XXIV. THE COAL, OR CARBONIFEROUS GROUP…

XXIV. THE COAL, OR CARBONIFEROUS GROUP

Carboniferous strata in the south-west of England--Superposition of Coal-measures to Mountain limestone--Departure from this type in North of England and Scotland--Section in South Wales--Underclays with Stigmaria--Carboniferous Flora--Ferns, Lepidodendra, Calamites, Asterophyllites, Sigillariae, Stigmariae--Coniferae--Endogens--Absence of Exogens--Coal, how formed--Erect fossil trees--Parkfield Colliery--St. Etienne, Coal-field--Oblique trees or snags--Fossil forests in Nova Scotia--Brackish water and marine strata--Origin of Clay-iron-stone.

The next group which we meet with in the descending order is the Carboniferous, commonly called "The Coal;" because it contains many beds of that mineral, in a more or less pure state, interstratified with sandstones, shales, and limestones. The coal itself, even in Great Britain and Belgium, where it is most abundant, constitutes but an insignificant portion of the whole mass. In the north of England, for example, the thickness of the coal-bearing strata has been estimated at 3000 feet, while the various coal-seams, 20 or 30 in number, do not in the aggregate exceed 60 feet.[308-A]

The carboniferous formation comprises two very distinct members: 1st, that usually called the Coal-measures, of mixed freshwater, terrestrial, and marine origin, often including seams of coal; 2dly, that named in England the Mountain or Carboniferous limestone, of purely marine origin, and containing corals, shells, and encrinites.

In the south-western part of our island, in Somersetshire and South Wales, the three divisions usually spoken of by English geologists are:

1. Coal-measures { Strata of shale, sandstone, and grit, with { occasional seams of coal, from 600 to 12,000 { feet thick.

2. Millstone grit { A coarse quartzose sandstone passing into a { conglomerate, sometimes used for millstones, with { beds of shale; usually devoid of coal; { occasionally above 600 feet thick.

3. Mountain or } A calcareous rock containing marine shells and Carboniferous } corals; devoid of coal; thickness variable, limestone } sometimes 900 feet.

The millstone grit may be considered as one of the coal sandstones of coarser texture than usual, with some accompanying shales, in which coal plants are occasionally found. In the north of England some bands of limestone, with pectens, oysters, and other marine shells, occur in this grit, just as in the regular coal-measures, and even a few seams of coal. I shall treat, therefore, of the whole group, as consisting of two divisions only, the Coal-measures and Mountain Limestone. The latter is found in the southern British coal-fields, at the base of the system, or immediately in contact with the subjacent Old Red Sandstone; but as we proceed northwards to Yorkshire and Northumberland it begins to alternate with true coal-measures, the two deposits forming together a series of strata about 1000 feet in thickness. To this mixed formation succeeds the great mass of genuine mountain limestone.[309-A] Farther north, in the Fifeshire coal-field in Scotland, we observe a still wider departure from the type of the south of England, or a more complete intercalation of dense masses of marine limestones with sandstones, and shales containing coal.

COAL-MEASURES.

In South Wales the coal-measures have been ascertained by actual measurement to attain the extraordinary thickness of 12,000 feet, the beds throughout, with the exception of the coal itself, appearing to have been formed in water of moderate depth, during a slow but perhaps intermittent depression of the ground, in a region to which rivers were bringing a never-failing supply of muddy sediment and sand. The same area was sometimes covered with vast forests, such as we see in the deltas of great rivers in warm climates, which are liable to be submerged beneath fresh or salt water should the ground sink vertically a few feet.

In one section near Swansea, in South Wales, where the total thickness of strata is 3246 feet, we learn from Sir H. De la Beche that there are ten principal masses of sandstone. One of these is 500 feet thick, and the whole of them make together a thickness of 2125 feet. They are separated by masses of shale, varying in thickness from 10 to 50 feet. The intercalated coal-beds, sixteen in number, are generally from 1 to 5 feet thick, one of them, which has two or three layers of clay interposed, attaining 9 feet.[309-B] At other points in the same coal-field the shales predominate over the sandstones. The horizontal extent of some seams of coal is much greater than that of others, but they all present one characteristic feature, in having, each of them, what is called its underclay. These underclays, co-extensive with every layer of coal, consist of arenaceous shale, sometimes called firestone, because it can be made into bricks which stand the fire of a furnace. They vary in thickness from 6 inches to more than 10 feet; and Mr. Logan first announced to the scientific world in 1841 that they were regarded by the colliers in South Wales as an essential accompaniment of each of the one hundred seams of coal met with in their coal-field. They are said to form the floor on which the coal rests; and some of them have a slight admixture of carbonaceous matter, while others are quite blackened by it.

All of them, as Mr. Logan pointed out, are characterized by inclosing a peculiar species of fossil vegetable called Stigmaria, to the exclusion of other plants. It was also observed that, while in the overlying shales or "roof" of the coal, ferns and trunks of trees abound without any Stigmariae, and are flattened and compressed, those singular plants in the underclays always retain their natural forms, branching freely, and sending out their slender leaves, as they were formerly styled, through the mud in all directions. Several species of Stigmaria had long been known to botanists, and described by them, before their position under each seam of coal was pointed out. It was conjectured that they might be aquatic, perhaps floating plants, which sometimes extended their branches and leaves freely in fluid mud, and which were finally enveloped in the same mud.

CARBONIFEROUS FLORA.

These statements will suffice to convince the reader that we cannot arrive at a satisfactory theory of the origin of coal till we understand the true nature of Stigmaria; and in order to explain what is now known of this plant, and of others which have contributed by their decay to produce coal, it will be necessary to offer a brief preliminary sketch of the whole carboniferous flora, an assemblage of fossil plants, with which we are better acquainted than with any other which flourished antecedently to the tertiary epoch. It should also be remarked that Goeppert has ascertained that the remains of every family of plants scattered through the coal-measures are sometimes met with in the pure coal itself, a fact which adds greatly to the geological interest attached to this flora.

Ferns.--The number of species of carboniferous plants hitherto described amounts, according to M. Ad. Brongniart, to about 500. These may perhaps be a fragment only of the entire flora, but they are enough to show that the state of the vegetable world was then extremely different from that now established. We are struck at the first glance with the similarity of many of the ferns to those now living, and the dissimilarity of almost all the other fossils except the coniferae. Among the ferns, as in the case of Pecopteris for example (fig. 351.), it is not always easy to decide whether they should be referred to different genera from those established for the classification of living species; whereas, in regard to most of the other contemporary tribes, with the exception of the coniferae, it is often difficult to guess the family, or even the class, to which they belong. The ferns of the carboniferous period are generally without organs of fructification, but in some specimens these are well preserved. In the general absence of such characters, they have been divided into genera, distinguished chiefly by the branching of the fronds, and the way in which the veins of the leaves are disposed. The larger portion are supposed to have been of the size of ordinary European ferns, but some were decidedly arborescent, especially the group called Caulopteris, by Lindley, and the Psaronius of the upper or newest coal-measures, before alluded to (p. 307.).

All the recent tree-ferns belong to one tribe (Polypodiaceae), and to a small number only of genera in that tribe, in which the surface of the trunk is marked with scars, or cicatrices, left after the fall of the fronds. These scars resemble those of Caulopteris (see fig. 353.). No less than 250 ferns have already been obtained from the coal strata; and even if we make some reduction on the ground of varieties which have been mistaken, in the absence of their fructification, for species, still the result is singular, because the whole of Europe affords at present no more than 50 indigenous species.

[3 Illustrations: Living tree-ferns of different genera. (Ad. Brong.)

Fig. 354. Tree-fern from Isle of Bourbon.

Fig. 355. Cyathea glauca, Mauritius.

Fig. 356. Tree fern from Brazil.]

[3 Illustrations: Lepidodendron Sternbergii. Coal-measures, near Newcastle.

Fig. 357. Branching trunk, 49 feet long, supposed to have belonged to L. Sternbergii. (Foss. Flo. 203.)

Fig. 358. Branching stem with bark and leaves of L. Sternbergii. (Foss. Flo. 4.)

Fig. 359. Portion of same nearer the root; natural size. (Ibid.)]

Lepidodendra.--These fossils belong to the family of Lycopodiums, yet most of them grew to the size of large trees. The annexed figures represent a large fossil Lepidodendron, 49 feet long, found in Jarrow Colliery, near Newcastle, lying in shale parallel to the planes of stratification. Fragments of others, found in the same shale, indicate, by the size of the rhomboidal scars which cover them, a still greater magnitude. The living club-mosses, of which there are about 200 species, are abundant in tropical climates, where one species is sometimes met with attaining a height of 3 feet. They usually creep on the ground, but some stand erect, as the L. densum, from New Zealand (fig. 360.).

In the carboniferous strata of Coalbrook Dale, and in many other coal-fields, elongated cylindrical bodies, called fossil cones, named by M. Adolphe Brongniart Lepidostrobus, are met with. (See fig. 361.) They often form the nucleus of concretionary balls of clay-iron-stone, and are well preserved, exhibiting a conical axis, around which a great quantity of scales were compactly imbricated. The opinion of M. Brongniart is now generally adopted, that the Lepidostrobus is the fruit of Lepidodendron.

Equisetaceae.--To this family belong two species of the genus Equisetites, allied to the living "horse-tail" which now grows in marshy grounds. Other species, which have jointed stems, depart more widely from Equisetum, but are yet of analogous organization. They differed from it principally in being furnished with a thin bark, which is represented in the stem of C. Suckowii (fig. 363.), in which it will be seen that the striped external pattern does not agree with that left on the stone where the bark is stripped off; so that if the two impressions were seen separately, they might be mistaken for two distinct species.

The tallest living "horse-tails" are only 2 or 3 feet high in Europe, and even in tropical climates only attain, as in the case of Equisetum giganteum, discovered by Humboldt and Bonpland, in South America, a height of about 5 feet, the stem being an inch in diameter. Several of the Calamites of the coal acquired the height and dimensions of small trees.

Asterophyllites.--In this family, M. Brongniart includes several genera, and among them Calamodendron, Asterophyllites, and Annularia. The graceful plant, represented in the annexed figure, is supposed to be the branch of a shrub called Calamodendron, a new genus, divided off by Brongniart from the Calamites of former authors. Its pith and medullary rays seem to show that it was dicotyledonous, and it appears to have been allied, by the nature of its tissue, to the gymnogens, or, still more, to the Sigillaria, which will next be mentioned.

Sigillaria.--A large portion of the trees of the carboniferous period belonged to this genus, of which about thirty-five species are known. The structure, both internal and external, was very peculiar, and, with reference to existing types, very anomalous. They were formerly referred, by M. Ad. Brongniart, to ferns, which they resemble in the scalariform texture of their vessels, and, in some degree, in the form of the cicatrices left by the base of the leafstalks which have fallen off (see fig. 365.). But with these points of analogy to cryptogamia, they combine an internal organization much resembling that of cycads, and some of them are ascertained to have had long linear leaves, quite unlike those of ferns. They grew to a great height, from 30 to 60, or even 70 feet, with regular cylindrical stems, and without branches, although some species were dichotomous towards the top. Their fluted trunks, from 1 to 5 feet in diameter, appear to have decayed rapidly in the interior, so as to become hollow, when standing; when, therefore, they were thrown prostrate on the mud, they were squeezed down and flattened. Hence, we find the bark of the two opposite sides (now converted into bright shining coal) to constitute two horizontal layers, one upon the other, half an inch, or an inch, in thickness. These same trunks, when they are placed obliquely or vertically to the planes of stratification, retain their original rounded form, and are uncompressed, the cylinder of bark having been filled with sand, which now affords a cast of the interior.

Stigmaria.--This fossil, the importance of which has already been pointed out, was formerly conjectured to be an aquatic plant. It is now ascertained to be the root of Sigillaria. The connection of the roots with the stem, previously suspected, on botanical grounds, by Brongniart, was first proved, by actual contact, in the Lancashire coal-field, by Mr. Binney. The fact has lately been shown, even more distinctly, by Mr. Richard Brown, in his description of the Stigmariae occurring in the underclays of the coal-seams of the Island of Cape Breton, in Nova Scotia.

In a specimen of one of these, represented in the annexed figure (fig. 366.), the spread of the roots was 16 feet, and some of them sent out rootlets, in all directions, into the surrounding clay.

The manner of attachment of the fibres to the stem resembles that of a ball and socket joint, the base of each rootlet being concave, and fitting on to a tubercle (see figs. 367 and 368.). Rows of these tubercles are arranged spirally round each root, which have always a medullary cavity and woody texture, much resembling that of Sigillaria, the structure of the vessels being, like it, scalariform.

Conifers.--The coniferous trees of this period are referred to five genera; the woody structure of some of them showing that they were allied to the Araucarian division of pines, more than to any of our common European firs. Some of their trunks exceeded 44 feet in height.

Endogens.--Hitherto but few monocotyledonous plants have been discovered in the coal-strata. Most of these consist of fruits referred by some botanists to palms. The three-sided nuts, called Trigonocarpum, seven species of which are known, appear to have the best claim to rank as palms, although M. Ad. Brongniart entertains some doubt even as to their being monocotyledons.

Exogens.

The entire absence, so far as our paleontological investigations have hitherto gone, of ordinary dicotyledons or exogens in the coal measures, is most remarkable. Hence, M. Adolphe Brongniart has called this period the age of acrogens, in consequence of the vast preponderance of ferns and Lepidodendra.[316-A] Nevertheless, a forest of the period, now under consideration, may have borne a considerable resemblance to those woody regions of New Zealand, in which ferns, arborescent and herbaceous, and lycopodiums, with many coniferae, abound.

The comparative proportion of living ferns and Araucariae, in Norfolk Island, to all the other plants, appears to be very similar to that formerly borne by these tribes respectively in a forest of the coal-period.

I have already stated that Professor Goeppert, after examining the fossil vegetables of the coal-fields of Germany, has detected, in beds of pure coal, remains of plants of every family hitherto known to occur fossil in the coal. Many seams, he remarks, are rich in Sigillaria, Lepidodendron, and Stigmaria, the latter in such abundance, as to appear to form the bulk of the coal. In some places, almost all the plants are calamites, in others ferns.[316-B]

Coal, how formed--Erect trees.--I shall now consider the manner in which the above-mentioned plants are imbedded in the strata, and how they may have contributed to produce coal. "Some of the plants of our coal," says Dr. Buckland, "grew on the identical banks of sand, silt, and mud, which, being now indurated to stone and shale, form the strata that accompany the coal; whilst other portions of these plants have been drifted to various distances from the swamps, savannahs, and forests that gave them birth, particularly those that are dispersed through the sandstones, or mixed with fishes in the shale beds." "At Balgray, three miles north of Glasgow," says the same author, "I saw in the year 1824, as there still may be seen, an unequivocal example of the stumps of several stems of large trees, standing close together in their native place, in a quarry of sandstone of the coal formation."[317-A]

Between the years 1837 and 1840, six fossil trees were discovered in the coal-field of Lancashire, where it is intersected by the Bolton railway. They were all in a vertical position, with respect to the plane of the bed, which dips about 15 deg. to the south. The distance between the first and the last was more than 100 feet, and the roots of all were imbedded in a soft argillaceous shale. In the same plane with the roots is a bed of coal, eight or ten inches thick, which has been ascertained to extend across the railway, or to the distance of at least ten yards. Just above the covering of the roots, yet beneath the coal seam, so large a quantity of the Lepidostrobus variabilis was discovered inclosed in nodules of hard clay, that more than a bushel was collected from the small openings around the base of the trees (see figure of this genus, p. 313.). The exterior trunk of each was marked by a coating of friable coal, varying from one quarter to three quarters of an inch in thickness; but it crumbled away on removing the matrix. The dimensions of one of the trees is 15-1/2 feet in circumference at the base, 7-1/2 feet at the top, its height being 11 feet. All the trees have large spreading roots, solid and strong, sometimes branching, and traced to a distance of several feet, and presumed to extend much farther. Mr. Hawkshaw, who has described these fossils, thinks that, although they were hollow when submerged, they may have consisted originally of hard wood throughout; for solid dicotyledonous trees, when prostrated in tropical forests, as in Venezuela, on the shore of the Caribbean Sea, were observed by him to be destroyed in the interior, so that little more is left than an outer shell, consisting chiefly of the bark. This decay, he says, goes on most rapidly in low and flat tracks, in which there is a deep rich soil and excessive moisture, supporting tall forest-trees and large palms, below which bamboos, canes, and minor palms flourish luxuriantly. Such tracts, from their lowness, would be most easily submerged, and their dense vegetation might then give rise to a seam of coal.[317-B]

In a deep valley near Capel-Coelbren, branching from the higher part of the Swansea valley, four stems of upright Sigillariae were seen, in 1838, piercing through the coal-measures of S. Wales; one of them was 2 feet in diameter, and one 13 feet and a half high, and they were all found to terminate downwards in a bed of coal. "They appear," says Sir H. De la Beche, "to have constituted a portion of a subterranean forest at the epoch when the lower carboniferous strata were formed.[318-A]

In a colliery near Newcastle, say the authors of the Fossil Flora, a great number of Sigillariae were placed in the rock as if they had retained the position in which they grew. Not less than thirty, some of them 4 or 5 feet in diameter, were visible within an area of 50 yards square, the interior being sandstone, and the bark having been converted into coal. The roots of one individual were found imbedded in shale; and the trunk, after maintaining a perpendicular course and circular form for the height of about 10 feet, was then bent over so as to become horizontal. Here it was distended laterally, and flattened so as to be only one inch thick, the flutings being comparatively distinct.[318-B] Such vertical stems are familiar to our miners, under the name of coal-pipes. One of them, 72 feet in length, was discovered, in 1829, near Gosforth, about five miles from Newcastle, in coal-grit, the strata of which it penetrated. The exterior of the trunk was marked at intervals with knots, indicating the points at which branches had shot off. The wood of the interior had been converted into carbonate of lime; and its structure was beautifully shown by cutting transverse slices, so thin as to be transparent. (See p. 40.)

These "coal-pipes" are much dreaded by our miners, for almost every year in the Bristol, Newcastle, and other coal-fields, they are the cause of fatal accidents. Each cylindrical cast of a tree, formed of solid sandstone, and increasing gradually in size towards the base, and being without branches, has its whole weight thrown downwards, and receives no support from the coating of friable coal which has replaced the bark. As soon, therefore, as the cohesion of this external layer is overcome, the heavy column falls suddenly in a perpendicular or oblique direction from the roof of the gallery whence coal has been extracted, wounding or killing the workman who stands below. It is strange to reflect how many thousands of these trees fell originally in their native forests in obedience to the law of gravity; and how the few which continued to stand erect, obeying, after myriads of ages, the same force, are cast down to immolate their human victims.

It has been remarked, that if, instead of working in the dark, the miner was accustomed to remove the upper covering of rock from each seam of coal, and to expose to the day the soils on which ancient forests grew, the evidence of their former growth would be obvious. Thus in South Staffordshire a seam of coal was laid bare in the year 1844, in what is called an open work at Parkfield Colliery, near Wolverhampton. In the space of about a quarter of an acre the stumps of no less than 73 trees with their roots attached appeared, as shown in the annexed plan (fig. 369.), some of them more than 8 feet in circumference. The trunks, broken off close to the root, were lying prostrate in every direction, often crossing each other. One of them measured 15, another 30 feet in length, and others less. They were invariably flattened to the thickness of one or two inches, and converted into coal. Their roots formed part of a stratum of coal 10 inches thick, which rested on a layer of clay 2 inches thick, below which was a second forest, resting on a 2-foot seam of coal. Five feet below this again was a third forest with large stumps of Lepidodendra, Calamites, and other trees.

In the account given, in 1821, by M. Alex. Brongniart of the coal-mine of Treuil, at St. Etienne, near Lyons, he states, that distinct horizontal strata of micaceous sandstone are traversed by vertical trunks of monocotyledonous vegetables, resembling bamboos or large Equiseta.[319-B] Since the consolidation of the stone, there has been here and there a sliding movement, which has broken the continuity of the stems, throwing the upper parts of them on one side, so that they are often not continuous with the lower.

From these appearances it was inferred that we have here the monuments of a submerged forest. I formerly objected to this conclusion, suggesting that, in that case, all the roots ought to have been found at one and the same level, and not scattered irregularly through the mass. I also imagined that the soil to which the roots were attached should have been different from the sandstone in which the trunks are enclosed. Having, however, seen calamites near Pictou, in Nova Scotia, buried at various heights in sandstone and in similar erect attitudes, I have now little doubt that M. Brongniart's view was correct. These plants seem to have grown on a sandy soil, liable to be flooded from time to time, and raised by new accessions of sediment, as may happen in swamps near the banks of a large river in its delta. Trees which delight in marshy grounds are not injured by being buried several feet deep at their base; and other trees are continually rising up from new soils, several feet above the level of the original foundation of the morass. In the banks of the Mississippi, when the water has fallen, I have seen sections of a similar deposit in which portions of the stumps of trees with their roots in situ appeared at many different heights.[320-A]

When I visited, in 1843, the quarries of Treuil above-mentioned, the fossil trees seen in fig. 370. were removed, but I obtained proofs of other forests of erect trees in the same coal-field.

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, when all 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 laminae 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 deg., 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.

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 from c to i are seen all dipping the same way, their average inclination being at an angle of 24 deg. S.S.W. The vertical height of the cliffs is from 150 to 200 feet; and between d and g, 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, between d and g, is about 2,500 feet, the erect trees consisting chiefly of large Sigillariae, 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 observed Stigmaria.

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 of Stigmaria, 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 at a, fig. 374., or in the bed e in 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 thick on which rests a seam of coal (b, fig. 374.) 1 foot thick. On this coal again stood two large trees (c and d), while at a greater height the trees f and g rest upon a thin seam of coal (e), and above them is an underclay, supporting the 4-foot coal.

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 deg.. He has published minute details of the whole series, showing at how many different levels erect trees occur, consisting of Sigillaria, 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 of Stigmaria in 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 of Cypris, Unio (?), Modiola, Microconchus carbonarius (see fig. 375.), and Spirorbis, 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 a Cyclas, a small Cypris (fig. 376.), and the microscopic shell of an annelid of an extinct genus called Microconchus (fig. 375.), allied to Serpula or Spirorbis.

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 feet thick, 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 genus Unio, and others of marine forms, such as Nautilus, Orthoceras, Spirifer, and Productus. 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]

[2 Illustrations: 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]]

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 genus Unio; but in the midst of the series there is one thin but very widely spread stratum, abounding in fishes and marine shells, such as Ammonites Listeri (fig. 377.), Orthoceras, and Avicula papyracea, Goldf. (fig. 378.)[325-D]

No similarly intercalated layer of marine shells has been noticed in the neighbouring coal-field of Newcastle, where, as in South Wales 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]

FOOTNOTES:

[308-A] Phillips; art. "Geology," Encyc. Britan.

[309-A] Sedgwick, Geol. Trans., Second Series, vol. iv.; and Phillips, Geol. of Yorksh. part 2.

[309-B] Memoirs of Geol. Survey, vol. i. p. 195.

[315-A] The trunk in this case is referred by Mr. Brown to Lepidodendron, but his illustrations seem to show the usual markings assumed by Sigillaria near its base.

[316-A] For terminology of classification of plants, see above, note, p. 223.

[316-B] Quart. Geol. Journ., vol. v., Mem., p. 17.

[317-A] Anniv. Address to Geol. Soc., 1840.

[317-B] Hawkshaw, Geol. Soc. Proceedings, Nos. 64. and 69.

[318-A] Geol. Report on Cornwall, &c. p. 143.

[318-B] Lindley and Hutton, Foss. Flo. part 6. p. 150.

[319-A] See papers by Messrs. Beckett and Ick. Proceed. in Geol. Soc., vol. iv. p. 287.

[319-B] Annales des Mines, 1821.

[320-A] Principles of Geol., 8th ed., p. 215.

[321-A] See figures of texture, Witham, Foss. Veget., pl. 3.

[321-B] See Lyell's Travels in N. America, vol. ii. p. 179.

[322-A] Quart. Geol. Journ., vol. ii. p. 177.

[324-A] Geol. Quart. Journ., vol. ii. p. 393.; and vol. vi. p. 115.

[325-A] Prestwich, Geol. Trans., 2d Series, vol. v. p. 440. Murchison, Silurian System, p. 105.

[325-B] Silurian System, p. 84.

[325-C] Trans. Roy. Soc. Edin. vol. xiii. Horner, Edin. New Phil. Journ., April, 1836.

[325-D] Phillips; art. "Geology," Encyc. Metrop., p. 590.

[326-A] Phillips; art. "Geology," Encyc. Metrop., p. 592.

[326-B] Memoirs of Geol. Survey, pp. 51. 255, &c.

Spotted a typo or a problem in this chapter? Tell us

XXV. CARBONIFEROUS GROUP--continued

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.

It was 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 understanding the 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.

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. and 224.), which are nearly horizontal. The next belt, from B to C, 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 (see p. 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, and B 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 ridges and troughs nearest the Atlantic, south-eastern dips predominate, in consequence of the beds having been folded back upon themselves, as in i, those on the north-western side of each arch having been inverted. The next set of arches (such as k) 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 at i and k.

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 region C is 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 (at C) 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 grain than 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 Stigmaria in situ under 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.

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. Let a a', fig. 380., be a mass of vegetable matter, capable, when condensed, of forming a 3-foot seam of coal. It rests on the underclay b b', filled with roots of trees in situ, and it supports a growing forest (C D). Suppose that part of the same forest D E had become submerged by the ground sinking down 25 feet, so that the trees have been partly thrown down and partly 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 lake D 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 forest C D will extend once more continuously over the whole area C F, as in fig. 381., and another mass of vegetable matter (g g'), forming 3 feet more of coal, may accumulate from C to F. We then find in the region F, two seams of coal (a' and g') each 3 feet thick, and separated by 25 feet of sandstone and shale, with erect trees based upon the lower coal, while, between D and C, 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 region C D to be thicker than the two distinct seams a' and g' at F; 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 seam a g, throughout the area C D, was equal to the two seams a' and g' at F.

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 (see fig. 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 (at a), 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 (at d 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 (from C to E, 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.

This coal formation, before its original limits were reduced by denudation, 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 region D 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. It is 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 (Stigmariae) 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 with Stigmaria.

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 sigillariae, 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]

CARBONIFEROUS REPTILES.

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 family Curculionidae, and of a neuropterous insect resembling the genus Corydalis, with another related to the Phasmidae.[335-C] In other coal-measures in Europe we find notice of a scorpion and of a moth allied to Tinea, 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 any now living, especially those of the family called Sauroid by Agassiz; as Megalichthys, 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 (see fig. 383.), the articulations of the spinous processes with the vertebrae, and other characters. Yet they do not form a family intermediate between fish and reptiles, but are true fish, though doubtless more highly organized than any living fish.[336-A]

The annexed figure represents a large tooth of the Megalichthys, found by Mr. Horner in the Cannel coal of Fifeshire. It probably inhabited an estuary, like many of its contemporaries, and frequented both rivers and the sea.

At length, in 1844, the first skeleton of a true reptile was announced from the coal of Muenster-Appel in Rhenish Bavaria, by H. von Meyer, under the name of Apateon pedestris, the animal being supposed to be nearly related to the salamanders. Three years later, in 1847, Prof. von Dechen found in the coal-field of Saarbrueck, at the village of Lebach, between Strasburg and Treves, the skeletons of no less than three distinct species of air-breathing reptiles, which were described by the late Prof. Goldfuss under the generic name of Archegosaurus. The ichthyolites and plants found in the same strata, left no doubt that these remains belonged to the true coal period. The skulls, teeth, and the greater portions of the skeleton, nay, even a large part of the skin, of two of these reptiles have been faithfully preserved in the centre of spheroidal concretions of clay-iron-stone. The largest of these lizards, Archegosaurus Decheni, must have been 3 feet 6 inches long. The annexed drawing represents the smallest of the three of the natural size. They were considered by Goldfuss as saurians, but by Herman von Meyer as most nearly allied to the Labyrinthodon, and therefore connected with the batrachians, as well as the lizards. The remains of the extremities leave no doubt that they were quadrupeds, "provided," says Von Meyer, "with hands and feet terminating in distinct toes; but these limbs were weak, serving only for swimming or creeping." The same anatomist has pointed out certain points of analogy between their bones and those of the Proteus anguinus; and Mr. Owen has observed to me that they make an approach to the Proteus in the shortness of their ribs. Two of these ancient reptiles retain a large part of the outer skin, which consisted of long, narrow, wedge-shaped, tile-like, and horny scales, arranged in rows (see fig. 385.).

Cheirotherian footprints in coal measures, United States.--In 1844, the very year when the Apateon or Salamander of the coal was first met with in the country between the Moselle and the Rhine, Dr. King published an account of the footprints of a large reptile discovered by him in North America. These occur in the coal strata of Greensburg, in Westmoreland County, Pennsylvania; and I had an opportunity of examining them in 1846. I was at once convinced of their genuineness, and declared my conviction on that point, on which doubts had been entertained both in Europe and the United States. The footmarks were first observed standing out in relief from the lower surface of slabs of sandstone, resting on thin layers of fine unctuous clay. I brought away one of these masses, which is represented in the accompanying drawing (fig. 386.). It displays, together with footprints, the casts of cracks (a, a') of various sizes. The origin of such cracks in clay, and casts of the same, has before been explained, and referred to the drying and shrinking of mud, and the subsequent pouring of sand into open crevices. It will be seen that some of the cracks, as at b, c, traverse the footprints, and produce distortion in them, as might have been expected, for the mud must have been soft when the animal walked over it and left the impressions; whereas, when it afterwards dried up and shrank, it would be too hard to receive such indentations.

No less than twenty-three footsteps were observed by Dr. King in the same quarry before it was abandoned, the greater part of them so arranged (see fig. 387.) on the surface of one stratum as to imply that they were made successively by the same animal. Everywhere there was a double row of tracks, and in each row they occur in pairs, each pair consisting of a hind and fore foot, and each being at nearly equal distances from the next pair. In each parallel row the toes turn the one set to the right, the other to the left. In the European Cheirotherium, before mentioned (p. 290.), both the hind and fore feet have each five toes, and the size of the hind foot is about five times as large as the fore foot. In the American fossil the posterior footprint is not even twice as large as the anterior, and the number of toes is unequal, being five in the hinder and four in the anterior foot. In this, as in the European Cheirotherium, one toe stands out like a thumb, and these thumb-like toes turn the one set to the right, and the other to the left. The American Cheirotherium was evidently a broader animal, and belonged to a distinct genus from that of the triassic age in Europe.[338-A]

We may assume that the reptile which left these prints on the ancient sands of the coal-measures was an air-breather, because its weight would not have been sufficient under water to have made impressions so deep and distinct. The same conclusion is also borne out by the casts of the cracks above described, for they show that the clay had been exposed to the air and sun, so as to have dried and shrunk.

The geological position of the sandstone of Greensburg is perfectly clear, being situated in the midst of the Appalachian coal-field, having the main bed of coal, called the Pittsburg seam, above mentioned (p. 331.), 3 yards thick, 100 feet above it, and worked in the neighbourhood, with several other seams of coal at lower levels. The impressions of Lepidodendron, Sigillaria, Stigmaria, and other characteristic carboniferous plants, are found both above and below the level of the reptilian footsteps.

Analogous footprints of a large reptile of still older date have since been found (1849), by Mr. Isaac Lea, in the lowest beds of the coal formation at Pottsville, near Philadelphia, so that we may now be said to have the footmarks of two reptilians of the coal period, and the skeletons of four.[340-A]

CARBONIFEROUS OR MOUNTAIN LIMESTONE.

We have already seen that this rock lies sometimes entirely beneath the coal-measures, while, in other districts, it alternates with the shales and sandstone of the coal. In both cases it is destitute of land plants, and usually charged with corals, which are often of large size; and several species belong to the lamelliferous class of Lamarck, which enter largely into the structure of coral reefs now growing. There are also a great number of Crinoidea (see fig. 388.), and a few Echinoderms, associated with the zoophytes above mentioned. The Brachiopoda constitute a large proportion of the Mollusca, many species being referable to two extinct genera, Spirifer (or Spirifera) (fig. 389.), and Productus (Leptaena) (fig. 390.).

Among the spiral univalve shells the extinct genus Euomphalus (see fig. 391.) is one of the commonest fossils of the Mountain limestone. In the interior it is often divided into chambers (see fig. 391. d); the septa or partitions not being perforated, as in foraminiferous shells, or in those having siphuncles, like the Nautilus. The animal appears, like the recent Bulimus decollatus, to have retreated at different periods of its growth, from the internal cavity previously formed, and to have closed all communication with it by a septum. The number of chambers is irregular, and they are generally wanting in the innermost whorl.

There are also many univalve and bivalve shells of existing genera in the Mountain limestone, such as Turritella, Buccinum, Patella, Isocardia, Nucula, and Pecten.[341-A] But the Cephalopoda depart, in general, more widely from living forms, some being generically distinct from all those found in strata newer than the coal. In this number may be mentioned Orthoceras, a siphuncled and chambered shell, like a Nautilus uncoiled and straightened. Some species of this genus are several feet long (fig. 392.). The Goniatite is another genus, nearly allied to the Ammonite, from which it differs in having the lobes of the septa free from lateral denticulations, or crenatures; so that the outline of these is continuous and uninterrupted (see a, fig. 393.). Their siphon is small, and in the form of the striae of growth they resemble Nautili. Another extinct generic form of Cephalopod, abounding in the Mountain limestone, and not found in strata of later date, is the Bellerophon (fig. 394.), of which the shell, like the living Argonaut, was without chambers.

FOOTNOTES:

[329-A] H. D. Rogers, Trans. Assoc. Amer. Geol., 1840-42, p. 440.

[333-A] Trans. of Ass. of Amer. Geol., p. 470.

[334-A] Lyell's Second Visit to the U. S., vol. ii. p. 245. American Journ. of Sci., 2d series, vol. v. p. 17.

[335-A] Principles of Geol., p. 696.

[335-B] For changes in climate, see Principles of Geol., chaps. vii. and viii.

[335-C] Geol. Trans., 2d series, vol. vi. p. 330.

[336-A] Agassiz, Poiss. Foss., lib. 4. p. 62. and liv. 5. p. 88.

[337-A] Goldfuss, Neue Jenaische Lit. Zeit., 1848; and Von Meyer, Quart. Geol. Journ., vol. iv. p. 51., memoirs.

[338-A] See Lyell's Second Visit, &c., vol. ii. p. 305.

[340-A] These impressions, found by Mr. Lea, were imagined to be in a rock as ancient as the old red sandstone; but, according to Mr. H. D. Rogers, they are in the lowest part of the coal formation.

[341-A] Phillips, Geol. of Yorksh., vol. ii. p. 208.

[342-A] Phillips, Geol. of Yorksh., pl. 20. fig. 65.

[342-B] Ibid., pl. 17. fig. 15.

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XXVI. OLD RED SANDSTONE, OR DEVONIAN GROUP

Old Red Sandstone of Scotland, and borders of Wales--Fossils usually rare--"Old Red" in Forfarshire--Ichthyolites of Caithness--Distinct lithological type of Old Red in Devon and Cornwall--Term "Devonian"--Organic remains of intermediate character between those of the Carboniferous and Silurian systems--Corals and shells--Devonian strata of Westphalia, the Eifel, Russia, and the United States--Coral reef at Falls of the Ohio--Devonian flora.

It was stated in Chap. XXII. that the Carboniferous formation is surmounted by one called the "New Red," and underlaid by another called the "Old Red Sandstone."[342-C] The British strata of the last mentioned series were first recognized in Herefordshire and Scotland as of great thickness, and immediately subjacent to the coal; but they were in general so barren of organic remains, that it was difficult to find paleontological characters of sufficient importance to distinguish them as an independent group. In Scotland, and on the borders of Wales, the "Old Red" consists chiefly of red sandstone, conglomerate, and shale, with few fossils; but limestones of the same age, peculiarly rich in organic remains, were at length found in Devonshire.

I shall first advert to the characters of the group as developed in Herefordshire, Worcestershire, Shropshire, and South Wales. Its thickness has been estimated at 8000 feet, and it has been subdivided into--

1st. A quartzose conglomerate passing downwards into chocolate-red and green sandstone and marl.

2d. Cornstone and marl--red and green argillaceous spotted marls, with irregular courses of impure concretionary limestone, provincially called Cornstone.

Here, as usual, fossils are extremely rare in the clays and sandstones in which the red oxide of iron prevails; but remains of fishes of the genera Cephalaspis and Onchus have been discovered in the Cornstone.

The whole of the northern part of Scotland, from Cape Wrath to the southern flank of the Grampians, has been well described by Mr. Miller as consisting of a nucleus of granite, gneiss, and other hypogene rocks, which seem as if set in a sandstone frame.[343-A] The beds of the Old Red Sandstone constituting this frame, may once perhaps have extended continuously over the entire Grampians before the upheaval of that mountain range; for one band of the sandstone follows the course of the Moray Frith far into the interior of the great Caledonian valley; and detached hills and island-like patches occur in several parts of the interior, capping some of the higher summits in Sutherlandshire, and appearing in Morayshire like oases among the granite rocks of Strathspey. On the western coast of Ross-shire, the Old Red forms those three immense insulated hills before described (p. 67.), where beds of horizontal sandstone, 3000 feet high, rest unconformably on a base of gneiss, attesting the vast denudation which has taken place.

But in order to observe the uppermost part of the Old Red, we must travel south of the Grampians, and examine its junction with the bottom of the Carboniferous series in Fifeshire. This upper member may be seen in Dura Den, south of Cupar, to consist of a belt of yellow sandstone, in which Dr. Fleming first discovered scales of Holoptychius, and in which species of fish of the genera Pterichthys, Pamphractus, and others, have been met with. (For genus Pterichthys, see fig. 400. p. 345.)

The beds next below the yellow sandstone are well seen in the large zone of Old Red which skirts the southern flank of the Grampians from Stonehaven to the Frith of Clyde. It there forms, together with trap, the Sidlaw Hills and the strata of the valley of Strathmore. A section of this region has been already given (p. 48.), extending from the foot of the Grampians in Forfarshire to the sea at Arbroath, a distance of about 20 miles, where the entire series of strata is several thousand feet thick, and may be divided into three principal masses: 1st, and uppermost, red and mottled marls, cornstone, and sandstone (Nos. 1. and 2. of the section); 2d, Conglomerate, often of vast thickness (No. 3. ibid.); 3d, Roofing and paving stone, highly micaceous, and containing a slight admixture of carbonate of lime (No. 4. ibid.). In the first of these divisions, which may be considered as succeeding the yellow sandstone of Fifeshire before mentioned, a gigantic species of fish of the genus Holoptychius has been found at Clashbinnie near Perth. Some scales (see fig. 395.) have been seen which measured 3 inches in length by 2-1/2 in breadth.

At the top of the next division, or immediately under the conglomerate (No. 3. p. 48.), there have been found in Forfarshire some remarkable crustaceans, with several fish of the genus named by Agassiz Cephalaspis, or "buckler-headed," from the extraordinary shield which covers the head (see fig. 396.), and which has often been mistaken for that of a trilobite, of the division Asaphus.

Species of the same genus are considered in England as characteristic of the second or Cornstone division (p. 343.).

In the same grey paving-stones and coarse roofing-slates, in which the Cephalaspis occurs, in Forfarshire and Kincardineshire, the remains of marine plants or fucoids abound. They are frequently accompanied by groups of hexagonal, or nearly hexagonal markings, which consist of small flattened carbonaceous bodies, placed in a slight depression of the sandstone or shale. (See figs. 397 and 398.) They much resemble in form the spawn of the recent Natica (see fig. 399.), in which the eggs are arranged in a thin layer of sand, and seem to have acquired a polygonal form by pressing against each other. The substance of the egg, if fossilized, might give rise to small pellicles of carbonaceous matter.

These fossils I have met with, both to the north of Strathmore, in the vertical shale beneath the conglomerate, and in the same beds in the Sidlaw hills, at all the points where fig. 4. is introduced in the section, p. 48.

Beds of red shale and red sandstone, sometimes associated with pudding-stone (older than No. 3., fig. 62. p. 48.), and destitute of organic remains, separate, in the region of Strathmore, the above-described fossiliferous strata from the older crystalline rocks of the Grampians. But, in the north of Scotland, we find, at the base of the Old Red, other grey slaty sandstones, in the counties of Banff, Nairn, Moray, Cromarty, Caithness, and in Orkney, rich in ichthyolites of peculiar forms, belonging to the genera Pterichthys (fig. 400.), Coccosteus, Diplopterus, Dipterus, Cheiracanthus, and others of Agassiz.

Five species of Pterichthys have been found in this lowest division of the Old Red. The wing-like appendages, whence the genus is named, were first supposed by Mr. Miller to be paddles, like those of the turtle; but Agassiz regards them as weapons of defence, like the occipital spines of the River Bull-head (Cottus gobio, Linn.); and considers the tail to have been the only organ of motion. The genera Dipterus and Diplopterus are so named, because their two dorsal fins are so placed as to front the anal and ventral fins, so as to appear like two pairs of wings. They have bony enamelled scales.

South Devon and Cornwall.--A great step was made in the classification of the slaty and calciferous strata of South Devon and Cornwall in 1837, when a large portion of the beds, previously referred to the "transition" or most ancient fossiliferous series, were found to belong in reality to the period of the Old Red Sandstone. For this reform we are indebted to the labours of Professor Sedgwick and Sir R. Murchison, assisted by a suggestion of Mr. Lonsdale, who, in 1837, after examining the South Devonshire fossils, perceived that some of them agreed with those of the Carboniferous group, others with those of the Silurian, while many could not be assigned to either system, the whole taken together exhibiting a peculiar and intermediate character. But these paleontological observations alone would not have enabled us to assign, with accuracy, the true place in the geological series of these slate-rocks and limestones of South Devon, had not Messrs. Sedgwick and Murchison, in 1836 and 1837, discovered that the culmiferous or anthracitic shales of North Devon belonged to the Coal, and not, as preceding observers had imagined, to the transition period.

As the strata of South Devon here alluded to are far richer in organic remains than the red sandstones of contemporaneous date in Herefordshire and Scotland, the new name of the "Devonian system" was proposed as a substitute for that of Old Red Sandstone.

The rocks of this group in South Devon consist, in great part, of green chloritic slates, alternating with hard quartzose slates and sandstones. Here and there calcareous slates are interstratified with blue crystalline limestone, and in some divisions conglomerates, passing into red sandstone.

The link supplied by the whole assemblage of imbedded fossils, connecting as it does the paleontology of the Silurian and Carboniferous groups, is one of the highest interest, and equally striking, whether we regard the genera of corals or of shells. The species are almost all distinct.

Among the more abundant corals, we find the genera Favosites and Cyathophyllum, common on the one hand to the Mountain limestone, and on the other to the Silurian system. Some few even of the species are common to the Devonian and Silurian groups, as, for example, Favosites polymorpha (fig. 401.), very abundant in South Devon.

The Cyathophyllum caespitosum (fig. 402.) and Porites pyriformis (fig. 424. p. 356.) are more peculiarly characteristic of the Devonian rocks.

In regard to the shells, all the brachiopodous genera, such as Terebratula, Orthis, Spirifer, Atrypa, and Productus, which are found in the Mountain limestone, occur, together with those of the Silurian system, except the Pentamerus. Some forms, however, seem exclusively Devonian, as for example, Calceola sandalina (fig. 403.) and Strygocephalus Burtini (fig. 404.), which have been met with both in the Eifel, in Germany, and in Devonshire, in the very lowest Devonian beds.

Among the peculiar lamellibranchiate bivalves, also common to Devonshire and the Eifel, we find Megalodon cucullatus (fig. 405.). Several spiral univalves are abundant, among which are many species of Pleurotomaria and Euomphalus. Among the Cephalopoda we find Bellerophon and Orthoceras, as in the Silurian and Carboniferous groups, and Goniatite and Cyrtoceras, as in the Carboniferous. In some of the upper Devonian beds, a shell, resembling a flattened Goniatite, occurs, called Clymenia, by Munster (Endosiphonites, Ansted.[347-A]).

A peculiar species of trilobite, called Brontes flabellifer (fig. 407.), is found in the Devonian strata of the Eifel and in South Devon. It should be observed, however, that the head in the specimen here figured by Goldfuss, the most perfect which could be obtained, is incomplete, and a restoration has been attempted by Mr. Salter in fig. 408., from data supplied by other species of the same genus occurring in older rocks.

For determining the true equivalents of the Devonian group in the Rhenish provinces and adjacent parts of Germany, we are indebted to the labours of Messrs. Sedgwick and Murchison, in 1839, from which it appears that rocks of that age emerge from beneath the coal-field of Westphalia, and are also found in troughs among the Silurian rocks in Nassau. Many of the limestones, particularly those on the river Lahn, are identical, both in structure and in coralline remains, with the beautiful marbles of Babbacombe, Torquay, and Plymouth.

The limestones of the Eifel, long ago celebrated for their fossils, and which lie in a basin supported by Silurian rocks, are found to be referable to the lower part of the Devonian system.

In Russia, also, Messrs. Murchison and De Verneuil have shown (1840) that the "Old Red" group occupies a wide area south from St. Petersburg. It was formerly supposed to be the New Red Sandstone, on account of its saliferous and gypseous beds; but it is now proved to be the Old Red by containing ichthyolites of genera which characterize this group in the British Isles, as, for example, Holoptychius, Coccosteus, Diplopterus, &c.[349-A], associated with mollusca found in the Devonian of Western Europe. Among the fish are also many species of sharks of the Cestraciont division, a fact worthy of notice, because the squaloid fishes of the present day offer the highest organization of the brain and of the generative organs, and make, in these respects, the nearest approach to the higher vertebrate classes.

Devonian Strata in the United States.

The position of this formation between the carboniferous rocks of Pennsylvania and Ohio, is pointed out in the section, fig. 379. p. 327., and it is a remark of M. de Verneuil that in no European country is there so complete and uninterrupted a development of the Devonian system as in North America. At the falls of the Ohio, at Louisville, in Kentucky, there is a grand display of one of the limestones of this period, resembling a modern coral reef. A wide extent of surface is exposed in a series of horizontal ledges, at all seasons, when the water is not high; and the softer parts of the stone having decomposed and wasted away, the harder calcareous corals stand out in relief, and many of them send out branches from their erect stems precisely as if they were living. Among other species I observed large masses, not less than 5 feet in diameter, of Favosites gothlandica, with its beautiful honeycomb structure well displayed, and, by the side of it, the Favistella, combining a similar honeycombed form with the star of the Astrea. There was also the cup-shaped Cyathophyllum, and the delicate network of the Fenestella, and that elegant and well-known European species of fossil, called "the chain coral," Catenipora escharoides, with a profusion of others (see fig. 423. p. 355.). These coralline forms were mingled with the joints, stems, and occasionally the heads, of lily encrinites. Although hundreds of fine specimens have been detached from these rocks, to enrich the museums of Europe and America, another crop is constantly working its way out, under the action of the stream, and of the sun and rain, in the warm season when the channel is laid dry. The waters of the Ohio, when I visited the spot in April, 1846, were more than 40 feet below their highest level, and 20 feet above their lowest, so that large spaces of bare rock were exposed to view.[349-B]

Devonian Flora.

With the exception of the fucoids above mentioned (p. 344.), but little is known with certainty of the plants of the Devonian group. Those found in the department of La Sarthe in France, and in various parts of Brittany, formerly referred to the Devonian era, have been shown (in 1850), by M. de Verneuil, to belong to the carboniferous series. The same may be said of the species of Lepidodendron, Knorria, Calamite, Sagenaria, and other genera recently figured (1850), by Mr. F. A. Roemer, from the formation called "Greywacke a Posodonomyes" in the Hartz.[350-A] They are accompanied by Goniatites reticulatus Phillips, G. intercostatus Phil., and other mountain limestone species, and had been previously assigned to the oldest part of the carboniferous series by Messrs. Murchison and Sedgwick.

If hereafter we should become well acquainted with the land plants of the Devonian era, we may confidently expect that nearly all of them will agree generically with those of the carboniferous period, but the species will be as different as are the Devonian vertebrate and invertebrate animals from the fossil species of the Coal.

FOOTNOTES:

[342-C] See section, fig. 318. p. 287.

[343-A] The Old Red Sandstone, by Hugh Miller, 1841.

[345-A] Old Red Sandstone. Plate 1. fig. 1. Mr. M.'s description of the fish is most graphic and correct.

[347-A] Camb. Phil. Trans., vol. vi. pl. 8. fig. 2.

[349-A] See Proceedings of Geol. Soc., and the anniversary speech of Dr. Buckland, P. G. S., for 1841.

[349-B] Lyell's Second Visit to the United States, vol. ii. p. 277.

[350-A] Memoir on the Hartz, Palaeontographica of Dunker and Von Meyer, part iii.

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XXVII. SILURIAN GROUP

Silurian strata formerly called transition--Term grauwacke--Subdivisions of Upper and Lower Silurian--Ludlow formation and fossils--Wenlock formation, corals and shells--Caradoc and Llandeilo beds--Graptolites--Lingula--Trilobites--Cystideae--Vast thickness of Silurian strata in North Wales--Unconformability of Caradoc sandstone--Silurian strata of the United States--Amount of specific agreement of fossils with those of Europe--Great number of brachiopods--Deep-sea origin of Silurian strata--Absence of fluviatile formations--Mineral character of the most ancient fossiliferous rocks.

We come next in the descending order to the most ancient of the primary fossiliferous rocks, that series which comprises the greater part of the strata formerly called "transition" by Werner, for reasons explained in Chap. VIII., pp. 91 and 92. Geologists have also applied to these older strata the general name of "grauwacke," by which the German miners designate a particular variety of sandstone, usually an aggregate of small fragments of quartz, flinty slate (or Lydian stone), and clay-slate cemented together by argillaceous matter. Far too much importance has been attached to this kind of rock, as if it belonged to a certain epoch in the earth's history, whereas a similar sandstone or grit is found sometimes in the Old Red, and in the Millstone Grit of the Coal, and sometimes in certain Cretaceous and even Eocene formations in the Alps.

The name of Silurian was first proposed by Sir Roderick Murchison, for a series of fossiliferous strata lying below the Old Red Sandstone, and occupying that part of Wales and some contiguous counties of England, which once constituted the kingdom of the Silures, a tribe of ancient Britons. The strata have been divided into Upper and Lower Silurian, and these again in the region alluded to admit of several well-marked subdivisions, all of them explained in the following table.

UPPER SILURIAN ROCKS.

Prevailing Thickness Organic Lithological in Feet. Remains. characters.

{ {Finely laminated } } {Tilestones. { reddish and }800? } { { green sandstones } } { { and shales. } } 1. Ludlow { }Marine mollusca of formation {Upper {Micaceous grey } } almost every order, {Ludlow. { sandstone. } } the Brachiopoda most { } } abundant. Serpula, {Aymestry {Argillaceous } } Corals, Sauroid fish, {limestone. { limestone. }2000 } Fuci. { } } {Lower {Shale, with } } {Ludlow. { concretions of } } { { limestone. } }

{Wenlock }Concretionary } {Marine mollusca of {limestone. } limestone. } { various orders as 2. Wenlock { } }1800 { before, Crustaceans formation. { } } { of the Trilobite { } } { family. {Wenlock }Argillaceous } {Oldest bones of {shale. } shale. } { fish yet known.

LOWER SILURIAN ROCKS.

{Flags of shelly } { { limestone and } {Crinoidea, Corals, 3. Caradoc {Caradoc { sandstone, thick }2500 { Mollusca, chiefly formation. {sandstones. { bedded white } { Brachiopoda, { freestone. } { Trilobites.

4. Llandeilo {Llandeilo }Dark coloured }1200 {Mollusca, formation. {flags. } calcareous flags. } { Trilobites.

UPPER SILURIAN ROCKS.

Ludlow formation.--This member of the Upper Silurian group, as will be seen by the above table, is of great thickness, and subdivided into four parts,--the Tilestone, the Upper and Lower Ludlow, and the intervening Aymestry limestone. Each of these may be distinguished near the town of Ludlow, and at other places in Shropshire and Herefordshire, by peculiar organic remains.

1. Tilestones.--This uppermost division was originally classed by Sir R. Murchison with the Old Red Sandstone, because they decompose into a red soil throughout the Silurian region. At the same time he regarded the tilestones as a transition group forming a passage from Silurian to Old Red. It is now ascertained that the fossils agree in great part specifically, and in general character entirely, with those of the succeeding formation.

2. Upper Ludlow.--The next division, called the Upper Ludlow, consists of grey calcareous sandstone, decomposing into soft mud, and contains, among other shells, the Lingula cornea, which is common to it and the lowest, or tilestone beds of the Old Red. But the Orthis orbicularis is peculiar to the Upper Ludlow, and very common; and the lowest or mudstone beds, are loaded for a thickness of 30 feet with Terebratula navicula (fig. 410.), in vast numbers. Among the cephalopodous mollusca occur the genera Bellerophon and Orthoceras, and among the crustacea the Homalonotus (fig. 418. p. 354.). A coral called Favosites polymorpha, Goldf. (fig. 401. p. 346.) is found both in this subdivision and in the Devonian system.

Among the fossil shells are species of Leptaena, Orthis, Terebratula, Avicula, Trochus, Orthoceras, Bellerophon, and others.[352-A]

Some of the Upper Ludlow sandstones are ripple-marked, thus affording evidence of gradual deposition; and the same may be said of the accompanying fine argillaceous shales which are of great thickness, and have been provincially named "mudstones." In these shales many zoophytes are found enveloped in an erect position, having evidently become fossil on the spots where they grew at the bottom of the sea. The facility with which these rocks, when exposed to the weather, are resolved into mud, proves that, notwithstanding their antiquity, they are nearly in the state in which they were first thrown down.

The scales, spines (ichthyodorulites), jaws, and teeth of fish of the genera Onchus, Plectrodus, and others of the same family, have been met with in the Upper Ludlow rocks.

3. Aymestry limestone.--The next group is a subcrystalline and argillaceous limestone, which is in some places 50 feet thick, and distinguished around Aymestry by the abundance of Pentamerus Knightii, Sow. (fig. 411.), also found in the Lower Ludlow. This genus of brachiopoda has only been found in the Silurian strata. The name was derived from +pente+, pente, five, and +meros+, meros, a part, because both valves are divided by a central septum, making four chambers, and in one valve the septum itself contains a small chamber, making five; but neither the structure of this shell, nor the connection of the animal with its several parts, are as yet understood. Messrs. Murchison and De Verneuil discovered this species dispersed in myriads through a white limestone of upper Silurian age, on the banks of the Is, on the eastern flank of the Urals in Russia.

Three other abundant shells in the Aymestry limestone are, 1st, Lingula Lewisii (fig. 412.); 2d, Terebratula Wilsoni, Sow. (fig. 413.), which is also common to the Lower Ludlow and Wenlock limestone; 3d, Atrypa reticularis, Lin. (fig. 414.), which has a very wide range, being found in every part of the Silurian system, except the Llandeilo flags.

4. Lower Ludlow shale.--A dark grey argillaceous deposit, containing, among other fossils, the new genera of chambered shells, the Phragmoceras of Broderip, and the Lituites of Breyn (see figs. 415, 416.). The latter is partly straight and partly convoluted, nearly as in Spirula.

The Orthoceras Ludense (fig. 417.), as well as the shell last mentioned, is peculiar to this member of the series. The Homalonotus delphinocephalus (fig. 418.) is common to this division and to the Wenlock limestone. This crustacean belongs to a group of trilobites which has been met with in the Silurian rocks only, and in which the tripartite character of the dorsal crust is almost lost.

A species of Graptolite, G. Ludensis, Murch. (fig. 419.), a form of zoophyte which has not yet been met with in strata newer than the Silurian, occurs in the Lower Ludlow.

Wenlock formation.--We next come to the Wenlock formation, which has been divided (see Table, p. 351.) into

1. Wenlock limestone, formerly well known to collectors by the name of the Dudley limestone, which forms a continuous ridge, ranging for about 20 miles from S.W. to N.E., about a mile distant from the nearly parallel escarpment of the Aymestry limestone. The prominence of this rock in Shropshire, like that of Aymestry, is due to its solidity, and to the softness of the shales above and below. It is divided into large concretional masses of pure limestone, and abounds in trilobites, among which the prevailing species are Phacops caudatus (fig. 422.) and Calymene Blumenbachii, commonly called the Dudley trilobite. The latter is often found coiled up like a wood-louse (see fig. 420.).

Leptaena depressa, Sow., is common in this rock, but also ranges through the Lower Ludlow, Wenlock shale, and Caradoc Sandstone.

Among the corals in which this formation is very rich, the Catenipora escharoides, Lam. (fig. 423.), or chain coral, may be pointed out as one very easily recognized, and widely spread in Europe, ranging through all parts of the Silurian group, from the Aymestry limestone to the bottom of the series.

Another coral, the Porites pyriformis, is also met with in profusion; a species common to the Devonian rocks.

Cystiphyllum Siluriense (fig. 425.) is a species peculiar to the Wenlock limestone. This new genus, the name of which is derived from +kystis+, a bladder, and +phyllon+, a leaf, was instituted by Mr. Lonsdale for corals of the Silurian and Devonian groups. It is composed of small bladder-like cells (see fig. 425. b.).

2. The Wenlock Shale, which exceeds 700 feet in thickness, contains many species of brachiopoda, such as a small variety of the Lingula Lewisii (fig. 412.), and the Atrypa reticularis (fig. 414.) before mentioned, and it will be seen that several other fossils before enumerated range into this shale.

LOWER SILURIAN ROCKS.

The Lower Silurian rocks have been subdivided into two portions.

1. The Caradoc sandstone, which abuts against the trappean chain called the Caradoc Hills, in Shropshire. Its thickness is estimated at 2500 feet, and the larger proportion of its fossils are specifically distinct from those of the Upper Silurian rocks. Among them we find many trilobites and shells of the genera Orthoceras, Nautilus, and Bellerophon; and among the Brachiopoda the Pentamerus oblongus and P. laevis (fig. 426.), which are very abundant and peculiar to this bed; also Orthis grandis (fig. 427.), and a fossil of well-defined form, Tentaculites annulatus, Schlot. (fig. 428.), which Mr. Salter has shown to be referable to the Annelids and to the same tribe as Serpula.

The most ancient bony remains of fish yet discovered in Great Britain are those obtained from the Wenlock limestones; but coprolites referred to fish occur still lower in the Silurian series in Wales.

2. The Llandeilo flags, so named from a town in Caermarthenshire, form the base of the Silurian system, consisting of dark-coloured micaceous grit, frequently calcareous, and distinguished by containing the large trilobites Asaphus Buchii and A. tyrannus, Murch., both of which are peculiar to these rocks. Several species of Graptolites (fig. 430.) occur in these beds.

In the fine shales of this formation Graptolites are very abundant. I collected these same bodies in great numbers in Sweden and Norway in 1835-6, both in the higher and lower shales of the Silurian system; and was informed by Dr. Beck of Copenhagen, that they were fossil zoophytes related to the genera Pennatula and Virgularia, of which the living species now inhabit mud and slimy sediment. The most eminent naturalists still hold to this opinion.

A species of Lingula is met with in the lowest part of the Llandeilo beds; and it is remarkable that this brachiopod is among the earliest, if not the most ancient animal form detected in the lowest Silurian of North America. These inhabitants of the seas, of so remote an epoch, belonged so strictly to the living genus Lingula, as to demonstrate, like the pteriform ferns of the coal, through what incalculable periods of time the same plan and type of organization has sometimes prevailed.

Among the forms of trilobite extremely characteristic of the Lower Silurian throughout Europe and North America, the Trinucleus may be mentioned. This family of crustaceans appears to have swarmed in the Silurian seas, just as crabs, shrimps, and other genera of crustaceans abound in our own. Burmeister, in his work on the organization of trilobites, supposes them to have swum at the surface of the water in the open sea and near coasts, feeding on smaller marine animals, and to have had the power of rolling themselves into a ball as a defence against injury. They underwent various transformations analogous to those of living crustaceans. M. Barrande, author of a work on the Silurian rocks of Bohemia, has traced the same species from the young state just after its escape from the egg to the adult form, through various metamorphoses, each having the appearance of a distinct species. Yet, notwithstanding the numerous species of preceding naturalists which he has thus succeeded in uniting into one, he announces a forthcoming work in which descriptions and figures of 250 species of Trilobite will be given.

Cystideae.--Among the additions which recent research has made to the paleontology of the oldest Silurian rocks, none are more remarkable than the radiated animals called Cystideae. Their structure and relations were first elucidated in an essay published by Von Buch at Berlin in 1845. They are usually met with as spheroidal bodies covered with polygonal plates, with a mouth on the upper side, and a point of attachment for a stem b (which is almost always broken off) on the lower. (See fig. 433.) They are considered by Professor E. Forbes as intermediate between the crinoids and echinoderms. The Sphaeronites here represented (fig. 433.) occurs in the Llandeilo beds in Wales.[358-A]

Thickness and unconformability of Silurian strata.--According to the observation of our government surveyors in North Wales, the Lower Silurian strata of that region attain, in conjunction with the contemporaneous volcanic rocks, the extraordinary thickness of 27,000 feet. One of the groups, called the trappean, consisting of slates and associated volcanic ash and greenstone, is 15,000 feet thick. Another series, called the Bala group, composed of slates and grits with an impure limestone rich in organic remains, is 9,000 feet thick.[359-A]

Throughout North Wales the Wenlock shales rest unconformably upon the Caradoc sandstones; and the Caradoc is in its turn unconformable to the Llandeilo beds, showing a considerable interval of time between the deposition of this group and that of the formations next above and below it. The Caradoc sandstone in the neighbourhood of the Longmynd Hills in Shropshire, appears to Professor E. Forbes to have been a deep-sea deposit formed around the margin of high and steep land. That land consisted partly of upraised Llandeilo flags and partly of rocks of still older date.[359-B]

Such evidence of the successive disturbance of strata during the Silurian period in Great Britain is what we might look for when we have discovered the signs of so grand a series of volcanic eruptions as the contemporaneous greenstones and tuffs of the Welsh mountains afford.

Silurian Strata of the United States.

The position of some of these strata, where they are bent and highly inclined in the Appalachian chain, or where they are nearly horizontal to the west of that chain, is shown in the section, fig. 379. p. 327. But these formations can be studied still more advantageously north of the same line of section, in the states of New York, Ohio, and other regions north and south of the great Canadian lakes. Here they are found, as in Russia, in horizontal position, and are more rich in well-preserved fossils than in almost any spot in Europe. The American strata may readily be divided into Upper and Lower Silurian, corresponding in age and fossils to the European divisions bearing the same names. The subordinate members of the New York series, founded on lithological and geographical considerations, are most useful in the United States, but even there are only of local importance. Some few of them, however, tally very exactly with English divisions, as for example the limestone, over which the Niagara is precipitated at the great cataract, which, with its underlying shales, agrees paleontologically with the Wenlock limestone and shale of Siluria. There is also a marked general correspondence in the succession of fossil forms, and even species, as we trace the organic remains downwards from the highest to the lowest beds.

Mr. D. Sharpe, in his report on the mollusca collected by me from these strata in North America[359-C], has concluded that the number of species common to the Silurian rocks, on both sides of the Atlantic, is between 30 and 40 per cent.; a result which, although no doubt liable to future modification, when a larger comparison shall have been made, proves, nevertheless, that many of the species had a wide geographical range. It seems that comparatively few of the gasteropods and lamellibranchiate bivalves of North America can be identified specifically with European fossils, while no less than two-fifths of the brachiopoda are the same. In explanation of these facts, it is suggested, that most of the recent brachiopoda (especially the orthidiform ones) are inhabitants of deep water, and may have had a wider geographical range than shells living near shore. The predominance of bivalve mollusca of this peculiar class has caused the Silurian period to be sometimes styled the age of brachiopods.

Whether the Silurian rocks are of deep-water origin.--The grounds relied upon by Professor E. Forbes, for inferring that the larger part of the Silurian Fauna is indicative of a sea more than 70 fathoms deep, are the following: first, the small size of the greater number of conchifera; secondly, the paucity of pectinibranchiata (or spiral univalves); thirdly, the great number of floaters, such as Bellerophon, Orthoceras, &c.; fourthly, the abundance of orthidiform brachiopoda; fifthly, the absence or great rarity of fossil fish.

It is doubtless true that some living Terebratulae, on the coast of Australia, inhabit shallow water; but all the known species, allied in form to the extinct Orthis, inhabit the depths of the sea. It should also be remarked that Mr. Forbes, in advocating these views, was well aware of the existence of shores, bounding the Silurian sea in Shropshire, and of the occurrence of littoral species of this early date in the northern hemisphere. Such facts are not inconsistent with his theory; for he has shown, in another work, how, on the coast of Lycia, deep-sea strata are at present forming in the Mediterranean, in the vicinity of high and steep land.

Had we discovered the ancient delta of some large Silurian river, we should doubtless have known more of the shallow, and brackish water, and fluviatile animals, and of the terrestrial flora of the period under consideration. To assume that there were no such deltas in the Silurian world, would be almost as gratuitous an hypothesis, as for the inhabitants of the coral islands of the Pacific to indulge in a similar generalization respecting the actual condition of the globe.[360-A]

Mineral Character of Silurian Strata.

In lithological character, the Silurian strata vary greatly when we trace them through Europe and North America. The shales called mudstones are as little altered from some deposits, found in recent submarine banks, as are those of many tertiary formations. We meet with red sandstone and red marl, with gypsum and salt, of Upper Silurian date, in the Niagara district, which might be mistaken for trias. The whitish granular sandstone at the base of the Silurian series in Sweden resembles the tertiary siliceous grit of Fontainebleau. The Calcareous Grit, oolite, and pisolite of Upper Silurian age in Gothland, are described by Sir R. Murchison as singularly like rocks of the oolitic period near Cheltenham; and, not to cite more examples, the Wenlock or Dudley limestone often resembles a modern coral-reef. If, therefore, uniformity of aspect has been thought characteristic of rocks of this age, the idea must have arisen from the similarity of feature acquired by strata subject to metamorphic action. This influence, seeing that the causes of change are always shifting the theatre of their principal development, must be multiplied throughout a wider geographical area by time, and become more general in any given system of rocks in proportion to their antiquity. We are now acquainted with dense groups of Eocene slates in the Alps, which were once mistaken by experienced geologists for Transition or Silurian formations. The error arose from attaching too great importance to mineral character as a test of age, for the tertiary slates in question having acquired that crystalline texture which is in reality most prevalent in the most ancient sedimentary formations.

CAMBRIAN GROUP.

Below the Silurian strata in North Wales, and in the region of the Cumberland lakes, there are some slaty rocks, devoid of organic remains, or in which a few obscure traces only of fossils have been detected (for which the names of Cambrian and Cumbrian have been proposed). Whether these will ever be entitled by the specific distinctness of their fossils to rank as independent groups, we have not yet sufficient data to determine.

* * * * *

TABULAR VIEW OF FOSSILIFEROUS STRATA,

Showing the Order of Superposition or Chronological Succession of the principal European Groups.

I. POST-TERTIARY.

A. POST-PLIOCENE.

Periods and Groups. Examples. Observations.

1. Recent. { Peat mosses and shell-marl, } All the imbedded shells, { with bones of land animals, } freshwater and marine, { human remains, and works } of living species, { of art. } with occasional { } human remains and { Newer parts of modern deltas } works of art. { and coral reefs. }

2. Post-Pliocene. { Clay, marl, and volcanic tuff } All the shells of living { of Ischia, p. 113. } species. No human { } remains or works { Loess of the Rhine, p. 117. } of art. Bones of { } quadrupeds, partly { Newer part of boulder } of extinct species. { formation, with erratics, } { p. 124. }

II. TERTIARY.

B. PLIOCENE.

3. Newer Pliocene { Boulder formation or drift of { Three-fourths of the or Pleistocene. { northern Europe and North { fossil shells of { America, chaps. 11. & 12. { existing species. { { { Cavern deposits and osseous { A majority of the { breccias, p. 153. { mammalia extinct; { { but the genera { Fluvio-marine crag of Norwich, { corresponding with { p. 148. { those now surviving in { { the same great { Limestone of Girgenti, { geographical and { in Sicily, p. 152. { zoological province, { p. 157. { { During part of this { period icebergs { frequent in the seas { of the northern { hemisphere, and { glaciers on hills { of moderate height.

4. Older Pliocene. { Red and Coralline crag of { A third or more of the { Suffolk, p. 162. { species of mollusca { { extinct. { Subapennine beds, p. 166. { { Nearly, if not all, the { mammalia extinct.

C. MIOCENE.

5. Miocene. { Faluns of Touraine, p. 168. { About two-thirds of the { { species of shells { Part of Bordeaux beds, p. 171. { extinct. { { { Part of Molasse of { The recent species of { Switzerland, p. 171. { shells often not { found in the { adjoining seas, but { in warmer latitudes. { { All the mammalia { extinct.

D. EOCENE.

6. Upper Eocene. { Upper marine of Paris basin, } Fossil shells of the { Fontainebleau sandstone, } Eocene period, with { p. 175. } very few exceptions, { } extinct. Those which { Upper freshwater and millstone } are identified with { of same. } living species rarely { Kleyn Spauwen beds, p. 176. } belong to neighbouring { } regions. { Hermsdorf tile-clay, near } { Berlin. } All the mammalia of { } extinct species, and { Mayence tertiary strata, } the greater part of { p. 177. } them of extinct { } genera. { Freshwater beds of Limagne } { d'Auvergne, p. 181. } Plants of Upper Eocene, } indicating a south 7. Middle Eocene. { Paris gypsum with } European or { Paleotherium, &c., p. 191. } Mediterranean climate; { } those of Lower Eocene, { Freshwater and fluvio-marine } a tropical climate. { beds of Headon Hill, Isle } { of Wight, p. 197. } { } { Barton beds, Hants, p. 198. } { } { Calcaire Grossier, Paris, } { p. 193. } { } { Bagshot and Bracklesham beds, } { Surrey and Sussex, p. 198. } } 8. Lower Eocene. { London clay proper of Highgate } { Hill and Sheppey,--Bognor } { beds, Sussex, p. 200. } { } { Sables inferieurs, and lits } { coquilliers of Paris basin, } { p. 196. } { } { Mottled and plastic clays and } { sands of the Hampshire and } { London basins, p. 203. } { } { Sables inferieurs and argiles } { plastiques of Paris basin, } { p. 196. } { } { Nummulitic formation of the } { Alps, p. 205. }

III. SECONDARY.

E. CRETACEOUS.

Sec. UPPER CRETACEOUS.

9. Maestricht { Yellowish white limestone of { Ammonite, Baculite, and beds. { Maestricht, p. 209. { Belemnite, associated { { with Cypraea, Oliva, { Coralline limestone of Faxoe, { Mitra, Trochus, &c. { Denmark, p. 210. { Large marine saurians.

10. Upper White { White chalk with flints of } Marine limestone Chalk. { North and South } formed in part of { Downs,--Surrey and Sussex, } decomposed corals. { p. 211. }

11. Lower White { Chalk without flints, and } Chalk. { chalk marl, ibid. }

12. Upper { Loose sand, with bright green } Greensand. { particles, ibid. } { } { Firestone of Merstham, Kent, } { p. 218. } { } { Marly stone, with layers of } { chert, south of Isle of } { Wight. }

13. Gault. { Dark blue marl at base of { Numerous extinct genera { chalk escarpment,--Kent { of conchiferous { and Sussex, p. 218. { cephalopoda, Hamite, { Scaphite, Ammonite, &c.

Sec.Sec. LOWER CRETACEOUS.

14. Lower { Sand with green matter,--Weald } Species of shells, &c., Greensand. { of Kent and Sussex, } nearly all distinct { p. 219. } from those of Upper { } Cretaceous; most of { White, yellowish, and } the genera the same. { ferruginous sand, with } { concretions of limestone and } { chert,--Atherfield, Isle } { of Wight. } { } { Limestone called Kentish Rag }

F. WEALDEN.

15. Weald Clay. { Clay with occasional bands of { Of freshwater origin. { limestone,--Weald of Kent, { Shells of { Surrey, and Sussex, p. 227. { pulmoniferous { mollusca, and of { Cypris. Land reptiles.

16. Hastings Sand. { Sand with calciferous grit and { Freshwater with { clay,--Hastings, Sussex, { intercalated bed of { Cuckfield, Kent, p. 229. { brackish and salt { water origin. Shells { of fluviatile and { lacustrine genera. { Reptiles of the genera { Pterodactyle, { Iguanodon, { Megalosaurus, { Plesiosaurus, Trionyx, { and Emys.

17. Purbeck Beds. Limestones, calcareous slates { Chiefly freshwater, and and marls, p. 231. { divisible into three { groups, each { containing distinct { species of freshwater { mollusca and of { entomostraca. { Alternations of { deposits formed in { fresh, brackish, and { marine water, and of { ancient soils formed { on land and retaining { roots of trees. { Plants chiefly cycads { and conifers, p. 231.

G. OOLITE.

18. Upper Oolite. { a. Portland building stone, } Ammonites and Belemnites { p. 259. } numerous. { } { b. Portland sand. } Large saurians, as { } Pterodactyles, { c. Kimmeridge clay, } Plesiosaurs, { Dorsetshire, p. 260. } Ichthyosaurs. } 19. Middle Oolite. { a. Coral Rag, p. 260. } No cetaceans yet known, { Calcareous freestones, } but three species of { oolitic, often full of } terrestrial mammalia, { corals. Oxfordshire. } p. 267, 268. { } Preponderance of { b. Oxford clay--Dark blue } ganoid fish. The { clay,--Oxfordshire and } plants chiefly cycads, { midland counties, p. 262. } conifers, and ferns, } with a few palms. 20. Lower Oolite. { a. Cornbrash and forest } { marble, Wiltshire, p. 263. } { } { b. Great oolite and } { Stonesfield slate,--Bath, } { Bradford, Stonesfield near } { Woodstock, Oxfordshire, } { p. 266. } { } { c. Fuller's earth,--Clay } { containing fuller's earth } { near Bath, p. 272. } { } { d. Inferior oolite, } { calcareous freestone, and } { yellow sands,--Cotteswold } { Hills, Dundry Hill, near } { Bristol, p. 272. }

H. LIAS.

21. Lias. { Argillaceous limestone, marl { Mollusca, reptiles, { and clay,--Lyme Regis, { and fish of genera { Dorsetshire, p. 273. { analogous to the { oolitic.

I. TRIAS.

22. Upper Trias. { Keuper of Germany, or } Batrachian reptiles, { variegated marls--Red, grey, } e.g. Labyrinthodon, { green, blue, and white marls } Rhyncosaurus, &c. { and sandstones with } Cephalopoda: { gypsum--Wuertemberg, bone-bed } Ceratites. No { of Axmouth, Dorset, p. 289. } Belemnites. Plants: } Ferns, Cycads, } Conifers.

23. Middle Trias { Compact greyish limestone } With Equisetites or { with beds of dolomite and } and Calamite. Muschelkalk. { gypsum,--North of Germany, } { p. 287. Wanting in } { England. }

24. Lower Trias. { Variegated or Bunter sandstone } Plants different for { of Germans--Red and white } the most part from { spotted sandstone with } those of the Upper { gypsum and rock-salt, P. 288 } Trias. { } { Part of New Red sandstone of } { of Cheshire with rock-salt, } { p. 294. }

IV. PRIMARY.

K. PERMIAN.

25. Upper Permian. { Yellow magnesian limestone, } Organic remains, both { Yorkshire and Durham, } animal and vegetable, { P. 301. } more allied to primary { } than to secondary { Zechstein of Thuringia, Upper } periods. { part of Permian beds, } { Russia. }

26. Lower Permian. { a. Marl slate of Durham and } Thecodont saurians. { Thuringia. } Heterocercal fish of { } genus Palaeoniscus, &c. { b. Lower New Red sandstone } { of north of England and } { Rothliegendes of Germany. } { } { a. and b. Lower part of } { Permian beds, Russia, } { p. 301. }

L. CARBONIFEROUS.

27. Coal measures. { a. Strata of sandstone and } Great thickness of { shale, with beds of } strata of { coal,--S. Wales and } fluvio-marine origin, { Northumberland, p. 309. } with beds of coal of { } vegetable origin, { b. Millstone grit,--S. } based on soils { Wales, Bristol coal-field, } retaining the roots { Yorkshire, p. 308. } of trees. } } Oldest of known reptiles } or Archegosaurus. } Sauroid fish.

28. Mountain { Carboniferous or mountain { Brachiopoda of genus limestone. { limestone, with marine { Productus. { shells and corals. { { { Cephalopoda of genera { Mendip Hills, and many parts { Cyrtoceras, Goniatite, { of Ireland, p. 340. { Orthoceras. { { Crustaceans of the { genus Phillipsia. { { Crinoideans abundant.

M. DEVONIAN.

29. Upper { a. Yellow sandstone of Dura } Tribe of fish with hard Devonian. { Den, Fife. } coverings like { } chelonians, { b. Red sandstone and marl } Pterichthys, { with cornstone of } Pamphractus, &c.; { Herefordshire and } also of genera { Forfarshire. } Cephalaspis, { } Holoptichius, &c. { Paving and roofing-stone, } { Forfarshire. } No reptiles yet known. { } { Upper part of Devonian beds } { of South Devon. }

30. Lower { Grey sandstone with } Fish, partly of same Devonian. { Ichthyolites,--Caithness, } genera, but of { Cromarty, and Orkney, Lower } distinct species from { part of Devonian beds of } those in Upper { South Devon, and green } Devonian; also { chloritic slates of } Osteolepis, { Cornwall, limestone of } Coccosteus, { Gerolstein, Eifel. } Glyptolepis, } Dipterus, &c.

N. SILURIAN.

31. Upper { a. Tilestone of Brecon and { Oldest of fossil fish Silurian. { Caermarthen. { yet discovered. { { { b. Limestone and shale, { Trilobites and { Ludlow, Shropshire. { Graptolites abundant. { { { c. Wenlock or Dudley { Brachiopoda very { limestone. { numerous. { { Cephalopoda: { Bellerophon, { Orthoceras.

{ Same genera of 32. Lower { a. Caradoc sandstone, Caer { invertebrate animals Silurian. { Caradoc, Shropshire. { as in Upper Silurian, { { but species chiefly { b. Llandeilo flags, { distinct. Trinucleus { calcareous flags and { caractaci, Cystideae, { schists,--Builth, { p. 358. { Radnorshire, Llandeilo, { { Caermarthenshire. { No land plants yet { known. { { Footprints of tortoise, { see note, p. 360.

FOOTNOTES:

[352-A] Murchison, Silurian System, p. 198, 199.

[354-A] Silurian System, pl. 7. bis. fig. 1. b.

[358-A] Quart. Geol. Journ., vol. ii. p. 11.; and Memoirs of Geol. Survey, vol. ii. p. 518.

[359-A] Quart. Geol. Journ., vol. iv. p. 300.

[359-B] Ibid., 299.

[359-C] Ibid., 145.

[360-A] Since this was written, Mr. Logan has discovered chelonian footprints in the lowest fossiliferous beds of the Silurian series, near Montreal, in Canada. Professor Owen inclines to refer them to the genus Emys.--Quart. Journ. G. S., vol. vii. p. lxxvi.

Spotted a typo or a problem in this chapter? Tell us

XXVIII. VOLCANIC ROCKS

Trap rocks--Name, whence derived--Their igneous origin at first doubted--Their general appearance and character--Volcanic cones and craters, how formed--Mineral composition and texture of volcanic rocks--Varieties of felspar--Hornblende and augite--Isomorphism--Rocks, how to be studied--Basalt, greenstone, trachyte, porphyry, scoria, amygdaloid, lava, tuff--Alphabetical list, and explanation of names and synonyms, of volcanic rocks--Table of the analyses of minerals most abundant in the volcanic and hypogene rocks.

The aqueous or fossiliferous rocks having now been described, we have next to examine those which may be called volcanic, in the most extended sense of that term. Suppose a a in the annexed diagram, to represent the crystalline formations, such as the granitic and metamorphic; b b the fossiliferous strata; and c c the volcanic rocks. These last are sometimes found, as was explained in the first chapter, breaking through a and b, sometimes overlying both, and occasionally alternating with the strata b b. They also are seen, in some instances, to pass insensibly into the unstratified division of a, or the Plutonic rocks.

When geologists first began to examine attentively the structure of the northern and western parts of Europe, they were almost entirely ignorant of the phenomena of existing volcanos. They also found certain rocks, for the most part without stratification, and of a peculiar mineral composition, to which they gave different names, such as basalt, greenstone, porphyry, and amygdaloid. All these, which were recognized as belonging to one family, were called "trap" by Bergmann, from trappa, Swedish for a flight of steps--a name since adopted very generally into the nomenclature of the science; for it was observed that many rocks of this class occurred in great tabular masses of unequal extent, so as to form a succession of terraces or steps on the sides of hills. This configuration appears to be derived from two causes. First, the abrupt original terminations of sheets of melted matter, which have spread, whether on the land or bottom of the sea, over a level surface. For we know, in the case of lava flowing from a volcano, that a stream, when it has ceased to flow, and grown solid, very commonly ends in a steep slope, as at a, fig. 435. But, secondly, the step-like appearance arises more frequently from the mode in which horizontal masses of igneous rock, such as b c, intercalated between aqueous strata, have, subsequently to their origin, been exposed, at different heights, by denudation. Such an outline, it is true, is not peculiar to trap rocks; great beds of limestone, and other hard kinds of stone, often presenting similar terraces and precipices: but these are usually on a smaller scale, or less numerous, than the volcanic steps, or form less decided features in the landscape, as being less distinct in structure and composition from the associated rocks.

Although the characters of trap rocks are greatly diversified, the beginner will easily learn to distinguish them as a class from the aqueous formations. Sometimes they present themselves, as already stated, in tabular masses, which are not divided into strata: sometimes in shapeless lumps and irregular cones, forming chains of small hills. Often they are seen in dikes and wall-like masses, intersecting fossiliferous beds. The rock is occasionally found divided into columns, often decomposing into balls of various sizes, from a few inches to several feet in diameter. The decomposing surface very commonly assumes a coating of a rusty iron colour, from the oxidation of ferruginous matter, so abundant in the traps in which augite or hornblende occur; or, in the felspathic varieties of trap, it acquires a white opaque coating, from the bleaching of the mineral called felspar. On examining any of these volcanic rocks, where they have not suffered disintegration, we rarely fail to detect a crystalline arrangement in one or more of the component minerals. Sometimes the texture of the mass is cellular or porous, or we perceive that it has once been full of pores and cells, which have afterwards become filled with carbonate of lime, or other infiltrated mineral.

Most of the volcanic rocks produce a fertile soil by their disintegration. It seems that their component ingredients, silica, alumina, lime, potash, iron, and the rest, are in proportions well fitted for vegetation. As they do not effervesce with acids, a deficiency of calcareous matter might at first be suspected; but although the carbonate of lime is rare, except in the nodules of amygdaloids, yet it will be seen that lime sometimes enters largely into the composition of augite and hornblende. (See Table, p. 377.)

Cones and Craters.--In regions where the eruption of volcanic matter has taken place in the open air, and where the surface has never since been subjected to great aqueous denudation, cones and craters constitute the most striking peculiarity of this class of formations. Many hundreds of these cones are seen in central France, in the ancient provinces of Auvergne, Velay, and Vivarais, where they observe, for the most part, a linear arrangement, and form chains of hills. Although none of the eruptions have happened within the historical era, the streams of lava may still be traced distinctly descending from many of the craters, and following the lowest levels of the existing valleys. The origin of the cone and crater-shaped hill is well understood, the growth of many having been watched during volcanic eruptions. A chasm or fissure first opens in the earth, from which great volumes of steam and other gases are evolved. The explosions are so violent as to hurl up into the air fragments of broken stone, parts of which are shivered into minute atoms. At the same time melted stone or lava usually ascends through the chimney or vent by which the gases make their escape. Although extremely heavy, this lava is forced up by the expansive power of entangled gaseous fluids, chiefly steam or aqueous vapour, exactly in the same manner as water is made to boil over the edge of a vessel when steam has been generated at the bottom by heat. Large quantities of the lava are also shot up into the air, where it separates into fragments, and acquires a spongy texture by the sudden enlargement of the included gases, and thus forms scoriae, other portions being reduced to an impalpable powder or dust. The showering down of the various ejected materials round the orifice of eruption gives rise to a conical mound, in which the successive envelopes of sand and scoriae form layers, dipping on all sides from a central axis. In the mean time a hollow, called a crater, has been kept open in the middle of the mound by the continued passage upwards of steam and other gaseous fluids. The lava sometimes flows over the edge of the crater, and thus thickens and strengthens the sides of the cone; but sometimes it breaks it down on one side, and often it flows out from a fissure at the base of the hill (see fig. 436.).[368-A]

Composition and nomenclature.--Before speaking of the connection between the products of modern volcanos and the rocks usually styled trappean, and before describing the external forms of both, and the manner and position in which they occur in the earth's crust, it will be desirable to treat of their mineral composition and names. The varieties most frequently spoken of are basalt, greenstone, syenitic greenstone, clinkstone, claystone, and trachyte; while those founded chiefly on peculiarities of texture, are porphyry, amygdaloid, lava, tuff, scoriae, and pumice. It may be stated generally, that all these are mainly composed of two minerals, or families of simple minerals, felspar and hornblende; some almost entirely of hornblende, others of felspar.

These two minerals may be regarded as two groups, rather than species. Felspar, for example, may be, first, common felspar, that is to say, potash-felspar, in which the alkali is potash (see table, p. 377.); or, secondly, albite, that is to say, soda-felspar, where the alkali is soda instead of potash; or, thirdly, Labrador-felspar (Labradorite), which differs not only in its iridescent hues, but also in its angle of fracture or cleavage, and its composition. We also read much of two other kinds, called glassy felspar and compact felspar, which, however, cannot rank as varieties of equal importance, for both the albitic and common felspar appear sometimes in transparent or glassy crystals; and as to compact felspar, it is a compound of a less definite nature, sometimes containing both soda and potash; and which might be called a felspathic paste, being the residuary matter after portions of the original matrix have crystallized.

The other group, or hornblende, consists principally of two varieties; first, hornblende, and, secondly, augite, which were once regarded as very distinct, although now some eminent mineralogists are in doubt whether they are not one and the same mineral, differing only as one crystalline form of native sulphur differs from another.

The history of the changes of opinion on this point is curious and instructive. Werner first distinguished augite from hornblende; and his proposal to separate them obtained afterwards the sanction of Hauey, Mohs, and other celebrated mineralogists. It was agreed that the form of the crystals of the two species were different, and their structure, as shown by cleavage, that is to say, by breaking or cleaving the mineral with a chisel, or a blow of the hammer, in the direction in which it yields most readily. It was also found by analysis that augite usually contained more lime, less alumina, and no fluoric acid; which last, though not always found in hornblende, often enters into its composition in minute quantity. In addition to these characters, it was remarked as a geological fact, that augite and hornblende are very rarely associated together in the same rock; and that when this happened, as in some lavas of modern date, the hornblende occurs in the mass of the rock, where crystallization may have taken place more slowly, while the augite merely lines cavities where the crystals may have been produced rapidly. It was also remarked, that in the crystalline slags of furnaces, augitic forms were frequent, the hornblendic entirely absent; hence it was conjectured that hornblende might be the result of slow, and augite of rapid cooling. This view was confirmed by the fact, that Mitscherlich and Berthier were able to make augite artificially, but could never succeed in forming hornblende. Lastly, Gustavus Rose fused a mass of hornblende in a porcelain furnace, and found that it did not, on cooling, assume its previous shape, but invariably took that of augite. The same mineralogist observed certain crystals in rocks from Siberia which presented a hornblende cleavage, while they had the external form of augite.

If, from these data, it is inferred that the same substance may assume the crystalline forms of hornblende or augite indifferently, according to the more or less rapid cooling of the melted mass, it is nevertheless certain that the variety commonly called augite, and recognized by a peculiar crystalline form, has usually more lime in it, and less alumina, than that called hornblende, although the quantities of these elements do not seem to be always the same. Unquestionably the facts and experiments above mentioned show the very near affinity of hornblende and augite; but even the convertibility of one into the other by melting and recrystallizing, does not perhaps demonstrate their absolute identity. For there is often some portion of the materials in a crystal which are not in perfect chemical combination with the rest. Carbonate of lime, for example, sometimes carries with it a considerable quantity of silex into its own form of crystal, the silex being mechanically mixed as sand, and yet not preventing the carbonate of lime from assuming the form proper to it. This is an extreme case, but in many others some one or more of the ingredients in a crystal may be excluded from perfect chemical union; and, after fusion, when the mass recrystallizes, the same elements may combine perfectly or in new proportions, and thus a new mineral may be produced. Or some one of the gaseous elements of the atmosphere, the oxygen for example, may, when the melted matter reconsolidates, combine with some one of the component elements.

The different quantity of the impurities or refuse above alluded to, which may occur in all but the most transparent and perfect crystals, may partly explain the discordant results at which experienced chemists have arrived in their analysis of the same mineral. For the reader will find that a mineral determined to be the same by its physical characters, crystalline form, and optical properties, has often been declared by skilful analyzers to be composed of distinct elements. (See the table at p. 377.) This disagreement seemed at first subversive of the atomic theory, or the doctrine that there is a fixed and constant relation between the crystalline form and structure of a mineral, and its chemical composition. The apparent anomaly, however, which threatened to throw the whole science of mineralogy into confusion, was in a great degree reconciled to fixed principles by the discoveries of Professor Mitscherlich at Berlin, who ascertained that the composition of the minerals which had appeared so variable, was governed by a general law, to which he gave the name of isomorphism (from +isos+, isos, equal, and +morphe+, morphe, form). According to this law, the ingredients of a given species of mineral are not absolutely fixed as to their kind and quality; but one ingredient may be replaced by an equivalent portion of some analogous ingredient. Thus, in augite, the lime may be in part replaced by portions of protoxide of iron, or of manganese, while the form of the crystal, and the angle of its cleavage planes, remain the same. These vicarious substitutions, however, of particular elements cannot exceed certain defined limits.

Having been led into this digression on the recent progress of mineralogy, I may here observe that the geological student must endeavour as soon as possible to familiarize himself with the characters of five at least of the most abundant simple minerals of which rocks are composed. These are, felspar, quartz, mica, hornblende, and carbonate of lime. This knowledge cannot be acquired from books, but requires personal inspection, and the aid of a teacher. It is well to accustom the eye to know the appearance of rocks under the lens. To learn to distinguish felspar from quartz is the most important step to be first aimed at. In general we may know the felspar because it can be scratched with the point of a knife, whereas the quartz, from its extreme hardness, receives no impression. But when these two minerals occur in a granular and uncrystallized state, the young geologist must not be discouraged if, after considerable practice, he often fails to distinguish them by the eye alone. If the felspar is in crystals, it is easily recognized by its cleavage: but when in grains the blow-pipe must be used, for the edges of the grains can be rounded in the flame, whereas those of quartz are infusible. If the geologist is desirous of distinguishing the three varieties of felspar above enumerated, or hornblende from augite, it will often be necessary to use the reflecting goniometer as a test of the angle of cleavage, and shape of the crystal. The use of this instrument will not be found difficult.

The external characters and composition of the felspars are extremely different from those of augite or hornblende; so that the volcanic rocks in which either of these minerals decidedly predominates, are easily recognized. But there are mixtures of the two elements in every possible proportion, the mass being sometimes exclusively composed of felspar, at other times solely of augite, or, again, of both in equal quantities. Occasionally, the two extremes, and all the intermediate gradations, may be detected in one continuous mass. Nevertheless there are certain varieties or compounds which prevail so largely in nature, and preserve so much uniformity of aspect and composition, that it is useful in geology to regard them as distinct rocks, and to assign names to them, such as basalt, greenstone, trachyte, and others, already mentioned.

Basalt.--As an example of rocks in which augite greatly prevails, basalt may first be mentioned. Although we are more familiar with this term than with that of any other kind of trap, it is difficult to define it, the name having been used so vaguely. It has been very generally applied to any trap rock of a black, bluish, or leaden-grey colour, having a uniform and compact texture. Most strictly, it consists of an intimate mixture of augite, felspar, and iron, to which a mineral of an olive green colour, called olivine, is often superadded, in distinct grains or nodular masses. The iron is usually magnetic, and is often accompanied by another metal, titanium. Augite is the predominant mineral, the felspar being in much smaller proportions. There is no doubt that many of the fine-grained and dark-coloured trap rocks, called basalt, contained hornblende in the place of augite; but this will be deemed of small importance after the remarks above made. Other minerals are occasionally found in basalt; and this rock may pass insensibly into almost every variety of trap, especially into greenstone, clinkstone, and wacke, which will be presently described.

Greenstone, or Dolerite, is usually defined as a granular rock, the constituent parts of which are hornblende and imperfectly crystallized felspar; the felspar being more abundant than in basalt; and the grains or crystals of the two minerals more distinct from each other. This name may also be extended to those rocks in which augite is substituted for hornblende (the dolorite of some authors), or to those in which albite replaces common felspar, forming the rock sometimes called Andesite.

Syenitic greenstone.--The highly crystalline compounds of the same two minerals, felspar and hornblende, having a granitiform texture, and with occasionally some quartz accompanying, may be called Syenitic greenstone, a rock which frequently passes into ordinary trap, and as frequently into granite.

Trachyte.--A porphyritic rock of a whitish or greyish colour, composed principally of glassy felspar, with crystals of the same, generally with some hornblende and some titaniferous iron. In composition it is extremely different from basalt, this being a felspathic, as the other is an augitic, rock. It has a peculiar rough feel, whence the name +trachys+, trachus, rough. Some varieties of trachyte contain crystals of quartz.

Porphyry is merely a certain form of rock, very characteristic of the volcanic formations. When distinct crystals of one or more minerals are scattered through an earthy or compact base, the rock is termed a porphyry (see fig. 437.). Thus trachyte is porphyritic; for in it, as in many modern lavas, there are crystals of felspar; but in some porphyries the crystals are of augite, olivine, or other minerals. If the base be greenstone, basalt, or pitchstone, the rock may be denominated greenstone-porphyry, pitchstone-porphyry, and so forth.

Amygdaloid.--This is also another form of igneous rock, admitting of every variety of composition. It comprehends any rock in which round or almond-shaped nodules of some mineral, such as agate, calcedony, calcareous spar, or zeolite, are scattered through a base of wacke, basalt, greenstone, or other kind of trap. It derives its name from the Greek word amygdala, an almond. The origin of this structure cannot be doubted, for we may trace the process of its formation in modern lavas. Small pores or cells are caused by bubbles of steam and gas confined in the melted matter. After or during consolidation, these empty spaces are gradually filled up by matter separating from the mass, or infiltered by water permeating the rock. As these bubbles have been sometimes lengthened by the flow of the lava before it finally cooled, the contents of such cavities have the form of almonds. In some of the amygdaloidal traps of Scotland, where the nodules have decomposed, the empty cells are seen to have a glazed or vitreous coating, and in this respect exactly resemble scoriaceous lavas, or the slags of furnaces.

The annexed figure represents a fragment of stone taken from the upper part of a sheet of basaltic lava in Auvergne. One half is scoriaceous, the pores being perfectly empty; the other part is amygdaloidal, the pores or cells being mostly filled up with carbonate of lime, forming white kernels.

Scoriae and Pumice may next be mentioned as porous rocks, produced by the action of gases on materials melted by volcanic heat. Scoriae are usually of a reddish-brown and black colour, and are the cinders and slags of basaltic or augitic lavas. Pumice is a light, spongy, fibrous substance, produced by the action of gases on trachytic and other lavas; the relation, however, of its origin to the composition of lava is not yet well understood. Von Buch says that it never occurs where only Labrador-felspar is present.

Lava.--This term has a somewhat vague signification, having been applied to all melted matter observed to flow in streams from volcanic vents. When this matter consolidates in the open air, the upper part is usually scoriaceous, and the mass becomes more and more stony as we descend, or in proportion as it has consolidated more slowly and under greater pressure. At the bottom, however, of a stream of lava, a small portion of scoriaceous rock very frequently occurs, formed by the first thin sheet of liquid matter, which often precedes the main current, or in consequence of the contact with water in or upon the damp soil.

The more compact lavas are often porphyritic, but even the scoriaceous part sometimes contains imperfect crystals, which have been derived from some older rocks, in which the crystals pre-existed, but were not melted, as being more infusible in their nature.

Although melted matter rising in a crater, and even that which enters rents on the side of a crater, is called lava, yet this term belongs more properly to that which has flowed either in the open air or on the bed of a lake or sea. If the same fluid has not reached the surface, but has been merely injected into fissures below ground, it is called trap.

There is every variety of composition in lavas; some are trachytic, as in the Peak of Teneriffe; a great number are basaltic, as in Vesuvius and Auvergne; others are andesitic, as those of Chili; some of the most modern in Vesuvius consist of green augite, and many of those of Etna of augite and Labrador-felspar.[374-A]

Trap tuff, volcanic tuff.--Small angular fragments of the scoriae and pumice, above mentioned, and the dust of the same, produced by volcanic explosions, form the tuffs which abound in all regions of active volcanos, where showers of these materials, together with small pieces of other rocks ejected from the crater, fall down upon the land or into the sea. Here they often become mingled with shells, and are stratified. Such tuffs are sometimes bound together by a calcareous cement, and form a stone susceptible of a beautiful polish. But even when little or no lime is present, there is a great tendency in the materials of ordinary tuffs to cohere together.

Besides the peculiarity of their composition, some tuffs, or volcanic grits, as they have been termed, differ from ordinary sandstones by the angularity of their grains. When the fragments are coarse, the rock is styled a volcanic breccia. Tufaceous conglomerates result from the intermixture of rolled fragments or pebbles of volcanic and other rocks with tuff.

According to Mr. Scrope, the Italian geologists confine the term tuff, or tufa, to felspathose mixtures, and those composed principally of pumice, using the term peperino for the basaltic tuffs.[374-B] The peperinos thus distinguished are usually brown, and the tuffs grey or white.

We meet occasionally with extremely compact beds of volcanic materials, interstratified with fossiliferous rocks. These may sometimes be tuffs, although their density or compactness is such as to cause them to resemble many of those kinds of trap which are found in ordinary dikes. The chocolate-coloured mud, which was poured for weeks out of the crater of Graham's Island, in the Mediterranean, in 1831, must, when unmixed with other materials, have constituted a stone heavier than granite. Each cubic inch of the impalpable powder which has fallen for days through the atmosphere, during some modern eruptions, has been found to weigh, without being compressed, as much as ordinary trap rocks, and to be often identical with these in mineral composition.

The fusibility of the igneous rocks generally exceeds that of other rocks, for there is much alkaline matter and lime in their composition, which serves as a flux to the large quantity of silica, which would be otherwise so refractory an ingredient.

It is remarkable that, notwithstanding the abundance of this silica, quartz, that is, crystalline silica, is usually wanting in the volcanic rocks, or is present only as an occasional mineral, like mica. The elements of mica, as of quartz, occur in lava and trap; but the circumstances under which these rocks are formed are evidently unfavourable to the development of mica and quartz, minerals so characteristic of the hypogene formations.

It would be tedious to enumerate all the varieties of trap and lava which have been regarded by different observers as sufficiently abundant to deserve distinct names, especially as each investigator is too apt to exaggerate the importance of local varieties which happen to prevail in districts best known to him. It will be useful, however, to subjoin here, in the form of a glossary, an alphabetical list of the names and synonyms most commonly in use, with brief explanations, to which I have added a table of the analysis of the simple minerals most abundant in the volcanic and hypogene rocks.

Explanation of the names, synonyms, and mineral composition of the more abundant volcanic rocks.

AMPHIBOLITE. See Hornblende rock, amphibole being Hauey's name for hornblende.

AMYGDALOID. A particular form of volcanic rock; see p. 372.

AUGITE ROCK. A kind of basalt or greenstone, composed wholly or principally of granular augite. (Leonhard's Mineralreich, 2d edition, p. 85.)

AUGITIC-PORPHYRY. Crystals of Labrador-felspar and of augite, in a green or dark grey base. (Rose, Ann. des Mines, tom. 8. p. 22. 1835.)

BASALT. Chiefly augite--an intimate mixture of augite and felspar with magnetic iron, olivine, &c. See p. 371. The yellowish green mineral called olivine, can easily be distinguished from yellowish felspar by its infusibility, and having no cleavage. The edges turn brown in the flame of the blow-pipe.

BASANITE. Name given by Alex. Brongniart to a rock, having a base of basalt, with more or less distinct crystals of augite disseminated through it.

CLAYSTONE and CLAYSTONE-PORPHYRY. An earthy and compact stone, usually of a purplish colour, like an indurated clay; passes into hornstone; generally contains scattered crystals of felspar and sometimes of quartz.

CLINKSTONE. Syn. Phonolite, fissile Petrosilex; a greenish or greyish rock, having a tendency to divide into slabs and columns; hard, with clean fracture, ringing under the hammer; principally composed of compact felspar, and, according to Gmelin, of felspar and mesotype. (Leonhard, Mineralreich, p. 102.) A rock much resembling clinkstone, and called by some Petrosilex, contains a considerable percentage of quartz and felspar. As both trachyte and basalt pass into clinkstone, the rock so called must be very various in composition.

COMPACT FELSPAR, which has also been called Petrosilex; the rock so called includes the hornstone of some mineralogists, is allied to clinkstone, but is harder, more compact, and translucent. It is a varying rock, of which the chemical composition is not well defined, and is perhaps the same as that of clay. (MacCulloch's Classification of Rocks, p. 481.) Dr. MacCulloch says, that it contains both potash and soda.

CORNEAN. A variety of claystone allied to hornstone. A fine homogeneous paste, supposed to consist of an aggregate of felspar, quartz, and hornblende, with occasionally epidote, and perhaps chlorite; it passes into compact felspar and hornstone. (De la Beche, Geol. Trans. second series, vol. 2. p. 3.)

DIALLAGE ROCK. Syn. Euphotide, Gabbro, and some Ophiolites. Compounded of felspar and diallage, sometimes with the addition of serpentine, or mica, or quartz. (MacCulloch. ibid. p. 648.)

DIORITE. A kind of greenstone, which see. Components, felspar and hornblende in grains. According to Rose, Ann. des Mines, tom. 8. p. 4., diorite consists of albite and hornblende.

DIORITIC-PORPHYRY. A porphyritic greenstone, composed of crystals of albite and hornblende, in a greenish or blackish base. (Rose, ibid. p. 10.)

DOLERITE. Formerly defined as a synonym of greenstone, which see. But, according to Rose (ibid. p. 32.), its composition is black augite and Labrador-felspar; according to Leonhard (Mineralreich, &c. p. 77.), augite, Labrador-felspar, and magnetic iron.

DOMITE. An earthy trachyte, found in the Puy de Dome, in Auvergne.

EUPHOTIDE. A mixture of grains of Labrador-felspar and diallage. (Rose, ibid. p. 19.) According to some, this rock is defined to be a mixture of augite or hornblende, and saussurite, a mineral allied to jade. (Allan's Mineralogy, p. 158.) See Diallage rock.

FELSPAR-PORPHYRY. Syn. Hornstone-porphyry; a base of felspar, with crystals of felspar, and crystals and grains of quartz. See also Hornstone.

GABBRO, see Diallage rock.

GREENSTONE. Syn. Dolerite and diorite; components, hornblende and felspar, or augite and felspar in grains. See above, p. 372.

GREYSTONE. (Graustein of Werner.) Lead grey and greenish rock, composed of felspar and augite, the felspar being more than seventy-five per cent. (Scrope, Journ. of Sci. No. 42. p. 221.) Greystone lavas are intermediate in composition between basaltic and trachytic lavas.

HORNBLENDE ROCK. A greenstone, composed principally of granular hornblende, or augite. (Leonhard, Mineralreich, &c., p. 85.)

HORNSTONE, HORNSTONE-PORPHYRY. A kind of felspar porphyry (Leonhard, ibid.), with a base of hornstone, a mineral approaching near to flint, differing from compact felspar in being infusible.

HYPERSTHENE ROCK, a mixture of grains of Labrador-felspar and hypersthene (Rose, Ann. des Mines, tom. 8. p. 13.), having the structure of syenite or granite; abundant among the traps of Skye. Some geologists consider it a greenstone, in which hypersthene replaces hornblende.

LATERITE. A red jaspery rock, composed of silicate of alumina and oxide of iron. Abundant in the Deccan, in India; and referred to the trap formation; from Later, a brick or tile.

MELAPHYRE. A variety of black porphyry, the base being black augite with crystals of felspar; from +melas+, melas, black.

OBSIDIAN. Vitreous lava like melted glass, nearly allied to pitchstone.

OPHIOLITE, sometimes same as Diallage rocks (Leonhard, p. 77.); sometimes a kind of serpentine.

OPHITE. A green porphyritic rock composed chiefly of hornblende, with crystals of that mineral in a base of the same, mixed with some felspar. It passes into serpentine by a mixture of talc. (Burat's d'Aubuisson, tom. ii. p. 63.)

PEARLSTONE. A volcanic rock, having the lustre of mother of pearl; usually having a nodular structure; intimately related to obsidian, but less glassy.

PEPERINO. A form of volcanic tuff, composed of basaltic scoriae. See p. 374.

PETROSILEX. See Clinkstone and Compact Felspar.

PHONOLITE. Syn. of Clinkstone, which see.

PITCHSTONE. Vitreous lava, less glassy than obsidian; a blackish green rock resembling glass, having a resinous lustre and appearance of pitch; composition various, usually felspar and augite; passes into basalt; occurs in veins, and in Arran forms a dike thirty feet wide, cutting through sandstone; forms the outer walls of some basaltic dikes.

PORPHYRY. Any rock in which detached crystals of felspar, or of one or more minerals, are diffused through a base. See p. 372.

POZZOLANA. A kind of tuff. See p. 36.

PUMICE. A light, spongy, fibrous form of trachyte. See p. 373.

PYROXENIC-PORPHYRY, same as augitic-porphyry, pyroxene being Hauey's name for augite.

SCORIAE. Syn. volcanic cinders; reddish brown or black porous form of lava. See p. 373.

SERPENTINE. A greenish rock, in which there is much magnesia; usually contains diallage, which is nearly allied to the simple mineral called serpentine. Occurs sometimes, though rarely, in dikes, altering the contiguous strata; is indifferently a member of the trappean or hypogene series.

SYENITIC-GREENSTONE; composition, crystals or grains of felspar and hornblende. See p. 372.

TEPHRINE, synonymous with lava. Name proposed by Alex. Brongniart.

TOADSTONE. A local name in Derbyshire for a kind of wacke, which see.

TRACHYTE. Chiefly composed of glassy felspar, with crystals of glassy felspar. See p. 372.

TRAP TUFF. See p. 374.

TRASS. A kind of tuff or mud poured out by lake craters during eruptions; common in the Eifel, in Germany.

TUFACEOUS CONGLOMERATE. See p. 374.

TUFF. Syn. Trap-tuff, volcanic tuff. See p. 374.

VITREOUS LAVA. See Pitchstone and Obsidian.

VOLCANIC TUFF. See p. 374.

WACKE. A soft and earthy variety of trap, having an argillaceous aspect. It resembles indurated clay, and when scratched exhibits a shining streak.

WHINSTONE. A Scotch provincial term for greenstone and other hard trap rocks.

ANALYSIS OF MINERALS MOST ABUNDANT IN THE VOLCANIC AND HYPOGENE ROCKS.

Silica. Alu- Mag- Lime. Pot- Soda. Iron. Manga- Remain- mina. nesia. ash. Oxide. nese. der.

Actinolite 64. -- 22. -- -- -- 3. a 43.05 C. (Bergman) trace

Albite (Rose) 68.84 20.53 -- a trace -- 9.12 -- -- -- --(mean of 4 69.45 19.44 0.13 0.22 -- 9.95 a -- -- analyses) trace

Augite (Rose) 53.36 -- 4.99 22.19 -- -- 17.38 0.09 -- --(mean of 4 53.57 1. 11.26 20.9 -- -- 10.75 0.67 -- analyses)

Carbonate of -- -- -- 56.33 -- -- -- -- -- Lime (Biot)

Chiastolite 68.49 30.17 4.12 -- -- -- 2.7 -- 0.27 W. (Landgrabe)

Chlorite 26. 18.5 8. -- -- 2. 43. -- -- (Vauquelin) --(mean of 3 27.43 17.9 14.56 0.50 1.56 -- 30.63 -- 6.92 W. analyses)

Diallage 60. -- 27.5 -- -- -- 10.5 -- -- (Klaproth) --(mean of 3 43.33 2.2 26.41 5.58 -- -- 11.53 -- 8.54 W. analyses)

Epidote 37. 21. -- 15. -- -- 24. 1.5 -- (Vauquelin)

Felspar, 62.83 17.02 -- 3. - 13. -- 1. -- -- common (Vauq.) --(Rose) 66.75 17.5 -- 1.25 12. -- 0.75 -- -- --(mean of 7 64.04 18.94 -- 0.76 13.66 -- 0.74 -- -- analyses)

Garnet 35.75 27.25 -- -- -- -- 36. 0.25 -- (Klaproth) --(Phillips) 43. 16. -- 20. -- -- 16. -- --

Hornblende 42. 12. 2.25 11. a -- 30. 0.25 -- (Klap.) trace --(Bonsdorff.) 45.69 12.18 18.79 13.85 -- -- 7.32 0.22 1.5 F.

Hypersthene 54.25 2.25 14. 1.5 -- -- 24.5 a 1. W. (Klaproth) trace

Labrador- 55.75 26.5 -- 11. -- 4. 1.25 -- 0.5 W. felspar (Klap.)

Leucite 53.75 24.62 -- -- 21.35 -- -- -- -- (Klap.)

Mesotype 54.64 19.70 -- 1.61 -- 15.09 -- -- 9.83 W. (Gehlen)

Mica 42.5 11.5 9. -- 10. -- 22. 2. -- (Klaproth) --(Vauquelin) 50. 35. -- 1.33 -- -- 7. -- -- --(mean of 3 45.83 22.58 -- -- 11.08 -- 14. 1.45 -- analyses)

Olivine 50. -- 38.5 -- -- -- 12. -- -- (Klaproth)

Schorl or 35.48 34.75 4.68 -- 0.48 1.75 17.44 1.89 4.02 B. Tourmaline (Gmelin) --(mean of 6 36.03 35.82 4.44 0.28 0.71 1.96 13.71 1.62 -- analyses)

Serpentine 43.07 0.25 40.37 0.5 -- -- 1.17 -- 12.45 W. (Hisinger) --(mean of 5 37.29 4.97 36.8 2.89 -- -- 3.14 -- 12.77 W. analyses)

Steatite 64. -- 22. -- -- -- 3. -- 5. W. (Vauquelin) --(mean of 3 48.3 6.18 26.65 -- -- -- 2. -- 9.5 W. anal. by Klap.)

Talc. 61.75 -- 30.5 -- 2.75 -- 2.5 -- -- (Klaproth)

In the last column of the above Table, the letters B. C. F. W. represent Boracic acid, Carbonic acid, Fluoric acid, and Water.

FOOTNOTES:

[368-A] For a description and theory of active volcanos, see Principles of Geology, chaps. xxiv. to xxvii.

[374-A] G. Rose, Ann. des Mines, tom. viii. p. 32.

[374-B] Geol. Trans. vol. ii. p. 211. 2d series.

Spotted a typo or a problem in this chapter? Tell us

XXIX. VOLCANIC ROCKS--continued

Trap dikes--sometimes project--sometimes leave fissures vacant by decomposition--Branches and veins of trap--Dikes more crystalline in the centre--Foreign fragments of rock imbedded--Strata altered at or near the contact--Obliteration of organic remains--Conversion of chalk into marble--and of coal into coke--Inequality in the modifying influence of dikes--Trap interposed between strata--Columnar and globular structure--Relation of trappean rocks to the products of active volcanos--Submarine lava and ejected matter corresponds generally to ancient trap--Structure and physical features of Palma and some other extinct volcanos.

Having in the last chapter spoken of the composition and mineral characters of volcanic rocks, I shall next describe the manner and position in which they occur in the earth's crust, and their external forms. Now the leading varieties, such as basalt, greenstone, trachyte, porphyry, and the rest, are found sometimes in dikes penetrating stratified and unstratified formations, sometimes in shapeless masses protruding through or overlying them, or in horizontal sheets intercalated between strata.

Volcanic dikes.--Fissures have already been spoken of as occurring in all kinds of rocks, some a few feet, others many yards in width, and often filled up with earth or angular pieces of stone, or with sand and pebbles. Instead of such materials, suppose a quantity of melted stone to be driven or injected into an open rent, and there consolidated, we have then a tabular mass resembling a wall, and called a trap dike. It is not uncommon to find such dikes passing through strata of soft materials, such as tuff or shale, which, being more perishable than the trap, are often washed away by the sea, rivers, or rain, in which case the dike stands prominently out in the face of precipices, or on the level surface of a country. (See the annexed figure.[378-A])

In the islands of Arran, Skye, and other parts of Scotland, where sandstone, conglomerate, and other hard rocks are traversed by dikes of trap, the converse of the above phenomenon is seen. The dike having decomposed more rapidly than the containing rock, has once more left open the original fissure, often for a distance of many yards inland from the sea-coast, as represented in the annexed view (fig. 440.). In these instances, the greenstone of the dike is usually more tough and hard than the sandstone; but chemical action, and chiefly the oxidation of the iron, has given rise to the more rapid decay.

There is yet another case, by no means uncommon in Arran and other parts of Scotland, where the strata in contact with the dike, and for a certain distance from it, have been hardened, so as to resist the action of the weather more than the dike itself, or the surrounding rocks. When this happens, two parallel walls of indurated strata are seen protruding above the general level of the country, and following the course of the dike.

As fissures sometimes send off branches, or divide into two or more fissures of equal size, so also we find trap dikes bifurcating and ramifying, and sometimes they are so tortuous as to be called veins, though this is more common in granite than in trap. The accompanying sketch (fig. 441.) by Dr. MacCulloch represents part of a sea-cliff in Argyleshire, where an overlying mass of trap, b, sends out some veins which terminate downwards. Another trap vein, a a, cuts through both the limestone, c, and the trap, b.

In fig. 442., a ground plan is given of a ramifying dike of greenstone, which I observed cutting through sandstone on the beach near Kildonan Castle, in Arran. The larger branch varies from 5 to 7 feet in width, which will afford a scale of measurement for the whole.

In the Hebrides and other countries, the same masses of trap which occupy the surface of the country far and wide, concealing the subjacent stratified rocks, are seen also in the sea cliffs, prolonged downwards in veins or dikes, which probably unite with other masses of igneous rock at a greater depth. The largest of the dikes represented in the annexed diagram, and which are seen in part of the coast of Skye, is no less than 100 feet in width.

Every variety of trap-rock is sometimes found in these dikes, as basalt, greenstone, felspar-porphyry, and more rarely trachyte. The amygdaloidal traps also occur, and even tuff and breccia, for the materials of these last may be washed down into open fissures at the bottom of the sea, or during eruptions on the land may be showered into them from the air.

Some dikes of trap may be followed for leagues uninterruptedly in nearly a straight direction, as in the north of England, showing that the fissures which they fill must have been of extraordinary length.

Dikes more crystalline in the centre.--In many cases trap at the edges or sides of a dike is less crystalline or more earthy than in the centre, in consequence of the melted matter having cooled more rapidly by coming in contact with the cold sides of the fissure; whereas, in the centre, the matter of the dike being kept long in a fluid or soft state, the crystals are slowly formed. In the ancient part of Vesuvius, called Somma, a thin band of half-vitreous lava is found at the edge of some dikes. At the junction of greenstone dikes with limestone, a sahlband, or selvage, of serpentine is occasionally observed.

On the left shore of the fiord of Christiania, in Norway, I examined, in company with Professor Keilhau, a remarkable dike of syenitic greenstone, which is traced through Silurian strata, until at length, in the promontory of Naesodden, it enters mica-schist. Fig. 444. represents a ground plan, where the dike appears 8 paces in width. In the middle it is highly crystalline and granitiform, of a purplish colour, and containing a few crystals of mica, and strongly contrasted with the whitish mica-schist, between which and the syenitic rock there is usually on each side a distinct black band, 18 inches wide, of dark greenstone. When first seen, these bands have the appearance of two accompanying dikes; yet they are, in fact, only the different form which the syenitic materials have assumed where near to or in contact with the mica-schist. At one point, a, one of the sahlbands terminates for a space; but near this there is a large detached block, b, having a gneiss-like structure, consisting of hornblende and felspar, which is included in the midst of the dike. Round this a smaller encircling zone is seen, of dark basalt, or fine-grained greenstone, nearly corresponding to the larger ones which border the dike, but only 1 inch wide.

It seems, therefore, evident that the fragment, b, has acted on the matter of the dike, probably by causing it to cool more rapidly, in the same manner as the walls of the fissure have acted on a larger scale. The facts, also, illustrate the facility with which a granitiform syenite may pass into ordinary rocks of the volcanic family.

The fact above alluded to, of a foreign fragment, such as b, fig. 444., included in the midst of the trap, as if torn off from some subjacent rock or the walls of a fissure, is by no means uncommon. A fine example is seen in another dike of greenstone, 10 feet wide, in the northern suburbs of Christiania, in Norway, of which the annexed figure is a ground plan. The dike passes through shale, known by its fossils to belong to the Silurian series. In the black base of greenstone are angular and roundish pieces of gneiss, some white, others of a light flesh-colour, some without lamination, like granite, others with laminae, which, by their various and often opposite directions, show that they have been scattered at random through the matrix. These imbedded pieces of gneiss measure from 1 to about 8 inches in diameter.

Rocks altered by volcanic dikes.--After these remarks on the form and composition of dikes themselves, I shall describe the alterations which they sometimes produce in the rocks in contact with them. The changes are usually such as the intense heat of melted matter and the entangled gases might be expected to cause.

Plas-Newydd.--A striking example, near Plas-Newydd, in Anglesea, has been described by Professor Henslow.[381-A] The dike is 134 feet wide, and consists of a rock which is a compound of felspar and augite (dolerite of some authors). Strata of shale and argillaceous limestone, through which it cuts perpendicularly, are altered to a distance of 30, or even, in some places, to 35 feet from the edge of the dike. The shale, as it approaches the trap, becomes gradually more compact, and is most indurated where nearest the junction. Here it loses part of its schistose structure, but the separation into parallel layers is still discernible. In several places the shale is converted into hard porcellanous jasper. In the most hardened part of the mass the fossil shells, principally Producti, are nearly obliterated; yet even here their impressions may frequently be traced. The argillaceous limestone undergoes analogous mutations, losing its earthy texture as it approaches the dike, and becoming granular and crystalline. But the most extraordinary phenomenon is the appearance in the shale of numerous crystals of analcime and garnet, which are distinctly confined to those portions of the rock affected by the dike.[382-A] Some garnets contain as much as 20 per cent. of lime, which they may have derived from the decomposition of the fossil shells or Producti. The same mineral has been observed, under very analogous circumstances, in High Teesdale, by Professor Sedgwick, where it also occurs in shale and limestone, altered by basalt.[382-B]

Antrim.--In several parts of the county of Antrim, in the north of Ireland, chalk with flints is traversed by basaltic dikes. The chalk is there converted into granular marble near the basalt, the change sometimes extending 8 or 10 feet from the wall of the dike, being greatest near the point of contact, and thence gradually decreasing till it becomes evanescent. "The extreme effect," says Dr. Berger, "presents a dark brown crystalline limestone, the crystals running in flakes as large as those of coarse primitive (metamorphic) limestone; the next state is saccharine, then fine grained and arenaceous; a compact variety, having a porcellanous aspect and a bluish-grey colour, succeeds: this, towards the outer edge, becomes yellowish-white, and insensibly graduates into the unaltered chalk. The flints in the altered chalk usually assume a grey yellowish colour."[382-C] All traces of organic remains are effaced in that part of the limestone which is most crystalline.

The annexed drawing (fig. 446.) represents three basaltic dikes traversing the chalk, all within the distance of 90 feet. The chalk contiguous to the two outer dikes is converted into a finely granular marble, m m, as are the whole of the masses between the outer dikes and the central one. The entire contrast in the composition and colour of the intrusive and invaded rocks, in these cases, renders the phenomena peculiarly clear and interesting.

Another of the dikes of the north-east of Ireland has converted a mass of red sandstone into hornstone.[382-E] By another, the slate clay of the coal measures has been indurated, and has assumed the character of flinty slate[383-A]; and in another place the slate clay of the lias has been changed into flinty slate, which still retains numerous impressions of ammonites.[383-B]

It might have been anticipated that beds of coal would, from their combustible nature, be effected in an extraordinary degree by the contact of melted rock. Accordingly, one of the greenstone dikes of Antrim, on passing through a bed of coal, reduces it to a cinder for the space of 9 feet on each side.[383-C]

At Cockfield Fell, in the north of England, a similar change is observed. Specimens taken at the distance of about 30 yards from the trap are not distinguishable from ordinary pit coal; those nearer the dike are like cinders, and have all the character of coke; while those close to it are converted into a substance resembling soot.[383-D]

As examples might be multiplied without end, I shall merely select one or two others, and then conclude. The rock of Stirling Castle is a calcareous sandstone, fractured and forcibly displaced by a mass of greenstone which has evidently invaded the strata in a melted state. The sandstone has been indurated, and has assumed a texture approaching to hornstone near the junction. In Arthur's Seat and Salisbury Craig, near Edinburgh, a sandstone which comes in contact with greenstone is converted into a jaspideous rock.[383-E]

The secondary sandstones in Skye are converted into solid quartz in several places, where they come in contact with veins or masses of trap; and a bed of quartz, says Dr. MacCulloch, found near a mass of trap, among the coal strata of Fife, was in all probability a stratum of ordinary sandstone, having been subsequently indurated and turned into quartzite by the action of heat.[383-F]

But although strata in the neighbourhood of dikes are thus altered in a variety of cases, shale being turned into flinty slate or jasper, limestone into crystalline marble, sandstone into quartz, coal into coke, and the fossil remains of all such strata wholly and in part obliterated, it is by no means uncommon to meet with the same rocks, even in the same districts, absolutely unchanged in the proximity of volcanic dikes.

This great inequality in the effects of the igneous rocks may often arise from an original difference in their temperature, and in that of the entangled gases, such as is ascertained to prevail in different lavas, or in the same lava near its source and at a distance from it. The power also of the invaded rocks to conduct heat may vary, according to their composition, structure, and the fractures which they may have experienced, and perhaps, also, according to the quantity of water (so capable of being heated) which they contain. It must happen in some cases that the component materials are mixed in such proportions as prepare them readily to enter into chemical union, and form new minerals; while in other cases the mass may be more homogeneous, or the proportions less adapted for such union.

We must also take into consideration, that one fissure may be simply filled with lava, which may begin to cool from the first; whereas in other cases the fissure may give passage to a current of melted matter, which may ascend for days or months, feeding streams which are overflowing the country above, or are ejected in the shape of scoriae from some crater. If the walls of a rent, moreover, are heated by hot vapour before the lava rises, as we know may happen on the flanks of a volcano, the additional caloric supplied by the dike and its gases will act more powerfully.

Intrusion of trap between strata.--In proof of the mechanical force which the fluid trap has sometimes exerted on the rocks into which it has intruded itself, I may refer to the Whin-Sill, where a mass of basalt, from 60 to 80 feet in height, represented by a, fig. 447., is in part wedged in between the rocks of limestone, b, and shale, c, which have been separated from the great mass of limestone and shale, d, with which they were united.

The shale in this place is indurated; and the limestone, which at a distance from the trap is blue, and contains fossil corals, is here converted into granular marble without fossils.

Masses of trap are not unfrequently met with intercalated between strata, and maintaining their parallelism to the planes of stratification throughout large areas. They must in some places have forced their way laterally between the divisions of the strata, a direction in which there would be the least resistance to an advancing fluid, if no vertical rents communicated with the surface, and a powerful hydrostatic pressure was caused by gases propelling the lava upwards.

Columnar and globular structure.--One of the characteristic forms of volcanic rocks, especially of basalt, is the columnar, where large masses are divided into regular prisms, sometimes easily separable, but in other cases adhering firmly together. The columns vary in the number of angles, from three to twelve; but they have most commonly from five to seven sides. They are often divided transversely, at nearly equal distances, like the joints in a vertebral column, as in the Giant's Causeway, in Ireland. They vary exceedingly in respect to length and diameter. Dr. MacCulloch mentions some in Skye which are about 400 feet long; others, in Morven, not exceeding an inch. In regard to diameter, those of Ailsa measure 9 feet, and those of Morven an inch or less.[385-A] They are usually straight, but sometimes curved; and examples of both these occur in the island of Staffa. In a horizontal bed or sheet of trap the columns are vertical; in a vertical dike they are horizontal. Among other examples of the last-mentioned phenomenon is the mass of basalt, called the Chimney, in St. Helena (see fig. 448.), a pile of hexagonal prisms, 64 feet high, evidently the remainder of a narrow dike, the walls of rock which the dike originally traversed having been removed down to the level of the sea. In fig. 449. a small portion of this dike is represented on a less reduced scale.[385-B]

It being assumed that columnar trap has consolidated from a fluid state, the prisms are said to be always at right angles to the cooling surfaces. If these surfaces, therefore, instead of being either perpendicular, or horizontal, are curved, the columns ought to be inclined at every angle to the horizon; and there is a beautiful exemplification of this phenomenon in one of the valleys of the Vivarais, a mountainous district in the South of France, where, in the midst of a region of gneiss, a geologist encounters unexpectedly several volcanic cones of loose sand and scoriae. From the crater of one of these cones called La Coupe d'Ayzac, a stream of lava descends and occupies the bottom of a narrow valley, except at those points where the river Volant, or the torrents which join it, have cut away portions of the solid lava. The accompanying sketch (fig. 450.) represents the remnant of the lava at one of the points where a lateral torrent joins the main valley of the Volant. It is clear that the lava once filled the whole valley up to the dotted line d a; but the river has gradually swept away all below that line, while the tributary torrent has laid open a transverse section; by which we perceive, in the first place, that the lava is composed, as usual in this country, of three parts: the uppermost, at a, being scoriaceous; the second, b, presenting irregular prisms; and the third, c, with regular columns, which are vertical on the banks of the Volant, where they rest on a horizontal base of gneiss, but which are inclined at an angle of 45 deg. at g, and then horizontal at f, their position having been every where determined, according to the law before mentioned, by the concave form of the original valley.

In the annexed figure (451.) a view is given of some of the inclined and curved columns which present themselves on the sides of the valleys in the hilly region north of Vicenza, in Italy, and at the foot of the higher Alps.[386-A] Unlike those of the Vivarais, last mentioned, the basalt of this country was evidently submarine, and the present valleys have since been hollowed out by denudation.

The columnar structure is by no means peculiar to the trap rocks in which hornblende or augite predominate; it is also observed in clinkstone, trachyte, and other felspathic rocks of the igneous class, although in these it is rarely exhibited in such regular polygonal forms.

It has been already stated that basaltic columns are often divided by cross joints. Sometimes each segment, instead of an angular, assumes a spheroidal form, so that a pillar is made up of a pile of balls, usually flattened, as in the Cheese-grotto at Bertrich-Baden, in the Eifel, near the Moselle (fig. 452.). The basalt, there, is part of a small stream of lava, from 30 to 40 feet thick, which has proceeded from one of several volcanic craters, still extant, on the neighbouring heights. The position of the lava bordering the river in this valley might be represented by a section like that already given at fig. 450. p. 385., if we merely supposed inclined strata of slate and the argillaceous sandstone called greywacke to be substituted for gneiss.

In some masses of decomposing greenstone, basalt, and other trap rocks, the globular structure is so conspicuous that the rock has the appearance of a heap of large cannon balls.

A striking example of this structure occurs in a resinous trachyte or pitchstone-porphyry in one of the Ponza islands, which rise from the Mediterranean, off the coast of Terracina and Gaeta. The globes vary from a few inches to three feet in diameter, and are of an ellipsoidal form (see fig. 453.). The whole rock is in a state of decomposition, "and when the balls," says Mr. Scrope, "have been exposed a short time to the weather, they scale off at a touch into numerous concentric coats, like those of a bulbous root, inclosing a compact nucleus. The laminae of this nucleus have not been so much loosened by decomposition; but the application of a ruder blow will produce a still further exfoliation."[387-A]

A fissile texture is occasionally assumed by clinkstone and other trap rocks, so that they have been used for roofing houses. Sometimes the prismatic and slaty structure is found in the same mass. The causes which give rise to such arrangements are very obscure, but are supposed to be connected with changes of temperature during the cooling of the mass, as will be pointed out in the sequel. (See Chaps. XXXV. and XXXVI.)

Relation of Trappean Rocks to the products of active Volcanos.

When we reflect on the changes above described in the strata near their contact with trap dikes, and consider how great is the analogy in composition and structure of the rocks called trappean and the lavas of active volcanos, it seems difficult at first to understand how so much doubt could have prevailed for half a century as to whether trap was of igneous or aqueous origin. To a certain extent, however, there was a real distinction between the trappean formations and those to which the term volcanic was almost exclusively confined. The trappean rocks first studied in the north of Germany, and in Norway, France, Scotland, and other countries, were either such as had been formed entirely under deep water, or had been injected into fissures and intruded between strata, and which had never flowed out in the air, or over the bottom of a shallow sea. When these products, therefore, of submarine or subterranean igneous action were contrasted with loose cones of scoriae, tuff, and lava, or with narrow streams of lava in great part scoriaceous and porous, such as were observed to have proceeded from Vesuvius and Etna, the resemblance seemed remote and equivocal. It was, in truth, like comparing the roots of a tree with its leaves and branches, which, although they belong to the same plant, differ in form, texture, colour, mode of growth, and position. The external cone, with its loose ashes and porous lava, may be likened to the light foliage and branches, and the rocks concealed far below, to the roots. But it is not enough to say of the volcano,

"quantum vertice in auras AEtherias, tantum radice in Tartara tendit,"

for its roots do literally reach downwards to Tartarus, or to the regions of subterranean fire; and what is concealed far below, is probably always more important in volume and extent than what is visible above ground.

We have already stated how frequently dense masses of strata have been removed by denudation from wide areas (see Chap. VI.); and this fact prepares us to expect a similar destruction of whatever may once have formed the uppermost part of ancient submarine or subaerial volcanos, more especially as those superficial parts are always of the lightest and most perishable materials. The abrupt manner in which dikes of trap usually terminate at the surface (see fig. 454.), and the water-worn pebbles of trap in the alluvium which covers the dike, prove incontestably that whatever was uppermost in these formations has been swept away. It is easy, therefore, to conceive that what is gone in regions of trap may have corresponded to what is now visible in active volcanos.

It will be seen in the following chapters, that in the earth's crust there are volcanic tuffs of all ages, containing marine shells, which bear witness to eruptions at many successive geological periods. These tuffs, and the associated trappean rocks, must not be compared to lava and scoriae which had cooled in the open air. Their counterparts must be sought in the products of modern submarine volcanic eruptions. If it be objected that we have no opportunity of studying these last, it may be answered, that subterranean movements have caused, almost everywhere in regions of active volcanos, great changes in the relative level of land and sea, in times comparatively modern, so as to expose to view the effects of volcanic operations at the bottom of the sea.

Thus, for example, the recent examination of the igneous rocks of Sicily, especially those of the Val di Noto, has proved that all the more ordinary varieties of European trap have been there produced under the waters of the sea, at a modern period; that is to say, since the Mediterranean has been inhabited by a great proportion of the existing species of testacea.

These igneous rocks of the Val di Noto, and the more ancient trappean rocks of Scotland and other countries, differ from subaerial volcanic formations in being more compact and heavy, and in forming sometimes extensive sheets of matter intercalated between marine strata, and sometimes stratified conglomerates, of which the rounded pebbles are all trap. They differ also in the absence of regular cones and craters, and in the want of conformity of the lava to the lowest levels of existing valleys.

It is highly probable, however, that insular cones did exist in some parts of the Val di Noto: and that they were removed by the waves, in the same manner as the cone of Graham island, in the Mediterranean, was swept away in 1831, and that of Nyoee, off Iceland, in 1783.[389-A] All that would remain in such cases, after the bed of the sea has been upheaved and laid dry, would be dikes and shapeless masses of igneous rock, cutting through sheets of lava which may have spread over the level bottom of the sea, and strata of tuff, formed of materials first scattered far and wide by the winds and waves, and then deposited. Trap conglomerates also, to which the action of the waves must give rise during the denudation of such volcanic islands, will emerge from the deep whenever the bottom of the sea becomes land.

The proportion of volcanic matter which is originally submarine must always be very great, as those volcanic vents which are not entirely beneath the sea, are almost all of them in islands, or, if on continents, near the shore. This may explain why extended sheets of trap so often occur, instead of narrow threads, like lava streams. For, a multitude of causes tend, near the land, to reduce the bottom of the sea to a nearly uniform level,--the sediment of rivers,--materials transported by the waves and currents of the sea from wasting cliffs,--showers of sand and scoriae ejected by volcanos, and scattered by the wind and waves. When, therefore, lava is poured out on such a surface, it will spread far and wide in every direction in a liquid sheet, which may afterwards, when raised up, form the tabular capping of the land.

As to the absence of porosity in the trappean formations, the appearances are in a great degree deceptive, for all amygdaloids are, as already explained, porous rocks, into the cells of which mineral matter, such as silex, carbonate of lime, and other ingredients, have been subsequently introduced (see p. 373.); sometimes, perhaps, by secretion during the cooling and consolidation of lavas.

In the Little Cumbray, one of the Western Islands, near Arran, the amygdaloid sometimes contains elongated cavities filled with brown spar; and when the nodules have been washed out, the interior of the cavities is glazed with the vitreous varnish so characteristic of the pores of slaggy lavas. Even in some parts of this rock which are excluded from air and water, the cells are empty, and seem to have always remained in this state, and are therefore undistinguishable from some modern lavas.[390-A]

Dr. MacCulloch, after examining with great attention these and the other igneous rocks of Scotland, observes, "that it is a mere dispute about terms, to refuse to the ancient eruptions of trap the name of submarine volcanos; for they are such in every essential point, although they no longer eject fire and smoke."[390-B] The same author also considers it not improbable that some of the volcanic rocks of the same country may have been poured out in the open air.[390-C]

Although the principal component minerals of subaerial lavas are the same as those of intrusive trap, and both the columnar and globular structure are common to both, there are, nevertheless, some volcanic rocks which never occur as lava, such as greenstone, clinkstone, the more crystalline porphyries, and those traps in which quartz and mica appear as constituent parts. In short, the intrusive trap rocks, forming the intermediate step between lava and the plutonic rocks, depart in their characters from lava in proportion as they approximate to granite.

These views respecting the relations of the volcanic and trap rocks will be better understood when the reader has studied, in the 33d chapter, what is said of the plutonic formations.

FORM, STRUCTURE, AND ORIGIN OF VOLCANIC MOUNTAINS.

The origin of volcanic cones with crater-shaped summits has been alluded to in the last chapter (p. 368.), and more fully explained in the "Principles of Geology" (chaps. xxiv. to xxvii.), where Vesuvius, Etna, Santorin, and Barren Island were described. The more ancient portions of those mountains or islands, formed long before the times of history, exhibit the same external features and internal structure which belong to most of the extinct volcanos of still higher antiquity.

The island of Palma, for example, one of the Canaries, offers an excellent illustration of what, in common with many others, I regard as the ruins of a large dome-shaped mass formed by a series of eruptions proceeding from a crater at the summit, this crater having been since replaced by a larger cavity, the origin of which has afforded geologists an ample field for discussion and speculation.

Von Buch, in his excellent account of the Canaries, has given us a graphic picture of this island, which consists chiefly of a single mountain (fig. 455.). This mountain has the general form of a great truncated cone, with a huge and deep cavity in the middle, about six miles in diameter, called by the inhabitants "the Caldera," or cauldron. The range of precipices surrounding the Caldera are no less than 4000 feet in their average height; at one point, where they are highest, they are 7730 feet above the level of the sea. The external flanks of the cone incline gently in every direction towards the base of the island, and are in part cultivated; but the walls and bottom of the Caldera present on all sides rugged and uncultivated rocks, almost completely devoid of vegetation. So steep are these walls, that there is no part by which they can be descended, and the only entrance is by a great ravine, or Barranco, as it is called (see b b', map, fig. 456.), which extends from the sea to the interior of the great cavity, and by its jagged, broken, and precipitous sides, exhibits to the geologist a transverse section of the rocks of which the whole mountain is composed. By this means, we learn that the cone is made up of a great number of sloping beds, which dip outwards in every direction from the centre of the void space, or from the hollow axis of the cone. The beds consist chiefly of sheets of basalt, alternating with conglomerates; the materials of the latter being in part rounded, as if rolled by water in motion. The inclination of all the beds corresponds to that of the external slope of the island, being greatest towards the Caldera, and least steep when they are nearest the sea. There are a great number of tortuous veins, and many dikes of lava or trap, chiefly basaltic, and most of them vertical, which cut through the sloping beds laid open to view in the great gorge or Barranco. These dikes and veins are more and more abundant as we approach the Caldera, being therefore most numerous where the slope of the beds is greatest.

Assuming the cone to be a pile of volcanic materials ejected by a long succession of eruptions (a point on which all geologists are agreed), we have to account for the Caldera and the great Barranco. I conceive that the cone itself may be explained, in accordance with what we know of the ordinary growth of volcanos[392-A], by supposing most of the eruptions to have taken place from one or more central vents, at or near the summit of the cone, before it was truncated. From this culminating point, sheets of lava flowed down one after the other, and showers of ashes or ejected stones. The volcano may, in the earlier stages of its growth, have been in great part submerged, like Stromboli, in the sea; and, therefore, some of the fragments of rock cast out of its crater may not only have been rolled by torrents sweeping down the mountain's side, but have also been rounded by the waves of the sea, as we see happen on the beach near Catania, on which the modern lavas of Etna are broken up. The increased number of dykes, as we approach the axis of the cone, agrees well with the hypothesis of the eruptions having been most frequent towards the centre.

There are three known causes or modes of operation, which may have conduced towards the vast size of the Caldera. First, the summit of a conical mountain may have fallen in, as happened in the case of Capacurcu, one of the Andes, according to tradition, in the year 1462, and of many other volcanic mountains.[393-A] Sections seem wanting, to supply us with all the data required for judging fairly of the tenability of this hypothesis. It appears, however, from Captain Vidal's survey (see fig. 456.), that a hill of considerable height rises up from the bottom of the Caldera, the structure of which, if it be any where laid open, might doubtless throw much light on this subject. Secondly, an original crater may have been enlarged by a vast gaseous explosion, never followed by any subsequent eruption. A serious objection to this theory arises from our not finding that the exterior of the cone supports a mass of ruins, such as ought to cover it, had so enormous a volume of matter, partly made up of the solid contents of the dikes, been blown out into the air. In that case, an extensive bed of angular fragments of stone, and of fine dust, might be looked for, enveloping the entire exterior of the mountain up to the very rim of the Caldera, and ought nowhere to be intersected by a dike. The absence of such a formation has induced Von Buch to suppose that the missing portion of the cone was engulphed. It should, however, be remembered, that in existing volcanos, large craters, two or three miles in diameter, are sometimes formed by explosions, or by the discharge of great volumes of steam.

There is yet another cause to which the extraordinary dimensions of the Caldera may, in part at least, be owing; namely, aqueous denudation. Von Buch has observed, that the existence of a single deep ravine, like the Great Barranco, is a phenomenon common to many extinct volcanos, as well as to some active ones. Now, it will be seen by Captain Vidal's map (fig. 456. p. 391.), that the sea-cliff at Point Juan Graje, 780 feet high, now constituting the coast at the entrance of the great ravine, is continuous with an inland cliff which bounds the same ravine on its north-western side. No one will dispute that the precipice, at the base of which the waves are now beating, owes its origin to the undermining power of the sea. It is natural, therefore, to attribute the extension of the same cliff to the former action of the waves, exerted at a time when the relative level of the island and the ocean were different from what they are now. But if the waves and tides had power to remove the rocks once filling a great gorge which is 7 miles long, and, in its upper part, 2000 feet deep, can we doubt that the same power may have cleared out much of the solid mass now missing in the Great Caldera?

The theory advanced to account for the configuration of Palma, commonly called the "elevation crater theory," is this. All the alternating masses of basalt and conglomerate, intersected in the Barranco, or abruptly cut off in the escarpment or walls of the Caldera, were at first disposed in horizontal masses on the level floor of the ocean, and traversed, when in that position, by all the basaltic dikes which now cut through them. At length they were suddenly uplifted by the explosive force of elastic vapours, which raised the mass bodily, so as to tilt the beds on all sides away from the centre of elevation, causing at the same time an opening at the culminating point. Besides many other objections which may be urged against this hypothesis, it leaves unexplained the unbroken continuity of the rim of the Caldera, which is uninterrupted in all places save one[394-A], namely, that where the great gorge or Barranco occurs.

As a more natural way of explaining the phenomenon, the following series of events may be imagined. The principal vent, from which a large part of the materials of the cone were poured or thrown out, was left empty after the last escape of vapour, when the volcano became extinct. We learn from Mr. Dana's valuable work on the geology of the United States' Exploring Expedition, published in 1849, that two of the principal volcanos of the Sandwich Islands, Mounts Loa and Kea in Owyhee, are huge flattened volcanic cones, 15,000 feet high (see fig. 457.), each equalling two and a half Etnas in their dimensions.

From the summits of these lofty though featureless hills, and from vents not far below their summits, successive streams of lava, often 2 miles or more in width, and sometimes 26 miles long, have flowed. They have been poured out one after the other, some of them in recent times, in every direction from the apex of the cone, down slopes varying on an average from 4 degrees to 8 degrees; but at some places considerably steeper.[394-B] Sometimes deep rents open on the sides of these cones, which are filled by streams of lava passing over them, the liquid matter in such cases probably uniting in the fissure with other lava melted in subterranean reservoirs below, and thus explaining the origin of one great class of lateral dikes, on Etna, Palma, and other cones.

If the flattened domes, such as those here alluded to in the Sandwich Islands, instead of being inland, and above water, were situated in mid-ocean, like the Island of St. Paul, and for the most part submerged (see figs. 458, 459, 460.), and if a gradual upheaval of such a dome should then take place, the denuding power of the sea could scarcely fail to play an important part in modifying the form of the volcanic mountain as it rose. The crater will almost invariably have one side much lower than all the others, namely, that side towards which the prevailing winds never blow, and to which, therefore, showers of dust and scoriae are rarely carried during eruptions. There will also be one point on this windward or lowest side more depressed than all the rest, by which the sea may enter as often as the tide rises, or as often as the wind blows from that quarter. For the same reason that the sea continues to keep open a single entrance into the lagoon of an atoll or annular coral reef, it will not allow this passage into the crater to be stopped up, but scour it out, at low tide, or as often as the wind changes. The channel, therefore, will always be deepened in proportion as the island rises above the level of the sea, at the rate perhaps of a few feet or yards in a century.

The island of St. Paul may perhaps be motionless; but if, like many other parts of the earth's crust, it should begin to undergo a gradual upheaval, or if, as has happened to the shores of the Bay of Baiae, its level should oscillate, with a tendency upon the whole to increased elevation, the same power which has cut away part of the cone, and caused the cliffs now seen on the north-east side of the island, would have power to undermine the walls of the crater, and enlarge its diameter, keeping open the channel, by which it enters into it. This ravine might be excavated to the depth of 180 feet (the present depth of the crater), and its length might be extended to many miles according to the size of the submerged part of the cone. The crater is only a mile in diameter, and the surrounding cliffs, where loftiest, only 800 feet high, so that the size of this cone and crater is insignificant when compared to those in the Sandwich Islands, and I have merely selected it because it affords an example of a class of insular volcanos, into the craters of which the sea now enters by a single passage. The crater of Vesuvius in 1822 was 2000 feet deep; and if it were a half submerged cone, like St. Paul, the excavating power of the ocean might in conjunction with gaseous explosions and co-operating with a gradual upheaving force, give rise to a caldera on as grand a scale as that exhibited by Palma.

If, after the geographical changes above supposed, the volcanic fires long dormant should recover their energy, they might, as in the case of Teneriffe, Vesuvius, Santorin, and Barren Island, discharge from the old central vent, long sealed up at the bottom of the caldera, new floods of lava and clouds of elastic vapours. Should this happen, a new cone will be built up in the middle of the cavity or circular bay, formed, partly by explosion, partly perhaps by engulphment, and partly by aqueous denudation. In the island of Palma this last phase of volcanic activity has never occurred; but the subterranean heat is still in full operation beneath the Canary Islands, so that we know not what future changes it may be destined to undergo.

FOOTNOTES:

[378-A] I have been favoured with this drawing by Captain B. Hall.

[381-A] Cambridge Transactions, vol. i. p. 402.

[382-A] Cambridge Trans., vol. i. p. 410.

[382-B] Ibid. vol. ii. p. 175.

[382-C] Dr. Berger, Geol. Trans., 1st series, vol. iii. p. 172.

[382-D] Geol. Trans., 1st series, vol. iii. p. 210. and plate 10.

[382-E] Ibid. p. 201.

[383-A] Geol. Trans., 1st series, vol. iii. p. 205.

[383-B] Ibid. p. 213.; and Playfair, Illust. of Hutt. Theory, p. 253.

[383-C] Geol. Trans., 1st series, vol. iii. p. 206.

[383-D] Sedgwick, Camb. Trans. vol. ii. p. 37.

[383-E] Illust. of Hutt. Theory, Sec. 253. and 261. Dr. MacCulloch, Geol. Trans., 1st series, vol. ii. p. 305.

[383-F] Syst. of Geol. vol. i. p. 206.

[384-A] Camb. Trans. vol. ii. p. 180.

[385-A] MacCul. Syst. of Geol. vol. ii. p. 137.

[385-B] Seale's Geognosy of St. Helena, plate 9.

[386-A] Fortis. Mem. sur l'Hist. Nat. de l'Italie, tom. i. p. 233. plate 7.

[387-A] Scrope, Geol. Trans. vol. ii. p. 205. 2d series.

[389-A] See Princ. of Geol., Index, "Graham Island," "Nyoee," "Conglomerates, volcanic," &c.

[390-A] MacCulloch, West. Isl., vol. ii. p. 487.

[390-B] Syst. of Geol., vol. ii. p. 114.

[390-C] Ibid.

[392-A] See Principles, chaps. xxiv-xxvii.

[393-A] See Principles, chaps. xxvi. and xxx.; 8th ed. p. 397-475.

[394-A] See Principles of Geol. ch. xxiv. (8th ed. p. 355.).

[394-B] See Lyell on Craters of Denudation, Quart. Geol. Journ. vol. vi. p. 232.

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