before remarked, this conclusion may be regarded as sufficiently proved even by the phenomena of the British area; but it may be 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) "Classification 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 “Classification of the Older Stratified Rocks of Devonshire and Cornwall" -'Trans. Geol. Soc.,' vol. v., 1840. Sedgwick and Murchison. (5) "On the Distribution and Classification of the Older or Palæozoic Rocks of North Germany and Belgium"-Geol. Trans.,' 2d ser., vol. vi., 1842. Sedgwick and Murchison.

6

(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 "Palæontological 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.

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

Phillips.

(19) Recherches sur les Poissons Fossiles.' Agassiz.

(20) Poissons de l'Old Red.' Agassiz.

(21)

"On the Classification of Devonian Fishes"-'Mem. Geol. Survey of Great Britain,' Decade X. Huxley.

(22) 'Monograph of the Fishes of the Old Red Sandstone of Britain' (Palæontographical Society). Powrie and Lankester.

(23) Fishes of the Devonian System, Palæontology of Ohio.' New

berry.

(24) Monograph of British Trilobites' (Palæontographical Society).

Salter.

(25) 'Monograph of British Merostomata' (Palæontographical Society). Henry Woodward,

(26) 'Monograph of British Brachiopoda' (Palæontographical Society). Davidson.

(27) 'Monograph of British Fossil Corals' (Palæontographical Society). Milne-Edwards and Haime.

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

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

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

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

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

(33) Reports on the Paleontology 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 Germaniæ.' Goldfuss.

(36) Versteinerungen der Grauwacken-formation,' &c. Geinitz. (37) Beitrag zur Palæontologie des Thüringer-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 (Jurassic, 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 principally a palæontological 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 principally arenaceous group (the "Millstone Grit"); and an Upper group, of alternating shales and sandstones, with workable seams of coal (the "Coalmeasures").

I. The Carboniferous, Sub-Carboniferous, or Mountain Limestone Series constitutes the general base of the Carboniferous system. As typically developed in Britain, the Carboniferous. Limestone is essentially a calcareous formation, sometimes consisting of a mass 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 coarse sandstones; and in Scotland generally, the group is composed of massive 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 locallydeveloped 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 66 passage-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 principally of shales and sandstones, with subordinate beds of limestone, and sometimes with thin 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 largegrained and coarse in texture as to properly constitute 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 coarse sandstones and grits, sometimes with regular conglomerates. Whilst the Carboniferous Limestone was undoubtedly deposited in a tranquil ocean of considerable depth, the coarse mechanical sediments of the Millstone Grit indicate the progressive shallowing of the Carboniferous seas, and the consequent supervention of shore-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 Coalmeasures 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 accumulated in the fresh or brackish waters of vast lagoons, estuaries, and marshes. The fossils of the Coalmeasures 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.

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