CHAPTER X.

The structure of the stratified part of the earth's crust conveniently studied by the examination of a single formation—A coal-field selected for this purpose—Illustration of the principles necessary to such an investigation—The antiquities of a country of value in compiling its pre-historic annals—Geological antiquities equally valuable and more satisfactorily arranged—Order of superposition of stratified formations—Each formation contains its own suite of organic remains—The age of the boulder defined by this test from fossils—Each formation as a rule shades into the adjacent ones—Mineral substances chiefly composing the stratified rocks few in number—Not of much value in themselves as a test of age—The Mid-Lothian coal-basin—Its subdivisions—The limestone of Burdiehouse—Its fossil remains—Its probable origin—Carboniferous limestone series of Mid-Lothian—Its relation to that of England—Its organic remains totally different from those of Burdiehouse—Structure and scenery of Roman Camp Hill—Its quarries of the mountain limestone—Fossils of these quarries indicative of an ancient ocean-bed—Origin of the limestones—Similar formations still in progress—Coral-reefs and their calcareous silt—Sunset among the old quarries of Roman Camp Hill.

Amongthe standard jokes of ancient Athens was that of the simpleton who, with the intent of selling his house, carried about a brick as a specimen. In this and the following chapter I propose to follow his example, and, for the purpose of giving my reader a correct notion of the structure displayed in the stratified portion of the earth's crust, to select therefrom a single formation whose details will connect together the subjects discussed in the previous pages. And in so doing it will, I trust, be found that what was ludicrous in the hands of the Greek becomes sober sense in those of the geologist. The "brick," then, which I would humbly present to the thoughtful consideration of the reader as really a specimen of the house of which it forms a part, has been termed the "Carboniferous System," and consists of a series of stratified rocks sometimesnearly 15,000 feet thick. The plants and animals found in these strata have been already described somewhat in detail, and we have turned aside to look at the processes whereby such masses of sedimentary rock came to be accumulated. But we shall probably better understand the habits of the animals and the general aspect of the vegetation, as well as the agencies at work in depositing vast beds of mineral matter, if we take a coal-field and analyse it stratum by stratum, marking as we go their varied and ever-changing character, and the corresponding diversity of the included organic remains. Such an examination will bring before us some of the more striking and important laws of geological research, and while of use to the young observer, may be not without some share of interest to the general reader. Before beginning, however, let me endeavour to illustrate the principles that will guide us by a simple though hypothetical story.

Suppose the bed of the Firth of Forth were raised above the level of the sea and covered over with verdure, and that, in ignorance of the previous topography of the locality, a mason were to excavate on the lately-born land the foundation for a dwelling-house. Immediately below the grass he would com? upon layers of hardened mud containing oyster-beds, with detached valves of cockles, mussels, fish-bones, and perhaps the tooth of an anchor or the timber of some old herring-boat. Now, if he were gifted with but ordinary intelligence, what would he at once conclude from these remains? Plainly, that the spot on which he stood had once been the bed of the sea. And if in place of appearing as dry mud and sand these deposits had got hardened into shale and sandstone, and the shells, too, had become hard and stony, this would not alter his convictions. He would still assert positively that he stood upon an old sea-bottom. And suppose further, that all this were far away from any sea, still such a circumstance could make no change in his opinion; he would rightly assert thatthe place of sea and land might vary, and that the ocean's being now many miles distant could be no argument against the waves having once rolled over the site of the intended dwelling-house. Let us further imagine that he continues his trench, and in sinking deeper comes to a bed of dark peat with snail-shells and bones of sheep, deer, and oxen. What will he infer from these? Clearly that they represent an old land-surface, once covered with vegetation and browsed over by ruminant animals, and that this old land-surface has at some distant period been submerged beneath the sea. Suppose, moreover, that below the peat there were a thin bed of reeds and rushes intermingled with the mouldering remains of fresh-water shells. He would in that case infer that before the formation of the peat the locality was occupied by a lake.

Putting now all these deductions together, our mason would have evolved a very interesting history. He would have ascertained that in a bygone age the spot on which he stood was the site of a lake, tenanted by delicate shells and fringed with reeds and rushes, where the coot and the mallard may have reared their young; that in process of time the vegetation gained upon the water, choking up the lake, so as gradually to form a soil firm enough to support sheep, deer, and oxen, and yielding shady coverts whither the antlered stag could retire and lay him down to die; that in after years the sea had encroached upon the peat-moss, and oyster-beds begun to form where cattle had been wont to browse; that again the ocean receded, and the land emerged to assume new verdure and receive new inhabitants.

Now, in all this reasoning there is no hypothesis or speculation. The mason proves himself an intelligent, honest fellow, and uses his eyes and his head where many other men would perchance see very little need for the use of either. There can be no setting aside of his story; he can appeal to facts. "There," says he, "is a layer of peat with the rush-stalks andmoss-fibres matted together in the soft brown mouldering substance, exactly as I have seen them a hundred times in the peat-cuttings on the moors, and I cannot but believe that they must both have had the same origin, that is, that they grew in swampy hollows of the land. There, too, lies a stratum of fresh-water shells identical with those that occur in our ponds and marshes. Although mouldering now, they are evidently not fragmentary, but entire and unbroken; some of them are young, others full-grown, and they lie grouped together as in our present lakes. Such shells could only live in fresh water, therefore the spot where I stand must have been at one time a fresh-water lake. There, again," he continues, "is a bed of oysters which cannot have been transported hither, for their valves are together, lying just as they do in our present oyster-beds. This green field, therefore, must have been at one period a muddy sea-bottom."

After this manner and upon this kind of evidence must all inquiries into the past changes of the earth's surface be conducted. And provided only we proceed cautiously, reasoning from positive facts, and striving as far as possible to exhaust what Bacon calls the "negative instances," our deductions possess all the certainty of truth. For in much the same fashion do we derive no small part of our acquaintance with the early history of our own land, as well as with the arts and customs of other nations. The scattered relics turned up by the operations of the farmer—wooden canoes, flint hatchets, gold torques, bronze pots, fragments of pottery, and rusty coins—all have their bearing upon the annals of the country, and so clear is the evidence which they read out that an eminent antiquary has divided the early ages of Scotland into three periods, distinguished, from the character of their relics, as the "Stone Period," the "Bronze Period," and the "Iron Period."[57]Butin such a classification the historian has little to guide him save the nature of the relics themselves. He places the rudest first, and groups the rest in succession, according to the degree of advancement in civilisation which they respectively indicate. And the grouping seems just, though in some cases objects belonging to two of these periods may have been to some extent contemporaneous, just as thatched roofs gave way to tiles, tiles to slates, and slates partly to lead, though at the present day a walk of half an hour in some localities will bring before us specimens of all these styles still in use. If, however, the relics of geological history lay scattered about like those of early Scottish history, all hope of ever attaining to anything like a correct chronology and arrangement would have to be abandoned in despair. In truth, it would then be impossible to conjecture whether any succession of ages preceded man, during which other tribes of plants and animals lived and died, or whether the whole mass of fossiliferous rocks had been accumulated since the human era, or perhaps created just as we find them. But all this uncertainty and confusion has been obviated simply by the fossils being ranged in beds vertically above each other, the oldest at the bottom and the latest at the top. So that if we find in a low cliff along the shore blown sand and broken whelks immediately beneath the vegetable mould, and oyster-valves in a clayey bed three feet below, we pronounce the oysters to have lived before the whelks, and that between their respective lifetimes a sufficient interval must have elapsed to allow three feet of sand, clay, and gravel, to accumulate. What is thus true on the small scale holds equally so on the large. The stratified formations in which organic remains occur are found to be grouped regularly over each other in a settled invariable order. If A be below B in England it will be below B all over the world, and if C be above D at the North Pole it will be so at the South Pole too, and at every locality where the two rocks lie together. Thisorder of superposition forms one of the grand tests for the age of different rock masses. By means of this simple rule the geologist has been enabled to arrange the different stratified formations, supplying the missing portions of one locality from the more complete series of another, so as to form a chronological table of no small part of our planet's primeval history.

[57]See Dr. Daniel Wilson's deeply interesting workThe Pre-historic Annals of Scotland.

But this is not all. We must attend to the character of the organisms as well as to their order of occurrence. We must distinguish the animal from the vegetable, the terrestrial from the marine, and scrupulously examine the peculiarities of each so as to recognise them again in other strata. By such careful scrutiny we may trace out the successive changes in the physical aspect of a district during past times, viewing in terrestrial plants (when clearly occupying their original site) evidence of an old land-surface; incyprides,unios, andpaludinæ, traces of a former lake; and in corals and marine shells, unmistakable proofs of an ancient sea-bottom. Still further, by marking the specific character of such fossils we obtain a key to the age of many rocks that otherwise would be unintelligible, for it is found that each of the stratified formations, from the oldest upwards, has its own peculiar and characteristic organisms recognisable all over the world. This test of the geological position and age of any fossiliferous rock has a peculiar value, for it can be applied with infallible success where every other fails. The order of superposition is often obscured by dislocations and other causes, and the mineralogical texture of a formation may change entirely in a short space; but if the imbedded fossils remain, we can be at no loss as to the relationship of the rock which contains them. And hence, if in some lone island of the Hebrides, haunted only by the screaming sea-fowl, we find a patch of shale containing ammonites, belemnites, and a host of other shells in large measure identical with those occurring among the clays and limestones of Gloucestershire, we infer that they must all belong to one series and be of the same age; that, aswe know the English beds to form part of a formation called Lias, of which, the exact place in the geological scale has been ascertained, so in like manner the Scottish beds must occupy a position in the same series; and that consequently there was a time when the site of Cheltenham and part of the Hebrides lay each beneath a sea which teemed with ammonites, belemnites, and many other mollusca, along, too, with the bulky saurians of the Lias. And yet no study of the surrounding rocks in the northern locality, even if carried on for a thousand years, could ever have thrown one ray of light upon the subject. In an earlier page our grey rounded boulder was introduced to the reader as a mass of sandstone belonging to the Carboniferous group of rocks. How could one be sure of the precise geological age of a loose water-worn block that might have journeyed all round the world? Simply by its included fossils. The calamite, lepidodendron, and stigmaria, revealed the date of the stone as clearly and unmistakably as if we had seen it lifted from its original bed by the lever and crane of the quarryman. These plants are peculiarly characteristic of the Carboniferous strata, and they consequently stamp as undoubtedly of carboniferous age the rock which contains them, whether it be sandstone or conglomerate, limestone or shale, and whether we meet with it among the newly-raised blocks of the quarry, or among the pebbles of the sea-shore. Each geological formation, I repeat, beginning at the oldest known to us, and ending with those that are still forming in our lakes and seas, has its own set of organic remains whereby we can detect it wherever it may chance to occur, from the equator to the poles. Each has itsstyle, so to speak, just as we can at once tell whether a drawing represents a Hindoo, Egyptian, Assyrian, Greek, or Gothic temple, simply from the generalstyleof the architecture.

Could we but voyage back in time as we can sail forward in space, we should find each of the geological formations not lessclearly defined than are the different nations and countries of the present day.[58]Were the reader suddenly set down in an out-of-the-way street of Paris, he would probably not be long in discovering that he stood on French ground. Or if spirited away in his sleep he should awake on the banks of the Nile, he would soon ascertain himself to be in the land of the Ptolemies. And so if you transported a geologist blindfold into a quarry where ammonites and belemnites abounded, mingled here and there with bones of ichthyosaurs and plesiosaurs, he would tell you at once that the quarry lay among liassic strata. Or if he were placed in a ravine where the rocks on either hand displayed fern-stems, lepidodendra, stigmariæ, and sigillariæ, he would tell you that the surrounding district was one of carboniferous rocks, and that probably at no great distance there might be found smoking engines and dozens of coal-pits. Or could you set him down in some dark night upon a wild coast-line, and show him, perchance by the flare of torch-light, bones and scales of osteolepis, pterichthys, and dipterus, lying on the rocks around, he would tell you that the grim crags which shot up into the gloom were as ancient as the era of the Old Red Sandstone. In any case the character of the rock would signify nothing, nor would he care about the general features of the landscape, though these too become important characteristics in certain cases. Show him but a few recognisable fossils, and you give him, as it were, an "Open Sesame" to which the rocks unfold their gates and reveal a store of wonders yet more varied than those in the cave of Ali Baba.

[58]Seeante, pp. 31, 32, and the Table of Rocks at the end of the volume.

But though the geological systems stand thus strongly marked off from each other when viewed as a whole, their boundary lines can often be only approximately drawn, thereby reminding us that the divisions are of man's device, and can have had no place in the plans of Him who needs not to chronicle His working by years and ages, but with whom there is no past and no future.One formation insensibly passes into another just as one nation merges into those around it. There are sometimes gaps, however, between the formations, serving to mark out strongly the limits of each,[59]precisely as intervening seas and mountain-chains serve to mark put the boundaries of different peoples and tribes.

[59]Such cases, however, are probably merely local, and may have originated from some features in the ancient physical geography of the districts where they occur. For instance, it has always been thought that palæozoic ages were marked off by a strong line of demarcation from succeeding secondary times. But the gap which occurs in England, France, and Germany, is being slowly filled up from the evidence furnished by other countries, and we shall probably find in the end that the Permian dovetailed with the Trias as closely as the Silurian with the Old Red, or the Lias with the Oolite. In truth, the longer we study the past history of our planet the less do we see of hiatus and chasm and sharp clearly defined boundary line; while the doctrine of a uniform system of laws and arrangements in the physical world, first philosophically propounded in the immortal "Principles" of Sir Charles Lyell, is ever receiving fresh confirmation.

The mineral substances of which these formations consist are comparatively few in number, being chiefly varieties of sandstone, shale, conglomerate, and limestone. One sandstone can often be scarcely distinguished from another, and so also with the other rocks; hence such tests as mineralogical texture supplies can seldom be relied on to determine the age of rocks. We can prove, for example, that a series of limestones in England may be identical in age with a set of sandstones in Sweden, and with a group of shales in America, because they all contain the same or representative genera and species of organic remains. They occupy the same position in the geological scale; that is, the animals whose fossilized remains lie buried in these rocks were all living at the same time, while lime was gathering at the sea-bottom over the site of part of England, and sand was being thrown down upon a portion of what is now Sweden, and mud was accumulating over a submerged area of America. In such cases the differences of mineralogical character go for nothing in determining the age of the rocks; we have to rely solely on the embedded fossils, and on the order of superposition.

Keeping in view, then, that the formations into which thegeologist has grouped the stratified portion of the earth's crust have a settled and invariable order of occurrence, that each of them contains its own peculiar and characteristic group of organic remains whereby it can be recognised in any part of the world, and that such remains form often the sole test at once of the geologic age and of the origin of the rocks wherein they lie, we may return to the plan above proposed and endeavour to understand the structure of a coal-field. For this purpose it may be well to select one of the northern coal-fields of Britain, since these perhaps display a greater variety in their organic contents, and bear evidence of more diversified changes in their mode of formation than can be seen in those of the south. The strata that compose the coal-basin of Mid-Lothian will probably best suit our purpose, as they are free from the disturbing effects of those igneous intrusions which play so important a part among similar rocks to the north and west.

The Mid-Lothian coal-field comprises a mass of stratified beds of sandstone, shale, coal, ironstone, and limestone, the united depth of the whole being above 3000 feet. By reference to the annexed Table it will be seen that the lowest beds of the section are chiefly sandstones and shales, extending downwards to an unknown depth, without any coal that can be profitably worked. These under-strata form the Lower Carboniferous group. Above them comes a middle zone in which the characteristic beds are of limestone, comprising the middle portion or Mountain Limestone of the Scottish Carboniferous rocks. The third and highest subdivision forms the Upper Carboniferous group or true Coal Measures, and constitutes the whole of what is properly the Mid-Lothian coal-field. For the sake of noting some of the remarkable changes exhibited in the character of the rocks, it may be well to begin our survey among the upper beds of the under group. Let us take as our base the famous limestone of Burdiehouse, and work our way upward through the four thousand feet of strata that lie piled above it.

VERTICAL SECTION OF THE MID-LOTHIAN COAL-FIELD.

The Burdiehouse limestone is twenty-seven feet thick, of a yellowish or bluish-grey colour, very compact, splintery, and often fissile in structure, with a finely striped and laminated appearance, which probably indicates a slow and tranquil origin. It is Crowded with fossils, every fragment when taken up showing its seed-cone, fern-stem, fish-scale, or minutecyprides. All the plants seem to belong to terrestrial species, and have a broken and often a macerated look. Manifestly they never grew where we now find their remains; they must have come drifting down from swamp, or jungle, or hill-side. And so we come to know that during the later ages of the Lower Carboniferous period, there lay somewhere in the neighbourhood of Burdiehouse a land clothed with ferns and club-mosses, and through whose swampy hollows there spread a network of stigmariæ, while sigillariæ waved their fronds high overhead. From what has been said on a previous page we may infer that the climate of the old land was moist and equable like that of New Zealand, nourishing a prolific growth of ferns and other plants comparatively low in the botanical scale. The scenery of the vegetation displayed perhaps no great variety of outline, but exhibited rather an endless succession of the same graceful forms.

But the limestone presents us with other remains than merely those of terrestrial plants. It displays in abundance the minute dissevered cases ofcypris, the small crustaceous animal described above. Recent species of this genus inhabit stagnant ponds or the bottoms of gently-flowing rivers, and we hence infer that the ancient species must in like manner have possessed a similar habitat, and consequently that the rocks which preserve their remains must have been deposited in fresh (or, perhaps, brackish) water. Tried by this test the Burdiehouse limestone must be regarded as a lacustrine, or more probably a fluviatile formation, which gathered slowly on an undisturbed bottom swarming with crustaceans and plentifully covered with leaves,branches, rootlets, and other fragments of terrestrial plants brought down by streams from the adjoining land. Thus the inferences drawn from the numerous plants, and from the countless multitude of cypris-cases, come to be mutually corroborative. The former tell us of some neighbouring forest-covered country; the latter lead us, as it were, into its river-mouths, whence we can descry the waving woods on either side.

Still we have not exhausted all the fossil remains of the Burdiehouse rocks. Mingled among the stems of ferns and lepidodendra, and the scattered valves of the cyprides, lie the scales, teeth, and bones, of several large ganoidal fishes, along with entire specimens of the smaller genera. The scales of holoptychius are especially abundant, often crowded together by dozens, and probably not far out of the arrangement they had when grouped on the body of the living animal. Detached teeth of the same fish also frequently occur along with disjointed internal bones. The remains of the contemporary megalichthys likewise abound, more particularly the scales, which have a fine nut-brown colour, and dot the surface of the rock with their bright glittering enamel. Several other smaller ganoids may be met with, especially a small and elegant species of Palæoniscus (P. Robisoni), and one of Eurynotus, a fish remarkable for the great size of its dorsal fin. Not uncommon, too, are the ichthyodoralites of a gigantic placoid—theGyracanthus formosus—with all their delicately-fretted ornament and a peculiar crystalline glistening surface when broken across, whereby the smallest fragment can be easily distinguished from any other bone in the limestone. Such are the ichthyic remains of the Burdiehouse beds; what deductions can be legitimately drawn from them?

As before, we must have recourse to the analogy of living nature. The existing ganoidal fishes chiefly inhabit lakes and rivers, especially near the confluence of the latter with the ocean. They feed on the decaying matter brought down fromthe land, or on the minute Crustacea that swarm upon the river-bottom. If, as seems probable, the ancient ganoids had habits similar to those of their present representatives, then the rocks wherein their remains occur abundantly may have originated on river-bottoms, and such may have been the case at Burdiehouse. So that here again we have corroborative evidence of the fluviatile origin of the limestone in question. But besides the remains of ganoidal fishes there occur the defensive spines of placoids. Now, the placoids are emphatically marine fishes, and the sole living representative of the most ancient genera of this order is the Port-Jackson shark, that haunts the seas round Australia. The ichthyodorulites of Burdiehouse, therefore, if we would apply analogy consistently, must be regarded as the relics of marine species. And this conclusion, too, will be found in entire harmony with those already obtained, for if we are right in assuming the Burdiehouse strata to have originated at a river-bottom, particularly near the sea, we may expect to find the remains of marine predaceous fishes imbedded in the sediment that gathered there, just as the teeth of the shark may be preserved among the mud forming in the upper reaches of many British estuaries, seeing that not a few instances are known where that fish has been stranded on such shores as those of the higher parts of the Firth of Forth. These Burdiehouse ichthyodorulites give positive proof that the limestone could not have originated in a lake, and the only explanation left is that of a river-bottom.

But it may perhaps be objected that, after all, these fish-remains are for the most part fragmentary, and may consequently be drifted specimens, so that no conclusion as to the source of the rock can be based on their occurrence there. The imbedded land-plants confessedly came from some distance, why may not the same have been the case with the bones and scales of the river-haunting ganoid fishes? And, indeed, did we regard these fish-bones and scales merely in themselves, the argumentmight not perhaps be very easily answered, although the great numbers and perfect outline of the bones, teeth, and scales, afford pretty strong evidence that the owners lived and died in the locality where their remains are found. But there is a curious kind of evidence to be gleaned from the rocks around them whereby this objection can be at once set aside. In the limestone itself, and especially in some of the shales above, there occur vast numbers of small oblong coprolitic concretions of a dirty yellow or brown colour, full of scales and fragments of bone. There can be no doubt that these are the excremental remains of predaceous animals, while their great number and perfect preservation assure us that they could not have been drifted from a distance, but must rather have been deposited on the spot where we now find them. And thus we conclude that the site of Burdiehouse must have been a favourite haunt of these bone-covered fishes; that the bulkier forms, armed with pointed teeth or barbed-spines, preyed upon their humbler congeners, while these in turn may have fed on the cyprides that swarmed by millions at the bottom of the estuary. I have often detected in these coprolites the peculiarly-sculptured scales of the palæoniscus. These graceful little animals must, therefore, have died that their lordlier brethren might dine.

On a survey, then, of the whole evidence from fossils, we are led to conclude that the Burdiehouse limestone was slowly elaborated at the bottom of an estuary, into which the remains of terrestrial plants were drifted from the land, while bone-covered fishes haunted the waters, and into these busy scenes huge sharks ascended from the sea to share in the decaying putrescent matter ever brought down from the interior.

The upper part of the limestone is shaly and argillaceous, and rests below a series of shales and thin sandstones. If the question were asked, what caused the change from limestone to shale, from the deposition of a calcareous to that of a muddy sediment, several answers might be given. The most probableseems to be the following. The limestone on weathered surfaces displays the mouldering casts of cypris-cases sometimes in such abundance as to show that the rock must be largely made up of them. The cyprides of the present day probably cast their shells annually; the integuments thus thrown off forming under favourable circumstances a thin mouldering calcareous marl at the bottom of the pond or marsh, along with the decaying shells ofpaludina,planorbis,limnea, or other fresh-water molluscs. We may conceive the Burdiehouse limestone to have had a similar origin. The cyprides, inhabiting water that contained little argillaceous matter, must have propagated by myriads, and during a long period of repose, in which the conditions of land and sea, and the directions of tidal currents and river-courses, appear not to have greatly varied in the neighbourhood of Burdiehouse, the calcareous exuviæ of these minute animals, along perhaps with the remains of other estuarine or fluviatile organisms,[60]would form each year a scarce appreciable stratum, until by slow aggregation a bed twenty-seven feet deep was elaborated. Each successive annual layer would hardly settle down more perceptibly or more rapidly than "the flickering dust that mottles the floor of some old haunted chamber."

[60]Though I have never observed molluscan remains in the limestone of Burdiehouse, they are abundant twelve miles to the west, in the equivalent strata around Mid-Calder, one little gastropod being especially plentiful near the base of the calcareous rock in a seam known to the quarrymen as the "Buckie fake." I have not met with specimens sufficiently perfect for identification, the hard splintery nature of the rock seldom allowing anything but a cross-section to be seen save on weathered specimens, where the general contour of the shells has sometimes reminded me ofPaludina multiformisgrouped together in a recent fresh-water marl. In the shales above the Burdiehouse limestone, Dr Hibbert states he found aunio(?), called by himU. nuciformis.Trans. Roy. Soc. Edin.vol. xiii. p. 245.

At last, however, this condition of things came to be modified. The direction of the river channel along some part of its course had varied, or some analogous change had taken place, so that muddy sediment transported from the land sank down amid the cyprides at the bottom. In circumstances so uncongenialthese tiny denizens of the estuary diminished in numbers until the silt and sand came down so rapidly and in such abundance that they eventually died out. Alluvial matter still darkened the water and covered the river-bottom, enveloping now the fronds of a delicate fern that had waved along the margin of some sequestered lake far inland, anon a seed-cone that had been shaken by the breeze from the spiky branches of some tall club-moss. Among these muddy beds occur numerous coprolites and fish-scales, along with cypriscases and a few shells of unio (?), showing that though the cyprides were decreasing, the water still presented the old estuary conditions and still swarmed with life.

Eventually there came other changes in the direction or rapidity of river currents, and the accumulations of mud and silt were succeeded by a long protracted deposition of yellow sand, now forming the sandstone of Straiton. It enclosed many stems of stigmaria, lepidodendron, &c., and in certain limited areas these plants matted together in such quantities that their remains now form thin irregular seams of coal. It would appear, therefore, that notwithstanding these changes in the matter transported and deposited at the locality in question, the estuary character of the locality remained essentially the same. The sand was at length replaced by fresh accumulations of mud and sandy silt, which went to form the beds of shale and shaly sandstone now found above the Straiton rock.

When in the course of many long centuries a depth of strata amounting to fully 300 feet had been amassed, the area of Mid-Lothian underwent a total change. Owing to a depression of the earth's crust, that seems to have been general over the whole of central Scotland, the estuary in which the Burdiehouse limestone and superincumbent strata were deposited became open sea. As the evidence of this change rests solely on the character of the imbedded organic remains, we shall pursue our induction by examining the beds somewhat in detail.

Rather more than 300 feet above the limestone of Burdiehouse there occurs in the Mid-Lothian coal-field a series of shales and seams of limestone. The former are sometimes black and hard, sometimes bluish-grey, soft, and frequently imbedding the remains of several genera of mollusca and other organic remains. The limestones vary considerably in the thickness and general aspect of their several seams, some being highly crystallized and about two or three feet in depth, others dull, compact, and ranging up to twenty and thirty feet thick. The shales and limestones are intercalated with and sometimes pass into each other, through the gradations of shaly limestone and calcareous shales. The whole series may measure 150 to 200 feet, resting on the Straiton sandstone below, and passing upwards into the under part of the coal-bearing strata of Mid-Lothian known as theEdge series. These limestones form the northernmarineequivalents of the mountain limestone of England, while the sandstones and shales on which they rest, including the Burdiehouse beds and all the Lower Carboniferous group, must probably be regarded asestuarineequivalents of the same formation. That is to say, while marine limestones were accumulating over the site of central England, sandstone, shale, and drifted plants, were slowly gathering in a wide estuary over what is now central Scotland, and only at the close of the period did marine limestones form simultaneously at both localities.

In examining these Mid-Lothian beds we are struck at once with the great dissimilarity that obtains between their organic remains and those of the underlying strata. All the land-plants disappear—ferns, lepidodendra, sigillariæ, and stigmariæ. The cyprides, too, no longer occur, though the shales seem, at a first glance, to differ in no respect from those underneath, in some of which the cypris-cases were seen to abound. Neither can we detect the glittering scales and teeth that stood out in such strong relief upon the rocks below. Yet the fossils arescarcely less numerous than they were in the lower beds. Nay, in some of the limestones they lie so crowded together that the rock seems entirely made up of them. Plainly such a total renovation of organic life points to some equally extensive change of a physical kind. Let us examine for a little some of the fossil remains occurring in the mountain limestone series of Mid-Lothian, and read off, if we can, the revolutions which they chronicle.

Fig. 32.—Section from Gilmerton to Crichton;a, Lower Carboniferous;b, Mountain Limestone;c, Edge Series;d, Roslyn Sandstone Group;e, Flat Coals;y, Drift.

The neighbourhood of Edinburgh affords many facilities for the study of these rocks. They can be seen, for instance, at many points along the ridge of the Roman Camp Hill, near Dalkeith, exposed in the operations of quarrying. That ridge is formed by what is known technically as an anticlinal axis (Fig. 32); in other words, the lower beds of the coal-measures rise up here into a sort of broad wave-like undulation, round the sides of which the higher parts of the series are folded. The elevated area has either been pushed up from below, or the more level country around has subsided into two trough-like hollows, so that now the strata, which geologically speaking are lowest, come to occupy the highest ground in the district. Seated on some of the opener spots of this woody eminence the observer has a noble prospect on which to expatiate. The ground around him is rich in historic associations, and links itself to many a varied page in the annals of Scotland. The hill on which he rests is crowned by the mouldering mounds of what tradition reports to have been a Romanstation, but which may perhaps belong to a still earlier era. A few hundred yards north rise the wooded slopes of Carberry Hill, where the hapless Mary surrendered to her rebel lords, and whence she was led into her own capital amid the insults of an infuriate rabble. Northward, too, lies the fatal field of Pinkie, and eastward the less deadly but not less decisive field of Prestonpans. To the west the eye can mark the grey smoke of the Scottish metropolis, with its dusky towers and its lion-shaped hill, and then the blue waving outline of the Pentlands that sweep away south and lose themselves among the distant hills which girdle in the coal-basin of Edinburgh and Haddington. The course of the Esk—thatfabulosus amnis—passes by many a time-honoured spot, from Habbie's Howe and the scene of the Gentle Shepherd down by the haunted scenery of Roslyn, the cliffs of Hawthornden, the grounds of Newbattle, and the old Roman station of Inveresk. East, west, and south, the broad expanse of green field and clustering wood swells upward to the distant hills that encircle the landscape with a wavy line of softest blue. Northward the eye rests on the Firth of Forth with its solitary sails, bounded by the bosky heights of Fife, and opening outwards by the May Island and the Bass Rock into the far-off hazy ocean. On every side objects of historic interest lie crowded together, about which many pleasant volumes have been and might still be written. If the observer be a lover of geological science he will find an examination of the structure of the hill to impart an additional interest to the scene. From the wide panorama of hill and dale, river and sea, with all its battle-fields, castles, and abbeys, and all its memories of the olden time, let him turn into one of the quarries that indent the flanks of the hill, and try to decipher there the records of a still older history. An hour or two thus spent will pass swiftly and pleasantly away, and on quitting the quarry he will have gained a new light in which to look on the landscape that lies spread out below.

The mountain limestone of Mid-Lothian consists, as has been mentioned, of several seams interbedded with black and calcareous shales. The quarries on Roman Camp Hill have been opened in several of the thickest of these seams. Let us enter one of the excavations. A vertical face of rock forms the background, overhung above by long dangling tufts of withered grass, and washed below by a pool of water having that milky green tint peculiar to old lime-quarries. The lowest rock visible is a dull grey limestone with a yellowish weathered surface. Above it rests a mass of hard yellow calcareous shale, known to the workmen as "bands." This rock is worthless as a source of lime, nor from its irregular laminations and shivery structure has it much value in any other way. A few inches of surface-soil form the upper part of the section. It requires but a glance over the weathered surface of the limestone to mark that the rock abounds in fossils. Of these by far the most numerous are the joints of the stone-lily, for the most part of small size, and when broken across, with their minute central apertures, looking like so many fractured stems of tobacco-pipes. Other organisms also occur, such as a small delicately-plaited productus, a larger and more boldly-ribbed spirifer, a small cyathophyllum or cup-coral, and the fragile interlacing meshes of one of the net-like bryozoa—the fenestella. Of rarer occurrence are the whorled shells called bellerophon, the long chambered shells of orthoceratites, and the grooved tapering shells of pinnæ. Many of the same fossils can be detected in the beds above, which thus evidently all form part of one series with the rock below. What, then, were the circumstances under which these strata originated?

The answer to such a question is not far to seek. The corals and crinoids are exclusively marine families, and so any stratum in which their remains occur must have had a submarine origin. It matters not in this case though the specimens be fragmentary, showing a broken and drifted appearance.For even supposing that they did not live at the spot where their petrified relics are now exhumed by the operations of the quarryman, granting that they were drifted from a distance, still they could only have been drifted from one part of the sea-bottom to another. The state of keeping of the specimen often tells vastly on the value of its evidence when it belongs to a land or fresh-water tribe. Thus, in one of the limestones of West-Lothian I have found a black carbonized stem of sigillaria. Now, the sigillaria was a land-plant as much as any of our hazels or willows, and where the evidence from the associated organisms coincides, furnishes its own testimony as to the origin of the rock which imbeds its remains. But the stem in question was a mere fragment, and showed moreover a worn macerated surface. Such a fossil had evidently no value as a test of the origin of the limestone, which might have been elaborated either in an inland lake or in open sea. That it had really a marine origin, and that the sigillaria actually was, as it seemed to have been, a drifted plant, I ascertained beyond a doubt by detecting on the same slab hundreds of encrinal stems along with the shells, and thin, delicate, silvery spines of productus. Thus, then, the organisms of the land may be carried into the sea, and in dealing with their fossilized remains in the deposits of former ages we must be very careful in the use of evidence derived from fragmentary and drifted specimens. But no such caution is needed in regard to the productions of the sea. If they be fragmentary and drifted, we may believe they were rolled about by tides and currents previous to their final entombment; but still they remain as good a test as ever of the marine character of the rock in which they occur.[61]

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