphosed mudstone or shale which has been hardened and given a slaty cleavage by pressure acting on it. The colour may vary from grey green to purple or black. The chief characteristic is the ease with which it can be split along the cleavage planes. These cleavage planes are quite distinct from the bedding planes, which, when well marked, are known as the 'Stripe.' Many slates contain fossils, and as these are never perfect but always shorn and distorted, their appearance shows that the rock has undergone 'shear' movement.