Chap09

CHAPTER IX.

CARBONIFEROUS SERIES.

CARBONIFEROUS ROCKS.—In the south and middle of England, the Carboniferous rocks consist chiefly of Limestone at the base and Coal-measures above. Including the South Wales, the Forest of Dean, the Somersetshire and other areas, a typical section of the beds is as follows:

The Yellow Sandstone beds often form a passage from the Old Red Sandstone to the Carboniferous rocks, and the plants have carboniferous affinities. The accompanying shales in Pembrokeshire and elsewhere, contain numerous fish-teeth, Spirifers, Productas, and a few Lingulas; and the Carboniferous Limestone, which is more than 2,000 feet thick in South Wales, and in Somersetshire, is so highly fossiliferous that it may be stated that the whole of the limestone once formed parts of animals. The lowest 500 feet consists chiefly of fragments of Encrinites. The Yoredale rocks of Yorkshire have no precise lithological parallel in

[120 Coal-measures.]

South Wales and Somerset. They consist chiefly of shales and sandstones, with marine shells and occasional land-plants. The Millstone grit of South Wales is comparatively unfossiliferous, but sometimes contains the remains of plants, and more rarely Orthoceras and other marine shells.

The Coal-measures and Millstone grit of Monmouthshire, Glamorganshire, and Pembrokeshire, lie in a great oval basin, encircled by a rim of Carboniferous Limestone, beneath which lies the Old Red Sandstone. The Coal-measure beds alone were estimated by Sir William Logan at from 10,000 to 12,000 feet thick. They consist of alternations of sandstone, shale, fireclay or underclay, coal, and ironstone. There are about 100 beds of coal in the field, many of which are workable, chiefly in the lower part of the series, where the principal ironstones also occur. In the shales and sandstones large stems of plants are sometimes found standing vertically, in the positions in which they grew. Underneath each bed of coal is a bed of underclay with Stigmaria, forming the soil in which the plants were rooted, by the decay of which, passing through the stage of peat, material was supplied for the subsequent production of coal. Stigmaria, once supposed to be a special plant, was first proved by Mr. Binney to be the root of Sigillaria, and about the same time Logan showed that the underclay was a soil that lay invariably underneath beds of coal, and indeed that these roots and rootlets are in every underclay. The plants (the decay of which formed beds of coal) consisted largely of gigantic club-mosses, such as Lepidodendron and Calamites (Equisitaceæ) of various species, and many other ferns, with a few Coniferæ, &c.

Passing from east to west in this coal-field, the coals

[Coal-measures. 121]

(sometimes the very same beds) gradually change from so-called bituminous to anthracitic varieties. It is remarkable that anthracite usually occurs in coalfields the strata of which have been much disturbed and contorted, as, for instance, in the mountains of Pennsylvania. Anthracite is simply a metamorphic variety of coal; and in Pembrokeshire, where the coals are most anthracitic, the strata have been violently contorted. There is a connection between the heat that produced metamorphism and the lateral pressure that produced contortion, for pressure with movement is convertible into heat. A line of disturbance passes from the banks of the Wye south of Builth, through the north part of the coal-field south of Liandeilo, and from thence westward into Pembrokeshire, where masses of igneous rocks appear in contact with the coal-field. In connection with this, it may be that the rocks of the coal-field remained a long time highly heated, and so, by a species of distillation, deep under ground, the bituminous were converted into anthracite coals.

FIG. 27.

1. Old Red Sandstone.
Carboniferous Series:
    2. Carboniferous Limestone.
    3. Millstone Grit.
    4. Coal measures with beds of coal.

Dean Forest may be looked on as an outlier of the South Wales coal-field. Fig. 27 may be supposed to represent the arrangement of the strata on the east side of this very perfect basin. The limestone is about 700

[122 coal-measures.]

feet thick, and the Coal-measures, according to De La Beche, 2,765 feet. The limestone contains brown hæmatite iron ore in cavernous holes. There are in the field 23 chief beds of coal.

The Bristol and Somersetshire coal-field was also originally joined to the South Wales Carboniferous rocks, till separated by denudation. The Carboniferous Limestone series near Bristol, and on the Mendip Hills, is about 2,500 feet thick, containing the usual marine fossils in great variety. The Coal-measures and Millstone grit of the Bristol and Somerset shire coalfield lie in a basin, the base of which is formed of this limestone. The Coal-measures are altogether about 7,000 feet thick, and consist of an upper and a lower series, separated by thick beds of grit, called the Pennant rock, about 2,000 feet in thickness, and which itself holds beds of coal, some of them of value. Altogether they contain about 46 beds of coal, with a total thickness of about 98 feet. A large part of this Carboniferous basin is unconformably covered by New Red marl and Liassic and Oolitic strata, and here and there portions of the coal-field are exposed by denudation of the New Red marl between Bristol and the Mendip Hills, where the beds rise rapidly, and a narrow strip of Coal-measures skirts the Mendip limestones, the whole dipping no:th at high angles. Similar Coal-measures probably underlie the marshes, and part of the secondary strata south of the Mendip Hills.

These three coal-basins, South Wales, Dean Forest, and Bristol, once united, have only been separated by denudation similar to that shown at p. 33. In the case of these coal-fields the intervening spaces are antidinal, and the basins synclinal curves, and therefore it is not only possible, but probable, that other coal-basins

[Coal-measures. 123]

may lie far to the east beneath the Oolitic, Cretaceous, and Eocene strata of the London basin.

The Culm-measures of Devonshire, though of true Carboniferous age, and probably representing muchof the series, are nearly unproductive of coal. Near their base there are intermittent thin streaks of limestone, which may feebly represent part of the great masses of Somerset and South Wales, just as the thin worthless coals represent the numerous seams of these coal-fields. But the conditions of deposition in the areas were apparently very different. In the Devonshire area the purely terrestrial intervals, marked by the growth of land plants in situ, seem to have been infrequent and transitory, and from bottom to top common aqueous strata prevail.

Further north, in the neighbourhood of Newent, narrow bands of poor Coal-measures are barely traceable between the Old Red and the New Red Sandstones, and still further north, round Bewdley, there lies the coalfield of the Forest of Wyre, consisting of strata by no means very productive of coal-beds. They lie directly on the Old Red Sandstone, the Carboniferous Limestone being absent. The Coalbrookdale coal-field joins that of the Forest of Wyre, and lies partly on a thin development of Carboniferous Limestone, and partly unconformably on Upper Silurian rocks. On the northwest, the lower part of the New Red Sandstone is faulted against it, and on the east it is overlaid by Permian strata. It contains several bands of good nodular ironstones, which often yield Producta, Conularia, Orbicula, Limulus, and other marine remains, and in some of the strata fossil beetles, dragonifies, and spiders have been found. There are in places 22 beds of coal in this field, about 10 of which are workable, some

[124 Coal-measures.]

of them from 3 to 6 feet thick, with beds of underclay, the whole being interstratified with shales and sandstones. The total thickness of these Coal-measures is about 1,000 feet. The adjoining coal-fields of Le Botwood and Shrewsbury are comparatively of minor importance. The North Wales coal-field in all essential geological points resembles that of South Wales, and lies on the Carboniferous Limestone, which is from 800 to 1,000 feet thick. South of Wrexham the whole dips east under the Permian rocks, and further north under the New Red Sandstone. The Denbighshire part contains at least 17 beds of coal, most of which are worked, and the Flintshire part at least 12 beds. A small fragment of the same strata occurs in the central part of Anglesey. It is underlaid by the Carboniferous Limestone, and on the south-east is faulted against the Cambrian rocks. Permian strata overlie it, but the smaller faults and a greenstone dyke which affect the coal do not pass through the Permian beds, which lie unconformably over all.

In the centre of England the basement beds of the South Staffordshire coal-field rest directly on the Wenlock Limestone of the Upper Silurian series. This field, in the northern part, contains 14 beds of coal. Getting closer to each other by degrees in the south, several of these coalesce to form the thick coal, in places 40 feet in thickness, with two thin partings. The rocks are pierced by basalts and a white felspathic-looking trap, which has charred the coals at the points of junction, and is undoubtedly connected with the great basaltic mass, called the Rowley Rag, that overlies the Coal-measures.

The New Red Sandstone on the east is faulted against the Warwickshire coal-field, and generally over

[Coal-measures. 125]

laid by the Permian rocks on the west. It contains six beds of workable coal, besides ironstone, and on the south, where the strata pass under the Lower Keuper Sandstones, several of these, as in South Staffordshire, coalesce to form two beds of coal. The lower part of the Coal-measures is traversed by several lines of intrusive dioritic greenstones running in the line of strike.

The Ashby-de-la-Zouch coal-field is overlaid by the New Red Sandstone, and partly underlaid by the Carboniferous Limestone, and partly, probably, by a continuation of the Cambrian rocks of Charnwood Forest. It is divided into two districts or minor basins-the eastern, containing 15 beds of coal, 11 of which are workable; and the western 11 beds. Nine are of superior quality. The Coalbrookdale, South Staffordshire, and Warwickshire coal-fields present so many points of resemblance, that undoubtedly they were all originally formed as one coal-field, and even now in great part may be continuous in the districts that lie between, concealed by Permian and New Red strata.

North of this coal-field the Carboniferous rocks are somewhat modified in details. Between Derbyshire and Berwick they stretch north and south without a break for 200 miles, by about 60 miles in width. At the southern end, near Derby, the New Red Sandstone overlies them. West of Cheadle, along the edge of the North Staffordshire coal-field, they are generally faulted against the Permian rocks, north of which lie the coalfields of Cheshire and Lancashire. The Carboniferous Limestone and Millstone grit rise between these coalfields, forming the hills of Derbyshire; and the Coal-measures are thrown off on either side of the anticinal axis, forming, in the east, the Derbyshire and Yorkshire coal-field, and on the west those of North Staffordshire,

[126 Coal-measures.]

Cheshire, and Lancashire. Three or four beds of igneous rock, called toadstone, lie in the limestone. The Millstone grit of these areas is much mingled with shale, and between it and the Carboniferous Limestone there are often thick beds of shale and sandstone, called the Upper Limestone Shale, or Yoredale rocks. North of the Ribble the Carboniferous Limestone itself is divided by numerous interstratifications of sandstone and shale, with occasional beds of thin coal, and this increasing in the northern parts of Northumberland, the equivalents of the southern mass of Carboniferous Limestone die away into a few subordinate beds of limestone, and fairly pass by degrees into a lower coal-field, with several poor beds of coal.

The Lancashire, Cheshire and North Staffordshire coal-fields, exclusive of the Millstone grit, vary from about 3,500 to 7,500 feet in thickness, counting from the beds on which the unconformable Permian strata happen to rest.. They include about 30 coal-beds in North Staffordshire, in Lancashire 14 good seams about St. Helens, 15 at Wigan, 16 between Manchester and Bolton, and 13 at Burnley. Many of these, which in different districts go by different names, are equivalent beds. Fish remains and many marine and estuarine or fresh water shells occur among the interstratified shales and sandstones. There are also many beds of ironstone. The Nottingham, Derbyshire, and Yorkshire coal-fields united give about 15 beds of workable coal. All these are ironstone areas, and North Staffordshire is the great pottery district of England. The finer clay is imported, only the coarser qualities for tiles, &c., being native.

The Newcastle coal-field is about 1,600 feet thick, and contains about 16 beds of coal throughout the

[igneous Rocks. 127]

district. The lower coal-field of Northumberland, as already stated, is of the age of the Carboniferous Limestone series of Wales, and the Berwickshire coals of Scotland are of the same general age. There is another much smaller coal-field near Ingleton in North Lancashire which contains 8 beds of coal, and in Cumberland the Whitehaven Coal-measures, which lie on the Carboniferous Limestone, have 14 beds.

The great Scottish coal-fields lie in a broad synclinal curve, in which are the valleys of the Clyde and Forth. Beneath the Calciferous Sandstone and Carboniferous Limestone series, Old Red Sandstone, underlaid by Silurian rocks, rises on the south-east between St. Abb's Head on the east and Girvan on the west; while on the north-west the Old Red Sandstone resting on the Lower Silurian rocks of the Highlands, rises from beneath the same Carboniferous strata between the Frith of Tay and the Clyde, near Dumbarton. The whole tract is about 100 miles in length, by 40 to 50 in breadth.

The lower Carboniferous strata are much intermingled with igneous rock, sometimes feispathic, sometimes augitic. Some of these are intrusive, but large masses consist of truly interbedded lavas, associated with strongly marked and thick strata of volcanic ashes and conglomerates, well seen, for example, on the cliffs between Dunbar and Belhaven. The Carboniferous Limestones, which in occasional bands overlie the Calciferous Sandstone, do not lie in a mass at the base of the Coal-measures, but, as in the North of England, the limestone occurs in several beds, chiefly in the lower part of the series, interstratified with beds of sandstone, shale, and occasionally of coal. In Linlithgowshire and the Campsie Hills limestones are interbedded with trap. Marine, fresh or brackish water, and terrestrial

[128 Coal-measures.]

alternations are of constant occurrence. In some cases in East Lothian, beds of fireclay, with Stigmaria, and thin layers of coal lying on old terrestrial soils, immediately underlie marine limestones with Productas. In the Dalkeith coal-field valuable beds of coal with shales, &c. are interstratified with a thick series of beds of Carboniferous Limestone. The Burdiehouse brackish water limestone in East Lothian is the lowest of the limestones, and yields many small bivalve Crustacea of the genus Estheria, besides fish of the genera Megalicthys and Holoptychius.

In the East and Mid Lothian coal-fields about 20 beds of workable coal occur, besides many smaller layers. Eleven workable beds of coal are known above the Millstone grit or Moor rock, and 17 associated with the Carboniferous Limestone beds below the Millstone grit. The Carboniferous strata of the Lothians cross the Firth of Forth beneath the sea, and form great part of Kinross and Fife, where there are 29 workable beds, one of which is 21 feet, and others from 5 to 9 feet in thickness. The western part of the basin in Lanarkshire and Ayrshire yields 8 or 10 workable coal seams. It is in these districts that the well-known black-band ironstones occur.

I have already said that the South Wales, Dean Forest, Bristol and Devonshire Carboniferous areas originally formed one, and have been separated by disturbance of the strata and subsequent denudation. The same kind of original continuity may be inferred concerning all tlie coal-fields of the middle of England, North Wales, and northward to Cumberland and Northumberland, and the latter was even probably joined to the great coal-field of central Scotland. After the close of the Carboniferous epoch, this large area was

[Carboniferous Limestone Fossls. 129]

also thrown into a series of undulating auticlinal and syndinal curves, great denudations occurred, and the result was that the individual coal-fields now lie in basins often separated from each other by intervening tracts of Millstone Grit and Carboniferous Limestone. Sometimes portions of these basins are concealed by unconformable overlying Permian and New Red strata. Thus, the Northumberland and Durham coal-field is probably a basin, partly out at sea, and the southern edge of which is overlaid by Maguesian Limestone. The Yorkshire and Derbyshire coal-field is in my belief another basin, the eastern half of which must crop up against the Magnesian Limestone, deep under ground, and miles to the east of where it first dips beneath that limestone. The Lancashire and North Wales coalfields also form parts of another great basin, in places probably 6,000 feet or more beneath the New Red Marl of Cheshire. These statements will be more easily understood by referring to figs. 63, p. 325, and 115, p. 601.

In the purely marine strata of the Carboniferous series, of which the Carboniferous Limestone forms the most important part, we find that more than 30 genera and about 100 species of Corals have been named. Among the most common are species of the genera Cyathophyllum, Clisiophyllum, Syringopora, Lithostrotion, and Zaphrentis. Crinoidea are numerous, the most common of which belong to the genera Actinocrinus, Cyathocrinus, Platycrinus, Woodocrinus, and Poteriocrinus; 3 species of Echithdæ also occur. Trilobites are scarce in the Carboniferous rocks, the most characteristic genera being Griffithides and Phillipsia. Among other Crustacea there are Estheria, Eurypterus, Prestwichia, Belinurus, and Limulus. Polyzoa are common. Brachiopoda are also exceedingly

[130]

FIG. 28.

Group of Carboniferous Limestone Fossils.

[Coal-measure Fossils. 131]

numerous, and comprise 18 genera and 160 species, the most strikingly characteristic of which are Productus, Spirifera, Rhynchonella, and Terebratula. The genus Orthis only yields 12 species, a great decrease when cornpared with its development in Silurian seas. There are 334 species belonging to 49 genera of Lamellibranchiata, which, unlike their comparative development in Silurian rocks, far exceed the Brachiopoda, both specifically and generically, indicating a remarkable approach to the types of Secondary times, in which Lamellibranchiate molluscs by far predominate. The most common of these are Aviculopecten, Posidonomya, Arca, Conocardium, Edmondia, Modiola, Nucula, and Sanguinolites. Of Gasteropoda, there are 29 genera and 206 species, among which are many species of Euomphalus and Pleurotomaria. Of the Nucleobranchiata, 23 species of Bellerophon are known, and 148 species of Cephalopoda the chief of which are Goniatites, Nautilus, and Orthoceras. Ninety-nine genera and 221 species of fish have been described, some of which probably lived alike in the sea and in fresh and brackish water.

In the Carboniferous rocks, chiefly in the Coal-measures, more than 500 species of fossil plants have been named, a large proportion of which are ferns, some of great size. The most common genera are Sphenopteris, Pecopteris, Neuropteris, Cyclopteris, Odontopteris, Caulopteris (tree-fern), &c. The remaining plants belong chiefly to genera of Calamites (Equisitæ of large size), Lepidodendron (tree Lycopodiums), and Sigillaria, Fig. 29. Coniferous trees, the fruit of which is Trigonocarpum, also occur. In the Coal-measure strata of Britain there have also been found many fresh-water Crustacea of the genus Cypris, freshwater bivalves, Anthracomya, Anthracosia, &c., wings

[132]

FIG. 29.

Group of Coal-measure Plants and Freshwater Shells.

[Physical Geography. 133]

and wing-cases of beetles and other insects, spiders, &c. Rain pittings on the shales are not infrequent, together with sun-cracks and footprints of Labyrinthodont Amphibia, Dendrerpeton, Anthracosaurus, and other genera. The rain pittings in this special case, tell of showers falling on surfaces of moist mud, exposed by the temporary retirement of fresh water, and the suncracks of the drying and shrinkage of that mud; and these joined with the footprints of Amphibia tell of daily events which by happy accidents got perpetuated, first, by baking in the sun's rays. Next, when the area was again overflowed, new layers of mud settled on these impressions, and afterwards becoming consolidated into shale; and thus we have, in a measure, fossilised sunshine, showers, and footsteps of old Amphibians, imprinted, during their occasional visits to the moist land, on the margin of the water in which they chiefly lived.

Before closing the subject I must endeavour to explain under what broad conditions of Physical Geography the Carboniferous series was formed.

It is impossible to have an intimate knowledge of the Carboniferous rocks, even within the limited area of the British Islands, without coming to the conclusion, first, that the various strata were formed in seas, some comparatively open and deep, some shallow, estuarine, and restricted in area, and some in fresh water; and second, that beds of coal were due to terrestrial vegetable growths that flourished and died on the land, and were buried with the soils on which they grew. To examine all of these points in full detail would require the writing of a special treatise, and I can here only glance at the proofs.

In the southernmost parts of South Pembrokeshire,

[134 Physical Geography.]

the limestone is about 2,500 feet thick. Going north to Haverfordwest it rapidly thins out, and finally disappears by overlap in a distance of twelve miles. A rapid thinning of the same strata also takes place between the shore of Bristol channel in Glamorganshire and the north side of the South Wales coal-field. In the Mendip Hills the limestone has also a thickness of about 2,500 feet. Traces of it are still seen south of Bideford Bay, at Cannington Park, a few miles northwest of Bridgewater, while on the northern borders of the Culm-measures of North Devon, it may be said to have almost entirely disappeared as a special formation. Among the limestone hills of Derbyshire it is of enormous thickness, and its base is unknown; but so indistinct is the bedding in part of the centre of that region, that it is often as hard to make out the details of stratification as it is in a large consolidated modern coral reef. North of Clitheroe the bosses of limestone are in places remarkably massive, and thin away in various directions so rapidly, that the incautious geologist is at first tempted to imagine faults where none exist. Further north, near Settle, Kendal, and round the sides of the Vale of Eden, it is well developed and distinctly bedded; but passing east and north, into Durham and Northumberland, it rapidly splits up into a few comparatively insignificant bands, separated by thick interstratifications of shale, sandstone, and minor beds of coal. The lower coal-fields in Scotland lie in equivalent strata.

In Ireland the phenomena are still more remarkable, for in the south and south-west, as described by Jukes, the same masses of limestone in a few miles sometimes thin away from some 2,000 to 200 or 300 feet in thickness.

[Physical Geography. 135]

The prevalence of corals in the thick masses of Carboniferous Limestone, and sometimes the rapid thinfling out of these masses in opposite directions, point to the conclusion that they were true coral reefs, of the nature of the Barrier Reefs of Australia and the Pacific Ocean, and that they thinned away on one side to a feather edge in the direction of the land, and on the other more steeply towards the deep sea. These lenticular masses were probably formed round outlying islands, large and small, undergoing a process of slow depression, or otherwise on the shores of some old continent, the details of the original shape of which are now lost to our knowledge. One part of this land, however, consisted of that area now known as the mountainous parts of Wales, and the adjacent Silurian and Cambrian territory that underlies the Coal-measures of South Staffordshire, Warwickshire, and Leicestershire, Derbyshire, Cumbria, and the South of Scotland, while far north the Grampian mountains and the whole of the North Highlands stood higher above the level of the sea than they do now, for ever since they have suffered from denudation.

But while in the south, coral reefs of the nature of Barrier Reefs or Atolls were being formed, in the north the case was different; for there, as in parts of the modern Pacific, volcanic action was rife, and this is witnessed by the lavas and ashes, intermingled and interstratified with the whole of the Carboniferous series in Scotland. This area, together with the north of what is now England, was therefore more or less an area of elevation, accompanied by oscillations of partial depression. Thus it happens that in these regions, the bands of Carboniferous Limestone are quite insignificant when compared with the thick interstratifications

[136 Physical Geography]

of shale and sandstone with occasional beds of coal that lie between them, and which, excepting the beds of coal, were of ordinary aqueous sediments.

This naturally leads to the question under what circumstances were the purely mechanical sediments and the beds of coal formed? The answer is, that after the close of the Carboniferous Limestone epoch in the south, the area got filled up by the sands of the Millstone Grit and the more muddy strata (now shales and sandstones) that overlie them, and this shallowing of the seas may have been aided by partial upheaval of the area, till part of it was nearly at, and at length a little above, the level of the sea. Through this flat continental land, great rivers ran, bordered by wide marshy flats, on which the vegetation grew that by its decay and death became transformed into peat. Then by gradual depression these areas were again covered with water, in the first instance salt or fresh, as the case might be, but in all cases resulting in the deposition of layers of sediment. The area was thus converted by degrees into low land, covered by vegetation, a new growth and decay took place, and it was again depressed beneath the water to receive newer sediments, and so on through a vast period of time, till, for example, all the 10,000 feet of the South Wales coal-field were accumulated, interstratified with the hundred beds of coal, great and small, that lie among the shales and sandstones; and in equal or less degree the same was the case with all the other coal-fields of England and 'Wales, as far north as those of Lancashire and Yorkshire.

But when we come to other Carboniferous areas, further north, the case is somewhat different. There we find, in Durham, Northumberland, and Scotland, no

[Physical Geogaphy. 137]

thick masses of limestone, but only thin bands, interstratified with thick deposits of shale and sandstone, similar in most respects to those of the Coal-measures of Wales, and, like these, interstratified with beds of coal. The inference is obvious, that in these areas the conditions that prevailed were such, that a given area during oscillations of level was at one time sea, as proved by the sea shells in the strata, at another fresh water, as witnessed by the shells Anthracosia, Anthracomya, &c., and at another time land, as shown by the beds of coal, each underlaid by its terrestrial soil of unclerclay with Stigmaria, the roots of Sigillaria.

If this be true, we get a hint of a new phase of the physical geography of an epoch immediately succeeding that of the Old Red Sandstone. I have often thought that if we might imagine the vast flat territory of Northern Asia, with all its mighty rivers, to face south, so that they might run into a sub-tropical sea, we would have something like a picture of our Carboniferous epoch, succeeding one, the chief character of which, was the presence of numbers of large continental lakes. This at all events seems certain, that beds of coal are not the result of woody matter drifted into, and waterlogged in, lake hollows, by rivers, as was once imagined; but rather, considering the magnitude of the areas which the beds of coal cover, that they bear witness to the existence of a vast continent, or, if we take the whole world into account, of vast continents, through which wandering rivers traversed flat areas, comparable to those of the largest river areas of the living world. Deltas of the present day offer many analogies. The mouth of the Whang-ho or Yellow river is now 250 miles north from where it entered the sea about twenty years ago. The modern delta of the

[138 Physical Geography.]

Mississippi has an area of more than 12,000 square miles, consisting chiefly of sands and clays, with much vegetable matter, and that of the Nile an area of about 21,000. The delta of the Ganges and Brahmapootra is more than 48,000 square miles in extent, has peaty beds interstratified with clays and sands, containing freshwater shells and freshwater tortoises, often much below the level of the neighbouring sea. The area of all England and Wales is 57,812 square miles, and the areas of all the coal-fields of Great Britain extended to their original size did not equal that of this great delta.

It is not to be supposed that, in each coal. field, each bed of coal extends over the whole area. On the contrary they thicken and thin out, and have their edges like many a modern peat moss, and the vegetation of the Carboniferous epoch flourished and decayed rapidly, on moist ground and in a moist atmosphere, not of excessive warmth, as has often been stated, but, in the opinion of Sir Joseph Hooker, 'in a moist and equable climate,' that could scarcely have been sub-tropical.