The Story of a Boulder; or, Gleanings from the Note-book of a Field Geologist

by Archibald Geikie

And just so is it with our boulder. We can easily believe, merely from looking at it as it lies on its clayey bed, that a long time must have elapsed between the time of its formation as part of a sandstone bed, and the period of its transportation and striation by an iceberg. The sand of which it is formed must have been washed down by currents, and other sediment would settle down over it. It would take some time to acquire its present hardness and solidity, while, in long subsequent times, after being broken up and well-rounded by breaker or current action, it may have lain on some old coast-line for centuries before it was finally frozen into an ice-floe, and so freighted to a distance. But the stone, with all its stories of the olden time, can tell us nothing of this intervening period. It leads us from a dreary frozen sea at once into a land of tropical luxuriance, and so, if we desire to know anything of the missing portion of the chronology, we must seek it elsewhere.

The Boulder-clay is one of the latest of geologic periods.[16] Beyond it we get into Tertiary times, and learn from the caves of Yorkshire how elephants, hyenas, rhinoceroses, hippopotami, bears, and wolves, prowled over the rich valleys; while, from the quarries of the Isle of Wight, we see how at an earlier time herds of uncouth palæotheres and slimly-built anoplotheres browsed the plains of Old England. Beyond the Tertiary ages come those of the Chalk, with its ocean that swarmed with sea-urchins, terebratulæ, pectens, sponges, and many other forms. Then arises the era of the Wealden, with its bosky land haunted by the unwieldy iguanodon; the Oolite, with its land rich in a coniferous flora, and tenanted by a race of small marsupial animals, and its seas abounding in corals, encrinites of many a form, cidares, cuttle-fishes, and ammonites. Further back still, come the times of the Lias, that strange era in the history of our country, when reptiles huger than those of the Nile swam the seas, and sped on wings through the air. Then come the times of the Trias, when a vegetation still further removed from existing types clothed the land, and frogs large as oxen waddled along the shores. Then the times of the Permian, with its deep sea tenanted by a meagre list of corals and shells, and by a type of fishes that was slowly passing away. We arrive at last at the Coal or Carboniferous period, to the older ages of which our boulder belongs.

[16] For the names and succession of the rocks of which the known part of the earth's crust is composed, see the Table at the end of the volume.

These eras may have been some longer, some shorter, but each had a duration which, when tried by human standards, must be regarded as immensely protracted. The cycles of astronomy are very vast, yet I have often thought that the cycles of geology, though probably of much less duration, impress us more forcibly with the antiquity of our planet. The astronomer tells us of light that has taken two millions of years to reach our earth, and of nebulæ that are millions upon millions of miles distant, but these numbers are so vast that we cannot bring ourselves to realize them. We know that there is a great difference between two millions and ten millions, but we cannot fully appreciate it, and so the periods of the astronomer, beyond a certain point, cease adequately to impress us. So long as they can be easily contrasted with our own standards of comparison, they have their full force; but after that, every additional million, or ten millions, or ten hundred millions, produces only a confused and bewildered sense of immensity, and the comparative amount of each addition fails to be realized. Will my reader forgive a homely illustration:—Some years ago, I stood at the pier-head of one of our smaller sea-port towns, and watched the sun as it sullenly sank behind the outline of the opposite hills. The breadth of the channel, in the direction of sunset, was several miles, but in the flush of evening one fancied he could almost have thrown a stone across. The water lay unruffled by a ripple, and reflected all the thousand varying tints that lighted up the sky. The harbour, that had been a busy scene all evening, began to grow less noisy, as one by one the herring-boats pushed out to sea. I found it not a little interesting to mark, as the boats gained the open firth, how the opposite coast-line gradually seemed to recede. The farther the dark sails withdrew, the more remote did the adjacent shores appear, until, as the last tinge of glory faded from the clouds, and a cold grey tint settled down over the landscape, the hills lay deep in shade and stretched away in the twilight as a dark and distant land from whose valleys there rose troops of stars. The coast-line, as seen in early evening, reminded me of the periods of the astronomer; as seen in early night, it reminded me of the periods of the geologist. We fail to appreciate the real duration of astronomical cycles, because they are presented to us each as one vast period. They are not subdivided into intervals, and contain no succession of events, by means of which, as by milestones, we might estimate their extent; and so their unvaried continuity tends to diminish the impression of their vastness, just as the firth, without any islet or vessel on its surface, seemed greatly narrower than it really was. For it is with time as it is with space—the eye cannot abstractly estimate distance, nor can the mind estimate duration. In either case, the process must be conducted by a comparison with known standards. The geological periods exemplify the same rule. They may not be greater, perhaps not so great, as those revealed by astronomy, yet their vastness impresses us more, because we can trace out their history, and see how step by step they progressed. Thus, that the interval between the boulder-clay and the coal-measurer was immense, we learn from the records of many successive ages that intervened, in the same way that one began to perceive the real breadth of the firth, by resting his eye on the succession of intervening herring-boats. In the former case, the mind has ever and anon a sure footing on which to pause in gauging bygone eternity; in the latter, the eye had likewise a succession of points on which to rest in measuring distance. Or, to return to a former illustration: Boadicea lived eighteen hundred years ago, but who does not feel that the last nine hundred years look a great deal longer than the first? The one set has few marked incidents to fix the thoughts; the other is replete with those of the most momentous kind. In the one, we have M meagre list of conquerors and kings, from Julius Cæsar down to Athelstan; in the other, events crowd upon us from the waning of the Saxon power down through the rising glory of our country to the present plenitude of its power and greatness. The early centuries, like the cycles of the astronomer, pass through our mind rather as one continuous period; the later centuries, like the cycles of the geologist, arrest our thoughts by a succession of minor periods, and hence the idea of duration is more vividly suggested by the diversified events of the one series, than by the comparatively unbroken continuity of the other.

Let us now open the volume and try to decipher the strange legends which it contains. On removing some of the upper layers of the boulder, I found, as I have said, well-preserved remains of several kinds of plants. One of them was ribbed longitudinally, with transverse notches every three or four inches, us though a number of slender threads had been stretched along a rod, and tied tightly to it at regular intervals. Another, sorely mutilated, was pitted all over somewhat after the fashion in which the confectioner punctures his biscuits. A third had a more regular pattern, being prettily fretted with small lozenge-shaped prominences that wound spirally round the stalk. Other plants seemed to be present, but in a very bad state of preservation. They were all jumbled together and converted into a black coaly substance, in which no structure could be discerned.

These plants assuredly once grew green upon the land; but where now is that land on which they flourished? Had it hills and valleys, rivers and lakes, such as diversify our country? Was it tenanted by sentient beings, and, if so, what were their forms? Did insects hum their way through the air, and cattle browse on the plains, and fish gambol in the rivers? Was the land shaded with forests, dark and rugged like those of Norway, or fragrant as the orange-groves of Spain? What, in fine, were its peculiar features, and how far did its scenery resemble that of any country of the present day?

That old land has not entirely disappeared. Traces of it are found pretty extensively in South Wales, in Staffordshire, around Newcastle, and through central Scotland. Strange as it may seem, its forests are still standing in many places. The fishes that disported in its lakes, the insects that fluttered amid its woods, and the lizards that crawled among its herbage, are still in part preserved to us. Nay, more; we may sometimes see the sea-beaches of that ancient land pitted with rain-drops, and roughened with ripple-marks, as freshly as if the shower had fallen and the tide had flowed only yesterday. The peasants along the Bay of Naples gathered grapes from the flanks of Vesuvius for well-nigh seventeen centuries, before it was ascertained that they daily walked over the site of buried cities, with temples, theatres, and private houses still erect. It was many more centuries ere the people of Great Britain discovered that not a few of their villages and towns stood on the site of buried forests, and lakes, and seas. We have now, however, become aware of the fact, and are making good use of it. We dig into the earth and exhume these old forests to supply us with light and fuel; we quarry into the ripple-marked shores which fringed that old land, and build our houses with the hardened sand; we calcine the ferruginous mud that gathered in its swampy hollows, and extract therefrom our most faithful ally both in peace and war—metallic iron; we burn the delicate corals and shells and lily-like zoophytes which lived in the sea of that far-distant era, to enable us to smelt our iron, to build our houses, and manure our fields; in short, every year we are discovering some new and valuable material in the productions of that period, or finding out some new use which can be made of the substances already known. A more than ordinary interest, therefore, attaches to the history of the land and sea which have furnished us with so many aids to comfort as well as power; and we shall find, as we go on, that that history is a very curious one.

I shall describe some of the more common plants and animals of the period, that we may be able, in some measure, to look back through the ages of the past, and see how these plants would appear when they cast their broad shadow over river and lake, and how these animals would have seemed to human eye in the twilight of the forest, in the sluggish flow of the river, and in the stagnant waters of the lagoon.

The Flora, or vegetation of the Carboniferous era, differed widely from any that now exists. With the exception of the highest or exogenous class, it possessed representatives of all the existing classes of the botanic scale, but in very strange proportions. The number of species of carboniferous plants already found in Great Britain amounts to about three hundred, amongst which the ferns are especially abundant. Some of them seem to have been low-growing plants, like the bracken of our hillsides, but others must have shot up to the height of forest trees. We can recognise a few coniferous and cycadaceous plants, a good many stems resembling the "horse-tail" of our marshy grounds, and some of large size akin to the creeping club-moss of our heaths; but there are still many to which there exist no living analogues.

When we examine the roof of a coal-pit, or split open plates of shale in a quarry of the coal-measures, we are struck with the similarity which the ferns in the stone bear to those among our woods and hills. One of the most common, and, at the same time, most elegant forms, is the Sphenopteris or wedge-leaved fern, of which a large list of species is known. One of them (S. crenata) had a strong stem, from which there sprung straight tapering branches richly dight with leaflets. The leaflets—somewhat like minute oak-leaves—were ranged like those of our modern ferns, along two sides of the stalk, in alternate order, and tapered gently away to its outer extremity. The effect of the whole is singularly rich, and one can well believe that a garland of this ancient fern would have wreathed as gracefully around a victor's brow as the parsley of Nemea or the laurel-leaves of Delphi.