Leaving the Ganoid fishes, it still remains to be noticed that the Devonian deposits have yielded the remains of a number of fishes more or less closely allied to the existing Sharks, Rays, and _Chimoeroe_ (the _Elasmobranchii_). The majority of the forms here alluded to are allied not to the true Sharks and Dog-fishes, but to the more peaceable "Port Jackson Sharks," with their blunt teeth, adapted for crushing the sh.e.l.ls of Molluscs. The collective name of "Cestracionts" is applied to these; and we have evidence of their past existence in the Devonian seas both by their teeth, and by the defensive spines which were implanted in front of a greater or less number of the fins. These are bony spines, often variously grooved, serrated, or ornamented, with hollow bases, implanted in the integument, and capable of being erected or depressed at will. Many of these "fin-spines" have been preserved to us in the fossil condition, and the Devonian rocks have yielded examples belonging to many genera. As some of the true Sharks and Dog-fishes, some of the Ganoids, and even some Bony Fishes, possess similar defences, it is often a matter of some uncertainty to what group a given spine is to be referred. One of these spines, belonging to the genus _Mach.o.e.racanthus_, from the Devonian rocks of America, has been figured in a previous ill.u.s.tration (fig.

102, f).

In conclusion, a very few words may be said as to the validity of the Devonian series as an independent system of rocks, preserving in its successive strata the record of an independent system of life. Some high authorities have been inclined to the view that the Devonian formation has in nature no actual existence, but that it is made up partly of beds which should be referred to the summit of the Upper Silurian, and partly of beds which properly belong to the base of the Carboniferous. This view seems to have been arrived at in consequence of a too exclusive study of the Devonian series of the British Isles, where the physical succession is not wholly clear, and where there is a striking discrepancy between the organic remains of those two members of the series which are known as the "Old Red Sandstone" and the "Devonian" rocks proper. This discrepancy, however, is not complete; and, as we have seen, can be readily explained on the supposition that the one group of rocks presents us with the shallow water and littoral deposits of the period, while in the other we are introduced to the deep-sea acc.u.mulations of the same period. Nor can the problem at issue be solved by an appeal to the phenomena of the British area alone, be the testimony of these what it may. As a matter of fact, there is at present no sufficient ground for believing that there is any irreconcilable discordance between the succession of rocks and of life in Britain during the period which elapsed between the deposition of the Upper Ludlow and the formation of the Carboniferous Limestone, and the order of the same phenomena during the same period in other regions. Some of the Devonian types of life, as is the case with all great formations, have descended unchanged from older types; others pa.s.s upwards unchanged to the succeeding period: but the fauna and flora of the Devonian period are, as a whole, quite distinct from those of the preceding Silurian or the succeeding Carboniferous; and they correspond to an equally distinct rock-system, which in point of time holds an intermediate position between the two great groups just mentioned. As before remarked, this conclusion may be regarded as sufficiently proved even by the phenomena of the British area; but it maybe said to be rendered a certainty by the study of the Devonian deposits of the continent of Europe--or, still more, by the investigation of the vast, for the most part uninterrupted and continuous series of sediments which commenced to be laid down in North America at the beginning of the Upper Silurian, and did not cease till, at any rate, the close of the Carboniferous.

LITERATURE.

The following list comprises the more important works and memoirs to which the student of Devonian rocks and fossils may refer:--

(1) 'Siluria.' Sir Roderick Murchison.

(2) 'Geology of Russia in Europe.' Murchison (together with De Verneuil and Count von Keyserling).

(3) "Cla.s.sification of the Older Rocks of Devon and Cornwall"--'Proc.

Geol. Soc.,' vol. iii., 1839. Sedgwick and Murchison.

(4) "On the Physical Structure of Devonshire;" and on the "Cla.s.sification of the Older Stratified Rocks of Devonshire and Cornwall"--'Trans. Geol. Soc.,' vol. v., 1840. Sedgwick and Murchison.

(5) "On the Distribution and Cla.s.sification of the Older or Palaeozoic Rocks of North Germany and Belgium"--'Geol. Trans.,' 2d ser., vol. vi., 1842. Sedgwick and Murchison.

(6) 'Report on the Geology of Cornwall, Devon, and West Somerset.'

De la Beche.

(7) 'Memoirs of the Geological Survey of Ireland and Scotland.'

Jukes and Geikie.

(8) "On the Carboniferous Slate (or Devonian Rocks) and the Old Red Sandstone of South Ireland and North Devon"--'Quart.

Journ. Geol. Soc.,' vol. xxii. Jukes.

(9) "On the Physical Structure of West Somerset and North Devon;"

and on the "Palaeontological Value of Devonian Fossils"--'Quart.

Journ. Geol. Soc.,' vol. iii. Etheridge.

(10) "On the Connection of the Lower, Middle, and Upper Old Red Sandstone of Scotland"--'Trans. Edin. Geol. Soc.,' vol. i.

part ii. Powrie.

(11) 'The Old Red Sandstone,' 'The Testimony of the Rocks,' and 'Footprints of the Creator.' Hugh Miller.

(12) "Report on the 4th Geological District"--'Geology of New York,'

vol. iv. James Hall.

(13) 'Geology of Canada,' 1863. Sir W. E. Logan.

(14) 'Acadian Geology.' Dawson.

(15) 'Manual of Geology.' Dana.

(16) 'Geological Survey of Ohio,' vol. i.

(17) 'Geological Survey of Illinois,' vol. i.

(18) 'Palaeozoic Fossils of Cornwall, Devon, and West Somerset.'

Phillips.

(19) 'Recherches sur les Poissons Fossiles.' Aga.s.siz.

(20) 'Poissous de l'Old Red.' Aga.s.siz.

(21) "On the Cla.s.sification of Devonian Fishes"--' Mem. Geol. Survey of Great Britain,' Decade X. Huxley.

(22) 'Monograph of the Fishes of the Old Red Sandstone of Britain'

(Palaeontographical Society). Powrie and Lankester.

(23) 'Fishes of the Devonian System, Palaeontology of Ohio.' Newberry.

(24) 'Monograph of British Trilobites' (Palaeontographical Society); Salter.

(25) 'Monograph of British Merostomata' (Palaeontographical Society).

Henry Woodward.

(26) 'Monograph of British Brachiopoda' (Palaeontographical Society).

Davidson.

(27) 'Monograph of British Fossil Corals' (Palaeontographical Society).

Milne-Edwards and Haime.

(28) 'Polypiers Foss. des Terrains Paleozoiques.' Milne-Edwards and Jules Haime.

(29) "Devonian Fossils of Canada West"--'Canadian Journal,' new ser., vols. iv.-vi. Billings.

(30) 'Palaeontology of New York,' vol. iv. James Hall.

(31) 'Thirteenth, Fifteenth, and Twenty-third Annual Reports on the State Cabinet.' James Hall.

(32) 'Palaeozoic Fossils of Canada,' vol. ii. Billings.

(33) 'Reports on the Palaeontology of the Province of Ontario for 1874 and 1875.' Nicholson.

(34) "The Fossil Plants of the Devonian and Upper Silurian Formations of Canada"--'Geol. Survey of Canada.' Dawson.

(35) 'Petrefacta Germaniae.' Goldfuss.

(36) 'Versteinerungen der Grauwacken-formation.' &c. Geinitz.

(37) 'Beitrag zur Palaeontologie des Thuringer-Waldes.' Richter and Unger.

(38) 'Ueber die Placodermen der Devonischen System.' Pander.

(39) 'Die Gattungen der Fossilen Pflanzen.' Goeppert.

(40) 'Genera et Species Plantarum Fossilium.' Unger.

CHAPTER XII.

THE CARBONIFEROUS PERIOD.

Overlying the Devonian formation is the great and important series of the _Carboniferous Rocks_, so called because workable beds of coal are more commonly and more largely developed in this formation than in any other. Workable coal-seams, however, occur in various other formations (Jura.s.sic, Cretaceous, Tertiary), so that coal is not an exclusively Carboniferous product; whilst even in the Coal-measures themselves the coal bears but a very small proportion to the total thickness of strata, occurring only in comparatively thin beds intercalated in a great series of sandstones, shales, and other genuine aqueous sediments.

Stratigraphically, the Carboniferous rocks usually repose conformably upon the highest Devonian beds, so that the line of demarcation between the Carboniferous and Devonian formations is princ.i.p.ally a palaeontological one, founded on the observed differences in the fossils of the two groups. On the other hand, the close of the Carboniferous period seems to have been generally, though not universally, signalised by movements of the crust of the earth, so that the succeeding Permian beds often lie unconformably upon the Carboniferous sediments.

Strata of Carboniferous age have been discovered in almost every large land-area which has been sufficiently investigated; but they are especially largely developed in Britain, in various parts of the continent of Europe, and in North America. Their general composition, however, is, comparatively speaking, so uniform, that it will suffice to take a comprehensive view of the formation without considering any one area in detail, though in each region the subdivisions of the formation are known by distinctive local names. Taking such a comprehensive view, it is found that the Carboniferous series is generally divisible into a _Lower_ and essentially calcareous group (the "Sub-Carboniferous" or "Carboniferous Limestone"); a _Middle_ and princ.i.p.ally arenaceous group (the "Millstone Grit"); and an Upper group, of alternating shales and sandstones, with workable seams of coal (the "Coal-measures").

I. The _Carboniferous, Sub-Carboniferous_, or _Mountain Limestone Series_ const.i.tutes the general base of the Carboniferous system.

As typically developed in Britain, the Carboniferous Limestone is essentially a calcareous formation, sometimes consisting of a ma.s.s of nearly pure limestone from 1000 to 2000 feet in thickness, or at other times of successive great beds of limestone with subordinate sandstones and shales. In the north of England the base of the series consists of pebbly conglomerates and coa.r.s.e sandstones; and in Scotland generally, the group is composed of ma.s.sive sandstones with a comparatively feeble development of the calcareous element. In Ireland, again, the base of the Carboniferous Limestone is usually considered to be formed by a locally-developed group of grits and shales (the "Coomhola Grits" and "Carboniferous Slate"), which attain the thickness of about 5000 feet, and contain an intermixture of Devonian with Carboniferous types of fossils. Seeing that the Devonian formation is generally conformable to the Carboniferous, we need feel no surprise at this intermixture of forms; nor does it appear to be of great moment whether these strata be referred to the former or to the latter series. Perhaps the most satisfactory course is to regard the Coomhola Grits and Carboniferous Slates as "pa.s.sage-beds" between the Devonian and Carboniferous; but any view that may be taken as to the position of these beds, really leaves unaffected the integrity of the Devonian series as a distinct life-system, which, on the whole, is more closely allied to the Silurian than to the Carboniferous. In North America, lastly, the Sub-Carboniferous series is never purely calcareous, though in the interior of the continent it becomes mainly so. In other regions, however, it consists princ.i.p.ally of shales and sandstones, with subordinate beds of limestone, and sometimes with this beds of coal or deposits of clay-ironstone.

II. _The Millstone Grit_.--The highest beds of the Carboniferous Limestone series are succeeded, generally with perfect conformity, by a series of arenaceous beds, usually known as the _Millstone Grit_. As typically developed in Britain, this group consists of hard quartzose sandstones, often so large-grained and coa.r.s.e in texture as to properly const.i.tute fine conglomerates. In other cases there are regular conglomerates, sometimes with shales, limestones, and thin beds of coal--the thickness of the whole series, when well developed, varying from 1000 to 5000 feet. In North America, the Millstone Grit rarely reaches 1000 feet in thickness; and, like its British equivalent, consists of coa.r.s.e sandstones and grits, sometimes with regular conglomerates. Whilst the Carboniferous Limestone was undoubtedly deposited in a tranquil ocean of considerable depth, the coa.r.s.e mechanical sediments of the Millstone Grit indicate the progressive shallowing of the Carboniferous seas, and the consequent supervention of sh.o.r.e-conditions.

III. _The Coal-measures_.--The Coal-measures properly so called rest conformably upon the Millstone Grit, and usually consist of a vast series of sandstones, shales, grits, and coals, sometimes with beds of limestone, attaining in some regions a total thickness of from 7000 to nearly 14,000 feet. Beds of workable coal are by no means unknown in some areas in the inferior group of the Sub-Carboniferous; but the general statement is true, that coal is mostly obtained from the true Coal-measures--the largest known, and at present most productive coal-fields of the world being in Great Britain, North America, and Belgium. Wherever they are found, with limited exceptions, the Coal-measures present a singular _general_ uniformity of mineral composition. They consist, namely, of an indefinite alternation of beds of sandstone, shale, and coal, sometimes with bands of clay-ironstone or beds of limestone, repeated in no constant order, but sometimes attaining the enormous aggregate thickness of 14,000 feet, or little short of 3 miles.

The beds of coal differ in number and thickness in different areas, but they seldom or never exceed one-fiftieth part of the total bulk of the formation in thickness. The characters of the coal itself, and the way in which the coal-beds were deposited, will be briefly alluded to in speaking of the vegetable life of the period. In Britain, and in the Old World generally, the Coal-measures are composed partly of genuine terrestrial deposits--such as the coal--and partly of sediments acc.u.mulated in the fresh or brackish waters of vast lagoons, estuaries, and marshes. The fossils of the Coal-measures in these regions are therefore necessarily the remains either of terrestrial plants and animals, or of such forms of life as inhabit fresh or brackish waters, the occurrence of strata with marine fossils being quite a local and occasional phenomenon. In various parts of North America, on the other hand, the Coal-measures, in addition to sandstones, shales, coal-seams, and bands of clay-ironstone, commonly include beds of limestone, charged with marine remains, and indicating marine conditions. The subjoined section (fig. 107) gives, in a generalised form, the succession of the Carboniferous strata in such a British area as the north of England, where the series is developed in a typical form.

As regards the _life_ of the Carboniferous period, we naturally find, as has been previously noticed, great differences in different parts of the entire series, corresponding to the different mode of origin of the beds. Speaking generally, the Lower Carboniferous (or the Sub-Carboniferous) is characterised by the remains of marine animals; whilst the Upper Carboniferous (or Coal-measures) is characterised by the remains of plants and terrestrial animals.

In all those cases, however, in which marine beds are found in the series of the Coal-measures, as is common in America, then we find that the fossils agree in their general characters with those of the older marine deposits of the period.

[Ill.u.s.tration: Fig. 107. GENERALIZED SECTION OF THE CARBONIFEROUS STRATA OF THE NORTH OF ENGLAND.]

Owing to the fact that coal is simply compressed and otherwise altered vegetable matter, and that it is of the highest economic value to man, the Coal-measures have been more thoroughly explored than any other group of strata of equivalent thickness in the entire geological series. Hence we have already a very extensive acquaintance with the _plants_ of the Carboniferous period; and our knowledge on this subject is daily undergoing increase. It is not to be supposed, however, that the remains of plants are found solely in Coal-measures; for though most abundant towards the summit, they are found in less numbers in all parts of the series. Wherever found, they belong to the same great types of vegetation; but, before reviewing these, a few words must be said as to the origin and mode of formation of _coal_.

The coal-beds, as before mentioned, occur interstratified with shales, sandstones, and sometimes limestones; and there may, within the limits of a single coal-field, be as many as 80 or 100 of such beds, placed one above the other at different levels, and varying in thickness from a few inches up to 20 or 30 feet.

As a general rule, each bed of coal rests upon a bed of shale or clay, which is termed the "under-clay," and in which are found numerous roots of plants; whilst the strata immediately on the top of the coal may be shaly or sandy, but in either case are generally charged with the leaves and stems of plants, and often have upright trunks pa.s.sing vertically through them. When we add to this that the coal itself is, chemically, nearly wholly composed of carbon, and that its microscopic structure shows it to be composed almost entirely of fragments of stems, leaves, bark, seeds, and vegetable _debris_ derived from _land-plants_, we are readily enabled to understand how the coal was formed.

The "_under-clay_" immediately beneath the coal-bed represents an old land-surface--sometimes, perhaps, the bottom of a swamp or marsh, covered with a luxuriant vegetation; the _coal bed_ itself represents the slow acc.u.mulation, through long periods, of the leaves, seeds, fruits, stems, and fallen trunks of this vegetation, now hardened and compressed into a fraction of its original bulk by the pressure of the superinc.u.mbent rocks; and the strata of sand or shale above the coal-bed--the so-called "roof" of the coal--represent sediments quietly deposited as the land, after a long period of repose, commenced to sink beneath the sea. On this view, the rank and long-continued vegetation which gave rise to each coal-bed was ultimately terminated by a slow depression of the surface on which the plants grew. The land-surface then became covered by the water, and aqueous sediments were acc.u.mulated to a greater or less thickness upon the dense ma.s.s of decaying vegetation below, enveloping any trunks of trees which might still be in an erect position, and preserving between their layers the leaves and branches of plants brought down from the neighbouring land by streams, or blown into the wafer by the wind. Finally, there set in a slow movement of elevation,--the old land again reappeared above the water; a new and equally luxuriant vegetation flourished upon the new land-surface; and another coal-bed was acc.u.mulated, to be preserved ultimately in a similar fashion. Some few beds of coal may have been formed by drifted vegetable matter brought down into the ocean by rivers, and deposited directly on the bottom of the sea; but in the majority of cases the coal is undeniably the result of the slow growth and decay of plants _in situ_: and as the plants of the coal are not _marine_ plants, it is necessary to adopt some such theory as the above to account for the formation of coal-seams. By this theory, as is obvious, we are compelled to suppose that the vast alluvial and marshy flats upon which the coal-plants grew were liable to constantly-recurring oscillations of level, the successive land-surfaces represented by the successive coal-beds of any coal-field being thus successively buried beneath acc.u.mulations of mud or sand. We have no need, however, to suppose that these oscillations affected large areas at the same time; and geology teaches us that local elevations and depressions of the land have been matters of constant occurrence throughout the whole of past time.

All the varieties of coal (bituminous coal, anthracite; cannel-coal, &c.) show a more or less distinct "lamination"--that is to say, they are more or less obviously composed of successive thin layers, differing slightly in colour and texture. All the varieties of coal, also, consist chemically of _carbon_, with varying proportions of certain gaseous const.i.tuents and a small amount of incombustible mineral or "ash." By cutting thin and transparent slices of coal, we are further enabled, by means of the microscope, to ascertain precisely not only that the carbon of the coal is derived from vegetables, but also, in many cases, what kinds of plants, and what parts of these, enter into the formation of coal. When examined in this way, all coals are found to consist more or less entirely of vegetable matter; but there is considerable difference in different coals as to the exact nature of this. By Professor Huxley it has been shown that many of the English coals consist largely of acc.u.mulations of rounded discoidal sacs or bags, which are unquestionably the seed-vessels or "spore-cases" of certain of the commoner coal-plants (such as the _Lepidodendra_). The best bituminous coals seem to be most largely composed of these spore-cases; whilst inferior kinds possess a progressively increasing amount of the dull carbonaceous substance which is known as "mineral charcoal," and which is undoubtedly composed of "the stems and leaves of plants reduced to little more than their carbon." On the other hand, Princ.i.p.al Dawson finds that the American coals only occasionally exhibit spore-cases to any extent, but consist princ.i.p.ally of the cells, vessels, and fibres of the bark, integumentary coverings, and woody portions of the Carboniferous plants.

The number of plants already known to have existed during the Carboniferous period is so great, that nothing more can be done here than to notice briefly the typical and characteristic _groups_ of these--such as the Ferns, the Calamites, the Lepidodendroids, the Sigillarioids, and the Conifers.