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.
Among the 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 sometimes nearly 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 that the 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 and moss-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] But in 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. This order 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 work The 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; in cyprides, unios, and paludinæ, 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, as we 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 its style, 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 general style of 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 less clearly 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] See ante, pp. 31, 32, and the Table of Rocks at the end of the volume.