Fig. 370.Section showing the erect position of fossil trees in coal sandstone at St.Etienne. (Alex.Brongniart.)
Fig. 370.
Section showing the erect position of fossil trees in coal sandstone at St.Etienne. (Alex.Brongniart.)
When I visited, in 1843, the quarries of Treuil above-mentioned, the fossil trees seen infig. 370.were removed, but I obtained proofs of other forests of erect trees in the same coal-field.
Fig. 371.Inclined position of a fossil tree, cutting through horizontal beds of sandstone, Craigleith quarry,Edinburgh. Angleof inclination fromatob27°.
Fig. 371.
Inclined position of a fossil tree, cutting through horizontal beds of sandstone, Craigleith quarry,Edinburgh. Angleof inclination fromatob27°.
Snags.—In 1830, a slanting trunk was exposed in Craigleith quarry, near Edinburgh, the total length of which exceeded 60 feet. Its diameter at the top was about 7 inches, and near the base it measured 5 feet in its greater, and 2 feet in its lesser width. The bark was converted into a thin coating of the purest and finest coal, forming a striking contrast in colour with the white quartzose sandstone in which it lay. The annexed figure represents a portion of this tree, about 15 feet long, which I saw exposed in 1830, whenall the strata had been removed from one side. The beds which remained were so unaltered and undisturbed at the point of junction, as clearly to show that they had been tranquilly deposited round the tree, and that the tree had not subsequently pierced through them, while they were yet in a soft state. They were composed chiefly of siliceous sandstone, for the most part white; and divided into laminæ so thin, that from six to fourteen of them might be reckoned in the thickness of an inch. Some of these thin layers were dark, and contained coaly matter; but the lowest of the intersected beds were calcareous. The tree could not have been hollow when imbedded, for the interior still preserved the woody texture in a perfect state, the petrifying matter being, for the most part, calcareous.[321-A]It is also clear, that the lapidifying matter was not introduced laterally from the strata through which the fossil passes, as most of these were not calcareous. It is well known that, in the Mississippi and other great American rivers, where thousands of trees float annually down the stream, some sink with their roots downwards, and become fixed in the mud. Thus placed, they have been compared to a lance in rest; and so often do they pierce through the bows of vessels which run against them, that they render the navigation extremely dangerous. Mr. Hugh Miller mentions four other huge trunks exposed in quarries near Edinburgh, which lay diagonally across the strata at an angle of about 30°, with their lower or heavier portions downwards, the roots of all, save one, rubbed off by attrition. One of these was 60 and another 70 feet in length, and from 4 to 6 feet in diameter.
Fig. 372.Section of the cliffs of the South Joggins, near Minudie, Nova Scotia.
Fig. 372.
Section of the cliffs of the South Joggins, near Minudie, Nova Scotia.
The number of years for which the trunks of trees, when constantly submerged, can resist decomposition, is very great; as we might suppose from the durability of wood, in artificial piles, permanently covered by water. Hence these fossil snags may not imply a rapid accumulation of beds of sand, although the channel of a river or part of a lagoon is often filled up in a very few years.
Nova Scotia.—One of the finest examples in the world of a succession of fossil forests of the carboniferous period, laid open to view in a natural section, is that seen in the lofty cliffs bordering the Chignecto Channel, a branch of the Bay of Fundy, in Nova Scotia.[321-B]
In the annexed section (fig. 372.), which I examined in July, 1842, the beds fromctoiare seen all dipping the same way, their average inclination being at an angle of 24° S.S.W. The vertical height of the cliffs is from 150 to 200 feet; and betweendandg, in which space I observed seventeen trees in an upright position, or, to speak more correctly, at right angles to the planes of stratification, I counted nineteen seams of coal, varying in thickness from 2 inches to 4 feet. At low tide a fine horizontal section of the same beds is exposed to view on the beach. The thickness of the beds alluded to, betweendandg, is about 2,500 feet, the erect trees consisting chiefly of largeSigillariæ, occurring at ten distinct levels, one above the other; but Mr. Logan, who afterwards made a more detailed survey of the same line of cliffs, found erect trees at seventeen levels, extending through a vertical thickness of 4,515 feet of strata; and he estimated the total thickness of the carboniferous formation, with and without coal, at no less than 14,570 feet, every where devoid of marine organic remains.[322-A]The usual height of the buried trees seen by me was from 6 to 8 feet; but one trunk was about 25 feet high and 4 feet in diameter, with a considerable bulge at the base. In no instance could I detect any trunk intersecting a layer of coal, however thin; and most of the trees terminated downwards in seams of coal. Some few only were based in clay and shale, none of them in sandstone. The erect trees, therefore, appeared in general to have grown on beds of coal. In some of the underclays I observedStigmaria.
Fig. 373.Fossil tree at right angles to planes of stratification. Coal measures, Nova Scotia.
Fig. 373.
Fossil tree at right angles to planes of stratification. Coal measures, Nova Scotia.
In regard to the plants, they belonged to the same genera, and most of them to the same species, as those met with in the distant coal-fields of Europe. In the sandstone, which filled their interiors, I frequently observed fern leaves, and sometimes fragments ofStigmaria, which had evidently entered together with sediment after the trunk had decayed and become hollow, and while it was still standing under water. Thus the tree,a b,fig. 373., the same which is represented ata,fig. 374., or in the bedein the larger section,fig. 372., is a hollow trunk 5 feet 8 inches in length, traversing various strata, and cut off at the top by a layer of clay 2 feet thickon which rests a seam of coal (b,fig. 374.) 1 foot thick. On this coal again stood two large trees (candd), while at a greater height the treesfandgrest upon a thin seam of coal (e), and above them is an underclay, supporting the 4-foot coal.
Fig. 374.Erect fossiltrees. Coal-measures,Nova Scotia.
Fig. 374.
Erect fossiltrees. Coal-measures,Nova Scotia.
If we now return to the tree first mentioned (fig. 373.), we find the diameter (a b) 14 inches at the top and 16 inches at the bottom, the length of the trunk 5 feet 8 inches. The strata in the interior consisted of a series entirely different from those on the outside. The lowest of the three outer beds which it traversed consisted of purplish and blue shale (c,fig. 373.), 2 feet thick, above which was sandstone (d) 1 foot thick, and, above this, clay (e) 2 feet 8 inches. But, in the interior, were nine distinct layers of different composition: at the bottom, first, shale 4 inches, then sandstone 1 foot, then shale 4 inches, then sandstone 4 inches, then shale 11 inches, then clay (f) with nodules of ironstone 2 inches, then pure clay 2 feet, then sandstone 3 inches, and, lastly, clay 4 inches. Owing to the outward slope of the face of the cliff, the section (fig. 373.) was not exactly perpendicular to the axis of the tree; and hence, probably, the apparent sudden termination at the base without a stump and roots.
In this example the layers of matter in the inside of the tree are more numerous than those without; but it is more common in the coal-measures of all countries to find a cylinder of pure sandstone,—the cast of the interior of a tree, intersecting a great many alternating beds of shale and sandstone, which originally enveloped the trunk as it stood erect in the water. Such a want of correspondence in the materials outside and inside, is just what we might expect if we reflect on the difference of time at which the deposition of sediment will take place in the two cases; the imbedding of the tree having gone on for many years before its decay had made much progress.
The high tides of the Bay of Fundy, rising more than 60 feet, are so destructive as to undermine and sweep away continually the whole face of the cliffs, and thus a new crop of erect trees is brought into view every three or four years. They are known to extend over a space between two and three miles from north to south, and more than twice that distance from east to west, being seen in the banks of streams intersecting the coal-field.
In Cape Breton, Mr. Richard Brown has observed in the Sydney coal-field a total thickness of coal-measures, without including the underlying millstone grit, of 1843 feet, dipping at an angle of 8°. He has published minute details of the whole series, showing at how many different levels erect trees occur, consisting ofSigillaria,Lepidodendron,Calamite, and other genera. In one place eight erect trunks, with roots and rootlets attached to them, were seen at the same level, within a horizontal space 80 feet in length. Beds of coal of various thickness are interstratified. Some of the associated strata are ripple-marked, with impressions of rain-drops. Taking into account forty-one clays filled with roots ofStigmariain their natural position, and eighteen layers of upright trees at other levels, there is, on the whole, clear evidence of at least fifty-nine fossil forests, ranged one above the other, in this coal-field, in the above-mentioned thickness of strata.[324-A]
The fossil shells in Cape Breton and in the Nova Scotia section (fig. 372.), consisting ofCypris,Unio(?),Modiola,Microconchus carbonarius(seefig. 375.), andSpirorbis, seem to indicate brackish water; but we ought never to be surprised if, in pursuing the same stratum, we come to a fresh or purely marine deposit; for this will depend upon our taking a direction higher up or lower down the ancient river or delta deposit. When the Purbeck beds of the Wealden were described in Chap. XVIII., I endeavoured to explain the intimate connection of strata formed at a river's mouth, or in the tranquil lagoons of the delta, or in the sea, after a slight submergence of the land, with its dirt-beds.
In the English coal-fields the same association of fresh, or rather brackish water with marine strata, in close connection with beds of coal of terrestrial origin, has been frequently recognized. Thus, for example, a deposit near Shrewsbury, probably formed in brackish water, has been described by Sir R. Murchison as the youngest member of the carboniferous series of that district, at the point where the coal-measures are in contact with the Permian or "Lower New Red." It consists of shales and sandstones about 150 feet thick, with coal and traces of plants; including a bed of limestone, varying from 2 to 9 feet in thickness, which is cellular, and resembles some lacustrine limestones of France and Germany. It has been traced for 30 miles in a straight line, and can be recognized at still more distant points. The characteristic fossils are a small bivalve, having the form of aCyclas, a smallCypris(fig. 376.), and the microscopic shell of an annelid of an extinct genus calledMicroconchus(fig. 375.), allied toSerpulaorSpirorbis.
In the lower coal-measures of Coalbrook Dale, the strata, according to Mr. Prestwich, often change completely within very short distances, beds of sandstone passing horizontally into clay, and clay into sandstone. The coal-seams often wedge out or disappear; and sections, at places nearly contiguous, present marked lithological distinctions. In this single field, in which the strata are from 700 to 800 feetthick, between forty and fifty species of terrestrial plants have been discovered, besides several fishes and trilobites of forms distinct from those occurring in the Silurian strata. Also upwards of forty species of mollusca, among which are two or three referred to the freshwater genusUnio, and others of marine forms, such asNautilus,Orthoceras,Spirifer, andProductus. Mr. Prestwich suggests that the intermixture of beds containing freshwater shells with others full of marine remains, and the alternation of coarse sandstone and conglomerate with beds of fine clay or shale containing the remains of plants, may be explained by supposing the deposit of Coalbrook Dale to have originated in a bay of the sea or estuary into which flowed a considerable river subject to occasional freshes.[325-A]
Freshwater Fossils—Coal.Fig. 375.a.Microconchus carbonarius.b.var. of same; nat. size, and magnified.Fig. 376.Cypris inflata, natural size, and magnified. Murchison.[325-B]
Freshwater Fossils—Coal.
Fig. 375.
Fig. 376.Cypris inflata, natural size, and magnified. Murchison.[325-B]
In the Edinburgh coal-field, at Burdiehouse, fossil fishes, mollusca, and cypris, very similar to those in Shropshire and Staffordshire, have been found by Dr. Hibbert.[325-C]In the coal-field also of Yorkshire there are freshwater strata, some of which contain shells referred to the genusUnio; but in the midst of the series there is one thin but very widely spread stratum, abounding in fishes and marine shells, such asAmmonites Listeri(fig. 377.),Orthoceras, andAvicula papyracea, Goldf. (fig. 378.)[325-D]
Fig. 377.Ammonites Listeri, Sow.
Fig. 377.
Ammonites Listeri, Sow.
Fig. 378.Avicula papyracea, Goldf. (Pecten papyraceus, Sow.)
Fig. 378.
Avicula papyracea, Goldf. (Pecten papyraceus, Sow.)
No similarly intercalated layer of marine shells has been noticed in the neighbouring coal-field of Newcastle, where, as in SouthWales and Somersetshire, the marine deposits are entirely below those containing terrestrial and freshwater remains.[326-A]
Clay-iron-stone.—Bands and nodules of clay-iron-stone are common in coal-measures, and are formed, says Sir H. De la Beche, of carbonate of iron, mingled mechanically with earthy matter, like that constituting the shales. Mr. Hunt, of the Museum of Practical Geology, instituted a series of experiments to illustrate the production of this substance, and found that decomposing vegetable matter, such as would be distributed through all coal strata, prevented the farther oxidation of the proto-salts of iron, and converted the peroxide into protoxide by taking a portion of its oxygen to form carbonic acid. Such carbonic acid, meeting with the protoxide of iron in solution, would unite with it and form a carbonate of iron; and this mingling with fine mud, when the excess of carbonic acid was removed, might form beds or nodules of argillaceous iron-stone.[326-B]
Coal-fields of the United States — Section of the country between the Atlantic and Mississippi — Position of land in the carboniferous period eastward of the Alleghanies — Mechanically formed rocks thinning out westward, and limestones thickening — Uniting of many coal-seams into one thick one — Horizontal coal at Brownsville, Pennsylvania — Vast extent and continuity of single seams of coal — Ancient river-channel in Forest of Dean coal-field — Absence of earthy matter in coal — Climate of carboniferous period — Insects in coal — Rarity of air-breathing animals — Great number of fossil fish — First discovery of the skeletons of fossil reptiles — Footprints of reptilians — Mountain limestone — Its corals and marine shells.
Coal-fields of the United States — Section of the country between the Atlantic and Mississippi — Position of land in the carboniferous period eastward of the Alleghanies — Mechanically formed rocks thinning out westward, and limestones thickening — Uniting of many coal-seams into one thick one — Horizontal coal at Brownsville, Pennsylvania — Vast extent and continuity of single seams of coal — Ancient river-channel in Forest of Dean coal-field — Absence of earthy matter in coal — Climate of carboniferous period — Insects in coal — Rarity of air-breathing animals — Great number of fossil fish — First discovery of the skeletons of fossil reptiles — Footprints of reptilians — Mountain limestone — Its corals and marine shells.
Itwas stated in the last chapter that a great uniformity prevails in the fossil plants of the coal-measures of Europe and North America; and I may add that four-fifths of those collected in Nova Scotia have been identified with European species. Hence the former existence at the remote period under consideration (the carboniferous) of a continent or chain of islands where the Atlantic now rolls its waves seems a fair inference. Nor are there wanting other and independent proofs of such an ancient land situated to the eastward of the present Atlantic coast of North America; for the geologist deduces the same conclusion from the mineral composition of the carboniferous and some older groups of rocks as they are developed on the eastern flanks of the Alleghanies, contrasted with their character in the low country to the westward of those mountains.
The annexed diagram (fig. 379.) will assist the reader in understandingthe phenomena now alluded to, although I must guard him against supposing that it is a true section. A great number of details have of necessity been omitted, and the scale of heights and horizontal distances are unavoidably falsified.
Fig. 379.Diagram explanatory of the geological structure of a part of the United States between the Atlantic and the Mississippi.Length from E. to W. 850 miles.A B. Atlantic plain.B C. Atlantic slope.C D. Alleghanies or Appalachian chain.D E. Appalachian coal-field west of the mountains.E F. Dome-shaped outcrop of strata on the Ohio, older than the coal.F G. Illinois coal-field.h.Falls and rapids of the rivers at the junction of the hypogene and newer formations.i,k,l,m. Parallel folds of Appalachians becoming successively more open, and flatter in going from E. to W.References to the different Formations.1. Miocene tertiary.2. Eocene tertiary.3. Cretaceous strata.4. Red sandstone with ornithichnites (new red or trias?) usually much invaded by trap.5. Coal-measures (bituminous coal).5' Anthracitic coal-measures.5'' Carboniferous limestone of the Illinois coal-field, wanting in the Appalachian.6. Old red or Devonian, Olive slate, &c.7. Primary fossiliferous or Silurian strata.8. Hypogene strata, or gneiss, mica schist, &c., with granite veins.Note.The dotted lines atiandkexpress portions of rock removed by denudation, the amount of which may be estimated by supposing similar lines prolonged from other points where different strata end abruptly at the surface.N.B.The lower section at ** joins on to the upper one at *.
Fig. 379.Diagram explanatory of the geological structure of a part of the United States between the Atlantic and the Mississippi.
Length from E. to W. 850 miles.
References to the different Formations.
Note.The dotted lines atiandkexpress portions of rock removed by denudation, the amount of which may be estimated by supposing similar lines prolonged from other points where different strata end abruptly at the surface.
N.B.The lower section at ** joins on to the upper one at *.
Starting from the shores of the Atlantic, on the eastern side of the Continent, we first come to a low region (A B), which was called the alluvial plain by the first geographers. It is occupied by tertiary and cretaceous strata, before described (pp.171.206.and224.), which are nearly horizontal. The next belt, fromBtoC, consists of granitic rocks (hypogene), chiefly gneiss and mica-schist, covered occasionally with unconformable red sandstone, No. 4. (New Red or Trias?), remarkable for its ornithichnites (seep. 327.). Sometimes, also, this sandstone rests on the edges of the disturbed paleozoic rocks (as seen in the section). The region (B C), sometimes called the "Atlantic Slope," corresponds nearly in average width with the low and flat plain (A,B), and is characterized by hills of moderate height, contrasting strongly, in their rounded shape and altitude, with the long, steep, and lofty parallel ridges of the Alleghany mountains. The outcrop of the strata in these ridges, like the two belts of hypogene and newer rocks (A B, andB C), above alluded to, when laid down on a geological map, exhibit long stripes of different colours, running in a N.E. and S.W. direction, in the same way as the lias, chalk, and other secondary formations in the middle and eastern half of England.
The narrow and parallel zones of the Appalachians here mentioned, consist of strata, folded into a succession of convex and concave flexures, subsequently laid open by denudation. The component rocks are of great thickness, all referable to the Silurian, Devonian, and Carboniferous formations. There is no principal or central axis, as in the Pyrenees and many other chains—no nucleus to which all the minor ridges conform; but the chain consists of many nearly equal and parallel foldings, having what is termed an anticlinal and synclinal arrangement (see above,p. 48.). This system of hills extends, geologically considered, from Vermont to Alabama, being more than 1000 miles long, from 50 to 150 miles broad, and varying in height from 2000 to 6000 feet. Sometimes the whole assemblage of ridges runs perfectly straight for a distance of more than 50 miles, after which all of them wheel round together, and take a new direction, at an angle of 20 or 30 degrees to the first.
We are indebted to the state surveyors of Virginia and Pennsylvania, Prof. W. B. Rogers and his brother Prof. H. D. Rogers, for the important discovery of a clue to the general law of structure prevailing throughout this range of mountains, which, however simple it may appear when once made out and clearly explained, might long have been overlooked; amidst so great a mass of complicated details. It appears that the bending and fracture of the beds is greatest on the south-eastern or Atlantic side of the chain, and the strata become less and less disturbed as we go westward, until at length they regain their original or horizontal position. By reference to the section (fig. 379.), it will be seen that on the eastern side, or in the ridgesand troughs nearest the Atlantic, south-eastern dips predominate, in consequence of the beds having been folded back upon themselves, as ini, those on the north-western side of each arch having been inverted. The next set of arches (such ask) are more open, each having its western side steepest; the next (l) opens out still more widely, the next (m) still more, and this continues until we arrive at the low and level part of the Appalachian coal-field (D E).
In nature or in a true section, the number of bendings or parallel folds is so much greater that they could not be expressed in a diagram without confusion. It is also clear that large quantities of rock have been removed by aqueous action or denudation, as will appear if we attempt to complete all the curves in the manner indicated by the dotted lines atiandk.
The movements which imparted so uniform an order of arrangement to this vast system of rocks must have been, if not contemporaneous, at least parts of one and the same series, depending on some common cause. Their geological date is well defined, at least within certain limits, for they must have taken place after the deposition of the carboniferous strata (No. 5.), and before the formation of the red sandstone (No. 4.). The greatest disturbing and denuding forces have evidently been exerted on the south-eastern side of the chain; and it is here that igneous or plutonic rocks are observed to have invaded the strata, forming dykes, some of which run for miles in lines parallel to the main direction of the Appalachians, or N.N.E. and S.S.W.
The thickness of the carboniferous rocks in the regionCis very great, and diminishes rapidly as we proceed to the westward. The surveys of Pennsylvania and Virginia show that the south-east was the quarter whence the coarser materials of these strata were derived, so that the ancient land lay in that direction. The conglomerate which forms the general base of the coal-measures is 1500 feet thick in the Sharp Mountain, where I saw it (atC) near Pottsville; whereas it has only a thickness of 500 feet, about thirty miles to the north-west, and dwindles gradually away when followed still farther in the same direction, till its thickness is reduced to 30 feet.[329-A]The limestones, on the other hand, of the coal-measures, augment as we trace them westward. Similar observations have been made in regard to the Silurian and Devonian formations in New York; the sandstones and all the mechanically-formed rocks thinning out as they go westward, and the limestones thickening, as it were, at their expense. It is, therefore, clear that the ancient land was to the east, where the Atlantic now is; the deep sea, with its banks of coral and shells to the west, or where the hydrographical basin of the Mississippi is now situated.
In that region, near Pottsville, where the thickness of the coal-measures is greatest, there are thirteen seams of anthracitic coal, several of them more than 2 yards thick. Some of the lowest of these alternate with beds of white grit and conglomerate of coarser grainthan I ever saw elsewhere, associated with pure coal. The pebbles of quartz are often of the size of a hen's egg. On following these pudding-stones and grits for several miles from Pottsville, by Tamaqua, to the Lehigh Summit Mine, in company with Mr. H. D. Rogers, in 1841, he pointed out to me that the coarse-grained strata and their accompanying shales gradually thin out, until seven seams of coal, at first widely separated, are brought nearer and nearer together, until they successively unite; so that at last they form one mass, between 40 and 50 feet thick. I saw this enormous bed of anthracitic coal quarried in the open air at Mauch Chunk (or the Bear Mountain), the overlying sandstone, 40 feet thick, having been removed bodily from the top of the hill, which, to use the miner's expression, had been "scalped." The accumulation of vegetable matter now constituting this vast bed of anthracite, may perhaps, before it was condensed by pressure and the discharge of its hydrogen, oxygen, and other volatile ingredients, have been between 200 and 300 feet thick. The origin of such a vast thickness of vegetable remains, so unmixed with earthy ingredients, can, I think, be accounted for in no other way, than by the growth, during thousands of years, of trees and ferns, in the manner of peat,—a theory which the presence of the Stigmariain situunder each of the seven layers of anthracite, fully bears out. The rival hypothesis, of the drifting of plants into a sea or estuary, leaves the absence of sediment, or, in this case, of sand and pebbles, wholly unexplained.
Fig. 380.
Fig. 380.
Fig. 381.
Fig. 381.
But the student will naturally ask, what can have caused so many seams of coal, after they had been persistent for miles, to come together and blend into one single seam, and that one equal in the aggregate to the thickness of the several separate seams? Often had the same question been put by English miners before a satisfactory answer was given to it by the late Mr. Bowman. The following is his solution of the problem. Leta a',fig. 380., be a mass of vegetable matter, capable, when condensed, of forming a 3-foot seam of coal. It rests on the underclayb b', filled with roots of treesin situ, and it supports a growing forest (C D). Suppose that part of the same forestD Ehad become submerged by the ground sinking down 25 feet, so that the trees have been partly thrown down andpartly remain erect in water, slowly decaying, their stumps and the lower parts of their trunks being enveloped in layers of sand and mud, which are gradually filling up the lakeD F. When this lake or lagoon has at length been entirely silted up and converted into land, say, in the course of a century, the forestC Dwill extend once more continuously over the whole areaC F, as infig. 381., and another mass of vegetable matter (g g'), forming 3 feet more of coal, may accumulate fromCtoF. We then find in the regionF, two seams of coal (a'andg') each 3 feet thick, and separated by 25 feet of sandstone and shale, with erect trees based upon the lower coal, while, betweenDandC, we find these two seams united into a 2-yard coal. It may be objected that the uninterrupted growth of plants during the interval of a century will have caused the vegetable matter in the regionC Dto be thicker than the two distinct seamsa'andg'atF; and no doubt there would actually be a slight excess representing one generation of trees with the remains of other plants, forming half an inch or an inch of coal; but this would not prevent the miner from affirming that the seama g, throughout the areaC D, was equal to the two seamsa'andg'atF.
The reader has seen, by reference to the section (fig. 379.p. 327.), that the strata of the Appalachian coal-field assume an horizontal position west of the mountains. In that less elevated country, the coal-measures are intersected by three great navigable rivers, and are capable of supplying for ages, to the inhabitants of a densely peopled region, an inexhaustible supply of fuel. These rivers are the Monongahela, the Alleghany, and the Ohio, all of which lay open on their banks the level seams of coal. Looking down the first of these at Brownsville, we have a fine view of the main seam of bituminous coal 10 feet thick, commonly called the Pittsburg seam, breaking out in the steep cliff at the water's edge; and I made the accompanying sketch of its appearance from the bridge over the river (seefig. 382.). Here the coal, 10 feet thick, is covered by carbonaceous shale (b), and this again by micaceous sandstone (c). Horizontal galleries may be driven everywhere at very slight expense, and so worked as to drain themselves, while the cars, laden with coal and attached to each other, glide down on a railway, so as to deliver their burden into barges moored to the river's bank. The same seam is seen at a distance, on the right bank (ata), and may be followed the whole way to Pittsburg, fifty miles distant. As it is nearly horizontal, while the river descends it crops out at a continually increasing, but never at an inconvenient, height above the Monongahela. Below the great bed of coal at Brownsville is a fire-clay 18 inches thick, and below this, several beds of limestone, below which again are other coal seams. I have also shown in my sketch another layer of workable coal (atd d), which breaks out on the slope of the hills at a greater height. Here almost every proprietor can open a coal-pit on his own land, and the stratification being very regular, he may calculate with precision the depth at which coal may be won.
The Appalachian coal-field, of which these strata form a part(fromCtoE, section,fig. 379.,p. 327.), is remarkable for its vast area; for, according to Professor H. D. Rogers, it stretches continuously from N.E. to S.W., for a distance of 720 miles, its greatest width being about 180 miles. On a moderate estimate, its superficial area amounts to 63,000 square miles.
Fig. 382.View of the great Coal Seam on the Monongahela at Brownsville, Pennsylvania, U. S.a.Ten-foot seam of coal.b.Black bituminous or carbonaceous shale, 10 feet thick.c.Micaceous sandstone.d d.Upper seam of coal, 6 feet thick.
Fig. 382.
View of the great Coal Seam on the Monongahela at Brownsville, Pennsylvania, U. S.
This coal formation, before its original limits were reduced bydenudation, must have measured 900 miles in length, and in some places more than 200 miles in breadth. By again referring to the section (fig. 379.,p. 327.), it will be seen that the strata of coal are horizontal to the westward of the mountains in the regionD E, and become more and more inclined and folded as we proceed eastward. Now it is invariably found, as Professor H. D. Rogers has shown by chemical analysis, that the coal is most bituminous towards its western limit, where it remains level and unbroken, and that it becomes progressively debituminized as we travel south-eastward towards the more bent and distorted rocks. Thus, on the Ohio, the proportion of hydrogen, oxygen, and other volatile matters, ranges from forty to fifty per cent. Eastward of this line, on the Monongahela, it still approaches forty per cent., where the strata begin to experience some gentle flexures. On entering the Alleghany Mountains, where the distinct anticlinal axes begin to show themselves, but before the dislocations are considerable, the volatile matter is generally in the proportion of eighteen or twenty per cent. At length, when we arrive at some insulated coal-fields (5',fig. 379.) associated with the boldest flexures of the Appalachian chain, where the strata have been actually turned over, as near Pottsville, we find the coal to contain only from six to twelve per cent. of bitumen, thus becoming a genuine anthracite.[333-A]
It appears from the researches of Liebig and other eminent chemists, that when wood and vegetable matter are buried in the earth, exposed to moisture, and partially or entirely excluded from the air, they decompose slowly and evolve carbonic acid gas, thus parting with a portion of their original oxygen. By this means, they become gradually converted into lignite or wood-coal, which contains a larger proportion of hydrogen than wood does. A continuance of decomposition changes this lignite into common or bituminous coal, chiefly by the discharge of carburetted hydrogen, or the gas by which we illuminate our streets and houses. According to Bischoff, the inflammable gases which are always escaping from mineral coal, and are so often the cause of fatal accidents in mines, always contain carbonic acid, carburetted hydrogen, nitrogen, and olefiant gas. The disengagement of all these gradually transforms ordinary or bituminous coal into anthracite, to which the various names of splint coal, glance coal, culm, and many others, have been given.
We have seen that, in the Appalachian coal-field, there is an intimate connection between the extent to which the coal has parted with its gaseous contents, and the amount of disturbance which the strata have undergone. The coincidence of these phenomena may be attributed partly to the greater facility afforded for the escape of volatile matter, where the fracturing of the rocks had produced an infinite number of cracks and crevices, and also to the heat of the gases and water penetrating these cracks, when the great movements took place, which have rent and folded the Appalachian strata. Itis well known that, at the present period, thermal waters and hot vapours burst out from the earth during earthquakes, and these would not fail to promote the disengagement of volatile matter from the carboniferous rocks.
Continuity of seams of coal.—As single seams of coal are continuous over very wide areas, it has been asked, how forests could have prevailed uninterruptedly over such wide spaces, without being oftener flooded by turbid rivers, or, when submerged, denuded by marine currents. It appears, from the description of the Cape Breton coal-field, by Mr. Richard Brown, that false stratification is common in the beds of sand, and some partial denudation of these, at least, must often have taken place during the accumulation of the carboniferous series.
In the Forest of Dean, ancient river-channels are found, which pass through beds of coal, and in which rounded pebbles of coal occur. They are of older date than the overlying and undisturbed coal-measures. The late Mr. Buddle, who described them to me, told me he had seen similar phenomena in the Newcastle coal-field. Nevertheless, instances of these channels are much more rare than we might have anticipated, especially when we remember how often the roots of trees (Stigmariæ) have been torn up, and drifted in broken fragments into the grits and sandstones. The prevalence of a downward movement is, no doubt, the principal cause which has saved so many extensive seams of coal from destruction by fluviatile action.
The purity of the coal, or its non-intermixture with earthy matter, presents another theoretical difficulty to many geologists, who are inclined to believe that the trees and smaller plants of the carboniferous period grew in extensive swamps, rather than on land not liable to be inundated. It appears, however, that in the alluvial plain and delta of the Mississippi, extensive "cypress swamps," as they are called, densely covered with various trees, occur, into which no matter held in mechanical suspension is ever introduced during the greatest inundations, inasmuch as they are all surrounded by a dense marginal belt of reeds, canes, and brushwood. Through this thick barrier the river-water must pass, so that it is invariably well filtered before it can reach the interior of the forest-covered area, within which, vegetable matter is continually accumulating from the decay of trees and semi-aquatic plants. In proof of this, I may observe, that whenever any part of a swamp is dried up, during an unusually hot season, and the wood set on fire, pits are burnt into the ground many feet deep, or as far down as the fire can descend without meeting with water, and it is then found that scarcely any residuum or earthy matter is left.[334-A]At the bottom of these "cypress swamps" of the Mississippi, a bed of clay is found, with roots of the tall cypress (Taxodium distichum), just as the underclays of the coal are filled withStigmaria.
Climate of Coal Period.—So long as the botanist taught that a tropical climate was implied by the carboniferous flora, geologists might well be at a loss to reconcile the preservation of so much vegetable matter with a high temperature; for heat hastens the decomposition of fallen leaves and trunks of trees, whether in the atmosphere or in water.[335-A]It is well known that peat, so abundant in the bogs of high latitudes, ceases to grow in the swamps of warmer regions. It seems, however, to have become a more and more received opinion, that the coal-plants do not, on the whole, indicate a climate resembling that now enjoyed in the equatorial zone. Tree-ferns range as far south as the southern part of New Zealand, and Araucarian pines occur in Norfolk Island. A great predominance of ferns and lycopodiums indicates warmth, moisture, equability of temperature, and freedom from frost, rather than intense heat; and we know too little of the sigillariæ, calamites, asterophyllites, and other peculiar forms of the carboniferous period, to be able to speculate with confidence on the kind of climate they may have required.
No doubt, we are entitled to presume, from the corals and cephalopoda of the mountain limestone, that a warm temperature characterized the northern seas in the carboniferous era; but the absence of cold may have given rise (as at present in the seas of the Bermudas, under the influence of the gulf stream) to a very wide geographical range of stone-building corals and shell-bearing cuttle-fish, without its being necessary to call in the aid of tropical heat.[335-B]
Where we have evidence in a single coal-field, as in that of Nova Scotia, or South Wales, of fifty or even a hundred ancient forests buried one above the other, with the roots of trees still in their original position, and with some of the trunks still remaining erect, we are apt to wonder that until the year 1844 no remains of contemporaneous air-breathing creatures, except a few insects, had been discovered. No vertebrated animals more highly organized than fish, no mammalia or birds, no saurians, frogs, tortoises, or snakes, were yet known in rocks of such high antiquity. In the coal-field of Coalbrook Dale mention had been made of two species of beetles of the familyCurculionidæ, and of a neuropterous insect resembling the genusCorydalis, with another related to thePhasmidæ.[335-C]In other coal-measures in Europe we find notice of a scorpion and of a moth allied toTinea, also of one air-breathing crustacean, or land-crab. Yet Agassiz had already described in his great work on fossil fishes more than one hundred and fifty species of ichthyolites from the coal strata, ninety-four belonging to the families of shark and ray, and fifty-eight to the class of ganoids. Some of these fish are very remote in their organization from anynow living, especially those of the family calledSauroidby Agassiz; asMegalichthys,Holoptychius, and others, which are often of great size, and all predaceous. Their osteology, says M. Agassiz, reminds us in many respects of the skeletons of saurian reptiles, both by the close sutures of the bones of the skull, their large conical teeth striated longitudinally (seefig. 383.), the articulations of the spinous processes with the vertebræ, and other characters. Yet they do not form a family intermediate between fish and reptiles, but are truefish, though doubtless more highly organized than any living fish.[336-A]