The study of Coal Mining, whether carried out in an elementary manner or in minute detail, requires an acquaintance with geology, and consequently some time must be devoted by the would-be student of mining to the acquisition of fundamental geological facts.

Only a few of the more important features that apply particularly to the needs of the young miner will be dealt with here, but every available opportunity should be taken to supplement what is here given by reading some elementary book devoted to the subject of geology alone, and carrying the study as far as possible by practical observations in the field.

Geology is a branch of nature study for which ample material is always at hand, and any coal-mining district will supply a wealth of opportunity for the examination of general features and specimens, which will awaken more interest and provide more real knowledge than it is possible to obtain by mere book study. For this reason, all of the features dealt with in this book should be looked for and carefully examined in the field. The benefit derived from a few afternoons in the field under the guidance of a good geologist is worth months of reading, and even if the help of the good geologist is not at hand, much can be done for oneself by perseverance and patient observation.

Geography tells us about the shape of the earth and the distribution of land and water as it exists at present. Long ages ago the earth was different from what we find it to be now; and it is generally believed by scientists that the earth was formed by the cooling and consolidation of a vast cloud of gaseous or meteoric matter which had previously been floating in space. As this cooling gradually proceeded, a crust was formed round the still molten interior, and the first rocks were thus brought into existence. As cooling went on, the crust gradually contracted and great wrinkling and folding of the rocks forming it took place. This bending and folding gave rise to great differences of level, and the condensation of vapours produced water, which occupied the depressions thus covering a great part of the surface. Further cooling and contraction caused redistribution of the land and water areas from time to time until the present configuration has been reached. The earth is thus formed of a vast stony ball with an irregular surface, the higher parts of which form the dry land this part is called the lithosphere. Resting in the hollows of the crust is a layer of water which covers a large portion of the total surface — this shell of water is called the hydrosphere. Finally a complete envelope of gas surrounds the whole this envelope stretches out to a considerable height from the surface and ultimately fades into space. This gaseous envelope is known as the atmosphere. Whatever may have been the position in the early days of the earth's history, it has for long been inhabited by various forms of life both animal and vegetable, and these have played their part in geological history. Indeed the whole study of geology is a study of changes produced by the action of heat, water, air, and life, so far as such changes affect the earth, and particularly the surface part of it which comes under our immediate observation.

Geological Change. — The changes taking place on the earth surface are of three kinds — destructive, transportive, and constructive — and are brought about by the following agencies: (1) The action of the atmosphere, (2) Changes of temperature, (3) The action of running water, (4) The action of the sea, (5) Ice action, and (6) Organic action, or the action of plants and animals.

Action of the Atmosphere. — The chief gases contained in the atmosphere — Nitrogen, Oxygen, and Carbon dioxide-are all more or less soluble in water, and are carried down into the rocks by rain, which, assisted by the action of these dissolved gases and the surrounding air, causes a chemical change to take place in certain of the constituents of the rocks, which is described as atmospheric weathering. Weathering is a purely destructive action.

Changes of atmospheric pressure produce wind, and high winds, especially in dry, arid countries, may carry fine sand grains against rocks with sufficient velocity to cause some abrasion. This again is destructive action; but sand may be blown from one place to another, which is transportive action, and its final settling down to form a bed of sand is constructive action. All over the world, and on many parts of our own western coastline, hillocks of blown sand have accumulated and are known as Sand Dunes.

Changes of Temperature. — Temperature changes cause rocks to expand and contract as the temperature rises or falls, and this movement causes the rapid breakage of a thin layer near the surface, especially in places where the daily range of temperature is considerable. In many parts of the world, especially along the equatorial belt, this action causes rapid disintegration.

Action of Running Water. — A considerable portion of the rainfall is immediately carried away by the natural drainage system formed by streams and rivers. During heavy rains much loose material is washed off the surface into the streams, and eventually finds its way to the sea. This rock material, carried by rivers during periods of flood, enables the water, by using it as an abrasive, to cut deeper into the bed of the river, thus forming a valley. In addition to the mechanically carried material, some mineral matter is dissolved by the water and passes away in solution. On the higher ground nearer its source, the work of a stream is largely destructive: the broken-down material is transported by the water to the lower reaches or to the sea, and there deposited, forming layers of sand and silt, which will ultimately consolidate into a 'stratum' or bed of rock.

Action of the Sea. — The work of the sea is chiefly that of erosion of the coastline. During storms great waves are formed, which driving inshore lash the cliffs or the beach, and, aided by loose sand and pebbles raised from the bottom, gradually wear into the land. Where this action is directed against a promontory or outstanding part of the land, the undermining of the rocks along the shore frequently causes large landslides. The material brought down by this means is rapidly carried back by the action of the waves and tide, and a fresh face laid bare for further action. Thus we have both erosion and transportation of the eroded material taking place. Where bays have been cut deep into the land, due to the presence of a comparatively soft rock at that part, the waves expend their force in rushing up the sloping face of the beach, and often during severe storms carry a great deal of sand and pebbles, which they deposit at or near high-water mark. In this way sea action at certain parts may become constructive.

Ice Action. — At high altitudes the temperature of the atmosphere is much lower than at sea level, and moisture thrown out above a certain height, varying from 16,000 feet at the equator to sea level in the polar regions, assumes the form of snow. In this way, the polar regions, and many of the lofty mountain ranges in other parts of the world, are perpetually covered with snow. The gradually accumulating mass is compressed into ice by the weight of the upper layers resting on the bottom ones, and this ice begins to move from the higher to the lower ground. In the polar regions the ice reaches the sea in the form of a vast ice-sheet, where it is broken off by the buoyancy of the sea water and drifted away as ice-floes and icebergs. In temperate and torrid regions, where only the higher mountains are snowclad, the excess accumulation finds its way down the mountain-sides as glaciers, which are simply rivers of ice. When a glacier comes down below the snowline it melts and gives rise to a river. During the passage of the ice from the higher to the lower part of its path, it scrapes its way over the rock surface, and by the aid of part of the material which it detaches, it scratches and polishes the surface over which it moves. The material thus broken out is carried down by the glacier and deposited at its terminus, the accumulation being called a terminal moraine. When debris is deposited along the margin of a glacier, the deposit is called a lateral moraine, and where two glaciers meet, their adjacent lateral moraines form a medial moraine. Large blocks are often carried for long distances by ice, and are left perched in peculiar situations by the ice melting: these blocks are called perched blocks or erratics. Finely divided clayey matter is also left by melting ice, and since this clay often contains a mixed assortment of rocks, derived from various sources, which have been rounded and worn in their journey with the ice, the deposit is known as boulder clay. The work done by ice is of much the same character as that accomplished by running water, and includes erosion, transportation, and deposition. At one time in the world's history the northern part of Britain was covered with an ice-sheet from which numerous glaciers pushed their way to the lower grounds, and we have interesting evidence of this in accumulations of moraine matter, boulder clay containing scratched and striated rocks and perched blocks of distant origin left standing along the sides of many of our valleys.

Action of Plants and Animals. — Living organisms of both animal and vegetable origin produce certain changes on rocks which, although in some cases apparently unimportant, are in the end far reaching in their effects.

Burrowing animals, such as rabbits and moles, undermine the ground, and expose the subsoil to the action of denuding agents. Earthworms grind up the soil and bring up a large quantity of it to the surface in the form of castings. Charles Darwin the great naturalist estimated that the quantity of finely divided earth brought to the surface in this way was in some places not less than ten tons per annum over an acre of ground. These fine particles are easily washed off the surface by rain, or, when dried, are blown about as dust by the wind: thus the humble earthworm is a powerful agent in producing geological changes. Trees split up rocks by the growth of their roots in tiny cracks and along joints. The roots of all plants open up the subsoil to the action of air and water, and the decay of plants produces acids which act on certain of the mineral constituents of the rocks, and thus cause further disintegration.

Certain rocks are formed from the remains of animals which, during life, made a protective covering for their soft bodies by extracting lime in the form of the carbonate from the water in which they lived. The death of the animal and the accumulation of the shells formed beds of carbonate of lime, which usually contain unquestionable evidence of their origin. Coral reefs are further examples of rocks formed by the direct action of living organisms.

Siliceous deposits formed by organic action are also found, but not in great quantity. Most of these rocks are composed of the skeletons of Radiolaria or of Diatoms, the former being minute animals, the latter plants, both having the power of secreting Silica from water holding it in solution. On the death of the animal or plant the siliceous skeletons accumulate and form beds of rock.

Plants by their growth and decay are continually adding to the soil, and play an important part in rock-building. Accumulations of plant growth form peat mosses and other swamps, and in course of time, under favourable circumstances, may be mineralised and converted into the black carbonaceous rock we call Coal.

The Earth's Internal Temperature. — Reference has already been made to the earth having been formed by the cooling and consolidation of a vast cloud of gaseous matter. If such were the origin of the earth then we would naturally expect to find some trace of heat still remaining, especially deep down where the heat has been confined by the outer layers of rock material, and prevented from radiating into space as rapidly as it would at first. Ample evidence exists to show that the mass of the earth is still at a high temperature with the exception of a small thickness of the crust. Very careful observations of the internal temperature of the earth's crust have been made in deep mines and boreholes all over the world, and as a result, it has been noticed that the influence of the surface temperature, summer or winter, is not felt beyond a depth of 100 feet. Within this zone of variable temperature, the surface temperature exercises a distinct influence ; below this depth the temperature of the rocks is unaffected by summer heat or winter cold, and gradually increases with an increase of depth. Below the line of constant temperature, 100 feet from the surface, where the average temperature is about 53 degrees Fahrenheit, the temperature increases about 1 degree Fahrenheit for each additional 60 feet of depth. Thus at a depth of 600 feet below this line of constant temperature, or 760 feet below the surface, the temperature will be about 63 degrees Fahrenheit, and 6000 feet below it will be 153 degrees Fahrenheit, and so on until a point is reached where the temperature would be far in excess of the melting-point of any known rock. It is, however, known that substances which solidify on cooling have their melting-points raised by an increase of pressure, and as a consequence it is almost certain that even at those high internal temperatures the rocks remain solid owing to the enormous pressure of the overlying mass. If a sudden release of pressure takes place, the rocks may assume the molten condition, and if a favourable opening is formed may reach the surface in the form of molten lava. This is what probably gives rise to the outflow of molten rocks from active volcanoes during periods of eruption.

Rocks. — The term rock is used in geology to describe all masses of mineral matter composing the earth's crust, whether hard and compact like granite or sandstone, or soft and incoherent like clay or sand. Rocks are found beneath the soil of a country, some kinds prevailing in one district and others in another. They consist, for the most part, of mineral aggregates which have been brought together by various means and have thereafter consolidated. A few rocks like limestone and quartzite are composed of a single mineral in a more or less pure state, but the majority are mixtures of two or more minerals.