as pigeons, larks, ducks, ravens, and snipes. All these were not only mingled together, but many of them had evidently been gnawed. From the great proportion of hyænas' bones, and the intermixture of its peculiar hard earthy dung, it is thought that those animals must have inhabited the cave for a very long period, and that the bones of the other animals are the remains of living prey, or dead carcasses dragged by those ravenous beasts into their den. Thus it appears that after the land had assumed its present form, Great Britain swarmed with wild beasts similar to those which now roam in the forests and swamps of Africa.

We have now completed, in a brief and therefore incomplete sketch, that general view of the structure of the crust of the globe, which we consider to be a necessary introduction to the accounts of the natural history of those mineral substances which enter into the business of common life.

Coal.

Among the many mineral treasures which the United Kingdom contains, coal is unquestionably the most valuable. It is the chief source of our wealth and power as the foundation of our manufacturing industry; and without such an abundant supply of fuel, our iron, lead, tin, and copper ores must have remained in their native beds.

Coal is a compound substance, consisting of charcoal, bitumen or mineral pitch, and earthy matter. Its various qualities depend on the proportion in which these ingredients are combined; a large quantity of bitumen produces the fat caking qualities common in the Newcastle mines; and, when it is in small proportion, that dull variety which burns almost without flame; if there be much earth, the quantity of ashes is proportionably increased. The specific gravity of coal compared with that of water is, on an average, as 1,250 to 1,000. When we say that coal is a combination of charcoal and bitumen, we employ rather the terms of a popular explanation of its composition than the strict language of chemical analysis; for it consists of a greater number of elementary substances, all of which are gases, with the exception of the carbon. Carbon, which is charcoal in a state of purity, constitutes the chief ingredient of all coals, amounting to from sixty to seventy per cent.; it is a simple elementary body. Bitumen, the other chief ingredient, is a compound substance, for it yields a large quantity of hydrogen gas, or inflammable air; and oxygen gas, that which constitutes the pure part of the air of the atmo sphere and sustains life, has also been found in considerable quantity in coal. When coal is strongly heated in a close iron vessel, the hydrogen gas is given out in combination with carbon, forming the gas used for lighting; and those coals which contain the most bitumen yield the largest quantity of gas. The flame of coal, in a common fire, is occasioned by a sort of distillation of the coal which is slowly going on; gas is given out in the process, and is set fire to. We often see the Newcastle coal, in our grates, swelling up like a soap-bubble, which is occasioned by the disengagement of gas in the softened bituminous coal; and when we hear a rushing sound issuing from the coals accompanied with smoke, if we bring a bit of lighted paper to the smoke, it catches fire on account of the large admixture of gas. The gas from coal may be exhibited in a very simple way, by putting some pounded coal into the bowl of a tobacco-pipe, closing it up well with clay, and placing it in a strong common fire; smoke will soon issue from the pipe, and, if a lighted candle be applied, it will catch fire and continue to flame for some time: what remains in the bowl is coke or charcoal.

The coals used in this country for fuel may be divided into three different kinds :-1. The stone-coal, or splent-coal, as it is sometimes called from its splintery fracture, has the least proportion of bitumen, and, being intermixed with much earthy matter, yields a large quantity of ashes. There is no precise name for this kind of coal, and there is consequently much confusion in descriptions of different coal-mines; what we speak of now is the prevailing quality in the Staffordshire

and Scotch coal-fields; 2. The caking-coal, which is the prevalent quality in the Northumberland and Durham mines,that used in London; and, 3. A variety called cannel-coal in England, and parrot-coal in Scotland, which has a very close compact texture, is hard and splintery, crackles in the fire, and burns with a very bright flame: it is found in comparatively small quantities. These different kinds are sometimes all met with in the same mine.

Coals are found under the surface of the ground, associated with beds of sandstone, of a hard slaty clay called shale, presenting great differences of composition, colour, and hardness, and occasionally with beds of limestone. These associated beds, or strata of coal, sandstones, clays, shales, and limestones, are usually called COAL-MEASURES by practical miners, and a tract of country containing the mines is a COAL-FIELD. There is no determinate order in which these strata occur in different coal-fields, but in different parts of the same coal-field they generally preserve a regular succession. Coal-fields are usually separated from each other by extensive tracts of country, composed of rocks in which no coal exists, and they vary in magnitude from a few acres to many square miles. The measures in the same field sometimes consist of 100 alternations of beds, all of very different degrees of thickness, from less than an inch to many feet; and this difference applies equally to the beds or seams of coal and to the rest, but the proportion of coal to the interstratified stones is always much inferior.

The rocks which are comprehended in what, as a whole, may be called the coal formation, are, beginning with the lowest

1. The old red sandstone (P). When this is present it forms the foundation of the whole; and when not present, the coal-measures rest on the older strata which lie beneath that sandstone.

2. A limestone, called by English geologists the mountain limestone, and also the carboniferous limestone. (O.)

3. Beds of coarse sandstone, composed of sand and flinty pebbles, sometimes fine-grained, but more generally very coarse, called the millstone grit, (N,) grit being a provincial name for sandstone, and millstones being got from some parts of it.

4. The coal-measures. (M.) (Diagram, p. 235.)

In the north-eastern, midland, and southern coal-fields of England this is the usual order, the coal being all above the millstone grit; but in the north-west of England the beds of coal are interstratified both with the millstone grit and with the carboniferous limestone.

Thin seams of coal are occasionally found in some of the superior deposits of the secondary strata; but all the great coalmeasures belong to the lowest part of the secondary series. (Diagram, p. 235.) Coal, such as we are now describing, has never been found in or below the old red sandstone, P, and never in or above the magnesian limestone, L, or rather a red sandstone which lies immediately beneath that limestone. No searches for coal, therefore, in the great series of strata which lie above the coal-measures, (Diagram,) or in the old red sandstone and the strata beneath, can ever turn to good account, and in ninetynine cases out of a hundred would be fruitless. Vast sums of money have been thrown away in such attempts; and it is much to be regretted that many of those persons who, in this country, follow the profession of what is called a mineral surveyor are extremely deficient in the knowledge requisite for the right understanding of their business. Men of property too often suffer themselves to be led into mining undertakings of vast expense by ignorant pretenders; and are often subjected to enormous losses which an application to a geologist might have saved them from.

The coal-measures do not lie horizontally as they are represented in the Diagram, p. 235. They must have been originally deposited, in most cases, on a level or nearly level bottom; but with very few exceptions indeed, the coal-measures have been thrown out of the horizontal line into highly-inclined positions, and frequently broken up and thrown about in the most extra

sometimes only a few inches, at others several fathoms, and even as much as 500 feet, so that the same seam of coal which on one side of the dike is perhaps not more than 20 feet from the surface, may be sunk to the depth of 500 feet on the other side of it. It is impossible to say whether it was the portion a which was depressed, or b which was upheaved. Sometimes several such dikes occur within a very short distance, as in the following diagram, which is a section of the coal-measures in Jarrow colliery, in the county of Durham, where there are five different dikes D, all producing changes in the levels of the strata on each side of them.

These dikes are clefts or fissures which often extend many miles; they penetrate in most cases to an unknown depth, and usually in a vertical direction. They are sometimes mere rents; at other times, and this is the more common case, they are filled with fragments of the disrupted strata imbedded in clay, which has subsequently filtered into them. In a part of the Newcastle coal-field, in Montagu colliery, there is a dike which is 22 yards wide.

The coal-measures are also disturbed by the passage of vast veins of trap, basalt, or whinstone, which have been ejected from the interior of the earth like lava, filling up vents either previously existing, or caused by the same force which threw the melted stone to the surface. They are also called dikes, with the addition of the name of the stone, whin dikes by miners, and basaltic dikes by geologists, and they produce the same effect of changing the planes of stratification on each side of them. They are very common in the coal-fields of Northumberland, Staffordshire, and Scotland.

Dikes occasion vast difficulties in mining, not only on account of their interrupting the regularity of the seams of coal, but because they very often are conduits for water; and when pierced, a flood drowns the mine. If this total destruction of the mine does not take place, they cause such a constant flow of water that it can only be drawn off by powerful steam-engines at the surface. On the other hand, faults are often a source of great benefit, for when filled with stiff clay they prevent the access of water from the other side, and by means of them a valuable seam of coal may be thrown up within reach of working, which would otherwise have been lost.

If we examine a piece of the fat caking quality from Newcastle we find it a compact, shining, stony body; but there are few fragments in which we may not discover some parts very like charcoal, and often with the distinct structure of wood or other vegetable matter. Such appearances are most frequently observed in the slaty coal of Staffordshire and Scotland. By an ingenious application of the microscope, Mr. Witham has exhibited a delicate cellular structure in fossil

woods, which, without such aid, presents only the appearance of compact stone; and he has detected the same in coal, by subjecting extremely thin slices of it to this very highly magnifying power. His researches have been followed up by Mr. William Hutton of Newcastle. Mr. Hutton states that, in all the varieties of coal found in the Newcastle coal-field, more or less of the fine, distinct, net-like structure of the original vegetable texture can always be discovered. The vegetable origin of coal is further illustrated by the vast quantities of fossil plants found in the sandstones and shales which are interstratified with the beds of coal. These are often in an extraordinary degree of preservation, for the most delicate leaves are spread out on the stone like the dried plants on the paper in the herbarium of a botanist.

About three hundred different species of plants have been discovered in the coal-measures of this and of other countries, and of these fully two-thirds have a close resemblance to ferns. Among the rest, one of the kinds most frequently found belongs to the tribe of plants called the equisetacea, of which the weed so common in our ditches, known by the name of horse-tails, is an example; but the stem of these weeds rarely exceeds the diameter of a goose-quill, and the fossil equiseta are sometimes as thick as a man's arm. Other fossil coal-plants resemble large reeds and canes; and bodies which appear to be fragments of the branches and stems of palms and other trees are of frequent occurrence. No remains either of grasses or of mosses have yet been observed in the coal-measures-plants which are not very often absent where vegetation is abundant, especially in moist situations; and the character of the whole fossil flora of the coal-fields shows that the plants must have grown in marshy or humid ground. These terrestrial plants are never mixed with any of those which grow in the sea. It is a very striking fact, too, that they are generally of such a size as to indicate a degree of luxuriance of growth that is now known to exist only in tropical regions. "It would hardly be credited," says Professor Lindley, in his Fossil Flora of Great Britain,' "by persons unacquainted with the evidence upon which such facts repose, that in the most dreary and desolate regions of the present day, there once flourished groves of tropical plants of coniferæ, like the Norfolk island and Araucarian pines, of bananas, tree ferns, huge cacti, and palms; that the marshes were filled with rush-like plants fifteen or twenty feet high, and the coverts with ferns like the undergrowth of a West India island."

In the greater proportion of the fossil plants of the coalmeasures there is little appearance of woody matter: stems of a foot and a half in diameter have been found with the external form perfectly preserved, but having only a coating of coaly matter of inconsiderable thickness, the interior part consisting of sandstone or clay, with now and then some more coaly matter in the centre, indicating, as it were, the pith. But trunks of trees, in which the woody texture was preserved nearly throughout the whole stem, have often been met with: they have been seen in the coal-mines of Westphalia sixty feet in length; and two remarkable instances of fossil trees in the coal-measures have occurred in Great Britain, which have been well described by Mr. Witham. In a bed of sandstone near Gosforth, about five miles north of Newcastle-upon-Tyne, a stem was found which measured seventy-two feet in length, four feet in width at its lower end, from which it tapered gradually, and was eighteen inches wide at the top. It was in a compressed state, as if flattened by great incumbent pressure, so that the above dimensions of the width are not the true diameter of the stem. The woody structure was, in this instance, only in part preserved, but in those places it was converted into a siliceous or flinty petrifaction, containing cavities lined with rock crystal; and this petrified portion was, in one place, nearly two feet in diameter. There were no roots attached to it, and no branches, but there were large knots and other places where branches appear to have been broken off. The other instance occurred in the great freestone quarries of Craigleith, near Edinburgh, from which the greater

part of the New Town of that city has been built, a sandstone belonging to the coal-field of Mid-Lothian, but underlying, it is believed, the regular coal-measures. It was a stem fortyseven feet long,-a large branchless trunk, in some parts very much flattened, the greatest diameter being five feet, the smallest nineteen inches. It was imbedded in the solid stone, with above a hundred feet of layers of rock above it, and lay across the strata, thus passing through several beds.

The following sketch of the appearance of the tree, as it was laid bare in the quarry, is copied from Mr. Witham's Me

moir :

[Fossil Tree at Craigleith Quarry.]

The bark was converted into 'coal; but, in the interior, the woody texture was in many places perfectly preserved. A large portion of this stem is in the Museum, and another in the Botanic Garden of the University of Edinburgh.

It is the general opinion of geologists that our beds of coal have been produced by vast quantities of plants carried down from the land and accumulated at the bottom of the sea, during a long succession of ages; the numerous alternations, amounting to many hundreds, sometimes of sandstone, shales, and beds of coal, proving a long duration of the process of deposition. The character of the vegetation indicates not only a tropical but also an insular climate; that is, the plants must have grown on islands in a very moist atmosphere, and in a heat as great as or even greater than that of the West Indies.

This mode of accounting for the deposition of our coal-beds is greatly in conformity with what is now going forward in many parts of the earth. Every river must carry down to the sea more or less of the trees and other plants which fall into it, or are swept from the banks by the force or undermining action of the stream; and the accumulation of such vegetable matter at the mouths of the larger rivers must be very great. In the case of the Mississippi, for instance, vast rafts, composed of trees held together by the interlacing of smaller plants, which have been washed from the banks by the main stream and its numerous tributaries, are floated down into the Gulf of Mexico, bearing upon them a luxuriant covering of plants. In many parts of the sea coast, by depressions of the land, great forests growing near the shore have been sunk below the level of the sea; the trees have been thrown down, and in process of time covered with mud and sand. Such submarine forests now exist on the coast of Lincolnshire, and near the mouth of the river Parrot in Somersetshire in the Bristol Channel.

But it may be thought that trees and other vegetable bodies, although carried down by the rivers to the sea, would continue to float, until, by the gradual process of decay, they would totally disappear. But wood swims in water only in consequence of the air contained in its cells; and it sinks as soon as the air is withdrawn from it. Very long soaking in water will expel the air, but this will take place more speedily when great pressure is applied at the same time. Captain Scoresby, in his Account of his Voyages to the Whale Fishery in the Arctic Regions, states that a whale, on being harpooned, ran

out all the line in the boat, and as the end of the rope was made fast, the boat was dragged by the fish under water. When the whale returned to the surface to breathe, it was killed; but it began to sink as soon as it was dead, in consequence of the weight of the boat, which was still attached to it by the line of the first harpoon remaining in its flesh. The sunken boat was raised with great difficulty; for so heavy was it, that, although before the accident it would have been buoyant when full of water, it now required a boat at each end to keep it from sinking. When they got it into the ship, the oaken planks were, Captain Scoresby says, as completely soaked in every pore, as if they had lain at the bottom of the sea since the flood." A piece of light firwood, about fifteen inches square, that had gone down with the boat, when thrown into the water again sank like a stone.

The conversion of vegetable matter into coal has been proved, by the observations of Dr. M'Culloch on peat-bogs, and by a series of experiments in the laboratory. Coal, freed from its adventitious earthy matter, which is merely mechanically mixed with it, is resolvable into the same ultimate elements as wood; and Dr. M'Culloch ascertained that the action of water on turf, or submerged wood, is sufficient to convert them into substances capable of yielding bitumen on distillation, and black and brittle like those varieties of coal called, by mineralogists, lignite and jet; and he is further of opinion that great pressure and long-continued action may have produced the other modifications. The coal so produced differs, however, very materially in appearance and properties as fuel from the coal of our mines; and the last link of the chain between a lump of Newcastle coal and a growing tree has yet to be found.

Previously to the researches of the English geologists, within the last fifteen years, very vague notions prevailed as to the extent to which coal was spread over England. But since the publication of the geological maps of Mr. Greenough and Mr. Smith, we know where our present coal-fields are situated, where there is a possibility that others may exist, and where the mineral structure of the ground is of such a nature as to make it certain that searches after coal in such situations can only end in disappointment. The annexed outline map gives a general view of all the coal-fields of England: and it will be seen that fully one half of the country is destitute of coal; for all that lies east and south of the double line Z Z, from the mouth of the Tees in Yorkshire to Lyme Regis in Dorsetshire, is composed of the superior secondary strata; and although some of these do sometimes contain thin beds of coal of a particular kind, it may be confidently said, that the kind of coal which we usually consume will never be found in those upper secondary strata; and, unless under very favourable circumstances, the inferior kind above alluded to can never be worked with profit. It will also be seen how comparatively small a space the coal-measures occupy. The spaces here marked with dark lines are the geological boundaries of the coal formations, which, as we have already explained in previous sections, consist of many different kinds of stone besides coal; workable coal is not spread over the whole space marked by the darker shade; but there is a very large part of all those spaces where not a trace of coal is to be seen.

The map also exhibits the boundaries of the country which each supplies with fuel. We are indebted for this information to the evidence given by Frederick Page, Esq., before the Committee of the House of Commons on the Coal Trade in 1830. In the annexed map, it is to be understood that all the space included within the line which surrounds a coal deposit is supplied from that source. These boundaries are of course not rigorously correct, but they are sufficiently so to give a tolerably accurate general view how far the market of each coal-field extends, independent of foreign export, and the supplies to Scotland from the Northumberland district, and to Ireland from the western coal-fields. The extent which the consumption of a coal-field reaches, depends upon a variety of circumstances, such as the facility of transport by sea or by