A Treatise on Geology/Chapter 6, part 1
HISTORICAL VIEW OF THE STRATIFIED ROCKS IN THE CRUST OF THE EARTH.
IN describing the successive phenomena visible in the crust of the earth, we may either begin at the surface, and pass from the operations of to-day, through the monuments of changes performed in historic periods, to those of earlier date; and thus, proceeding from the known to the unknown, approach by an easy gradation to the remote eras and obscure conditions of our planet, which were once degraded by the misapplied title of "Chaos;"—or take our departure at the most ancient recognisable point of geological time, and trace the events which happened on the globe in the order of their occurrence.
The former process offers some advantages to the student who, unaided by or distrustful of the generalisations already arrived at, is desirous of acquiring by his own labours a correct view of the relation of the present to earlier conditions of the globe: for thus, proceeding from diurnal operations to primeval phenomena, he is able to classify his observations with reference to causes really acting, to assemble partial truths into laws of phenomena, and by mere comparison of these with the actual condition of nature to arrive at the knowledge he is in quest of.
But for the purpose of clearly unfolding the series of geological phenomena whose laws are known, the contrary method is to be preferred. To describe what is known of the structure of the earth in the order of the occurrence of the phenomena—to present a series of pictures of its successive conditions—to exhibit these conditions as influencing others which succeeded them, till the present aspect of nature appears as a consequence of all the previous changes, is in fact to write the physical history of our planet upon the same plan as that universally adopted for histories of its human inhabitants.
This geological history of the earth must necessarily commence with the earliest (i.e. lowest) stratified formations; and the first things to be determined are, the extent to which they can be traced, and the nature of the basis on which they rest. Sufficient information is already gathered on these points to allow of a distinct affirmation, that below all the series of strata existing in any country, masses of crystallised but unstratified rocks exist so as to form a general floor, most irregular in surface and of unknown thickness, on which the strata successively rest. These rocks are generally of the nature of granite, that is to say, largely crystallised aggregates of felspar with variable admixtures of mica and quartz—or more rarely quartz and hornblende— or quartz and hypersthene. Examples of the first kind of granitic basis of the crust of the earth, are almost universal in mountainous regions; e. g. the Grampians, the Mourne and Wicklow mountains, Cumbria, Cornwall, Pyrenees, Alps, &c. Sienitic granite (holding hornblende with or instead of mica) occurs about Strontian and in Ben Cruachan; and hypersthenic granite shows itself in the Val di Fassa (Alps), gradually changing to common micaceous granite.
Seeing then the probably universal extent of the granitic floor beneath the stratified parts of the earth's crust, it becomes of great importance to ascertain if the law which is allowed to hold for all stratified rocks (viz. that the lowest are the oldest), is extensible to the subjacent granite, so that it may be ranked as an olderrock than any of the strata which rest upon it. A
striking change has taken place in respect to this matter of late years: formerly, when granite was by many geologists thought to be of aqueous origin, its inferiority of position was held to be sufficient proof of anterior production; now, when it is known to have been formed by the action of heat, this argument is of no value; and other circumstances have been observed which leave no doubt that in very many cases the granite has been in a state of fusion since the deposition of several of the older formations, so that it has actually been injected into fissures and cracks of these strata, or been raised up in a fluid mass amongst them. (Diagram No. 33.) Granite veins, as these injected portions, once thought so rare, are called, have now been observed in almost every region of old strata; entering hornblende slates, and primary limestones in Glen Tilt (Blair Athol), and clay slates in Arran, Skiddaw, and Cornwall. Granite which we thus see irrupted through and into the stratified rocks, is, in fact, of no one particular or determinate age, but is the local result and evidence of many independent excitements or periods of critical action among the subterranean agencies of heat. We may, therefore, consistently admit granite, as well as other igneous rocks, to be of any, that is, of all ages; some of that which is visible in the crust of the globe may have been solidified from fusion before the production of any of the strata; other granite has been melted or re-melted at various later periods; granite may yet be forming in the deeper parts of the earth, round the centres of volcanic fires; but, in general, we must look on this rock as characteristic of particular circumstances accompanying igneous action, not as belonging to particular periods of geological history.
These circumstances appear, however, to have occurred universally, if not simultaneously; and the tendency to produce fluid compounds which subsequently admit of granitic crystallisation, is a characteristic effect of subterranean heat in all past geological periods. It appears, therefore, a probable inference, that the formation of granite was a process which began before the production of any of the strata; was continued during the accumulation of primary, secondary, and tertiary rocks; and is yet in action under particular circumstances in the deep parts of the earth. One of the most remarkable speculations of modern geology is that advocated by Mr. Lyell, who, in his "Principles of Geology," defends the somewhat startling speculation, that the granitic floor of the stratified crust of the earth, is nothing else than the fused and re-consolidated materials of older strata than any which are now visible,—that at this time granite is forming in the same manner by the fusion of the lower portions of the strata,—and that as new stratified rocks, the fruit of water, are slowly deposited above, the older ones which they cover are slowly re-absorbed by the antagonist element of interior heat, and converted to crystallised granite.
Those who adopt this view must of necessity look on the stratified rocks as an incomplete series of monuments of watery action, the earliest being wholly consumed by heat. According to them, the history of the globe must unavoidably be imperfect; it can, as Dr. Hutton remarked, show no trace of a beginning, no prospect of an end; the appointed agencies of terrestrial nature are bound in a perpetual circle of compensation, and not united by a continuous chain of effects flowing from some one primal condition toward a determinate and permanent state. It is certain, that the study of igneous rocks alone will never enable us to decide how far this speculation is well founded, since they are not characteristic of time, nor capable of giving the least information as to the organic enrichment and atmospheric investment of the globe, except by combination with the data afforded by a study of the stratified rocks. To these, therefore, we must immediately apply.
HYPOZOIC SYSTEMS OF STRATA.
Gneiss and Mica Schist System.
Composition. It is a general truth, that to every principal mass of stratified rocks belong some remarkable mineral types of composition. The hypozoic and palæozoic strata are distinguished by the super-abundance of hard siliceous and argillaceous rocks, with crystallised or concretionary limestones; the secondary rocks have more variety of arenaceous and calcareous members; in the tertiary strata loose sands, marls, and clays, abound remarkably, while these scarcely occur at all among the older rocks.
The same truth is, perhaps, even more clearly perceived by comparing the successive systems and formations, and deserves more attention than has of late been given to it, since the study of organic remains has opened so many brilliant views of another kind, though equally related to, and characteristic of, geological time.
The materials of the rocks which enter into the composition of the gneiss and mica schist systems, are such as to form siliceous, argillaceous, and calcareous aggregates, somewhat resembling those of the later systems of rocks; but they are usually in a very different state of molecular aggregation. The siliceous strata of these ancient rocks (gneiss, mica schist, &c.) consist of the same minerals as those which abound in secondary sandstones, viz., quartz, felspar, and mica, principally; but these minerals, little or not at all worn or decomposed, compose a rock allied to granite. The argillaceous rocks, which often accompany them (clay slate, grauwacke slate, &c.), have nearly the same chemical composition as common clays and shales among the secondary rocks; but the degree of induration and the whole structure of the rocks require the supposition of their having undergone the influence of very different circumstances. In the same way the primary calcareous rocks, though chemically indistinguishable from secondary limestones, are so crystallised in texture as to leave no doubt that modifying agencies of great importance have operated on them since their deposition.
If we seek to ascertain the origin of the materials of the oldest or lowest of all the known systems of strata, and take characteristic specimens of gneiss and mica schist for the purpose, we shall be struck with the great resemblance they offer to granite, in the kind, proportionate abundance and admixture, even colour and aspect, of the constituent quartz, felspar, mica, hornblende, &c. So close is the resemblance, that some writers appear disposed to allow for these stratified granitoid rocks, an origin not very distinct from the igneous origin of granite; but careful attention discloses points of disagreement which are equally important, and tend to a different opinion. Let any one, for example, compare in well characterised granite and gneiss the constituents, felspar and mica: in granite these are always perfectly crystallised within, and have regular external geometrical figure; in gneiss the internal crystallisation remains, but the felspar is granular and disseminated like sand or the parts of broken crystals, and the mica is bent and contorted by irregular pressure among the felspar and quartz. Add to these circumstances the lamination of the masses, and we see clearly that the ingredients of gneiss and mica schist resemble granite, because they have been derived from granitic rocks; but they differ because they were accumulated under the mechanical influence of water, and not aggregated by chemical forces from a state of igneous fusion.
The divisions of the gneiss and mica schist system are, to a considerable degree, based on the mineralogical differences of the ingredients in the predominant rocks. Gneiss, for instance, is principally composed of the same materials as common granite, viz. quartz, felspar, mica (occasionally hornblende, augite, garnets occur in it); mica schist is principally formed of mica and quartz, with garnets, hornblende, &c.): in both, the ingredients are arranged in laminæ; the mica forming generally continuous sheets in mica schist, but interrupted patches in gneiss. Chlorite schist differs from mica schist by the substitution of chlorite for mica. In hornblende schist the mineral associated with quartz is hornblende or actynolite. In quartz rock, only a little felspar or mica is mixed with the granular quartz, and not generally arranged in layers.
In gneiss, mica schist, chlorite schist, and hornblende schist, the magnitude of the grains is indefinite; and it consequently happens that all of them admit of numerous variations, to which it is useless to give names, from largely granular or even conglomerated gneiss (Zetland), to a fine-grained nearly uniform admixture of mica, quartz, and felspar—mica and quartz—felspar and quartz—(with or without chlorite, hornblende, &c.) In this state these siliceous rocks become very similar to certain argillaceous slates, which, in fact, in some cases, seem to bear exactly the same relation to gneiss, mica schist, &c., that common clays do to common sandstones: there is every gradation between them; their origin is undoubtedly similar—it may even be called the same; since one land flood or sea storm will form both stratified sands and laminated clays from the same wasted land or broken cliff, according merely to the difference of circumstances under which the materials are accumulated. Now it is impossible to doubt that clay slates and grauwacke slates have been deposited in water: it is equally certain that the gneiss and other felspathic or quartzose rocks, which are associated with it, and occasionally with clay slate, are also of aqueous production; and the composition of gneiss, &c., completes the evidence wanted to prove that the primary strata analogous to sandstones and clays were formed from the waste of granitic rocks.
The structure of the rocks which compose the gneiss and mica schist system varies considerably, both in relation to lamination and stratification, which depend on the mode of aqueous deposition, and to joints and fissures, which are the result of subsequent agencies.
Lamination prevails amongst all the varieties of gneiss, mica schist, chlorite schist, hornblende schist, &c. It is often observable in primary limestone and sometimes in quartz rock. In gneiss, mica schist, and chlorite schist, but especially in the former, the laminæ are usually contorted, sometimes excessively so, indicating a troubled condition of the water from which the ingredients fell, or a source of agitation in the still yielding sediment which seems scarcely ever to have occurred among the secondary and later strata. The only plausible explanation of this remarkable circumstance
which has occurred to us, is the agitation of the sea, or the soft sediment on its bed by heat; exactly as in the bottoms of steam boilers, the calcareous sediment is formed in irregular undulating laminæ, which appear on a cross section very similar to the flexures in the laminæ of gneiss. It will appear hereafter that this speculation derives some corroboration from other circumstances, tending to show what was the condition, as to heat, of the ocean in which the ancient rocks were formed.
Dr. M'Culloch informs us (Memoir on Map of Scotland, p. 65.), "wherever there are numerous and conspicuous curvatures the gneiss is granitic; and it is the same, with little exception, where the position is angular. It is the same also, almost universally, when the beds are in the vicinity of granite.
"On the contrary, extensive and prolonged beds are very generally schistose or laminar: the strata, also, are of this character when alternating and continuous with mica slate and quartz rock."
Stratification, or bedding, independent of lamination, is less easily traceable in the gneiss and mica schist system than in most other aqueous rocks: yet sometimes in the gneiss of Strontian, the mica schist of the Trosachs, the chloritic schists of Loch Lomond, it is sufficiently plain, to be satisfactory proof of intermitting deposition of the rocks. This intermission of deposit is, perhaps, the true cause of the bedded or stratified structure in all rocks. When different sorts of matter are alternately deposited the bedding is most perfect; but the reality of aqueous deposition is often satisfactorily shown by mere variation of colour in a mass of rocks, otherwise of continuous and uniform character. Quartz rock (Balachulish) and limestone (Loch Earn, Inverary) associated with gneiss and mica schist, generally show stratification, but less perfectly than among more recent strata. A full examination of primary tracts will, probably in every instance, satisfy a candid inquirer that the gneiss and mica schist rocks are stratified; but he will certainly notice cases where the bedding of gneiss is lost, the lamination of mica schist unintelligible, and the proofs of aqueous deposition far more obscure than among later rocks. Does this prove a difference of condition in the agencies concerned in accumulating these earliest strata, or can it be explained by considering the original structures of deposition to have undergone partial or entire obliteration through the pervading influence of heat, or local proximity of igneous rocks? for both these causes are known to have produced important effects in this respect.
Superposed Structures.—So many circumstances have occurred to change the condition of rocks since their first deposition, that it is probable few or none of them now appear with their original characters of texture, structure, or position. If we represent to ourselves an extended mass of arenaceous, argillaceous, or calcareous sediment, becoming gradually consolidated under the pressure of water, partially dried by the superposition of other strata, and further subject to the influence of molecular aggregation (aided, perhaps, by subterraneous heat), we shall clearly perceive that the induration of the rock must be accompanied by such a degree of shrinking in a horizontal direction, as well as compression vertically, that numerous fissures and cracks must be formed. According to some peculiar circumstances in the different sorts of rocks, the cracks and fissures assume different appearances in them; there are distinct though not easily defined characters for these divisional planes in arenaceous, argillaceous, and calcareous rocks; and in each of these the fineness, or coarseness of grain, and the thickness of the beds make important difference.
Considering further, that rocks have been, according to difference of age, proximity to the surface of the sea, and other causes, unequally subject to the modifying influences alluded to, we must be prepared to find some characteristic differences of the cracks, joints and fissures, according to the antiquity of the strata.
In the rocks of the gneiss and mica schist system, we find these general results perfectly exemplified—the coarse grained gneiss and mica schists show very little of either cracks or fissures across the beds; fine grained examples of these rocks are, however, crossed by many regular divisional planes. The thick beds of crystallised primary limestones (Inverary, Glen Tilt) are less perfectly and regularly jointed than the thin bedded limestones of Loch Earn and the Crinan canal. Argillaceous schists, included among the gneiss or mica schist rocks, are always much more completely or symmetrically fissured than any others of the series, apparently because they are of finer grain. It might appear from these observations that divisional planes were, upon the whole, less common in the oldest systems of strata than in those of more recent date; but it would be a more correct inference, that the rocks are generally not of a nature to admit these structures.
In the British islands we have but few opportunities of beholding a complete section of the gneiss and mica schist system; the Scottish Highlands, in fact, must alone be appealed to and from these, perhaps, the most satisfactory result which can be gathered, is that which is derived from a general view rather than from any one district, like Braemar, Loch Sunart, or Loch Tay.
In the accompanying diagram the two great formations of which the system consists, viz. gneiss and mica schist, are placed in their order of succession above the granite. The gneiss is generally the lowest, thickest, and most extensive: it includes primary limestone (Iona, Assynt) quartz rock (Assynt, Loch Eribol), hornblende schist (Glen Tilt). Estimates of its thickness must be wholly conjectural; but we may believe it to exceed many thousand yards.
Passing to the south-east from the granite of Strontian King's House, or Cairn Gorum, we traverse the gneiss and reach the mica schist, near the base of which (Schihallion, Ben y gloe, Balachulish) quartz rock usually occurs. In different parts of the mica schist, primary limestone occurs in stratified masses, of limited extent and, sometimes, lenticular shape (Balachulish, Killin, Loch Earn, Inverary); and it seems probable these might be employed to subdivide the great mass of mica schist, were it likely to be of any use or interest where no organic remains and few mineral variations are to be recorded. The mica schist rocks are some thousand yards in thickness.
The upper parts of the mica schist (Loch Earn, Loch Lomond) become chloritic, and might, perhaps, deserve to be considered as a separate formation of less extent and thickness than the others.
Organic Life.—In all the enormously thick masses of gneiss and mica schist, and in all the included limestones and quartz rocks, we find probably no traces of organic beings. To judge from this extraordinary, and, perhaps, complete deficiency, we should say there were neither plants nor animals in existence on the globe at the time of the deposition of these rocks. But, before admitting this conclusion, it is necessary to determine whether any thing that is known of the history of these rocks would justify a suspicion that the traces of organic remains were peculiarly liable to be extinguished in them by heat or any other cause. It is a favourite speculation among a certain class of modern geologists, that the peculiar mineral, and structural characters of gneiss and mica schist are not original, but derived from the influence of heat upon common sandstones and shales—a greater effect of which heat would convert the gneiss to granite; and they suppose that such transformation of the substance of the rocks was accompanied by a complete extinction of the substance and impressions of the imbedded organic fossils.
Were this speculation of the origin and metamorphism of gneiss true to the extent stated, the supposition depending upon it, with regard to the contemporaneous extinction of all traces of organic fossils, would become plausible, perhaps probable; but if the view which we have given of the origin of gneiss, from disintegrated granite, be correct, there is no need of supposing any considerable change of the texture of the rock by heat, and the supposition concerning organic remains is of no authority. Independent of this circumstance, we know, first, that the forms of plants, crinoidea, and shells, do remain among limestones rendered completely saccharine by heat (Teesdale); among shales indurated to a great degree (Coley Hill Dyke); among coarse and fine slaty rocks (Snowdon, Coniston,) which have undergone metamorphosis; and, secondly, as we ascend in the series of strata, organic remains gradually appear (at first very few), and become continually more and more numerous, as the circumstances of the land and sea more approximated to the present. In the actual state of knowledge the most probable conclusion is, that during the deposition of these most ancient rocks the globe was so circumstanced with regard to heat, or some other agency, that organic life, if it had commenced at all, was exhibited at very few points en the surface of the globe. (See Table, p. 80.)
Extent of Country.—Within the British islands, it is to the highlands and western isles of Scotland, and to the mountains in the north-west and south-east of Ireland, that we must look for the great masses of gneiss and mica schist. The Hebrides, with Coll and Iona, and nearly all the north-western highlands from Sutherland to the Sound of Mull, a length of 120 miles, are composed of gneiss: if lines be drawn from the head of Loch Awe to Aberdeen, and to the Moray frith, the greater part of the large included area is filled with gneiss resting irregularly on the granites of Ben Cruachan, Loch Rannoch, Dalwhinnie, Cairn Gorum, Aberdeen, and Peterhead. Mica schist lies along the south-east side of the great valley from Fort Augustus to Lismore, spreading around Ben Nevis; a much larger space is filled by this formation on the south east flank of the gneiss, from Stonehaven by Killicrankie and Dunkeld, to the head of Loch Awe and the mouth of Loch Long; it fills all Cowal, the north side of Loch Fyne, Colonsay, and great part of Cantire, appearing likewise in Arran, Bute, and the south-western sides of Islay and Jura. Quartz rocks occupy large spaces (north-east and southwest) in Islay, Jura, and Scarba—range in a narrow line (north-east and south-west) through Breadalbane by Loch Lyon and Schihallion. From Ben y Gloe to Braemar, and between the Spey and the Doveran to Cullen, is a mass of quartz rocks, ramified among the gneiss and granite. An interrupted line of quartz rocks borders the western side of the gneiss tract from Loch Eribol to the southern parts of Skye. The primary limestones occupy but little surface.
From the Argyleshire highlands the mica schist may be considered as crossing the Channel, south-westward, to Derry and Donegal, where it expands into the large area adjoining the sea, from Lough Foyle to Ballyshannon, and stretches inland nearly to border the basaltic platform of Antrim, being associated with granite, quartz rock, limestone, and old red sandstone. From Donegal Bay to the Bay of Sligo, and from this nearly to the Bay of Galway, mica schist with quartz rocks occupies a great part of the mountainous borders of the Atlantic. The Wicklow granites are bordered by narrow belts of gneiss. Excepting very insignificant traces in Skiddaw, there is hardly any real gneiss or mica schist in England or Wales.
To describe the extent of country occupied by gneiss and mica schist on the continent of Europe, would, perhaps, be impracticable, and certainly, in an English treatise, of little use; the Pyrenees, the Alps, and the great chains of Bohemia and Scandinavia, are full of these rocks, which have much the same characters as in the Grampians and Connemara; rest in the same way on granite (which enters them in veins); are similarly associated with limestone, quartz rocks, and serpentine; and are equally deficient of organic remains. Most of the great mountain chains of the world contain these rocks, and they may be considered as the most nearly universal strata that we are acquainted with.
Physical Geography.—Usually exhibited at high angles of inclination along the axes or flanks of great mountain elevations, gneiss and mica schist, with their associated rocks, derive from this circumstance a grandeur of position which gives full effect to the bold summits, abrupt precipices, deep glens and lakes, which abound in these tracts. The pointed gneiss rocks near Mont Blanc (Aiguilles),—the conical tops of the quartz mountains of Schihallion,—the Paps of Jura, —the Sugarloaf in Wicklow,—the wildly broken crags of mica schist in the Trosachs, are too familiar to need description; but, picturesque effects of this high order depend on a combination of circumstances; the position and hardness of the rocks—relative depth of valleys and other causes; and large tracts of gneiss in Ross and Sutherland, and of mica schist in Argyle, can by no fancy be transformed into the sublime or beautiful. Yet, even in the dreariest wastes around the heads of the highland glens the hills of gneiss, mica schist or quartz rock, contain elements of form and colour which the artist knows how to value. Monotonous as they sometimes are, the irregularity of their outline prevents formality; the immensity of the mountains fills while it saddens the mind; and if the scarcity of wood gives a wildness to the fairest lakes, the partial herbage, lichens and mosses, cover the hills with tints suitable to the other features of the landscape. It is not prettiness nor gentle beauty, nor antithetic effect of colour or outline, which reward the wanderer among the Grampian Hills; but a deep feeling of the grand and awful harmonies of nature is sure to steal into his mind, and linger there even after he has climbed the snowy Alps or sunny Pyrenees.
Igneous Rock.—Granite, as was stated before, is found almost universally beneath gneiss and mica schist,— sometimes touching one (gneiss most frequently), some. times the other. It generally appears to have been in a state of fusion since the deposition of these superincumbent strata, since veins of it are injected into their cracks and fissures. (Examples may be seen in Glen Tilt, in Arran, in Skiddaw, in Wicklow, &c.) Porphyritic dykes divide mica schist under Ben Cruachan, and gneiss in Glen Coe. A mass of porphyry has perforated the granite and mica schist of Ben Nevis. Greenstone and other trap dykes are frequent (Perthshire). Serpentine occurs at Portsoy, in Iona, Lewis, and Zetland, in Connemara, &c. Very long and remarkable trap dykes run east and west through the mica schist and carboniferous limestones of Mayo and Sligo. Mineral veins are not so abundant in these rocks in Scotland, as in Saxony, Bohemia, &c.: it is generally near the granitic masses that they occur at all. The lead mine of Strontian is one of the most remarkable; it may be looked upon as a metalliferous dyke. Neither hot springs nor mineral waters are common in the British tracts of gneiss or mica schist.
General Inference.—The preceding statements are sufficient to allow of our forming an incomplete notion of the origin and formation of the rocks contained in the gneiss and mica schist system. On a first view of the phenomena, granitic rocks of various composition appear to have been disintegrated, the separated minerals, quartz, felspar, mica, &c., agitated in a peculiar manner in water, re-aggregated in laminæ, and partially collected into beds. At intervals in this process there was formed in the water a chemical precipitate, limestone, seldom in extended strata, frequently in limited lenticular masses, implying a merely local agency. There is no proof, nor any very high degree of probability, that organic beings had been created—no proof of the emergence of land; but evidence of watery movements, different from the agitation of currents or the tide.
To connect all these circumstances together, the least unreasonable speculation appears to be that the globe had cooled at the surface, so as to allow of the ocean collecting itself over the granitic basis of the strata; that this ocean was warm, agitated by somewhat like ebullition, traversed by certain gases from below, which aided in the general disintegration of the granite and in the partial precipitation of limestone; and that the general surface of the earth was hotter than the limits of temperature within which organic life has been restricted by Providence.
The general condition upon which all this explanation might be made to depend is the hypothesis that the earth at the time of the production of this earliest system of strata, retained within, and communicated to the surface, a much larger portion of its original heat than is now experienced.
But to this speculation, and indeed to almost all the partial inferences which it is intended to embrace, there is the general objection made, that the present mineral aspect of the gneiss and mica schist does not prove their origin from granite, but their partial re-conversion to that rock; that the absence of organic remains in these ancient strata is a fruit of such re-aggregation of the mass of the rocks; and that thus the whole basis of the reasoning and speculation changes, gneiss and mica schist become types of metamorphic rocks, and the monuments of the origin of watery action and organic life on the globe are wholly and irrecoverably lost. It must be confessed, that the doctrine of metamorphism of rocks has well explained the changes near trap dykes, in sandstones, shales, and limestones,—has fully explained the production of crystallised minerals among sedimentary strata (Teesdale, Plas Newydd); but the condition of the grains in mica schist and gneiss is not such, nor is the manner of their aggregation such as to justify a belief that these strata have undergone so complete a metamorphosis as Mr. Lyell's doctrine teaches. They are generally indurated; near granite rocks specially changed: every where they have suffered the influence of pervading heat, always enough to agglutinate, sometimes to recrystallise, the fragmentary mica, quartz, and felspar. Moreover there are cases where organic remains do occur (Dauphiné), among strata of analogous composition though different antiquity. The absence of organic remains in these ancient strata is still a fact to be explained otherwise than by the action of heat. The watery origin of these rocks is a truth; the alterations which they have since undergone are intelligible; and, thus, we appear to be justified in rejecting the doctrine which denies the power of discovering monuments of the commencement of watery action and organic life upon the earth.
Lower Cambrian System of Sedgwick.
In several parts of the world the Hypozoic Strata of gneiss and mica schist are succeeded immediately, or almost immediately, by the Silurian strata, as they were defined in 1836 by sir R. Murchison (Scandinavia, Bohemia, &c.), but in others, (Wales, Cumberland) the Llandeilo rocks, which were assumed as the original base of the Silurians, rest upon a great thickness of arenaceous and argillaceous deposits, among which clay slate is very abundant, and in which organic life is rare. Professor Sedgwick regarded these from 1832 to 1836 as the Cumbrian and lower part of his Cambrian systems; and it not having then been ascertained that the limestone of Bala, which both he and sir R. Murchison appear to have regarded as far below the Silurian strata, was really the same formation as the limestone of Llandeilo, a partial discordance of nomenclature arose, which is not yet well settled, and which cannot be entirely settled without a full discussion of first principles. Whether we have yet sufficiently investigated all the bearings of the case may be doubted; but these things at least are clear. If by the term "system" we mean to collect large groups of mineral deposits, with certain general physical relations, and certain general relations to life, then both in Wales and in Cumberland such a system intervenes between the characteristic Silurians of Murchison and the true hypozoic strata. But if, following views which have lately become prevalent, we fix our attention exclusively on the successive great groups of life, and regard the whole "upper and lower" Silurian series of Murchison as such a group, then certainly there is no other older group known to palæontology, and we may, in the words of an earlier edition of this treatise (1837), "consistently view the organic remains of the clay-slate and Silurian periods as belonging to one long succession of creative energy the first, if our views as to the origin of the gneiss and mica schist be correct, "which was established upon the globe." But there is yet another aspect of this question. Sir R. Murchison, when he proposed the Silurian system, was perfectly aware of its divisible character, and did in fact divide it into upper and lower. To each of these divisions belongs a large suite of fossils, and they are so distributed that only a small percentage passes from the one to the other. If upon this basis we were to cut the original Silurian series into two systems, the Lower Silurians would attach to themselves all the strata above the hypozoic series, and this is really the Cambrian system of Sedgwick. But this will hardly be done without hesitation, and without a more full discussion of evidence from all quarters of the palæozoic strata, than has yet been attempted. In the Malvern district, though upon the whole the fossils of the upper and lower Silurians are very distinct, limestones which occur in the lower group are full of fossils which are generally viewed as belonging to the upper, but they occur below sandstones which contain the fossils of the lower series.
Perhaps no conclusion now to be adopted can be entirely satisfactory. As the matter stands at present we prefer to keep the Silurian system within its original limits, and to treat separately the strata between it and the hypozoic rocks, as a series in which the rarity of life contrasts strongly with the rich variety of organization which fills the true Silurians. It is to professor Sedgwick we are mainly indebted for our knowledge of them. In his nomenclature they are ranked as lower Cambrian.
Composition. The type of these rocks is upon the whole eminently argillaceous, as that of the older systems is arenaceous: but between these two terms the difference is not always very clear. Some proportion of alumina must, indeed, be present in argillaceous rocks, but it is seldom absent from arenaceous compounds: such a substance as felspar, reduced to fine particles in water, might make a good substitute for clay; if left in a state of granulation it might constitute an arenaceous rock, and be even called sandstone. The former is, perhaps, almost really true with respect to clay slate; for this substance is not very distinct, chemically speaking, from decomposed felspar which has lost or changed the condition of its potash by the operation of water: hence it happens under particular circumstances (which permit the access of alkali and the agency of great heat), that powdered blue slate is actually transformed to white and glassy crystalline grains of felspar. This is one of the results of the yet uncompleted experiments on the effects of long continued heat, instituted by Mr. W. V. Harcourt in Yorkshire.
Clay slate, the simplest form of argillaceous fissile rock, is so uniform in its appearance, fineness of grain, colour, hardness and chemical composition, that mineralogists have often included it in their arrangements as a peculiar mineral species. Imbedded in it we sometimes find certain crystallised minerals, as chiastolite or hornblende (in Skiddaw), cubic pyrites (Dunolly, near Oban, Ingleton, in Yorkshire); its colour is black (Skiddaw), purple (Snowdon), green (Langdale), yellow (Charnwood Forest), mottled (near Ambleside): some varieties (Westmoreland) are translucent at the edges: others (N. Wales) opaque: there are variations of hardness, from the soft perishing slate of Skiddaw to the hard durable rocks of Langdale.
If we imagine the substance of clay slate diffused amongst and around grains of quartz, felspar, mica, bits of jasper or other minerals, and the whole indurated considerably, the general title applicable to the whole series of rocks thus composed is Grauwacke,—which varies in fineness of grain from what emulates clay slate to a conglomerate with quartz pebbles half an inch in diameter. Examples maybe found in Ben Ledi,—the Lammermuir,—the Cavan district,—at Llanberis.
Structures.—Amongst these rocks the evidence of successive deposition is sometimes most clear and decisive, especially amongst the arenaceous and calcareous compounds; in other cases, particularly among the thick masses of uniformly fine grained clay slate, very obscure. Yet, in no case, have our personal investigations among the slates of Wales or Cumbria been unsuccessful in verifying the statements of Sedgwick, and detecting certain, though not obvious, proofs of consecutive depositions among all the complication introduced by later agencies.
As a general rule it may be stated that lamination prevails most in the rocks of finest grain; beds are most distinct and continuous among the coarser grauwackes; but the laminæ observed in slate rocks are not always, nor indeed frequently, the effect of intermitting subsidence of the particles from water; for, in almost all clay slates, the predominant lamination and fissility arise from a change of molecular arrangement by influences acting since the deposition of the rock. To illustrate this, let the subjoined diagrams represent portions of clay slate and grauwacke slate, alike in all respects of structure, except the nature and direction of the lamination, D being in each a plane of stratification. In grauwacke slate (No. 37.) the laminæ of deposition show on all the vertical planes, being all parallel or nearly so to the plane of stratification; in clay slate laminæ of deposition are not seen;
other laminæ, viz. those of cleavage, induced by some process since the deposition of the rock, cross the planes of stratification (seldom at right angles), so that the stone may be split by wedges almost indefinitely into thin plates, nearly in a vertical direction. In some cases, as shown in the lower part of No. 36., the laminæ of deposition remain in clay slate; and instances occur in grauwacke slates where one or more fine grained bands have the cleavage structure, and other coarser bands have not. But the most obvious and constant marks of interrupted deposition from water traceable across cleavage planes are stripes of colour different from the mass, or thin bands of harder matter, or layers of coarser ingredients. The most perfectly cleanable slate rock, though it be almost a crystal in respect of its regular structure, shows in the quarry indubitable marks of stratified deposition; arid where fine grained and coarse grained slate rocks alternate, a very common circumstance about Snowdon, the fact is perfectly obvious.
Cleavage must be viewed as a structure imposed on the rock by agencies subsequent to its accumulation as sediment. What was exactly the nature of these is a problem of some delicacy, which may be better discussed hereafter: in the mean time, the following laws have been established with respect to it. First, it is never so perfectly exhibited as in the ancient argillaceous strata; is most conspicuous among those of finest grain and most uniform nature; disappears in very coarse rocks; ranges in almost exact parallels over many square miles of country; preserves these parallels even across contorted stratification (as Plate IV. fig. 17. "Guide to Geology," edit. 3.); and mostly coincides in horizontal direction with the great axes of elevation and depression of strata in the region observed. Finally, imperfect cleavage structures are produced in argillaceous rocks of later date near trap dykes (Coley Hill, Newcastle), and near great granitic irruptions (Vale of Chamouni).
Succession of the Strata.—It is only of late years that the real nature of the proof of the stratification of slate rocks has been sufficiently understood, to permit of its application to particular districts for the purpose of constructing a section of the series of the strata. As yet only two districts in Great Britain can be considered as at all completely investigated, viz. the region of the Cumbrian Lakes, and North Wales; but they are, perhaps, the very best for the purpose that are anywhere known.
Between Skiddaw and Saddleback the base of the clay slate series is found resting on very thin mica schist and gneiss: and declining to the south-east from an axis of elevation which ranges N.E. and S.W. The dip, judged of by appearance on Derwent Water, in Borrowdale and Grasmere, appears to be considerable, yet not very steep: probably not exceeding, on an average of many miles, 10°.
The above diagram represents the whole series of rocks of this system, in their real order of superposition. The following reference will be sufficient to aid the reader's conception.
C. Slaty group of Langdale, (Borrowdale), 10,OOO feet or more.
In the upper part (4) are dark, flaggy, and slaty rocks; the middle (2) abounds with fine green slates; near the bottom (2) most of the rocks are mottled, amygdaloidal, or fragmentary: 1 is a red argillaceous mottled rock, which sometimes appears like a conglomerate.
B. Slaty group of Skiddaw, 1000 yards or more.
It consists almost wholly of dark, soft, useless slate: toward the lower parts chiastolite abounds in it (2), and near the base hornblende (Graptolites).
|A. Of the gneiss and mica schist system is a mere trace, over granite.|
In Wales, also, these very ancient strata may be traced on the west of Snowdonia and between Cader Idris and Moel Siabod. Mr Jukes and Mr. Selwyn have presented a section of the latter district (Geological Proceedings, 1848.). The former is familiar to English geologists, in consequence of the interest inspired by the well known previous researches of Sedgvvick; and as the whole of Wales has at length been coloured on the Ordnance maps by Sir H. De la Beche, Mr. Ramsay, and their friends, and illustrated by measured sections, the data for reasoning are gradually growing complete. The section from Snowdonia westward to the Menai is, on many accounts, one cf the finest in Wales. Commencing in Anglesea, we have undulations of micaceous and chloride schists with quartz rocks, apparently the lowest (probably hypozoic) strata visible in Wales. Then follow sandstones and conglomerates—black shales of Bangor—other sandstones and conglomerates interstratified with trappean rocks—then the fine purple slates of Llanberis and Nant Francon. In all this vast series no fossils have been discovered.
Above are sandstones and slates beneath the trappean group of Snowdon, on the summit of which are the fossils of Bala. These sandstones, however, are much better seen in the section from Cader Idris to Moel Siabod. They are thus described by Sir H. De la Bechet:—
"A series of sandstones and conglomerates, with some beds of purple and blue slates, and occasionally trap rocks, about 3000 feet thick, constitute the base of that part of Wales. These are known as the 'Barmouth and Harlech Sandstones,' and they are brought up by an axis of elevation called by Sedgwick the 'Great Merioneth Anticlinal.'
"A trappean group succeeds above, containing contemporaneous igneous rocks, some felspathic, some hornblendic, with beds of 'ash,' probably ejected into the air, and falling in water, arranged like ordinary detritus by tides and currents, about 15,000 feet thick. This is formed in two divisions; the lower, containing blue and gray slates and flagstones, is known as 'the Lingula beds,' from the abundance of that shell which, with some others, occurs in them. In the upper division are many interstratified beds of black slate, often occurring as irregular and lenticular masses, and graduating into 'ash.' Lingula and graptolites occur in these beds, though not abundantly."
Upon these rests the Bala group. This is in accordance with the early general views of Sedgwick. Thus by combining the information which these researches yield we have strata of slate, sandstone, conglomerate, and trappean ash, many thousand yards in thickness, below the lowest band of what was considered, in 1836, the true lower Silurian base. In all this thickness only a few organic remains occur either in Wales or Cumberland, and all the very lowest beds in Wales appear devoid of them. Perhaps this deficiency of the remains of life is the main character by which this "Lower Cambrian" system is distinguished from the strata above.
Organic Remains.—By whatever name these ancient strata shall in future be called, they must always bear to the philosophic student of geology a peculiar and paramount interest, as the oldest rocks in which organic remains are certainly known to occur. It may surprise the speculators in Cosmogony to hear that these, the most ancient forms of life known to us, are not plants, but animals: not even the lowest grades of their respective classes, but perfectly developed zoophyta, echinodermata, brachiopoda, and trilobites. No gasteropods or cephalopods are indeed as yet mentioned in these rocks, but we must be careful not to attach to this merely negative evidence more value than really belongs to it. Whether at the time of the formation of these ancient rocks in the sea, plants were growing on the land (whether, indeed, there were any neighbouring dry land), we must only conjecture: that plants might be growing in the sea, which nourished the shells and trilobites and zoophyta, may be a probable but not a certain inference; since sea-weeds do not alone constitute the food of conchifera or zoophyta. We found no satisfactory trace of plants in the fossiliferous rocks of Snowdon, nor are they common in Cumberland.
We cannot, from the few scattered species of fossils (yet very imperfectly known) which have been obtained from these ancient rocks, learn the conditions under which they lived; but they are of great value as the oldest monuments yet discovered of the creation of living things. The very rarity of their occurrence and the paucity of species confirm the general views advanced as to the cause of the absence of organic fossils from the still older systems of gneiss and mica schist; for these few remains, scattered through as many miles of stratified rocks of different nature, appear to indicate that only at a few exceptional points were the conditions established which allowed of organic life being developed.
No clear or general differences of form distinguish the fossils of the lowest palaeozoic rocks from those of the Silurian system next above: the same predominance of brachiopoda among the shells, the same comparative abundance of zoophyta, and the same rarity of plants, appear to show that the circumstances affecting organic life differed only by degrees. We may, perhaps, consistently view the organic beings of the clay slate and Silurian periods, as belonging to one long succession of creative energy,—the first, if our views as to the origin of the gneiss and mica schist be correct, which was established upon the globe.
Geographical Extent.—In Great Britain, fortunately, this valuable system of rocks is extensively developed along the flanks of the great mountain ranges. A narrow band of clay slate and grauwacke accompanies the south-eastern flank of the Grampians, from Stonehaven, by Dunkeld, Comrie, Loch Venacher, Luss on Loch Lomond, the lower end of Loch Long, part of Bute, and the north-eastern part of Arran. Detached portions occur in Aberdeenshire, at Balachulish, about Dalmally, &c.
As already observed, the greater part of the mountainous region which incloses the Cumbrian Lakes is formed of these rocks, and the slate exists there in great variety of colour, composition, and quality. The slaty cleavage often crosses the interposed trappean masses, as well as the coarsely aggregated argillaceous beds.
In Wales the horizontal area which they cover is greatest on the western side of Snowdonia—about Harlech and Barmouth, and at points in the S. W. extension of the principality toward Prescelly and St. David's.
The Isle of Man is principally composed of slaty rocks, amongst which, perhaps, these oldest rocks have a place. The slate of Charnwood Forest is perhaps (we think probably) to be regarded as of the same period as the green slate of Coniston. It is unsafe at present to quote the foreign localities, though perhaps the Scandinavian schists containing Oleni, like those of Malvern, North Wales, and Angers, may be of coeval date, and the lowest palæozoics of North America may be put on the same parallel.
Physical Geography.—The slate system, though but a very inferior feature along the Grampian ranges of Scotland, forms the most elevated points of land in England. Supported by granite, and mixed with igneous masses, the slaty rocks of the English lakes rise to more than 3000 feet in height (Sea fell is 3160, Skiddaw 3022), and present a variety of outline, and intricacy of combination, which, in connection with clear lakes and considerable waterfalls, leave to Switzerland little superiority, except that beauty and grandeur imparted by the mighty summits of snow, which is perfectly inconceivable to an English tourist, who might shudder by his fireside at the very mention of a wintry view of Helvellyn.
Each of the slate formations of Cumbria has its own characters of scenery: broad swelling forms accompany the Skiddaw rocks; enormous crags and fearful precipices, with broken waterfalls, characterise the middle division, and the upper has, generally, a number of serrated hills of very inferior effect in the scenery. The lakes of the Cumbrian region are often so deep as to preclude wholly the notion of their having been eroded by water. The valleys are, according to Sedgwick, usually accompanied by great dislocations, radiating from the central elevations of the rocks.
The slaty regions of North Wales are superior in the breadth and grandeur of their effects, though not in picturesque beauty, to the districts of the English lakes. Their effective height is greater, from the entrance of many arms of the sea into the midst of the mountains: there is, besides, something deeper and richer in all the colouring, a greater expanse of surface, largeness of feature, and freedom of outline, which reminds us of the best parts of the Grampians; while the valleys, sometimes richly wooded and watered (Festiniog, Dolgelly,) and sometimes dreary and solitary (Llanberis, Beddgelert), furnish, even without their wild lakes and rough cascades, every possible variety of pictorial accompaniment.
If we consider how all these circumstances depend upon the general conditions of a forcible elevation of rocks of different qualities into an atmosphere competent to produce upon them unequal chemical effects, and on their disintegrated particles to nourish correlative vital phenomena, we shall see how trifling is the enjoyment of beautiful nature which they experience who are satisfied to gaze on effects with the painter, and seek not their appointed causes with the geologist.
Igneous Rocks are associated with the argillaceous slates in every district where they appear in the British islands. Granites touch the slates in Cumberland, in Cavan and Arran; porphyry and greenstone are abundant in the Cumbrian mountains and Snowdonia, both in dykes and partially stratified masses. Mineral veins are found nowhere so abundantly as near the granites, or other igneous rocks; a circumstance which combines with many other facts to demonstrate the dependence of mineral veins on some peculiar agency of subterranean heat. Quartz, the most frequent matrix or vein stuff of these mineral veins, is very often found ramifying into the fissures and cracks of the slate rocks, without any metallic admixtures.
Judging from personal observation, as well as recorded phenomena, we should say the effects of locally developed igneous agency are much more frequent among the rocks of the slate system, than in those of earlier date, and that dykes, veins, and interspersed beds of porphyry and greenstone, are more abundant and varied. The circumstances observed in Cumbria and North Wales of the porphyry beds being subject to the same flexures and inclinations as the slates, leads to the inference that they were effused, as lava sometimes is, on the bed of the sea, at intervals during the deposition of the slates; the dykes, of course, are of later origin, and the same is generally (we think) true of the mineral and quartz veins, but some metallic and quartzose masses are probably contemporaneous with the rocks which enclose them.
Besides these local effects of subterranean heat, the whole structure of the slaty rocks appears to be the result of a general pervading heat, operating on the argillaceous sediments so as to overcome their natural horizontal lamination, and induce a new, almost crystalline, fissility in vertical or highly-inclined planes, having one constant direction. We may consistently view this remarkable polarity of the cleavage as a characteristic effect of some very general agency, directing the results of molecular attraction: it is certain that this directive energy was never displayed in the same general way in the argillaceous rocks of later systems, though, as in Devonshire, South Wales, and the South of Ireland, local effects of this kind are seen in strata of the old red period, and in the Alps lias shales are decidedly subject to slaty cleavage. We may adopt, subject to a few exceptions, the rule first stated by Sedgwick—that the strikes of cleavage correspond to the strikes of the strata, though their inclination differs in amount, and even in direction; and this leads almost positively to the inference that the one is dependent on the other. My own observations have led me formerly to adopt the opinion that the divisional planes of slate were due to a molecular re-arrangement with polarity; another view ascribes the structure to pressure. It is certain that such pressure has operated, for shells and trilobites have been changed in form by it.
Silurian System of Murchison.
Composition.—The rocks of the Silurian system, as it is exhibited in the country first surveyed by Murchison (the whole Welsh border and large tracts in South Wales) may be said to contain types of the usual sedimentary aggregates—argillaceous, arenaceous, calcareous; nor is there any very clear or exact definition by which they can be discriminated in the cabinet, as a mass or individually, though in the field they are easily and accurately traceable, for limited ranges of country. Compared with the older strata, the argillaceous rocks, in general less indurated, less complicated by divisional planes, and only locally endowed with cleavage, retain their original lamination: the arenaceous rocks deviate from the character of grauwacke, toward ordinary sandstone and conglomerate; the calcareous rocks are not usually so crystalline as in gneiss, nor of so earthy a substance as many of the later secondary limestones, but have a concretionary sub-crystalline texture.
Examined in detail, however, considerable variations appear among the different members of the Silurian system: some of the argillaceous beds are black, others of a liver or grey colour: some arenaceous beds fine grained, and argillaceous (Ludlow) were aptly named by Mr. Murchison "mudstone": others are like common hard gritstone (in the Caradoc): some appear to be principally composed of volcanic ashes, or the disintegrated particles of trap rocks, and are called "volcanic sandstones" (Malvern hills, the Caradoc, &c.). Some of the limestones (Llandeilo) are broad, flaggy, and argillaceous, as usual in shale; others (Aymestry, Wenlock) are purer, more concretionary, and more analogous to the calcareous rocks of the carboniferous system above.
Structure.—In general the accumulation of these rocks appears to have been regular and tranquil; the whole series of argillaceous and most of the arenaceous rocks are full of laminæ of deposition: beds are very distinct in the sandstones: the limestones are also regularly stratified, though nodular and uneven on their surfaces, and sometimes partially lenticular or included among shales, like other calcareous rocks supposed to have originated as coral reefs. According to what we have found to be a general law, that divisional planes abound and are regular in proportion to the regularity of the laminæ of deposition, the argillaceous beds of this system are seen to be very exactly divided by joints and fissures, some of which seem frequently to coincide with the axis of elevation of the country (in Wales this is frequently N. E.), and others to be rectangulated thereto. In this respect, however, variations occur, so that Mr. Murchison has often found two sets of joints, both forming oblique angles with the strike of the strata. In the Wren's Nest at Dudley the long joints in the limestone) observed with care, July, 1837, appeared to me to be nearly parallel to the axis of elevation, which is N. and S., or, on the east side of the hill, N. 10° E. The joints 1) in other directions were few and irregular; and though cracks were not infrequent in the thinner layers of " Baven" (argillo-calcareous and fossiliferous), the most general direction of the long joints in this also was N. 10 E. The planes of these joints are not always even nearly rectangled to the stratification they are sometimes waving in their outline, and present other circumstances of interest, particularly striations and associated strings of calcareous spar. The faults in this hill obey the law of displacement, which is given in the "Geology of Yorkshire," and illustrated p. 40. sup.
The planes of the joints and fissures are stated by Mr. Murchison to be nearly rectangled to the planes of the strata. In the country near Llandovery the lower Silurian rocks are metamorphic slates; the slaty cleavage, induced since their aggregation, predominating over the yet traceable surfaces of deposition. This is sometimes expressed by the convenient though somewhat unlearned word, "slatified."
The nodular uneven surface of the limestones of this system is so remarkable, as to be of great importance in reasoning on the circumstances concerned in their production. It is extremely difficult to resist the notion that in many instances the limestone has been collected by molecular attraction from a mingled mass of argillaceous and calcareous sediment, round corals, and other organic marine exuviæ. The great abundance of corals in these rocks (Aymestry, Dudley, Wenlock, &c.) leads further to the supposition of their being really formed, like a coral reef, in the present seas. If this were correct, the whole of the substance of the rock must be supposed to have been abstracted from sea-water, by that vital action which dissolves the strongest chemical aggregations, and fixes the unwilling elements in new combinations. The perfectly laminated or bedded structure of the rock requires, further, the admission that the materials were arranged in obedience to the fluctuations of water: this, which implies the removal and partial drifting of the corals and shells, is strongly confirmed by the worn and rounded forms of some corals (Aymestry), the unattached condition of almost all (Dudley), the broken and crushed condition of many. If, therefore, we must compare the origin of the Wenlock and Aymestry limestones to that of a modern coral island or group of islands, the Bermuda group, where vital action furnishes the substance, and oceanic currents determine
Succession and Thickness of Strata.
The best, or rather the only, complete British series of these rocks is that of the Welsh border, of which the above section is a sketch: below is Mr. Murchison's summary.
|Ludlow Formation||2000||Upper Ludlow rock.||Slightly micaceous, gray coloured, thin-bedded sandstone.|
|Aymestry limestone.||Subcrystalline gray or blue argillaceous limestone.|
|Lower Ludlow rock.||Sandy, liver and dark coloured shale and flag, with concretions of earthy limestone.|
|Wenlock Formation.||1800||Wenlock, limestone.||Highly concretionary subcrystalline gray and blue limestone.|
|Wenlock shale||Argillaceous shale, liver and dark gray coloured, rarely micaceous, with nodules of earthy limestone.|
|Caradoc Formation.||2500||Flags.||Thin-bedded, impure, shelly limestone, and finely laminated, slightly micaceous, greenish sandstone.|
|Sandstones grits, and limestones.||Thick-bedded, red, purple, green, and white freestones; conglomeritic quartzose grits, sandy and gritty limestones.|
|Llandeilo Formation.||1200||Dark-coloured flags, mostly calcareous, with some sand stone and schist.|
On a careful examination of the vicinity of Ludlow, all the upper parts of the Silurian rocks may be perfectly traced and clearly discriminated: it is in the vale of the Towey (Dinevawr) that the lower formation is best exhibited. The farther west we go in Wales, the less obvious is the division of the formations; the greatest degree of distinctness is observable in the eastern districts; we may by selecting examples find that the characters of the whole series graduate from one group to another. There is so great and real an analogy between the Llandeilo, Caradoc, Wenlock, and Ludlow formations, as to permit our viewing them all as one varied series of deposits effected by one general system of mechanical, chemical, and vital agencies. Sir R. Murchison, by his long researches on the border of Wales, added to the magnificent series of English strata and palaeozoic life this one of the most important of their terms. He thus advanced the principles of W. Smith to a new and most fruitful conquest, and established results which have and must retain a powerful and beneficial influence on future geological discovery.
Organic Remains.—In his great work, "the Silurian System," (1836) Murchison records 351 species of fossils which had come under his notice, and describes in respect of each its geological and geographical ranges as then determined. A similar investigation made by the author of this work on the materials collected by the Geological Survey of Great Britain (1848), gave 353 species for the districts between the Severn and St. Bride's Bay. The following table represents the geographical distribution of these 353 species in thirteen districts.
The two most prolific districts in this table are those of Malvern and Llandeilo, which are the two containing the most complete series of Silurian strata. The two least productive are those of Cardigan and Caermarthen, in which only the lowest group appear. The eastern districts are more prolific than the western, because they exhibit a greater variety of mineral character in the deposits.
|Western Districts.||Eastern Districts.||Total Number of Species in each Class.|
|Number of distinct species in each districts.||5||81||29||73||4||187||72||128||30||76||143||107||195|
If, taking the same 353 species, we inquire into their distribution through the strata of the eastern region (in which deposits are most sharply defined and present the greatest variety of mineral composition) we have the following table:—
|Number of distinct species in each districts.|
|Caradoc (Upper)||2 or 3||4||1||3||—||10||1||29||2||13||66|
The maximum of variety appears in the Wenlock limestones. The series of life appears at first small or null (the lower strata possibly belong to a period older than the true Silurians); then swells to a full development in Upper Caradoc; sinks to a small amount in Wenlock shale; expands to its fullest extent in Wenlock limestone; contracts again; amplifies again to the Upper Ludlow, and dies away in the Downton sandstone, which unites the Silurian rocks to the superjacent old red sandstones. Viewed on a larger scale it appears that only a small portion of the fossils of the Silurian system has been found both in the upper and lower divisions— between which undoubtedly, as sir R. Murchison first remarked, a rather strong line is traceable, especially in the eastern regions, where the upper division is calcareo-argillaceous, and the lower one arenaceous. The difference is less obvious in the extreme west, where the limestones nearly vanish. These limestones mark intervals of rest, in the generally descending movement to which the bed of the Silurian sea was subject, a movement which was continued through later periods, so as to cause the accumulation of many thousands of feet of other sediments brought from other points of the earth's surface by other currents, charged with other forms of marine life.
To obtain these 353 species 313 localities were searched diligently. The specimens obtained at almost every locality, even in a limited district, offer something peculiar to and characteristic of place; still greater is the difference between different districts; only per cent being found in each of the two neighbouring districts of Abberley and Woolhope, which contain the same strata equally exposed. A few species have very wide geographical ranges: such are:—
Now it is remarkable that many of these are fossils which occur in more than one stratum,—often, as the brachiopoda, in several—so that they are thus found to be truly characteristic of the Silurian system, or of large thicknesses of it. Other shells are most abundant in particular deposits—as Serpulites longissimus in Upper Ludlow, Pentamerus Knightii in Aymestry rock, Graptolithus Ludensis in Lower Ludlow, Cyathophyllum dianthus in Wenlock limestone, Trinucleus Caractaci in Caradoc, Ogygia Buchii in Llandeilo flags.
The results obtained by M. Barrande in the small Silurian basin of Bohemia are very similar—the same rather strong division of Upper and Lower Silurians being admissible.
In North America we have a numerous list of fossils, derived from a greater number of beds, and the general parallelism of all the deposits from the base of the carboniferous to the top of the hypozoic series, together with the gradual shading of one group of fossils into another, renders the division into Devonian, Upper Silurian, Lower Silurian, and Cambrian somewhat objectionable. Hall (Geol. of New York) admits what he supposes to be Devonians into the same system as the Upper and Lower Ludlow. This classification may be useful for reference to English geologists.
|Subdivisions of the Rocks of the New York System.||Subdivisions of the Silurian and Old Red System in Great Britain.|
|1. Chemung group||Devonian||Upper and Lower Ludlow Rocks including the Devonian System|
|2. Portage group|
|3. Genesse slate|
|4. Tully limestpne|
|5. Hamilton group|
|6. Marcellus shale|
|7. Corniferous limestone||Possibly
|8. Onondaga limestone|
|9. Schoarie grit|
|10. Cristagalli grit|
|11. Oriskany sandstone|
|12. Upper Pentamerus limestone|
|13. Encrinal limestone
|Subdivisions of the Rocks of the New York System.||Subdivisions of the Silurian and Old Red System in Great Britain.|
|14. Delthyris shaly limestone||Wenlock Rocks.|
|15. Pentamerus limestone|
|16. Waterlime group|
|17. Onoudaga salt group|
|18. Niagara group|
|19. Clinton group||Caradoc sandstone.|
|20. Medina sandstone|
|21. Oneida conglomerate|
|22. Grey sandstone|
|23. Hudson River group|
|24. Utica slate||Llandeilo flags.|
|25. Trenton limestone||These formations are not as fully recognised in Great Britain as in New York|
|26. Bird's eye and Black River limestones|
|27. Chazy limestone|
|28. Calciferous sand rock|
|29. Potsdam sandstone|
The 'lowest of the fossiliferous rocks'—the Potsdam sandstone—like some of the lower 'Cambrians,' contains only or almost only a Lingula. In this rock, however, Mr. Logan reports 'traces' of a Chelonian reptile (1851). It requires further study. In the next groups 2, 3. the number increases, but is still small. In the Trenton limestone (4.) organic remains of the Lower Silurian types become plentiful.
The distinctness of the organic fossils of the Silurian rocks from those of the carboniferous formation, as far as regards the marine races, is an important truth which has received further and exact confirmation from Mr. Murchison's researches. To what extent the few fossils of the slate system are analogous to the Silurian reliquiæ is not accurately known; but there appears a sufficient resemblance between them to justify a belief that the physical conditions of the ocean were not greatly changed, though evidently rendered more favourable to the development of a varied system of organic life. Mr. Murchison showed that each of the four formations of the Silurian system contains distinct suites and characteristic species of fossils. The following are among the most common or remarkable:—
Ogygia Buchii, fig. 1. Pentamerus Knightii. fig. 2. Dalmannia caudata, fig. 3. Euomphalus rugosus, fig. 4. Leptæna depressa, fig. 5. Calymene Blumenbachii, fig. 6. Orthoceras pyriforme, fig. 7. Orbicula rugata, fig. 8. Palaeopora interstincta, figs. 10, 10 a magnified. Cateniporalabyiinthica,fig. 11. Cyathophyllum cyathus, fig. 12.
Geographical Extent.—Ranging on each side of the Vale of Clwydd, the Silurian system continues by Llangollen, widening southwards to the valley of the Severn, which runs in it from Newton to the plain of Shrewsbury: it borders on the south the coal fields near Shrewsbury, and the Longmont and Stiperstones hills (of older rocks), and enters between these hills and the Clee hills in a long tongue directed N. E. to the Severn at Buildwas. This strike of the Silurian rocks, prolonged in the other direction to the S. W., passes by Knighton and Builth to Llandovery, Llangadock, and Llandeilo: it hence turns in the vale of Towy in a narrow course nearly west to Caermarthen, and with the same range passes Haverfordwest to St. Bride's Bay. From this central line the system expands on the south-east to Ludlow, Aymestry, and Knighton; and this straight south-eastern border extends parallel to the range from Builth to Llandovery, into a curious narrow tongue or broken anticlinal ridge, which crosses the Wye between Builth and Hay, and ranges towards Trecastle. (From Mr. Murchison's Observations.)
About Dudley and Walsall the Ludlow formation is admirably exhibited in singular narrow and short anticlinal ridges, rising in the midst of the coal formation of South Staffordshire, near the hills of basalt called Rowley Rag. These anticlinal ridges run north and south, in parallel courses (Sedgley, Hurst Hill, the Wren's Nest, and Dudley Castle Hill make four such ridges, the two latter being extremely clear), and against them all the coal strata rest at considerable angles of inclination. The diagrams fig. 40. and fig. 41. are intended to illustrate the curious structure of this region.
In Westmoreland and Yorkshire the upper Ludlow formation occurs near Kirkby Lonsdale, yielding fossils, and what I have supposed to represent the Llandeilo rocks in Ribblesdale, a conclusion placed on good evidence by a late investigation of Sedgwick (Geol. Proc. 1852).
Ireland contains the Silurian system, especially in
a. Sedgley ridge, or anticlinal, containing the Aymestry limestone.
b. Hurst ridge.
c. Wren's Nest, Wenlock limestone.
d. Dudley Castle ridge, Wenlock limestone.
e. The Hayes, upper Ludlow formation.
f. Rowley hills of basalt.
g. Barrow hill of basalt.
h. Coal deposit, resting somewhat unconformedly on the Wenlock formation, and partly resting on, partly passing under, the Rowley basalt which chars it.
Tyrone. The Llandeilo limestone forms a line from Broughton by Coniston Waterhead to Hougill fells, and between this and Kirkby Lonsdale are fossiliferous schists. The Lammermuir Hills present on their northern side a considerable exhibition of Silurian strata (Sedgwick, in Brit. Assoc. Rep. for 1850). In Brittany Silurian fossils occur: the limestones of Christiania and Gothland belong to this system. Perhaps the most remarkable and complete series of Silurian deposits on the continent of Europe is that of Bohemia, worked out by M. Barrande, from whose patient hands a valuable memoir is expected. It is said by Mr. Strickland to occur about Smyrna with asaphi. It is found near Oporto with coal (Sharpe). Perhaps it is from these rocks that trilobites are obtained near the Cape of Good Hope, by Sir J. Herschel. In North America the Silurian system is largely developed: its classification contains many more terms than have been found necessary in Europe. The distribution of organic remains in it is in many respects very similar (Hall, Geol. of New York).
Physical Geography.—Lying on the sloping sides of the slate systems of Wales and Cumberland, the principal masses of the Silurian rocks show but little boldness of feature, compared to these older rocks: frequently, as in the beautiful neighbourhood of Ludlow and Aymestry, and in the Vale of the Towy, they are richly covered with woods, as ancient, perhaps, as Caractacus. The limestone country about Dudley is pleasingly varied.
Igneous Rocks.—These are abundantly exhibited in irruptive axes and points throughout the Silurian formations. Lilleshall Hill, the Wrekin, and other points about it, consist of compact felspar, ranging N.E. and S.W., and they convert sandstones into quartz rock. Caer Caradoc, the Lickey, Helmeath, &c. constitute a similar and parallel group of hills, in which greenstone, actynolitic trap, &c. occur: similar changes happen to the sandstones which touch the trap: the argillaceous rocks are indurated and much altered. The Breiddin group of hills consists of porphyries, compact felspar, greenstone, &c.; and near these the strata of the Silurian system are indurated and fissured. The same range (N.E. to S.W.) is noticed in the trap rocks near Old Radnor, Builth, and Baxter's Bank, near Llandrindod. Hypersthene abounds in traps near Old Radnor, and great changes happen to the Ludlow and Wenlock rocks near them: limestone becomes crystallised; shale is indurated; anthracite, copper ore, iron pyrites, and bad serpentine, are generated at the contact. The large trap district of Llandegley, Llandrindod, and Builth, presents a variety of such phenomena, and the mineral springs of Builth, Llandrindod, &c., are supposed to be residual effects of the same igneous agency. In Brecknockshire and Caermarthenshire, similar phenomena are repeated around several erupted masses of trap.
The Malvern hills consist of Silurian rocks resting on a granito-syenitic base, which has been uplifted with them, so that the beds are vertical or even retroverted, and bent in anticlinal and synclinal axes. The Abberley Hills, Woolhope Forest and May Hill, are very interesting groups of Silurian strata.
Mineral Veins.—In the Shelve district of Shropshire, and at Nant y Moen, seven miles north of Llandovery, the lead mines are so related to the axis of irruption of the igneous rocks, as to leave no doubt of the propriety of classing them as an effect of the same volcanic excitement, not perhaps contemporaneous with the irruption of trap, but certainly and strictly associated with it, and dependent upon it. Sulphate of barytes, sulphuret of iron and carbonate of lime, accompany the ores of lead.
There is almost a total absence of proof, in the mineral composition and organic contents of the Silurian strata, of the contemporaneous existence of dry land: for all the early periods at least, the absence of land plants, and the infrequency of conglomerates, seem to justify a doubt whether the sea of that period was subject, in the regions now dried, to any thing of the nature of violent land flood, or great littoral agitation. Yet it is not only probable but proved by some instances in Wales, that the bed of the Silurian sea had been somewhat disturbed before the completion of the system. For, along considerable lengths of the boundary of the Caradoc sandstone, this littoral rock is found to rest unconformedly on previously disturbed rocks of the Llandeilo and earlier groups. There is, however, nothing to contradict the assumption that, till the close of the lower palæozoic period, nearly all the strata of the British Isles and the continent of Europe were covered by the sea in which they were formed: indeed, it may be doubted, whether any certain proof can be shown that any part of the European region was subjected to great displacement during this period.
It is true that a survey of the porphyries, greenstones, and other igneous rocks, so strangely interlaminated among the clay slates and grauwacke slates of Snowdon, and the middle Cumbrian region, from Black Comb to Ulswater, appears to prove that at certain periods during the formation of these rocks, eruptions of melted rock occurred over a great extent of the oceanic bed; and such we must suppose were accompanied by considerable, if only transient, movements of the solid crust of the globe. Elie de Beaumont has supposed that some of the most considerable displacements of primary strata which are observed in Europe, happened before the completion of the newest of those strata; but it cannot be satisfactorily proved by examples taken from the British Islands. Indeed, every fresh inquiry into the geological dates of particular disturbances of the strata, shows the difficulty of arriving at accurate conclusions on this important subject.
The evidence is sometimes insufficient; in other instances complicated with the effects of convulsions of later date, but similar geographical positions; and however strange it may appear, it is nevertheless true, that the strongest arguments in favour of the convulsions having occurred within particular limits of geological time, have been based on comprehensive views of a whole physical region, rather than on a minute scrutiny and complete survey of the details of the position of the strata, at the line of junction of the displaced and the undisturbed rocks.
After the lapse of most part of the Silurian and before the commencement of the old red period (whatever the interval of time was), great disturbances happened, which uplifted large parts of the bed of the sea, and either raised them above the surface into dry land, or, at least, placed them in such situations that no further deposit of strata was spread upon them at later periods. In many instances the Silurian and old red strata are unconformedly situated with respect to one another, as in the subjoined section (fig. 4-2).
and the geological map of the country shows superficial unconformity of direction and dip of strata as in fig. 43.
The position of the secondary strata is discordant with respect to the primary, both in dip and direction; because these latter were disturbed from their original position by subterranean forces, and the bed of the sea upon which the secondary rocks were subsequently spread entirely altered in form. The unconformity, above exemplified, is the geological proof that the older strata had been disturbed previously to the formation of the newer; and the reason for thinking they had been in many cases actually raised into dry land, is the total absence of any later deposit upon them: the former is a most certain conclusion; the latter is frequently a highly probable inference.
In the British islands we have magnificent examples of these ancient disturbances. The range of the Grampian mountains from Aberdeen to Cantire, and indeed most of the Highlands, appear to have been uplifted at this early period, if not to the surface, yet so as to prevent any depositions upon them; though round the east and west coasts of Scotland, the south border of the Grampians, and in the great valley of the Caledonian canal, the old red sandstone rocks abound. It is supposed that about the same period the Lammermuir hills were raised; and the Cumbrian mountains received one of their great upward movements. It is important to remark in connection with this subject, that along the borders of the Grampian, Lammermuir and Cumbrian ranges, the red conglomerates contain enormous quantities of pebbles, which appear to have been gathered by inundations from the surface of the broken rocks of the neighbouring slates, gneiss, &c.: if in addition we remark the fact that, especially in Cumbria, these conglomerates fill valleys at the border of the tract of the slate mountains, we shall see the probability that the slate rocks were raised above the surface to be washed by atmospheric rains, or else so near the surface as to be exposed to the agitation of shallow water.— The former is the most probable view. The slate and mica schist tracts of the Isle of Man, Donegal, Galway, Wexford, Wicklow, Cavan, and Down, appear to have been similarly raised; and the same is supposed to be true for the Snowdon and Berwyn ranges in North Wales, and the Ocrynian chain of Devon and Cornwall. We must, however, remark on these last-mentioned cases that, on the south-east border of Wales certainly, and in Cornwall probably, there is no observable unconformity between the old red and the Silurian rocks.
Were the displacements thus shown to have happened in the bed of the sea over so large a portion of the British islands, sudden or gradual? To decide whether violent uplifting, or a gentle intumescence of the rocks, lifted the Grampians or the Cumbrian mountains, would be difficult in the present state of our knowledge; yet there are considerations which would render it probable that a considerable time elapsed in the process. Amongst others, this appears worthy of notice: the secondary strata, around these and other tracts, dip at high angles from the centre or axis of the older rocks, the most modern rocks occupying the lowest; ranges; and thus appear to teach us that the elevatory action, whatever might be its first violence, was continually exerted in the same localities, late into the secondary period.
The surface of the earth has, however, undergone since so many changes, that it is difficult to say how far this argument can be safely trusted. Another highly interesting problem arises out of the admission that all the displacements of rocks, previously noticed, were nearly contemporaneous: they are found to be all raised on axes nearly parallel to a line from S. W. to N. E.; and it is required to be determined whether this proximate parallelism of contemporaneous axes of elevation is a general law of the phenomena. M. E. de Beaumont is the geologist who has most strenuously advocated the affirmative of this question; but it is certain that more rigorous investigations are needed on the subject, before any physical theory, like Mr. Hopkins's ingenious view, can be safely applied to the data. It is extremely difficult to assure ourselves that the elevations above noticed, as on parallel axes, were really contemporaneous, or even very quickly succeeding, because nothing can be more complete than our ignorance of the duration of past geological periods; and, in order to render the explanation of such parallelism consistent with Mr. Hopkins's demonstrations, the occurrence of parallel elevation must be really synchronous.
The elevations on the continent of Europe of or about this ancient period (anterior to the formation of the carboniferous rocks) are located in Brittany, the Harz, the Hundsrück, the Eifel, the Ardennes.
Whence came the materials of the great mass of deposits which rest upon the primary gneiss and mica schist?
Probably the true answer to this, though we cannot now give adequate proof of it, is that the disintegration of granitic and other igneous rocks, to which, on what seem good grounds, we have already ascribed the origin of gneiss and mica schist, has been the prolific source of all these sedimentary strata. Analysis of the principal rocks of the slaty systems does certainly not contradict this view; which neither those who admit with Leibnitz the first solid covering of the globe to have been a mass of rocks cooled from fusion, or, with Lyell, that strata added above, are melted and reabsorbed into granite below, have any reason to deny
Moreover, we see daily, on the slopes and at the foot of hills composed of trap rocks, considerable quantities of loosely aggregated sands, which to all appearance, if agitated in water, might be indistinguishable from various secondary or Silurian sandstones. The abundant detritus which surround the basaltic hills of Rowley, the sienites of Mount Sorrel, and the granites of Arran, are in this respect very worthy of attention, and may suggest to those who have the opportunity a train of valuable research, which might elucidate many points now obscure in the history of the disintegrated materials of igneous rocks.
The composition of this group of strata is formed upon one general model in all the parts of Britain north of the Severn; but we may distinguish two varieties in this plan. First, the series of the Grampians, Lammermuirs, and Cumbrian region, may be thus stated in general terms:—
On the borders of the English Lake district we have this combination reduced to a small thickness, and deprived of its upper term. The best sections are seen about the foot of Ulswater, in valleys near Kendal, and at Kirkby Lonsdale. At this latter place we have
A little further to the S.S.E. the old red series is totally deficient (Craven), and nearly so in Flintshire.
A second combination of the similar elements appears on the border of Wales, and acquires distinctness in the region south of Shrewsbury. An excellent general section is obtained in the country between Dean Forest and the Silurian Vale of Usk. It may be thus expressed:—
In all the strata enumerated, and in all their localities, the rarity of in vertebral remains and of vegetable fragments is remarkable. Only a small number of bivalves (not brachiopoda, possibly freshwater shells,) has occurred to Mr. Miller in Scotland. Fishes have been found abundantly in many parts of Scotland, and rarely in Herefordshire and Breconshire.
We now pass the Severn, and find a great change. North Devon exhibits both the old red series and the true carboniferous series in an aspect much different from their northern types. The series stands thus:—
We may now proceed to South Devon. The groups here observed are not to be placed in strict mineral or structural affinity with the coeval series in the north. The best section is afforded in Plymouth Sound, but enormous contortions prevail.
By tracing these beds farther westward Silurian strata appear beneath them.
Organic Remains.—By uniting in one summary the fossils of the Devonian and Caledonian types of old red in Britain we have a pretty full catalogue of invertebrate.
|No. of Species.|
Of these, 25 species are also found in the Silurian system; and 51 in the carboniferous limestone series, 57 occur in the Devonian strata of the Eifel, a limestone district wkich corresponds with the Plymouth group.
As characteristic Devonian forms we may mention—
- Cystiphyllum damnoniense.—a. Lamellæ,
- Strombodes vermicularis.—a. Lamellæ.
- Leptæna nodulosa.
- Calceola sandalina.
- Palaeopora pyriformis.
- Strigocephalus Burtini.
- Cyrtoceras tredecimale.—a. Aperture.
- Clymenia laevigata.—a. Aperture.
- Brontes flabellifer.
The fishes of the old red sandstone which have been systematized by Agassiz, and described with much effect by Miller, are very characteristic of this palæozoic period. The classification of Agassiz, presented to the British Association in 1843, includes 63 British species, viz.:
Placoid.—Ichthyodorulites 6; Cestraciontes 1.
Ganoid.—Lepidoid 33; Sauroid 13; Coelacanthoid 10.
Some of these, as well as a considerable number of the invertebrate, occur also in Russia, the only region which presents something like a fair example of thewhole fauna of the middle palæozoic period. Mantell
has recently described a batrachian? reptile (Telerpeton Elginense) from these strata, and eggs of the same group are reported from the old red shales of Forfarshire.
Geographical Extent.—It is in Scotland that the old red formation is thickest, most varied in composition, and most extensively distributed. It ranges on the N.N.W. coast in interrupted patches from near Cape Wrath to Loch Carron, Skye, and Rum; on the N.E. it forms a large surface in Caithness, skirts the Dornoch and Moray Friths, passes up the great valley to Meal Favournie, and spreads by Nairn and Elgin to the Vale of the Spey. A large belt of red conglomerates borders the Grampians, from Stonehaven to the islands in the lower part of Loch Lomond, and occupies much of the sea coast to the Frith of Tay. Red sandstones border the northern flanks of the Lammermuir hills, expand in the Vale of Tweed, and margin the slate tracts of Dumfriesshire and Kirkcudbright. Arran, Bute, Cantire, and the coast about Largs and Ardrossan show the same formation.
In England the old red sandstone sometimes appears associated by alternation of beds with the lower part of the mountain limestone series, especially on the eastern border of the Lake district about Penrith. Its conglomerates appear at the foot of Ulswater, and in valleys about Kendal, Kirkby Lonsdale, and Ulverston. It is enormously developed, and acquires more variety of composition in the counties of Hereford, Monmouth, Brecon, Carmarthen, and Pembroke; and is slightly exposed in connexion with the limestones of Mendip, Bristol, and Wickwar.
The old red changes its character, but occupies a large space, on the southern coast of the Severn, and again in a parallel course along the shore of South Devon. Devonian rocks appear in the Eifel, in the Rhine and Moselle, and are largely developed in Russia. 'They pass Livonia by the Lakes of Ilmen and the Waldai hills, and are extended over a vast region to the N. E., where they constitute a large portion of the shores of the White Sea.'
Analogous in mineral aspect to the old red formations of England, they contain, together with the characteristic fishes of Scotland and the brachiopoda of the formations as seen in Devonia, Westphalia, and Belgium; but they contain, in addition, salt springs and gypsum. A dome-like elevation of Devonian rocks (800 feet) occurs in the centre of European Russia, (Orel, Voroneje), full of fishes and mollusca imbedded in yellow and white marl stone and limestone.
The changes to which geological classification is reasonably liable appear in nothing more conspicuous than in the division of the great carboniferous system, as it was expounded by Conybeare (1829), into two systems, the lower being formed of the old red sandstone, as it appears on the borders of Wales, Cumberland, the Lammermuirs, and Grampians, and in a quite different form in North and South Devon. In the former districts the old red is still very poor in organic life, except in the class of fishes; but the Devonian series, poor in fishes, is rich in zoophyta, mollusca, and crustacean. As it is mainly by palæontological evidence that modern geology is guided, the now prevalent term for these strata is the Devonian system, which is found to be represented extensively in European Russia and North America, and probably in Africa. The fauna of the Devonian beds is certainly distinguishable from that of the rocks above and below: it has enough of an intermediate character to complete the harmony of the palæozoic groups, and yet enough of distinctiveness to demand an independent place.
This place was not assigned till after many examinations of the characteristic fossils of South Devon, of which some were first figured by Mr. Sowerby (Min. Conchology). In 1830, when composing the article Geology for the Encyclopædia Metropolitana, (published 1833,) my attention was caught by the singular fact of the classification of these Devonian rocks among the transition (Silurian) strata, while the fossils then known bore a great analogy to those of the mountain limestone; and I could not avoid expressing a strong doubt that these strata were really of so high antiquity. The doubt was more than confirmed (in 1837-40) by the united labours of Murchison, Sedgwick, and Lonsdale, the result being the establishment., by Mr. Lonsdale, of an intermediate group of fossils in an intermediate group of strata,—the now well-known 'Devonian system' (Geol. Trans., 1837 to 1840).
In 1837, Murchison, in his 'Silurian System,' raised, for the first time, the old red to the rank of a system, and in 1842 completed the evidence on this subject by an examination of Russia. Since that time the Devonian system has been generally adopted.
Yet still we must not forget that a strong physical relationship binds together the peroxidated old red to the protoxidated blue shales of the mountain limestone; that in all the districts of Britain they are in parallel deposits; and that beds of red sandstone enter into the composition of the lower limestone series in the north of England.
Composition.—Six substances are interstratified in this system: arenaceous, argillaceous, and calcareous rocks form the principal masses, and are associated with beds of chert, ironstone, and coal. Some of the arenaceous rocks are conglomerates, as millstone grit, which is partially filled with quartz, felspar, and fragments of shale, the mingled spoils of granitic rocks, quartz veins, and schists; others are freestones of an open grain and equal texture, breaking equally in all directions; others are compact close grits, called hazle; or still finer grained, called calliard; or laminated with mica, or carbonaceous matter, as flagstone. In colour these rocks are white, brown, grey, greenish, yellow, or red. There is almost every possible gradation between the sandstones and argillaceous deposits; which latter are frequently much laminated, and are then called plate, or bass; less remarkable lamination causes shale; deficiency of lamination belongs to some varieties, associated with coal, called clunch, bind, and other local names: most of them are more or less bituminous; colour blackish, greyish, bluish, yellowish. The limestones are compact or oolitic, or granularly crystallised; mostly pure carbonate of lime (except the granular sorts, which usually contain magnesia), white (rarely yellowish), grey, blue, black, red, or mottled. Some beds contain quartz pebbles. Nearly all are of marine origin, but some exceptions occur.
Chert nodules and beds, of white, black, yellow, or red colour, lie in the limestone, like lumps and layers of flint in chalk; and require similar suppositions to explain their occurrence. Some considerable beds of chert occur in the north of England (Swaledale), and many sandstones are of a cherty nature (Harrogate).
Ironstone (a carbonate of iron) often accompanies the thick dark plates and shales, in rows or layers of nodules (see Diag. No. 21, p. 61.), aggregated round shells (unio), fern branches, &c. Coal lies always in beds. Its quality varies from nearly pure carbon to a consumable mixture of carbon, hydrogen, oxygen, and azote; and it is often mixed with layers of woody fibre, like charcoal, and laminæ of earthy matter.
Structure.—Through out all this mass of varied deposits in the carboniferous system, the most decided proofs of aqueous deposits constantly present themselves. Lamination belongs, but not equally, to every one of the six constituent members; being often conspicuous in sandstones (flagstones), almost always so in argillaceous rocks and coal; frequent in black limestones, but rare in ironstone. Real beds occur in all these rocks; but in the argillaceous plates and shales they are often indiscernible; in sandstones they are commonly irregular; thick-bedded limestones have nodular or uneven surfaces.
The coarse sandstones (as millstone grit) frequently present oblique lamination, which, added to the irregularity
of the beds, renders it often embarrassing to say what is the true dip of such rocks. (Diag. No. 45.)
The divisional structures or cracks, joints, and fissures, vary much in relation to the nature of the rock— its fineness or coarseness of grain, the thickness or thinness of its beds, and the position of the point with regard to axes of elevation and perhaps other causes.
In the accompanying diagram, L may represent limestone., P plate, G gritstone. The joints in L are generally rectangular to the bed (in thin-bedded limestones L', the joints are more numerous).
In plate they are often oblique to the bed; in gritstone less regularly formed, being mostly cracks: this is especially the case where the beds are thick. The principal fissures F, which sometimes go through many beds, are most open and regular in the limestone.Coal has sometimes joints of the same kind, (called
'ends' or ' backs,') and, in addition, a minute fissility, generally in one certain direction across the bed, which does not occur in the shales above or below. It is a sort of crystallisation. Ironstone sometimes shows concentric laminæ, and often sparry divisions, when it becomes a septarium.
A very singular structure is frequently noticed in theargillaceous iron ores of a coal district, without however
The substance of the iron ore is formed into conical sheaths, involving one another, and marked by concentric undulations and radiating striae. Large spheroidal masses of iron ore, weighing at least a ton, are thus found, in connexion with the coal, at Ingleton, in Yorkshire; and in the coal fields of Staffordshire and South Wales it is a well known form of aggregation. This structure also occurs in many other formations, as in the slate of Skiddaw, the lias, oolites, &c., though with considerable variations. It is usually called 'cone in cone,' 'cone coralloid,' conical limestone, conical ironstone, &c.
A different but yet closely allied phenomenon, noticed by Mr. Dillwyn in the substance of the coal of Swansea and other parts of South Wales, which we have also seen at Ingleton, is represented in fig. 48. Such a mass of coal, however solid, is found to separate not along a plane, parallel to the bed, but with deep hollows, and acute sinuous ridges, sulcated on their slopes, and undulated on their edges. The striations on the slopes are very similar to those on the conical ironstone; and though the differences are in other respects great, they both probably depend on some general law of concretionary action, modified in operation by the nature of the substances acted on: but we are quite ignorant of the circumstances which determine this peculiar structure in coal.
Succession and Thickness of Strata.
Considered in its greatest generality, and with reference to countries where the masses appear in the greatest simplicity (as in the south of England), the carboniferous system consists of three formations: viz.—
This triple system becomes modified in the north of England, so as to constitute, in Derbyshire, a quadruple system, without any red sandstone, thus:—
Further north, viz. in the north-western parts of Yorkshire, the series is still more complicated and varied: as under:—
Pursuing the system to Northumberland, we find the scar limestone broken up into very many parts by inter positions of grits, shale, and abundance of coal; one of the grits being pebbly. Thus the whole method of variation of the system of carboniferous strata becomes known and appears nearly as in the diagram (fig. 18. p. 59.).
We may here notice the remarkable section presented in the Island of Arran, where, according to Murchison and Sedgwick, the new and old red formations are merely separated by a thin zone of limestone and coal, or, as from a careful examination we should be disposed to express it, where only small and diminished members of the mountain limestone formation (in one place yielding coal) appear buried in masses of red conglomerate, sandstone and shale, of very great thickness, there being no certain criterion for deciding that any of this series belongs to the new red sandstone. This section is, however, much in accordance with the views of Hoffman, who, in north-western Germany, finds the carboniferous limestone and coal buried in a great body of red sandstones; the lower ones being attributed to old red, the upper ones to new red.
The total thickness of coal workable in the English and Scottish coal fields, is generally about 50 or 60 feet: this is, in most districts, divided into 20 or more beds, of a thickness from 6 feet to a few inches, alternating with from 20 to 50 or 100 times as great a quantity of sandstones and shales. But in some districts (Cumnock in Ayrshire, Dudley and Bilston in Staffordshire) many beds of coal, deposited one upon another with but little intervening earthy matter, constitute one mass 30 or 40 feet in thickness, in which the different beds are easily traced, and possess different qualities, probably depending on the original differences of the component vegetables, and the manner of their accumulation.
In the Newcastle coal district, the coal beds are arranged in the following order by Mr. Westgarth Forster:—
|Brown post, or grindstone sill||24||0||0|
|Coal (High Main)||2||0||0|
|Coal (Metal Coal)||0||1||7
|Coal (Stone Coal)||0||1||2|
|Coal (Yard Coal)||1||0||0|
|Coal (Low Main)||2||0||6|
|Coal (Wbickham St.)||2||0||0|
|Various rock measures||50||2||0|
In Mr. Buddle's excellent sections, published in the 'Transactions of the Natural History Society of Newcastle,' the extent of the several alternations of coal, sandstone, shale, &c., in the upper parts of this series are clearly shown. There is not much ironstone in the coal tracts of the Tyne and Wear. In Yorkshire, the total thickness of the coal formation is from 1000 to 1500 yards. In Lancashire arid Wales greater thickness must be ascribed to it. In South Staffordshire (Dudley), it does not exceed 1000 feet. The most variable parts, in all coal tracts, are the sandstones and shales; the most regular parts are the coal beds and ironstones.
Organic Remains.—The forms of life buried in the carboniferous system of strata are exceedingly numerous and varied, and, being generally in an excellent state of preservation, allow of a most strict comparison with existing types. They consist of very many races of plants, abundance of zoophyta, multitudes of mollusca, some Crustacea, many fishes, but, as far as we yet know, neither reptiles, birds, nor mammalia. Many of the plants, indeed by far the greater number, are of terrestrial growth: all the zoophyta, and nearly all the mollusca, Crustacea, and fishes, are marine. The excepted mollusca occur among the remains of plants swept down from the land: the excepted Crustacea are those referred to by Dr. Hibbert, in his account of the Burdiehouse limestones, with which also a few fishes are found, which, by this author, are referred to a freshwater origin.
The plants are partly very similar to existing races, as the large group of ferns generally, and partly appear altogether unlike them, as the large-furrowed stems of sigillaria, the quincuncially ornamented stigmaria, &c. On making the most close comparison which the subject admits, we find that among the fossil ferns are arborescent species, to which we can only find parallels in warm or else Australian regions; that the same analogy to the productions of a warm climate is suggested by fossil equiseta, and confirmed by the lepidodendra, which seem related to existing lycopodiaceæ in structure, though enormously surpassing them in dimensions. Even the sigillariæ, when carefully studied, though they be not cacti, nor euphorbias, nor arborescent ferns, are so much like those singular plants of hot climates, as to add considerably to the accumulating evidence in this direction.
The following is a brief summary of the plants:—
|Cryptogamia vasculosa —||Equisetaceæ||above||90||species|
|Fig.||1. Stem of a sigillaria always denuded of leaves.|
|2. Stem of a large catamites.|
|3. Stem and leaves of asterophyllites.|
|4. Branch and leaves of lepidodendron.|
Of the accumulated remains of these plants coal seams are really composed, and one cause of the differences amongst them is the different structural composition of the original plants. How far the above fossil flora is to be taken as exhibiting the true proportions of the tribes of plants living on the globe, at the time of the production of the rocks of the carboniferous system, is uncertain: since, when plants are swept down from the land into the sea, it depends on many unknown conditions what sorts of them shall escape the floods, or perish by maceration in the waters.
As a general rule, it may be said that the plants are confined to arenaceous and argillaceous deposits: they abound in the upper parts of the carboniferous system, where coal abounds; they also occur in the midst of the millstone grits, and in sandstones and shales among limestones, especially where coal beds also are found; but they are almost unknown in the midst of the undivided limestone, where, however, a few algæ occur. In the true coal formation they are often accompanied by estuary, if not fresh water, shells, (unionidæ); but in Coalbrook Dale marine shells (such as lie in the mountain limestone) take their place. The large trees are sometimes found upright above the coal, while below it spread what seem to be their roots. Much of the substance of coal contains the structure of coniferous trees.
The suppositions which best connect the whole of the phenomena are, that the plants grew in and around swampy tracts by estuaries; that the land was sinking continually or by intermission; that thus the vegetable matter was gradually accumulated at and near the place of growth, and gradually or by intermission covered by marine estuary or river sediments.
The zoophyta of the carboniferous system are almost (perhaps wholly) absent from the coal formation: they are almost confined to the mountain limestone formation and to its calcareous portions, thus offering us most clear proof of the marine origin of that rock. When to this we add the absence of land reliquiæ from these limestones, it is evident that the materials of which these rocks are formed were not swept from the land like the substance of the arenaceous rocks, but elaborated from the salts of lime diffused in sea water. The zoophyta are partly of families almost extinct, as crinoidea; and partly of tribes yet abundant in the sea, as lamelliferous corals: the genera of corals often but not always (e.g. astræa, lithodendron) differ from those now living. The following summary is extracted from the 'Geology of Yorkshire,' vol. ii. p. 241.:—
|Fig.||1. Syringopora ramulosa. Goldfuss.|
|2. Cyathophyllum (or Lithostrotion) basaltiforme. Phillips.|
|3. Actinocrinus triaconta dactylus. Miller.|
|4. Platycrinus laevis. ? Miller.|
The molluscous reliquiæ are numerous; 326 species being described in the 'Geology of Yorkshire,' without noticing about a dozen others from the coal formation, which are included in the following general summary:
Of these, only about 10 can by any means be considered as of freshwater, or even estuary, origin: and these all belong to the coal formation (unio, anodon, &c.). Many of the genera are the same as those now existing (e. g. nucula, lingula, isocardia); but others are quite different, (as pleurorhynchus, producta, euomphalus, goniatites, &c.), and seem to belong to another order of creation. About 60 per cent, of the species belong to extinct genera; and it is very remarkable, that brachiopodous bivalves, which, in existing nature, are perhaps to other shells as 10 in 1000, were in these ancient periods as 10 in 34. The goniatites are most beautiful and characteristic features of this system, being found in none of the more recent strata.
Crustacea existed during the accumulation of these rocks, but bore little resemblance to the present forms of the class: the trilobites of these rocks are, however, less numerous and varied than in the older Silurian rocks, where they are remarkably plentiful.
Annelida have left many as yet undescribed remains on the surfaces of the flagstones in the millstone grit and limestone series. Some are enough like Nereis to be referred to the wandering setigerous tribes.
The fishes of the carboniferous system (Burdiehouse, Leeds, Bradford, Manchester, Bristol, Wales,
EXPLANATION OF FIGURES, p. 175.
From this general review, the reader will infer that most of the forms of plants and animals of the carboniferous system are very distinct from existing types, but yet comparable with them and intelligible by them; but that genera are mixed with them, which cannot be, or at least have not been, at all discriminated from recent; and among plants in particular, some fossil forms (ferns) have a resemblance to recent species which is quite surprising.
Physical Geography.—Much of the most picturesque contracted scenery of England is situated among the deep-cleft valleys and rock-breasted hills of the mountain limestone, which, in Cheddar cliffs, on the banks of the Wye, in Derbyshire, the Yorkshire dales, and parts of Cumberland, Westmoreland, Lancashire, Flintshire, and Glamorganshire, offers most attractive features to the artist. In Ireland, this rock is the source of very fine effects, about Sligo and Enniskillen. The Meuse flows from Namur to Huy through a succession of precipices of limestone comparable to those of the Wye, Coal deposits are generally found in countries deficient of beauty of form and luxuriance of vegetation; yet the undulations of the large coal tracts of Yorkshire and South Wales, with the noble oak woods which fill some of the valleys, are worthy of notice.
The millstone grit and Yoredale rocks form in the north of England a peculiar order of scenery; for resting in detached masses upon broad, bare surfaces of scar limestone, their bold craggy tops and edges, and abrupt precipices, produce often a grand, though sometimes a formal effect, and their combinations are frequently fine. To this country belong also many beautiful waterfalls, originating in the decay of soft shales and grits below ledges of limestone, over which the stream flings itself, in a free and lofty leap, into a dark and precipitous glen. (Hardrow force, in Wensley Dale; Ashgill force, in Aldstone Moor.)
Another thing worthy of notice in the scenery of the limestone districts in the north of England, especially Derbyshire, is the difference of herbage on the millstone grit, limestone shale, and limestone. On the latter (l'), a fine green turf—on the shale (s), bluish green sedgy pastures—on the grit rocks (m), brown or purple heath, enable a geologist to mark out the leading features of districts with great facility, suggest to the botanist many interesting inquiries, and demonstrate to the agriculturist the dependence of the quality of soils on the rocks which they cover.
The surface of country occupied by the rocks of the carboniferous system is proportionably much larger in the British islands than in other parts of the globe. In Ireland the greater part of the plains and broadly undulated interior consists of the mountain limestone, in places covered by coal measures, and in other parts supported by the old red sandstone. In fact, excluding the parts previously described as gneiss, mica schisf, clay slate, and grauwacke slate, and a large tract of later strata (red sandstone, green sand, chalk, &c., capped by basalt) extending from Lough Neagh to Lough Foyle, and to the sea-coast of Antrim, much of the rest of Ireland belongs to the carboniferous system. But the quantity of coal yielded by the coal fields about Lough Earn and Lough Allen, Monaghan, Dungannon, Newcastle, the counties of Clare, Kerry, and Limerick, about Cashel and Kilkenny, is not very considerable, nor is the coal of good quality. The Kilkenny coal is nearly pure carbon. A great part of the space in the interior of Ireland within its mountain border, Kerry, Mayo, Galway, Donegal, Down, Cavan, Wicklow, Carlow, Wexford, is filled by mountain limestone. (Mr. Griffith's map.)
In Scotland the mountain limestone is, on the contrary, very slightly developed, in connection with the large coal field which stretches from St. Andrews to Ardrossan, and from Haddington to Ayr, filling large spaces in the valleys of the Forth, Clyde, Ayr, Irvine, &c. (M'Culloch's map.)
The mountain limestone formation occupies an immense tract in Northumberland, Durham and Yorkshire, from which country it runs out in a curve, to encircle on the north, and partially on the south, the group of Cumbrian slate mountains. It also appears in great force in Derbyshire; ranges through Flint and Denbigh, to St. Orme's Head and Anglesea; shows slightly round the Clee Hills in Shropshire; and presents picturesque cliffs on the Wye, near Monmouth. There is a long belt of mountain limestone on the north and east sides of the coal fields of South Wales, from Narberth by Abergavenny to Caerphilly; and it is prolonged on the south side by Bridgend, Swansea, and Tenby, to Milford Haven. Detached masses of limestone appear about Bristol, and in the Mendip Hills, and, according to Messrs. Murchison's and Sedgwick's recent researches, the limestones of Barnstaple are classed in the same series.
The carboniferous limestone is supposed to occur in a narrow band below the coal formation of the Clee Hills, arid this is probably the correct explanation of the phenomena visible under Knowle Hill, at Orelton, &c.: but we must call attention to the fact that the white (sometimes internally blue) oolitic limestone there occurring is associated not only with dark shales (clunch), and light marly beds., altogether of a considerable thickness, at least 100 feet, but is also overlaid by an important deposit of red, whitish, and greenish argillaceous strata, altogether of the same nature as the "old red formation" of the vicinity. The whole series of the south Clee hills may be thus expressed in general terms:—
Calcareous layers, sandy or marly.
Admitting the limestone and shale beds (b, c, d, e) to be the equivalent of the lower scar limestone (Derbyshire limestone) of the north of England, the quartzose conglomerate (g) may be ranked as millstone grit; and the red and white clays (f) must be considered as a recurring bed of the old red marl, interpolated among the carboniferous rocks, just as the red grits and clays of Orton and Ravenstone dale have been described as marking one form of a transition between the old red sandstone and the carboniferous formation, on the border of the primary districts of Westmoreland.
The millstone grit is an important deposit in the north of England, from the Coquet to the Tyne, and on the hills between the dales of Durham and Yorkshire, from the Tyne to the Aire and the Kibble. A large mass of these rocks occupies the higher parts of Bolland; and a far larger tract extends from the Yore at East Witton, nearly S. W. to Ormskirk in Lancashire, and spreads from this line to the east, under the magnesian limestone of Yorkshire, from Masham to Aberford, and under the coal of Yorkshire, by Leeds and Bradford, to Penistone. Near this place it divides into two branches, one of which separates the limestone of Derbyshire from the coal of Yorkshire, Nottinghamshire, and Derbyshire; the other in like manner divides that limestone from the coal of Manchester and Congleton. In the south of England the millstone grit is feebly represented by the "Farewell rock" of the Forest of Dean, South Wales, and Somersetshire; but in Ireland it appears in great force on Kulkeagh, Belmore, and other mountains about Enniskillen.
The coal formation of Northumberland and Durham extends from the Coquet across the Tyne, Derwent, and Wear, to Cockfield, where it suddenly breaks off, and ends against the valley of the Tees; and no more appears between the magnesian limestone and the millstone grit till the south side of Wharfdale. Here from Aberford to Bradford it runs out, in a counterpart of the Durham recession, and then returns by Halifax and Huddersfield to Sheffield, Dronfield, Chesterfield, Alfreton and Belper, and ends near Nottingham. On the western side of the Cumbrian mountains is a narrow belt of coal formation, about Workington and Whitehaven: a small field of coal lies at the foot of Ingleborough (corresponding to one at Hartley Burn, on the South Tyne). The coal deposits of Lancashire form a considerable breadth, ranging east and west, from Manchester by Prescot and Wigan to near Liverpool, and appear to be connected underground with the coal tract of Flintshire, and, perhaps, of Shrewsbury. The detached coal fields of Ashby de la Zouch, Coventry, Dudley, and Colebrook Dale, are very valuable: some smaller fields are known south of Shrewsbury, in the Clee Hills, and at Newent. The Forest of Dean is a rich though small tract, and the disunited patches of coal in Kingswood, and south of the Bath Avon, are valuable. Almost the largest coal field in Great Britain is the great oval elongated tract of South Wales, from Pontypool to St. Bride's Bay, which furnishes fuel to the great iron works of Merthyr, Tredegar, Neath, &c. Murchison and Sedgwick show the culm of Devonshire (Bideford, &c.) to be in a deposit of the same Age as the culm of Swansea and other parts of South Wales, which is known to belong to the true coal formation.
When we recollect, that, in addition to this large expansion of rich coal tracts, in most of which 50 feet of coal (in many beds) exist, the millstone grit and mountain limestone tracts, north of Derbyshire, also yield some coal, it is easy to see that the popular opinion of the extraordinary abundance of coal in Great Britain is perfectly well founded. But does it follow that the supply of British coal is inexhaustible? will it last for one thousand or five hundred years, and during that period meet the hourly enlarging consumption at home, and the augmenting demands from abroad? This question has often been replied to, never answered. Nor have the replies been often dictated by a comprehensive view of the subject. If indeed the only data required were the superficial area of a coat tract, and the sum of the thickness of the several coal beds, nothing could be more easy than to convert this into a term of years, by assuming some fixed or regularly varying rate of annual consumption. But to this it must be objected, that all the coal in a given district cannot be worked, in consequence of natural impediments (thinness, bad quality, disturbed position, &c.), and of the wasteful and unscientific method of establishing coal works. It is not here meant to speak otherwise than with praise of the working of the collieries, in which much judgment and humanity are often to be noticed, but in the irregular and accidental manner (depending on distribution of property, private interests, &c.) in which the sites of collieries are chosen, and their field of work defined. Many portions of country are thus left full of unattainable coal; others untouched from dread of the water in long-abandoned works: beds of coal, of inferior quality or thickness, are abandoned till the future scarcity of fuel shall render it profitable to work them, under great disadvantages. Finally, as the thickness of the entire coal series often exceeds 1000 yards, and it is only in the Newcastle and Durham tract that pits descend even to 500, and then brave great dangers and difficulties, it is clear that, however long the coal of Great Britain may last, its price must gradually rise, because the cost of its production, relative to that of other articles of consumption, is necessarily on the increase. It is thus that coal will become scarce; and if the country be not yet sufficiently enlightened in this matter to prepare the way for some act of legislative wisdom, the time of trial may not be far remote.
It is a striking fact that no known coal district in the British islands (excepting, perhaps, a small part of Ayrshire) is unwrought: most of them are covered by manufactures; and ere long the geologist will be called upon to decide as to the propriety of sinking for coal in situations where it does not appear on the surface, yet is really spread beneath our feet in areas, perhaps, not less extensive than some of our largest coal fields. It may exist, for instance, beneath the plains of Cheshire, but who will have the boldness to penetrate the red sandstone, in search of that which may be placed by nature at an unattainable depth?
On the continent of Europe the carboniferous system is variously and locally developed in France, Belgium, Westphalia, Saxony, Bohemia, on the north of the Carpathians, &c. One of the most important deposits of coal and mountain limestone begins at Hardingen, near Boulogne, and, passing under the chalk and green sand, continues in an easterly direction by Valenciennes, Mons, Charleroi, and Namur, to Liege and Eschweiler, near Aix-la-Chapelle. On the right bank of the Rhine, the coal tract near Elberfeld may be viewed as a prolongation of this great Belgian deposit.
Some traces of millstone grit, and more of Aluminous shales, divide the coal from the limestone, in the valley of the Meuse, and also in Westphalia, at Lintdorf, and between Werden and Velbert. These representatives of the millstone grit group (flotzleerer) sandstein acquire, farther east, a great development about Arnsberg, Meschede, and Warstein. No old red sandstone is known in Westphalia, but red conglomerates represent it on the Meuse. The Saarbruck coal field contains thick red sandstones in its upper part, resembling the South Lancashire section. The same, on a greater scale, appears in Lower Silesia, and there, as in Lancashire, the true hunter sandstein covers unconformedly the coal. In Upper Silesia the coal without either limestone or old red sandstone rests on grauwacke. (Von Dechen.) The coal of Saxony, about Zwickau and Dresden, rests on igneous rocks.
At Litry near Bayeux, and between Angers and Nantes, coal occurs under relations to the older rocks, which appear like those of the Devonshire culm. "In the centre and south of France are some limited coal deposits, lying in the valleys of the Loire, the Allier, the Creuse, and the Dordogne, the Aveyron, and the Ardeche, between ridges proceeding from the primary central group connected with the Cevennes." (These coal fields are devoid of mountain limestone.) Coal is mentioned as occurring in eight places in Catalonia, in three in Aragon, and one in New Castile. (Mr. Conybeare, in "Geology of England and Wales.")
In Russia (provinces of Tula and Kalouga), in Syria, in the basin of the Indus, at Batavia, and in China, in Van Diemen's Land and New South Wales, in Virginia, and at several points west of the Alleghany Mountains, are extensive coal fields.Æ
Igneous Rocks.—A very considerable proportion of the trap rocks for which Scotland has long been celebrated is found amongst the strata of the carboniferous system. About Stonehaven, Bervie, Montrose, Arbroath, the Sidlay hills, south of Dunkeld, at Perth, KinnouL, and Moncrieff, felspathic, basaltic, and amygdaloidal rocks (at Kinnoul yielding various agates) appear among the old red sandstones. The Ochill ranges from the mouth of the Frith of Tay to Stirling, continued in the Campsie hills to Dumbarton, and thence expanding to Greenock and Ardrossan, divide the red sandstone from the coal formation of the Forth and Clyde. From Greenock to Kilmarnock and the Haughshaw hills is a prodigious mass of trap: detached portions occur in Ayrshire; a long range extends from Tinto by the Pentlands to Edinburgh. North Berwick Law, Tantallan, and the Bass, are the extremities of a large body of trap in Haddingtonshire: these rocks abound between Linlithgow and Bothwell; and a great variety of igneous masses occur about Kinghorn, the Lomond hills, and between Cupar and Largo. A considerable proportion of all these extended igneous rocks is connected with the coal formation.
The variety of composition among these rocks is so great, as to defy description in any moderate compass. These rocks, felspathic (porphyry, claystone, clinkstone, &c.), felspatho-pyroxenic (greenstone, basalt, wacke), produce at many points remarkable changes on the adjacent sandstones and shales; hardening both to an extraordinary degree, so as to resemble jasper of different colours. (Salisbury Craig, Stirling Castle, hill of Kinnoul, &c.) At Cumnock, coal is converted to anthracite and plumbago. (See Bouè, p. 122. et seq.)
Perhaps the most remarkable variety of igneous rocks yet known in a small compass appears in the island of Arran, generally associated with the red sandstones, and conglomerates. Pitchstone, claystone, hornstone, trachytic porphyry, clay porphyry, basalt, and greenstone, appear in many dikes, and form interposed beds of great interest in the theory of the formation of such rocks. (Jameson, M'Culloch, &c.)
In the north of England, the porphyritic masses of the Cheviot hills, the range of greenstone and basalt in Northumberland from Belford, by Alnwick, Rothbury, Whelpington, and the Roman Wall to the South Tyne, and thence along the west front of the Penine chain, to Hilton, near Appleby, and down the Tees to Middleton, with dykes passing through the mountain limestone, coal and newer strata, are the principal masses of trap rock associated with the carboniferous system. Dykes of basalt are common in the coal fields of Northumberland and Durham, but totally unknown in those of Yorkshire, Derbyshire, Nottinghamshire, and Lancashire. In Derbyshire, the limestones are separated by an irregular mass of interposed amygdaloidal trap, called "toadstone;" (indeed, more than one such bed can be proved to occur in certain districts.)
Mr. Murchison has described the trap rocks which penetrate the coal measures of the Titterstone and Clee hills, and cut and injure the coal: at Kinlet, Arley, and Shatterford the coal based on old red is divided by eruptive masses and dykes of trap. The trap rocks which rise in bosses within the coal fields of Colebrook Dale do not appear to have charred the coal: they never appear as dykes, or enter into the fissures of the rocks. (Mr. Prestwich.)
Basaltic hills adjoin coal and limestone at Rowley, near Dudley, and at Griffe, in the Warwickshire coal field: a dyke of basalt appears in Birchhill colliery, Walsall.
It is impossible in many cases to refer the igneous rocks, associated with the carboniferous system, to their true geological date. The bedded rocks of Northumberland, Teesdale, and Derbyshire, are certainly of the same age as the mountain limestone; but the dykes of Northumberland, Durham, and Walsall, and the other basaltic excrescences and ridges, are not easily determinable in age. This difficulty belongs to almost all cases of dykes, except when, as in the Quarrington dyke, in Durham, the igneous rock cutting through one formation (coal) is overlaid by another (magnesian limestone), which it does not divide. Even here the conclusion of the anteriority of the dyke to the overlying rock is somewhat insecure; because the extent of the dykes in the coal formation itself is very irregular and accidental.
Trap rocks are associated with the Irish mountain limestone between Limerick and Tipperary.
General View of the Circumstances under which the Carboniferous System was deposited.
If in the early part of the formation of the primary strata the ancient ocean was in a peculiar state, both as to temperature and extent, never since experienced, the effect of partial eruptions of igneous rocks, and perhaps of great displacements of the crust of the globe, was to vary the depths and localise the currents of the original ocean. But the effects of this change, apparent among the sedimentary deposits of the upper "transition" strata, were augmented to a vast degree, after the completion of the whole primary period, and the decided movements to which large parts of the globe were then subjected. The Northern Ocean, at the commencement of the carboniferous era, was certainly divided into basins, varied by islands, bounded by shores, supplied by inundations from extended land. The agitation on its shores is proved by conglomerates; the amount of inundations from the land is demonstrated by abundance of argillaceous and arenaceous sediments, plants, and beds of coal; while in the more tranquil laboratory of the deeper water limestone rocks were generated in great abundance.
The carboniferous and Devonian formations are, as compared with the older primary rocks, very limited in area, broken into many detached parts, and characterised by local conditions. Hence the red conglomerates of the Grampians, the Lammermuirs, and the Cumbrian valleys, hold fragments of the neighbouring and but lately uplifted rocks; hence the absence of old red sandstone in Derbyshire, its great predominance and complication on the south-east border of Wales; hence the unmingled oceanic character of the limestone of Derbyshire and Ingleborough, contrasted with the divided, sandy, shaly, carbonaceous littoral group of Northumberland. The small extent of coal in many countries is merely a fact indicative of the previous revolutions which affected the primary strata there; while the abundance of coal in Great Britain confirms to us the conclusion drawn from other considerations, that in this region of the globe, soon after the formation of primary strata, much land had been raised above the sea.
But there is yet to be explained the excessive abundance of the vegetation of that early land, which should be capable, whether overwhelmed in sitû or drifted to sea, of collecting into so enormous a mass of coal. On this point, if we turn our eyes on existing nature, nothing appears so likely to aid our conception as the damp forests on the Oronoko, Maranon, or Mississippi, from whose mere waste the mighty rivers roll every year to the Atlantic an immeasurable mass of trees and herbs, with soil, sand, and clay, which are in process of time arranged on the bed of the ocean, as we find the coal and its accompanying sands and clays to be. The analogy is strengthened by the general consent of botanists, in regarding the plants of which coal was formed to be decidedly analogous (though differing much) to tropical vegetation, and especially to the vegetation of a tropical region contiguous to the sea, where palms, cacteaceæ, and lycopodiaceæ might abound, and yet varied with mauntain slopes on which tree ferns and pines might flourish. If further we suppose, with M. Brongniart, that the atmosphere of that early time might be loaded with an extra proportion of carbonic acid, against which no law of nature militates, (for we know not if the proportion of carbonic acid be now constant in the air, and must admit that a reconversion of all the coal to carbonic acid gas would give a very large addition of this gas to the atmosphere,) we shall understand how the vegetation of the carboniferous period might be even more abundant than that now seen between the tropics, and at the same time comprehend the possibility of few land animals existing on the globe. Within what limits of proportion of carbonic acid in the air plants and animals can live, we do not know; but in this respect they are reciprocally circumstanced, —plants require most, animals require least.
De Luc, Brongniart, and other writers, prefer to explain the origin of coal from somewhat like peat-bogs, or from the decay or overwhelming of forests in sitû: if we admit further so much of water drift as the case requires, we have a general explanation. In most coal districts are from 20 to 60 seams of coal, alternating with sandy and argillaceous strata; each of these coal seams is composed of many parallel layers of different quality and structure, often separated by scattered patches and fragments of woody fibre. A bed or seam of coal is, in fact, an aggregate of many successive deposits of vegetable matter. Under almost every bed of coal, as Mr. Logan in particular has shown in South Wales, is a peculiar fire-clay, or a fine-grained sandstone (ganister in Yorkshire) traversed by the rootlets of a plant (stigmaria): over some beds of coal stand erect the stems of sigillaria and lepidodendron; and finally, at Dixonfold, near Manchester, and in other places, lepidodendra and sigillariæ rise erect out of a bed of coal, the former being connected below with roots which are stigmaria. In such cases, we seem to behold a nearly level swamp, often many miles in breadth, and of great longitudinal extent, covered with a peculiar deposit from water (the under clay), on which grew lepidodendra and other trees of the period. Spreading their symmetrical roots and rootlets through this mud, and rising to a considerable height above, a peaty accumulation happened around them, partly by the growth of cryptogamia, and the accumulations of leaves, branches, and fruits, the decay of local vegetation, partly by water-floated additions from not far distant surfaces. The land on which this accumulation was proceeding is then seen to subside in level, so as to be covered by river, estuary, or sea deposits, on which a new series of growths and water drifts accumulate a new bed of coal. In some cases the accumulation may be wholly from plants in sitû', in other cases, all the mass may have been water-drifted; often these causes have concurred or alternated. What is here sketched as the most probable general theory, is quite in harmony with the facts observable in connexion with the often buried forests of late Cainozoic and even historical age. Chat Moss, Waghen Fen, Thorn Moor, the Holderness lakes and river channels, yield plenty of cases so analogous that we cannot doubt of their illustrating the principles on which a large part of the ancient coal was accumulated. Inundations from the upraised land, littoral action of the sea, chemical decomposition of the oceanic waters, eruptive action of subterranean heat, vital action on the land and in the water,—these are the causes to which the formation of the whole carboniferous system is clearly traceable; and by comparing the effects of all these causes in that ancient period with what happen at this day, we shall find modern effects precisely comparable in kind, but altogether inferior in magnitude.
Where then was situated that ancient land, from which, according to our view, were swept the materials of the 1000 yards of sandstones and shales which inclose the coal deposits in most parts of England, and the continent of Europe? And recollecting that in the series of millstone grit and carboniferous limestone in the north of England occur other beds of coal, and several hundred yards in thickness of other sandstones and shales, again we ask from what land were the plants and earthy sediments drifted in such abundance over this limited area? In the discussion of this important question, which appears in my 'Illustrations of the Geology of Yorkshire,' I have found it necessary to analyse the phenomena, so as to be able to inquire separately into the local origin of the three substances of principal importance—limestone, sandstone, shale: the former is of oceanic origin, for it contains only marine exuviæ, and when in greatest thickness and purity, was evidently deposited by water in a state of great tranquillity, or slow decomposition. In the same south-eastern direction that the limestone grows thicker from a certain point in the district, the sandstone and shales grows thinner: in the opposite direction they thicken, but not equally; the sandstones thicken toward the north, the shales toward the west, and in this direction certain limestones and sandstones totally vanish. With these sandstones the coal beds also vanish; where the sandstones thicken and grow numerous toward the north, the coal beds also augment in number and thickness; and the limestones change gradually from an undivided mass to many distinct members, separated by sandstones, shales, coal, and ironstone.
Thus to any point A, in diagram No. 52., where a series of limestone, sandstone, shales, coal, ironstone, occurs,
the limestone may be supposed to have been brought by diffusion in the ocean from an area situated to the south-east; the shale transported from the west, ami the sandstone, plants, &c., drifted from the north. We may imagine two rivers, one flowing from the west, and bringing across the regions where now are Ireland, Lancashire, Derbyshire, and South Yorkshire, a vast body of argillaceous sediments, slightly charged with sand, and but little varied by floating trees and plants; the other rushing from the north, loaded with sandy matter, and bearing abundance of trees of different kinds, but not many ferns or delicate herbaceous plants. Alternately or contemporaneously, these rivers might fill the sea with deposits, such as we behold and in the manner that we see them, united with the proper calcareous deposit of the ocean.
This explanation of different sediments coming to the same part of the sea from various quarters, may probably be applied to every system of stratified rocks, containing, as constituent members, limestone, sandstone, and clay; but it is necessary previously to investigate the directions in which the agencies concerned in producing each sort of sediment were most powerful; i.e. the points or lines of their greatest intensity.
In some cases it appears highly probable that one such irregular fluviatile action, modifying the continuous depositions from the sea, would sufficiently explain the phenomena of the association of sandstone, shale, and limestone; because, by such action, the shores would be margined by a sandy deposit, beyond which clay would predominate in the sediments, and at a greater distance calcareous matter would be nearly unmixed with the effects of littoral agitation.
In the diagram No. 53. S represents the sandy accumulation near the shore, passing by gradation to thedeposit of clay, c, which extends further, and is finally
replaced by nearly pure carbonate of lime, b, which grows thicker farther from shore.
Still the question recurs, where was the land from which the materials were drifted? The slaty mountains of Cumberland, the Isle of Man, Cavan, &c., were perhaps above the water; but could they alone yield the materials for the argillaceous sediments, 1000 feet thick, of Enniskillen, Derbyshire, and Craven, even if we suppose them to have been much diminished by the operation? The Lammermuir mountains, to the north, seem not to be of such composition as would yield the coarse quartzose sandstones; we must therefore appeal to the Grampians or Scandinavian ranges, or finally close all further discussion, by admitting that tracts of land which supplied part of the sediments, mixed with the limestones of the carboniferous period, have disappeared from the Northern and Western Oceans.
The coal formation, lying above these limestones, appears in many cases (Yorkshire, Lancashire, &c.) to have been accumulated, or according to the other hypothesis, submerged, in estuaries or lakes: if so, the local origin of the materials must be sought around those lakes, and in one or more directions from those estuaries. If, as seems probable, the coal fields of Yorkshire and Lancashire were once united, as those of Durham and Newcastle still are, the margins of the estuary in which they were formed are lost, except toward the mountains of Lancashire and Westmoreland. In like manner, no margin can be fixed for the estuary of the coal fields of Durham and Newcastle, except the Lammermuir range; and thus we are again conducted to the conclusion, that, unless those mountains be thought to have yielded all the sediments, great displacements of the crust of the globe have confused the ancient boundaries of the carboniferous sea, and reduced to mere conjecture the extent of the bordering land, and the circumstances of its drainage. This important, though dark inquiry, will, however, again arrest our attention.
Extent of British Coal Fields under superior Strata.—Disturbances of the Carboniferous System.
To what extent the relative level of land and sea was disturbed during the period which elapsed in the production of the carboniferous rocks cannot be known: to judge from the universal conformity of all the strata which compose it, and the rarity of coarse conglomerates (except at the base of the system), it might appear that no considerable displacement of the crust of the globe happened any where near the British Islands, during the whole carboniferous period. Yet the occurrence of a marine conchiferous bed among the estuary or freshwater strata of the Yorkshire coal field, seems absolutely to require the admission of considerable disturbing movements at a distance.
After, however, the deposition of this whole system, and before, at least, any considerable part of the next (magnesian system) was laid upon it, the scene was totally changed, and the carboniferous rocks of the British islands broken and contorted by subterranean movements of an extensive and complicated description. Every coal field in these islands is remarkably dislocated by faults, often traversed by rock dykes, sometimes ridged or furrowed by anticlinal or synclinal dips, which cause great trouble and expense to the coal worker, and call forth all the resources of his art. Into the history of these disturbances we shall only enter, so far as to present a fair basis of comparison with physical theories. One of the most remarkable great faults or dislocations yet known in the world belongs to this period; viz. that great and continuous fracture of the earth's crust from Cullercoats, near Newcastle, westward along the valley of the South Tyne to Brampton; thence southward to Brough, Kirkby-Stephen, Dent, and Kirkby-Lonsdale; arid afterwards eastward to near Grassington, in Wharfdale, a distance of 110 miles.whole of the somewhat
(Tynedale), southern (Craven), and middle (Penine) portions of this fault, is elevated above the corresponding strata in the depressed surrounding regions, not less than from 1200 to 4000 feet; in consequence of which silurian rocks show themselves along the Penine and Craven portions, while small coal fields appear on the parts at h and i thrown down 2000 feet below the summits of millstone grit!
On the south side of the Craven branch of this great fault are found many anticlinal ridges, severally ranging north-east and south-west, or nearly, and throwing the whole Craven country into a series of parallel undulations. Through Derbyshire runs an axis, from which the rocks dip eastward and westward; and this ridge, continued northwards towards Colne, effects a complete disunion of the great coal field on the east (Yorkshire, Derbyshire, Nottinghamshire), from that on the west (Lancashire, Cheshire) which it appears most probable were once united on the bed of the sea. It is only by considering the effects of subterranean movements, that we can at all account for the disjointed and fragmentary condition of the central coal fields of England. Their disunion is sometimes real, but very frequently only apparent, since they often dip towardseach other, as c c, and would perhaps be seen to unite
but for the covering of red sandstone, which conceals the coal along the middle of the basin.
The great South Wales coal field is a vast double trough, having an included anticlinal axis, ranging east and west; as diag. 56.
In the Barnstaple and Bideford beds belonging to this system, the principal dislocations range also east and west: this is, perhaps, the most general line of movement in the Somersetshire tracts, where dislocations are numerous and remarkable: it is renewed in the north of France and Belgium (at Mons and Namur), and about Elberfeld. Without now stopping to discuss the bearing of these results on M. de Beaumont's views, we shall observe, that a careful study of the phenomena in the north of England has left but slight doubt on our mind that the application of Mr. Hopkins's mechanical theory (see Cambridge Transactions) to the dislocations of the carboniferous system, will be successful. Mineral veins commonly range a little N. of E. and a little W. of N. in the carboniferous system of the north of England.
Magnesian Limestone Series of England.
We now enter upon the last great member of the palæozoic series of strata—the magnesian limestone—long anked as a part of the new red or saliferous system. In the Encyclopædia Metropolitana (article Geology, 1830, et seq.) I stated, as the result of a general examination of the fossil conchifera of the saliferous system, that in the upper strata a general analogy to the oolitic æra can be recognised by the trigoniæ, plagiostomata, ostreæ, &c.; and by their productæ and spiriferæ the lower strata as distinctly claim affinity with the carboniferous limestone. While composing my work on the Palæozoic Fossils of Devon and Cornwall (published 1841) the lower group came necessarily under review, and I concluded that in respect of organic life its affinity was really with the carboniferous limestone, though its mineral association was with the new red. This view is now generally adopted; and Murchison, De Verneuil and Keyserling, by a large examination in Russia of strata which are the equivalents there of the magnesian limestone of England and Germany, have founded on these strata the Permian system.
Composition.—After examining the carboniferous rocks, the red sandstones and the associated strata present themselves with an air of novelty and freshness, not less striking to the geologist than a new country to the traveller. Instead of the black, blue, or grey limestone, full of crinoidal columns, products, &c., we have now yellow, sandy, or granular rocks, with few organic remains: the dark shales of the coal series are exchanged for red, green, and blue marls, and the micaceous yellow, ochraceous or brown grits, for red or white sandstones. On a general view the contrast is sharp and defined, even where the strata of the two groups are parallel, but close examination points out several instances (Manchester and Salop) of transition coal deposits, in which red grits and clays inclose coal, with shales and limestones of a peculiar aspect.
The limestones of this system vary much. They are often loaded with magnesia, and in general called "magnesian limestone;" but there are many beds in which little or no foreign admixture deteriorates the carbonate of lime. The colours are white, grey, smoky, but more frequently yellow; and in some districts reddened, or even very red. In texture, a few limestones are compact, some oolitic, many cellular, the cells lined with crystallised carbonate of lime, a large proportion of a fine sandy grain, some quite powdery, with crystallised balls included; and in Nottinghamshire, considerable tracts yield granular crystallised limestones. Near Sunderland laminated rocks are really of sparry texture. Strings and plates of spar are very common, and render buildings of the magnesian limestone very irregular in their decay, from the unequal perishing of the stone between the ribs of spar.
Structures of Deposition.—Stratification is distinct in all these rocks; but in all of them some peculiarities appear in this respect.
The fine-grained upper limestones of Knottingley are thin-bedded: the granular rocks of Nottinghamshire are either thick-bedded or flag-like; it is sometimes difficult to trace the beds at all in the powdery magnesian rocks; and in certain sparry rocks near Sunderland, the bedded structure is almost overlooked in admiration of the coralloidal forms of the concretionary masses, which sometimes are enveloped in soft yellow powder (Building Hill).
Divisional Planes.—The fine-grained limestones of Knottingley are traversed by vertical divisions from top to bottom, which in some places are open to a foot in width, or filled with clay and rolled pebbles; in other cases they are merely thin cuts in the rock; always their regularity, parallelism, and polarity (if we may so term their direction to N. or N. N. W., and its rectangle E. or E. N. E.), are remarkable. In other thick bedded limestones, the joints are less symmetrical, though always numerous: most of the rocks are traversed by small secret cracks, which, on being exposed by fracture, are found covered by dendritical markings of a dark colour. The joints are often coated by carbonate of lime, sometimes by carbonate of copper, or sulphuret of lead.
Succession and Thickness of Strata.—The most, or rather the only, complete series of the Permian system in the British islands, is that of the north of England, where alone certain lower members are clearly exhibited. The following synopsis is founded on the views of Professor Sedgwick. ('On Magnesian Limestone', Geological Transactions.)
Magnesian conglomerates border the Staffordshire and Salopian coal fields, and have a lower red sandstone beneath them. At Manchester, the magnesian limestone is somewhat better defined; at Kirkby Stephen, it is represented by a brecciated limestone rock; and at St. Bee's Head is a complicated formation of considerable thickness, in which the calcareous part is an important feature.
|Stinkstein, rauchwacke, &c.||Upper limestone.|
|Gypseous marls.||Gypseous marls.||Gres de Vosges.|
|Kupfer schiefer.||Marl slate.|
|Rothetodteliegende.||Lower red sandstone.||Gres rouge.|
The Organic Remains of this system, though not numerous, are exceedingly interesting to the naturalist and geologist, from the strong testimony they offer of the successive changes of the living creation, according to the new circumstances of the land and sea. The fossil plants, shells, fishes, and reptiles of this system appear to partake more of the character of those in the older carboniferous, than newer oolitic deposits. The few plants which occur in the rotheliegende are of the carboniferous, and especially Lepidendroid type. Products, so common in mountain limestone, occur in the zechstein with terebratulæ, and spirifera abundantly. Fishes of the genus palæoniscus here occur for the last time, in ascending the series of strata; above come in Labyrinthodon Thecodontosaurus, &c. These interesting relations appear in the following table, which also contains the names of some fossils, which are found in only one of the three systems:—
According to the organic remains, the lower half of these rocks must be ranked with the carboniferous, the upper with the oolitic rocks: but, by its own mineral characters, it is one great series of deposits which happened at the period when a decided change was taking place in the conditions which determine the forms of life upon the globe.
|1. Retepora flustracea. Phillips.|
|2. Mytilus acuminatus. Sowerby.|
|3. Avicula gryphaeoides. Sedgwick.|
|4. Axinus obscurus. Sowerby.|
|5. Producta horrida. Sowerby.|
All these are from the magnesian limestone of Durham and Yorkshire.
The fauna of the magnesian limestone in England has been fully treated by King, who presents in a synoptic table the following summary of British and foreign species. (See table at top of next page.)
In regard to the Reptilia we should prefer to omit the British fossils (Thecodontosaurus and Palæosaurus) as really belonging to a triassic conglomerate.
The plants of the Permian system have been examined by Morris and others. They are in a considerable degree marine; but a certain number of ferns, lopidodendra, and calamities, are identical with, or else very nearly allied to, known carboniferous species.
The table which follows was drawn up as the result of the labours of Murchison, Dr. Verneuil and Keyserling,in 1844. (Geol. Proceedings.)
FAUNA OF THE MAGNESIAN LIMESTONE.
|Totality of Genera||Totality of Species||Species occurring in England and Ireland||Species peculiar to England or Ireland||Species peculiar to Russia||Species peculiar to Germany|
|Gasteropoda||? 10||26||21||? 18||3||2|
|Pisces||? 14||? 45||16||? 16||2 (— ?) more.||27|
FAUNA OF THE PERMIAN SYSTEM IN EUROPE.
|Genera||Totality Number of Species in Europe||Species exclusively peculiar to the Permian System in Europe||Species found in older formations||Species found in Russia.|
|Peculiar to that country||Previously found elsewhere.|
|In the Permian and older formations.||In the Permian beds exclusively||In older formation exclusively|
|Total||66||166||148||18||32||3 or 4||12||5|
Geographical Extent.—England, Germany, and Russia are the three main centres of these latest palæozoic strata, and the limestones are the most characteristic parts. The range of magnesian limestone is very limited in England. Commencing at Cullercoats, north of Tynemouth, the lower limestone (zechstein) ranges through a part of Northumberland and across Durham to Coniscliffe on the Tees, being in this course penetrated by many coal pits. Below it is the fish-bed or marl slate, and a coarse yellow pebbly sand (rotheliegende). In the North of Yorkshire the magnesian limestone is poorly developed in patches at Catterick, Crakenhall, &c., but grows prominent near Masham, and then continues uninterruptedly by Knaresborough, Wetherby, Ferrybridge, Doncaster, Tickhill, Barlborough, and Bolsover to Nottingham.
Through the whole of this range it is the lower limestone which prevails and makes the general feature, a low regular continuous terrace; but it is only in the middle part between the river Wharfe and the vicinity of Tickhill that the whole series is visible. As already observed, a few detached parts of magnesian conglomerate (probably coeval with zechstein) occur under the Penine chain near Brough and Ingleton, and along the border of the Salopian coal field, while a more developed section appears in St. Bee's Head, and a mass of the oldest red conglomerate of Malvern may be referred to this period. The magnesian conglomerate of South Wales rests unconformedly upon the coal.
Zechstein is exhibited along the Thuringerwald, in Hesse Cassel, on the southern and eastern sides of the Harz, between the Elster and the Saale, and about Waldeck. In Russia coeval rocks are spread out in the west of the Ural so as to occupy the whole of the ancient kingdom of Perm (whence the name). They consist of limestones, marls, masses of gypsum, rock salt, and repeated alternations of cupriferous strata, and contain a fauna and flora of characters intermediate between those of the carboniferous and triassic periods. The cupreous element is attributed to springs flowing from the Ural in the ancient zechstein period: to similar causes we may probably ascribe the copper in the Kupferschiefer of Maarfeld, and in the limestone at a few localities in Yorkshire: the fishes extended in a perfect form on this shale may have been the victims of such infusions.
In Thuringia and Hesse Cassel the zechstein is surmounted by red and spotted sandstones (Lower Bunter) with calamities arenarius, a coal measure plant; these sandstones are referred to the Permian series, which thus becomes triple, viz.:
Lower Bunter. Zechstein. Rotheliegende.
- The notices of orteoceratites at Loch Eribol, by M'Culloch, have not been confirmed by later inquiries.
- Mem. of Geol. Survey, vol. ii. pt. i. pp. 73—75.
- British Palæozoic Rocks, 1851.
- The reader will remark that the terms "clay slate," and "grauwacke," are used to designate rocks, not formations.
- Sedgwick, in Geol. Proceedings.
- Address to Geol. Soc. 1849.
- Along the side of the Craven fault in Giggleswick Scar (Settle), the mountain limestone is crossed by many divisional planes, which cause it to split parallel to the fault, with a kind of rude cleavage. By the side of the Coleyhill dyke, heat has produced a similar effect on shale; in Mr. Fox's experiment, electrical polarity has given a like result. (Sedgwick, Geol, Trans.; Phillips, Geol. of Yorkshire; British Association Reports, 1843; Sharpe, Geol. Proceedings, 1846, 1849).
- See Guide to Geology, 3d edit., for an example of joints reduced by calculation to the plane of stratification, from observations made with Mr. Murchison on the ridge of Corn y Vaen, near Brecon.
- Exclusive of fishes which occur, indeed, in the upper Silurians, but not so frequently or regularly as to be of use in arguments depending on numerical data. They are all of small size, incompletely known, probably confined to the placoid group, and perhaps to the upper Silurian strata.
- From notices by M. Barrande and sir R. I. Murchison.
- Murchison, Proc. of Geol. Soc. 1834.
- Ramsay and Aveline, in Geol. Proceedings, 1849.
- Sedgwick, in Address and Memoirs to the Geological Society.
- Miller's Old Red Sandstone.
- See Murchison's Silurian System for a full account (the earliest) of the old red in these districts.
- Phillips in Palaeozoic Fossils of Devon and Cornwall. See also the earlier investigations of Lonsdale (Geol. Trans.), and the later researches of M'Coy (British Palaeozoic Fossils).
- The Old Red Sandstone.
- Murchison's Geology of Russia,
- Geol. Proceedings, 1852.
- Murchison, in Geol. Proceedings, 1841.
- Ibid, 1842.
- See separate Memoirs by Sedgwick and Phillips in Geological Transactions; also Geology of Yorkshire, vol. ii., and Geol. Proceedings, Dec. 1851.
- Sedgwick, British Palæozoic Rocks (1851).
- Memoir in Geol. Proceedings, 1842, 1843, &c.
- Trans. of Palæontol. Soc. 1850.
- Murchison's Address to Geol. Soc., 1842. See also Geology of Russia.
- Murchison and De Verneuil in Geol. Proc., 1844, p. 329.