Fig. 63Fig. 63.—Sphenophyllum restored.
Fig. 63.—Sphenophyllum restored.
In the family of the Sigillarias we have already presented the bark ofS. lævigata, at page 138; on page 157 we give a drawing of the bark ofS. reniformis, one-third the natural size (Fig. 65).
Fig. 64Fig. 64.—Calamites cannæformis. One-third natural size.
Fig. 64.—Calamites cannæformis. One-third natural size.
In the family of the Asterophyllites, the leaf ofA. foliosa(Fig. 66); and the foliage ofAnnularia orifolia(Fig. 67) are remarkable. In addition to these, we present, inFig. 63, a restoration of one of these Asterophyllites, theSphenophyllum, after M. Eugene Deslongchamps. This herbaceous tree, like the Calamites, would present the appearance of an immense asparagus, twenty-five to thirty feet high. It is represented here with its branches andfronds, which bear some resemblance to the leaves of the ginkgo. The bud, as represented in the figure, is terminal, and not axillary, as in some of the Calamites.
Fig. 65Fig. 65.—Sigillaria reniformis.
Fig. 65.—Sigillaria reniformis.
If, during the Coal-period, the vegetable kingdom had reached its maximum, the animal kingdom, on the contrary, was poorly represented. Some remains have been found, both in America and Germany, consisting of portions of the skeleton and the impressions of the footsteps of a Reptile, which has received the name of Archegosaurus. InFig. 68is represented the head and neck ofArchegosaurus minor, found in 1847 in the coal-basin of Saarbruck between Strasbourg and Trèves. Among the animals of this period we find a few Fishes, analogous to those of the Devonian formation. These are theHoloptychiusandMegalichthys, having jaw-bonesarmed with enormous teeth. Scales ofPygopterushave been found in the Northumberland Coal-shale at Newsham Colliery, and also in the Staffordshire Coal-shale. Some winged insects would probably join this slender group of living beings. It may then be said with truth that the immense forests and marshy plains, crowded with trees, shrubs, and herbaceous plants, which formed on the innumerable isles of the period a thick and tufted sward, were almost destitute of animals.
Fig. 66Fig. 66.—Asterophyllites foliosa.
Fig. 66.—Asterophyllites foliosa.
Plate XIXI.—Ideal view of a marshy forest of the Coal Period.
XI.—Ideal view of a marshy forest of the Coal Period.
On the opposite page (Pl. XI.) M. Riou has attempted, under the directions of M. Deslongchamps, to reproduce the aspect of Nature during the period. A marsh and forest of the Coal-period are here represented, with a short and thick vegetation, a sort of grass composed of herbaceous Fern and mare’s-tail. Several trees of forest-height raise their heads above this lacustrine vegetation.
On the left are seen the naked trunk of aLepidodendronand aSigillaria, an arborescent Fern rising between the two trunks. At the foot of these great trees an herbaceous Fern and aStigmariaappear, whose long ramification of roots, provided withreproductive spores, extend to the water. On the right is the naked trunk of anotherSigillaria, a tree whose foliage is altogether unknown, aSphenophyllum, and aConifer. It is difficult to describe with precision the species of this last family, the impressions of which are, nevertheless, very abundant in the Coal-measures.
Fig. 67Fig. 67.—Annularia orifolia.
Fig. 67.—Annularia orifolia.
In front of this group we see two trunks broken and overthrown. These are aLepidodendronandSigillaria, mingling with a heap of vegetable débris in course of decomposition, from which a rich humus will be formed, upon which new generations of plants will soon develop themselves. Some herbaceous Ferns and buds ofCalamitesrise out of the waters of the marsh.
A few Fishes belonging to the period swim on the surface of the water, and the aquatic reptileArchegosaurusshows its long and pointed head—the only part of the animal which has hitherto been discovered (Fig. 68). AStigmariaextends its roots into the water, and the prettyAsterophyllites, with its finely-cut stems, rises above it in the foreground.
A forest, composed ofLepidodendraandCalamites, forms the background to the picture.
Fig. 68Fig. 68.—Head and neck of Archegosaurus minor.
Fig. 68.—Head and neck of Archegosaurus minor.
Coal, as we have said, is only the result of a partial decomposition of the plants which covered the earth during a geological period of immense duration. No one, now, has any doubt that this is its origin. In coal-mines it is not unusual to find fragments of the very plants whose trunks and leaves characterise the Coal-measures, or Carboniferous era. Immense trunks of trees have also been met with in the middle of a seam of coal. In the coal-mines of Treuil,[44]at St. Etienne, for instance, vertical trunks of fossil trees, resembling bamboos or large Equiseta, are not only mixed with the coal, but stand erect, traversing the overlying beds of micaceous sandstone in the manner represented in the engraving, which has been reproduced from a drawing by M. Ad. Brongniart (Fig. 69).
Fig. 69Fig. 69.—Treuil coal-mine, at St. Etienne.
Fig. 69.—Treuil coal-mine, at St. Etienne.
In England it is the same; entire trees are found lying across the coal-beds. Sir Charles Lyell tells us[45]that in Parkfield Colliery, South Staffordshire, there was discovered in 1854, upon a surface of about a quarter of an acre, a bed of coal which has furnished as many as seventy-three stumps of trees with their roots attached, some of the former measuring more than eight feet in circumference; their roots formed part of a seam of coal ten inches thick, resting on a layer of clay two inches thick, under which was a second forest resting on a band of coal from two to five feet thick. Underneath this, again, was a third forest, with large stumps ofLepidodendra,Calamites, and other trees.[46]
In the lofty cliffs of the South Joggins, in the Bay of Fundy, in Nova Scotia, Sir Charles Lyell found in one portion of the coal-field 1,500 feet thick, as many as sixty-eight different surfaces, presenting evident traces of as many old soils of forests, where the trunks of the trees were still furnished with roots.[47]
We will endeavour to establish here the true geological origin of coal, in order that no doubt may exist in the minds of our readers on a subject of such importance. In order to explain the presence of coal in the depths of the earth, there are only two possible hypotheses. This vegetable débris may either result from the burying of plants brought from afar and transported by river or maritime currents, forming immense rafts, which may have grounded in different places and been covered subsequently by sedimentary deposits; or the trees may have grown on the spot where theyperished, and where they are now found. Let us examine each of these theories.
Can the coal-beds result from the transport by water, and burial underground, of immense rafts formed of the trunks of trees? The hypothesis has against it the enormous height which must be conceded to the raft, in order to form coal-seams as thick as some of those which are worked in our collieries. If we take into consideration the specific gravity of wood, and the amount of carbon itcontains, we find that the coal-deposits can only be about seven-hundredths of the volume of the original wood and other vegetable materials from which they are formed. If we take into account, besides, the numerous voids necessarily arising from the loose packing of the materials forming the supposed raft, as compared with the compactness of coal, this may fairly be reduced to five-hundredths. A bed of coal, for instance, sixteen feet thick, would have required a raft 310 feet high for its formation. These accumulations of wood could never have arranged themselves with sufficient regularity to form those well-stratified coal-beds, maintaining a uniform thickness over many miles, and that are seen in most coal-fields to lie one above another in succession, separated by beds of sandstone or shale. And even admitting the possibility of a slow and gradual accumulation of vegetable débris, like that which reaches the mouth of a river, would not the plants in that case be buried in great quantities of mud and earth? Now, in most of our coal-beds the proportion of earthy matter does not exceed fifteen per cent. of the entire mass. If we bear in mind, finally, the remarkable parallelism existing in the stratification of the coal-formation, and the state of preservation in which the impressions of the most delicate vegetable forms are discovered, it will, we think, be proved to demonstration, that those coal-seams have been formed in perfect tranquillity. We are, then, forced to the conclusion that coal results from the mineralisation of plants which has taken place on the spot; that is to say, in the very place where the plants lived and died.
It was suggested long ago by Bakewell, from the occurrence of the same peculiar kind of fireclay under each bed of coal, that it was the soil proper for the production of those plants from which coal has been formed.[48]
It has, also, been pointed out by Sir William Logan, as the result of his observations in the South Wales coal-field, and afterwards by Sir Henry De la Beche, and subsequently confirmed by the observations of Sir Charles Lyell in America, that not only in this country, but in the coal-fields of Nova Scotia, the United States, &c., every layer of true coal is co-extensive with and invariably underlaid by a marked stratum of arenaceous clay of greater or less thickness, which, from its position relatively to the coal has been long known to coal-miners, among other terms, by the name ofunder-clay.
The clay-beds, “which vary in thickness from a few inches to more than ten feet, are penetrated in all directions by a confused andtangled collection of the roots and leaves, as they may be, of theStigmaria ficoides, these being frequently traceable to the main stem (Sigillaria), which varies in diameter from about two inches to half a foot. The main stems are noticed as occurring nearer the top than the bottom of the bed, as usually of considerable length, the leaves or roots radiating from them in a tortuous irregular course to considerable distances, and as so mingled with the under-clay that it is not possible to cut out a cubic foot of it which does not contain portions of the plant.” (Logan “On the Characters of the Beds of Clay immediately below the Coal-seams of South Wales,” Geol. Transactions, Second Series, vol. vi., pp. 491-2. An account of these beds had previously been published by Mr. Logan in the Annual Report of the Royal Institution of South Wales for 1839.)
From the circumstance of the main stem of the Sigillaria, of which theStigmaria ficoideshave been traced to be merely a continuation, it was inferred by the above-mentioned authors, and has subsequently been generally recognised as probably the truth, that the roots found in the underclay are merely those of the plant (Sigillaria), the stem of which is met with in the overlying coal-beds—in fact, that theStigmaria ficoidesis only the root of theSigillaria, and not a distinct plant, as was once supposed to be the case.
This being granted, it is a natural inference to suppose that the present indurated under-clay is only another condition of that soft, silty soil, or of that finely levigated muddy sediment—most likely of still and shallow water—in which the vegetation grew, the remains of which were afterwards carbonised and converted into coal.[49]
In order thoroughly to comprehend the phenomena of the transformation into coal of the forests and of the herbaceous plants which filled the marshes and swamps of the ancient world, there is another consideration to be presented. During the coal-period, the terrestrial crust was subjected to alternate movements of elevation and depression of the internal liquid mass, under the impulse of the solar and lunar attractions to which they would be subject, as our seas are now, giving rise to a sort of subterranean tide, operating at intervals, more or less widely apart, upon the weaker parts of the crust, and producing considerable subsidences of the ground. It might, perhaps, happen that, in consequence of a subsidence produced in such a manner, the vegetation of the coal-period would be submerged, and the shrubsand plants which covered the surface of the earth would finally become buried under water. After this submergence new forests sprung up in the same place. Owing to another submergence, the second forests were depressed in their turn, and again covered by water. It is probably by a series of repetitions of this double phenomenon—this submergence of whole regions of forest, and the development upon the same site of new growths of vegetation—that the enormous accumulations of semi-decomposed plants, which constitute the Coal-measures, have been formed in a long series of ages.
But, has coal been produced from the larger plants only—for example, from the great forest-trees of the period, such as the Lepidodendra, Sigillariæ, Calamites, and Sphenophylla? That is scarcely probable, for many coal-deposits contain no vestiges of the great trees of the period, but only of Ferns and other herbaceous plants of small size. It is, therefore, presumable that the larger vegetation has been almost unconnected with the formation of coal, or, at least, that it has played a minor part in its production. In all probability there existed in the coal-period, as at the present time, two distinct kinds of vegetation: one formed of lofty forest-trees, growing on the higher grounds; the other, herbaceous and aquatic plants, growing on marshy plains. It is the latter kind of vegetation, probably, which has mostly furnished the material for the coal; in the same way that marsh-plants have, during historic times and up to the present day, supplied our existing peat, which may be regarded as a sort of contemporaneous incipient coal.
To what modification has the vegetation of the ancient world been subjected to attain that carbonised state, which constitutes coal? The submerged plants would, at first, be a light, spongy mass, in all respects resembling the peat-moss of our moors and marshes. While under water, and afterwards, when covered with sediment, these vegetable masses underwent a partial decomposition—a moist, putrefactive fermentation, accompanied by the production of much carburetted hydrogen and carbonic acid gas. In this way, the hydrogen escaping in the form of carburetted hydrogen, and the oxygen in the form of carbonic acid gas, the carbon became more concentrated, and coal was ultimately formed. This emission of carburetted hydrogen gas would, probably, continue after the peat-beds were buried beneath the strata which were deposited and accumulated upon them. The mere weight and pressure of the superincumbent mass, continued at an increasing ratio during a long series of ages, have given to the coal its density and compact state.
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 aredrierand 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.
Passing 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 passing, in short, into an imperfect kind ofcoal. 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 beconsidered a constituent 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 masses, 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. 70gives 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 analogous mode of occurrence.
Fig. 70Fig. 70.—Stratification of coal-beds.
Fig. 70.—Stratification of coal-beds.
The frequent presence of carbonate of iron in the coal-measures is a most fortunate circumstance 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:—
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:—
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 quantity produced by Great Britain.
The Coal-measures of England and Scotland cover a large area; and attempts have been made to estimate the quantity 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 quantity 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.
Fig. 71Fig. 71.—Contortions of Coal-beds.
Fig. 71.—Contortions of Coal-beds.
Fig. 72Fig. 72.—Cycas circinalis (living form).
Fig. 72.—Cycas circinalis (living form).
The coal-basin of Belgium and of the north of France forms a nearly continuous zone from Liége, Namur, Charleroi, and Mons, to Valenciennes, Douai, and Béthune. 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 Francepossess beds fewer in number, but individually thicker and less regularly stratified. The two basins of the Saône-et-Loire, the principal 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 ofthe 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 Saône-et-Loire and of the Loire—the principal basins in France are those of the Allier, where very important beds are worked at Commentry and Bezenet; the basin of Brassac, 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 principal 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.
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 Palæozoic Series.[50]
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 classification 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 theSilurian System.
Plate XIIXII.—Ideal landscape of the Permian Period.
XII.—Ideal landscape of the Permian Period.
The Permian rocks have of late years assumed 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 mass 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 masses 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-shore, and left exposed by the retiring tide, are Molluscs and Zoophytes peculiar to the period, such asProducta,Spirifera, andEncrinites; pretty plants—theAsterophyllites—which we have noticed in our description of the Carboniferous age, are growing at the water’s edge, not far from the shore.
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 Corncockle 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.
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 Palæontographical 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:—
At the base of the system lies a band oflower sandstone(No. 6) of various colours, separating the Magnesian Limestone from the coal in Yorkshire and Durham; sometimes associated 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 theRothliegende, orred-lyer, a great deposit of red sandstone and conglomerate, associated 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 surroundedby dense masses 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—Palæoniscus,Pygopterus,Cœlacanthus, andPlatysomus—genera which all belong to the Carboniferous system, and which Professor King thinks probably lived at no great distance from the shore; but the Permian species of the marl-slate of England are identical with those of the copper-slate of Thuringia. Agassiz 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 “homocercal” (even-tail), that is, either rounded, or, if forked, divided into two equal lobes. In Palæoniscus, and most Palæozoic 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 Agassiz termed “heterocercal” (unequal-tail) is only in a very general way distinctive of Palæozoic fishes, since this asymmetry exists, though in a minor degree, in many living genera besides those just mentioned. The compact limestone (No. 4) is rich in Polyzoa. The fossiliferous limestone (No. 3), Mr. King considers, is a deep-water formation, from the numerous Polyzoa which it contains. One of these,Fenestella retiformis, found in the Permian rocks of England and Germany, sometimes measures eight inches in width.
Many species of Mollusca, and especially Brachiopoda, appear in the Permian seas of this age,SpiriferaandProductabeing the most characteristic.
Fig. 73Fig. 73.—Strophalosia Morrisiana.
Fig. 73.—Strophalosia Morrisiana.
Other shells now occur, which have not been observed in strata newer than the Permian.Strophalosia(Fig. 73) is abundantly represented in the Permian rocks of Germany, Russia, and England, and much more sparingly in the yellow magnesian limestone, accompanied bySpirifera undulata, &c.S. Schlotheimiiis widely disseminated both in England, Germany, and Russia, withLingula Credneri, and other Palæozoic Brachiopoda. Here also we note the first appearance of the Oyster, but still in small numbers.Fenestellarepresents the Polyzoa.Schizodushas been found by Mr. Binney in the Upper Red Permian Marls of Manchester; but no shells of any kind have hitherto been met with in the Rothliegende of Lancashire, or in the Vale of Eden.
The brecciated limestone (No. 2) and the concretionary masses (No. 1) overlying it (although Professor King has attempted to separate them) are considered by Professor Sedgwick as different forms of the same rock. They contain no foreign elements, but seem to be composed of fragments of the underlying limestone,No. 3. Some of the angular masses at Tynemouth cliff are two feet in diameter, and none of them are water-worn.
Fig. 74Fig. 74.—Cyrtoceras depressum.
Fig. 74.—Cyrtoceras depressum.
The crystalline or concretionary limestone (No. 1) formation is seen upon the coast of Durham and Yorkshire, between the Wear and the Tees; and Mr. King thinks that the character of the shells and the absence of corals indicate a deposit formed in shallow water.
The plants also found in some of the Permian strata indicate the neighbourhood of land. These are land species, and chiefly of genera common in the Coal-measures. Fragments of supposed coniferous wood (generally silicified) are occasionally met with in the Permian red beds of many parts of England.
Fig. 75Fig. 75.—Walchia Schlotheimii.
Fig. 75.—Walchia Schlotheimii.
Among the Ferns characteristic of the period may be mentionedSphenopteris dichotomaandS. Artemisiæfolia;Pecopteris lonchiticaandNeuropteris gigantea, figured on pp. 143, 144. “If we are,” says Lyell, “to draw a line between the Secondaryand Primary fossiliferous strata, it must be run through the middle of what was once called the ‘New Red.’ The inferior half of this group will rank as Primary or Palæozoic, while its upper member will form the base of the Secondary or Mesozoic series.”[52]Among theEquisetaof the Permian formation of Saxony, Colonel Von Gutbier foundCalamites gigasand sixty species of fossil plants, most of them Ferns, forty of which have not been found elsewhere. Among these are several species ofWalchia, a genus of Conifers, of which an example is given inFig. 75.
In their stems, leaves, and cones, they bear some resemblance to theAraucarias, which have been introduced from North America into our pleasure-grounds during the last half-century.