The heat emanating from the interior of the globe would, also, exercise a great influence upon the final result. It is to these two causes--that is to say, to pressure and to the central heat--that we may attribute the differences which exist in the mineral characters of various kinds of coal. The inferior beds are _drier_ and more compact than the upper ones; or less bituminous, because their mineralisation has been completed under the influence of a higher temperature, and at the same time under a greater pressure.

An experiment, attempted for the first time in 1833, at Sain-Bel, afterwards repeated by M. Cagniard de la Tour, and completed at Saint-Etienne by M. Baroulier in 1858, fully demonstrates the process by which coal was formed. These gentlemen succeeded in producing a very compact coal artificially, by subjecting wood and other vegetable substances to the double influence of heat and pressure combined.

The apparatus employed for this experiment by M. Baroulier, at Saint-Etienne, allowed the exposure of the strongly compressed vegetable matter enveloped in moist clay, to the influence of a long-continued temperature of from 200 to 300 Centigrade. This apparatus, without being absolutely closed, offered obstacles to the escape of gases or vapours in such a manner that the decomposition of the organic matters took place in the medium saturated with moisture, and under a pressure which prevented the escape of the elements of which it was composed. By placing in these conditions the sawdust of various kinds of wood, products were obtained which resembled in many respects, sometimes brilliant shining coal, and at others a dull coal. These differences, moreover, varied with the conditions of the experiment and the nature of the wood employed; thus explaining the striped appearance of coal when composed alternately of shining and dull veins.

When the stems and leaves of ferns are compressed between beds of clay or pozzuolana, they are decomposed by the pressure only, and form on these blocks a carbonaceous layer, and impressions bearing a close resemblance to those which blocks of coal frequently exhibit. These last-mentioned experiments, which were first made by Dr. Tyndall, leave no room for doubt that coal has been formed from the plants of the ancient world.

Pa.s.sing from these speculations to the Coal-measures:--

This formation is composed of a succession of beds, of various thicknesses, consisting of sandstones or gritstones, of clays and shales, sometimes so bituminous as to be inflammable--and pa.s.sing, in short, into an imperfect kind of _coal_. These rocks are interstratified with each other in such a manner that they may consist of many alterations. Carbonate of protoxide of iron (clay-ironstone) may also be considered a const.i.tuent of this formation; its extensive dissemination in connection with coal in some parts of Great Britain has been of immense advantage to the ironworks of this country, in many parts of which blast-furnaces for the manufacture of iron rise by hundreds alongside of the coal-pits from which they are fed. In France, as is frequently the case in England, this argillaceous iron-ore only occurs in nodules or lenticular ma.s.ses, much interrupted; so that it becomes necessary in that country, as in this, to find other ores of iron to supply the wants of the foundries. Fig. 70 gives an idea of the ordinary arrangement of the coal-beds, one of which is seen interstratified between two parallel and nearly horizontal beds of argillaceous shale, containing nodules of clay iron-ore--a disposition very common in English collieries. The coal-basin of Aveyron, in France, presents an a.n.a.logous mode of occurrence.

[Ill.u.s.tration: Fig. 70.--Stratification of coal-beds.]

The frequent presence of carbonate of iron in the coal-measures is a most fortunate circ.u.mstance for mining industry. When the miner finds, in the same spot, the ore of iron and the fuel required for smelting it, arrangements for working them can be established under the most favourable conditions. Such is the case in the coal-fields of Great Britain, and also in France to a less extent--that is to say, only at Saint-Etienne and Alais.

The extent of the Coal-measures, in various parts of the world, may be briefly and approximately stated as follows:--

ESTIMATED AREA OF THE COAL-MEASURES OF THE WORLD.

Square Miles.

United States 220,166 } 420,166 Lignites and inferior Coals 200,000 } British Possessions in North America 2,200 Great Britain 3,000 France 2,000 Belgium 468 Rhenish Prussia and Saarbruck 1,550 Westphalia 400 Bohemia 620 Saxony 66 The Asturias, in Spain 310 Russia 11,000 Islands of the Pacific and Indian Ocean Unknown.

The American continent, then, contains much more extensive coal-fields than Europe; it possesses very nearly two square miles of coal-fields for every five miles of its surface; but it must be added that these immense fields of coal have not, hitherto, been productive in proportion to their extent. The following Table represents the annual produce of the collieries of America and Europe:--

Tons.

British Islands (in 1870) 110,431,192 United States 14,593,659 Belgium (in 1870) 13,697,118 France (in 1864) 10,000,000 (in 1866) 11,807,142 Prussia (in 1864) 21,197,266 Na.s.sau (in 1864) 2,345,459 Netherlands (in 1864) 24,815 Austria (in 1864) 4,589,014 Spain 500,000

We thus see that the United States holds a secondary place as a coal-producing country; raising one-eleventh part of the out-put of the whole of Europe, and about one-eighth part of the quant.i.ty produced by Great Britain.

The Coal-measures of England and Scotland cover a large area; and attempts have been made to estimate the quant.i.ty of fuel they contain.

The estimate made by the Royal Commission on the coal in the United Kingdom may be considered as the nearest; and, in this Report, lately published, it is stated that in the ascertained coal-fields of the United Kingdom there is an aggregate quant.i.ty of 146,480,000,000 tons of coal, which may be reasonably expected to be available for use. In the coal-field of South Wales, ascertained by actual measurement to attain the extraordinary thickness of 11,000 feet of Coal-measures, there are 100 different seams of coal, affording an aggregate thickness of 120 feet, mostly in thin beds, but varying from six inches to more than ten feet. Professor J. Phillips estimates the thickness of the coal-bearing strata of the north of England at 3,000 feet; but these, in common with all other coal-fields, contain, along with many beds of the mineral in a more or less pure state, interstratified beds of sandstones, shales, and limestone; the real coal-seams, to the number of twenty or thirty, not exceeding sixty feet in thickness in the aggregate. The Scottish Coal-measures have a thickness of 3,000 feet, with similar intercalations of other carboniferous rocks.

[Ill.u.s.tration: Fig. 71.--Contortions of Coal-beds.]

[Ill.u.s.tration: Fig. 72.--Cycas circinalis (living form).]

The coal-basin of Belgium and of the north of France forms a nearly continuous zone from Liege, Namur, Charleroi, and Mons, to Valenciennes, Douai, and Bethune. The beds of coal there are from fifty to one hundred and ten in number, and their thickness varies from ten inches to six feet. Some coal-fields which are situated beneath the Secondary formations of the centre and south of France possess beds fewer in number, but individually thicker and less regularly stratified. The two basins of the Saone-et-Loire, the princ.i.p.al mines of which are at Creuzot, Blanzy, Montchanin, and Epinac, only contain ten beds; but some of these (as at Montchanin) attain 30, 100, and even 130 feet in thickness. The coal-basin of the Loire is that which contains the greatest total thickness of coal-beds: the seams there are twenty-five in number. After those of the North--of the Saone-et-Loire and of the Loire--the princ.i.p.al basins in France are those of the Allier, where very important beds are worked at Commentry and Bezenet; the basin of Bra.s.sac, which commences at the confluence of the Allier and the Alagnon; the basin of the Aveyron, known by the collieries of Decazeville and Aubin; the basin of the Gard, and of Grand'-Combe.

Besides these princ.i.p.al basins, there are a great many others of scarcely less importance, which yield annually to France from six to seven million tons of coal.

The seams of coal are rarely found in the horizontal position in which their original formation took place. They have been since much crumpled and distorted, forced into basin-shaped cavities, with minor undulations, and affected by numerous flexures and other disturbances.

They are frequently found broken up and distorted by faults, and even folded back on themselves into zigzag forms, as represented in the engraving (Fig. 71, p. 167), which is a mode of occurrence common in all the Coal-measures of Somersetshire and in the basins of Belgium and the north of France. Vertical pits, sunk on coal which has been subjected to this kind of contortion and disturbance, sometimes traverse the same beds many times.

PERMIAN PERIOD.

The name "Permian" was proposed by Sir Roderick I. Murchison, in the year 1841, for certain deposits which are now known to terminate upwards the great primeval or Palaeozoic Series.[50]

[50] See "Siluria," p. 14. _Philosophical Mag._, 3rd series, vol.

xix., p. 419.

This natural group consists, in descending order, in Germany, of the Zechstein, the Kupfer-schiefer, Roth-liegende, &c. In England it is usually divided into Magnesian Limestone or Zechstein, with subordinate Marl-slate or Kupfer-schiefer, and Rothliegende. The chief calcareous member of this group of strata is termed in Germany the "Zechstein," in England the "Magnesian Limestone;" but, as magnesian limestones have been produced at many geological periods, and as the German Zechstein is only a part of a group, the other members of which are known as "Kupfer-schiefer" ("copper-slate"), "Roth-todt-liegende" (the "Lower New Red" of English geologists), &c., it was manifest that a single name for the whole was much needed. Finding, in his examination of Russia in Europe, that this group was a great and united physical series of marls, limestones, sandstones, and conglomerates, occupying a region much larger than France, and of which the Government of Perm formed a central part, Sir Roderick proposed that the name of Permian, now in general use, should be thereto applied.

Extended researches have shown, from the character of its embedded organic remains, that it is closely allied to, but distinct from, the carboniferous strata below it, and is entirely distinct from the overlying Trias, or New Red Sandstone, which forms the base of the great series of the Secondary rocks.

Geology is, however, not only indebted to Sir Roderick Murchison for this cla.s.sification and nomenclature, but also to him, in conjunction with Professor Sedgwick, for the name "_Devonian_," as an equivalent to "Old Red Sandstone;" whilst every geologist knows that Sir R. Murchison is the sole author of the SILURIAN SYSTEM.

[Ill.u.s.tration: XII.--Ideal landscape of the Permian Period.]

The Permian rocks have of late years a.s.sumed great interest, particularly in England, in consequence of the evidence their correct determination affords with regard to the probable extent, beneath them, of the coal-bearing strata which they overlie and conceal; thus tending to throw a light upon the duration of our coal-fields, one of the most important questions of the day in connection with our industrial resources and national prosperity.

On the opposite page an ideal view of the earth during the Permian period is represented (PL. XII.). In the background, on the right, is seen a series of syenitic and porphyritic domes, recently thrown up; while a ma.s.s of steam and vapour rises in columns from the midst of the sea, resulting from the heat given out by the porphyries and syenites.

Having attained a certain height in the cooler atmosphere, the columns of steam become condensed and fall in torrents of rain. The evaporation of water in such vast ma.s.ses being necessarily accompanied by an enormous disengagement of electricity, this imposing scene of the primitive world is illuminated by brilliant flashes of lightning, accompanied by reverberating peals of thunder. In the foreground, on the right, rise groups of Tree-ferns, Lepidodendra, and Walchias, of the preceding period. On the sea-sh.o.r.e, and left exposed by the retiring tide, are Molluscs and Zoophytes peculiar to the period, such as _Producta_, _Spirifera_, and _Encrinites_; pretty plants--the _Asterophyllites_--which we have noticed in our description of the Carboniferous age, are growing at the water's edge, not far from the sh.o.r.e.

During the Permian period the species of plants and animals were nearly the same as those already described as belonging to the Carboniferous period. Footprints of reptilian animals have been found in the Permian beds near Kenilworth, in the red sandstones of that age in the Vale of Eden, and in the sandstones of Cornc.o.c.kle Moor, and other parts of Dumfriesshire. These footprints, together with the occurrence of current-markings or ripplings, sun-cracks, and the pittings of rain-drops impressed on the surfaces of the beds, indicate that they were made upon damp surfaces, which afterwards became dried by the sun before the flooded waters covered them with fresh deposits of sediment, in the way that now happens during variations of the seasons in many salt lakes.[51] M. Ad. Brongniart has described the forms of the Permian flora as being intermediate between those of the Carboniferous period and of that which succeeds it.

[51] A. C. Ramsay, "On the Red Rocks of England." _Quart. Jour. Geol.

Soc._, vol. xxvii., p. 246.

Although the Permian flora indicates a climate similar to that which prevailed during the Carboniferous period, it has been pointed out by Professor Ramsay, as long ago as 1855, that the Permian breccia of Shropshire, Worcestershire, &c., affords strong proofs of being the result of direct glacial action, and of the consequent existence at the period of glaciers and icebergs.

That such a state of things is not inconsistent with the prevalence of a moist, equable, and temperate climate, necessary for the preservation of a luxuriant flora like that of the period in question, is shown in New Zealand; where, with a climate and vegetation approximating to those of the Carboniferous period, there are also glaciers at the present day in the southern island.

Professor King has published a valuable memoir on the Permian fossils of England, in the Proceedings of the Palaeontographical Society, in which the following Table is given (in descending order) of the Permian system of the North of England, as compared with that of Thuringia:--

NORTH OF ENGLAND. THURINGIA. MINERAL CHARACTER.

1. Crystalline, earthy, } compact, and oolitic } 1. Stinkstein 1. Oolitic limestones.

limestones }

2. Brecciated and pseudo- } 2. Rauchwacke 2. Conglomerates.

brecciated limestones }

3. Fossiliferous { 3. Upper Zechstein, or } 3. Marlstones.

limestone { Dolomit-Zechstein }

4. Compact limestone 4. Lower Zechstein 4. Magnesian limestones.

5. Marl-slate { 5. Mergel-Schiefer or } 5. Red and green grits { Kupferschiefer } with copper-ore.

{ 6. White limestone with 6. Lower sandstones, and } 6. Todteliegende { gypsum and white sands of various colours } { salt.

At the base of the system lies a band of _lower sandstone_ (No. 6) of various colours, separating the Magnesian Limestone from the coal in Yorkshire and Durham; sometimes a.s.sociated with red marl and gypsum, but with the same obscure relations in all these beds which usually attend the close of one series and the commencement of another; the imbedded plants being, in some cases, stated to be identical with those of the Carboniferous series. In Thuringia the _Rothliegende_, or _red-lyer_, a great deposit of red sandstone and conglomerate, a.s.sociated with porphyry, basaltic trap, and amygdaloid, lies at the base of the system.

Among the fossils of this age are the silicified trunks of Tree-ferns (_Psaronius_), the bark of which is surrounded by dense ma.s.ses of air-roots, which often double or quadruple the diameter of the original stem; in this respect bearing a strong resemblance to the living arborescent ferns of New Zealand.

The marl-slate (No. 5) consists of hard calcareous shales, marl-slates, and thin-bedded limestone, the whole nearly thirty feet thick in Durham, and yielding many fine specimens of Ganoid and Placoid fishes--_Palaeoniscus_, _Pygopterus_, _Clacanthus_, and _Platysomus_--genera which all belong to the Carboniferous system, and which Professor King thinks probably lived at no great distance from the sh.o.r.e; but the Permian species of the marl-slate of England are identical with those of the copper-slate of Thuringia. Aga.s.siz was the first to point out a remarkable peculiarity in the forms of the fishes which lived before and after this period. In most living fishes the trunk seems to terminate in the middle of the root of the tail, whose free margin is "h.o.m.ocercal" (even-tail), that is, either rounded, or, if forked, divided into two equal lobes. In Palaeoniscus, and most Palaeozoic fishes, the axis of the body is continued into the upper lobe of the tail, which is thus rendered unsymmetrical, as in the living sharks and sturgeons. The latter form, which Aga.s.siz termed "heterocercal"

(unequal-tail) is only in a very general way distinctive of Palaeozoic fishes, since this asymmetry exists, though in a minor degree, in many living genera besides those just mentioned. The compact limestone (No.