CHAPTER VI.
Organisms of the Boulder-clay not unequivocal—First Impressions of the Boulder-clay—Difficulty of accounting for its barrenness of Remains—Sir Charles Lyell's reasoning—A Fact to the contrary—Human Skull dug from a Clay-bank—The Author's Change of Belief respecting Organic Remains of the Boulder-clay—Shells from the Clay at Wick—Questions respecting them settled—Conclusions confirmed by Mr. Dick's Discoveries at Thurso—Sir John Sinclair's Discovery of Boulder-clay Shells in 1802—Comminution of the Shells illustrated—Cyprina islandica—Its Preservation in larger Proportions than those of other Shells accounted for—Boulder-clays of Scotland reformed during the existing Geological Epoch—Scotland in the Period of the Boulder-clay "merely three detached groups of Islands"—Evidence of the Subsidence of the Land in Scotland—Confirmed by Rev. Mr. Cumming's conclusion—High-lying Granite Boulders—Marks of a succeeding elevatory Period—Scandinavia now rising—Autobiography of a Boulder desirable—A Story of the Supernatural.
For the greater part of a quarter of a century I had been finding organisms in abundance in the boulder-clay, but never anything organic that unequivocally belonged to its own period. I had ascertained that it contains in Ross and Cromarty nodules of the Old Red Sandstone, which bear inside, like so many stone coffins, their well laid out skeletons of the dead; but then the markings on their surface told me that when the boulder-clay was in the course of deposition, they had been exactly the same kind of nodules that they are now. In Moray, it incloses, I had found, organisms of the Lias; but they also testify that they present an appearance in no degree more ancient at the present time than they did when first enveloped by the clay. In East and West Lothian too, and in the neighborhood of Edinburgh, I had detected in it occasional organisms of the Mountain Limestone and the Coal Measures; but these, not less surely than its Liasic fossils in Moray, and its Old Red ichthyolites in Cromarty and Ross, belonged to an incalculably more ancient state of things than itself; and—like those shrivelled manuscripts of Pompeii or Herculaneum, which, whatever else they may record, cannot be expected to tell aught of the catastrophe that buried them up—they throw no light whatever on the deposit in which they occur. I at length came to regard the boulder-clay—for it is difficult to keep the mind in a purely blank state on any subject on which one thinks a good deal—as representative of a chaotic period of death and darkness, introductory, mayhap, to the existing scene of things.
After, however, I had begun to mark the invariable connection of the clay, as a deposit, with the dressed surfaces on which it rests, and the longitudinal linings of the pebbles and boulders which it incloses, and to associate it, in consequence, with an ice-charged sea and the Great Gulf Stream, it seemed to me extremely difficult to assign a reason why it should be thus barren of remains. Sir Charles Lyell states, in his "Elements," that the "stranding of ice-islands in the bays of Iceland since 1835 has driven away the fish for several successive seasons, and thereby caused a famine among the inhabitants of the country;" and he argues from the fact, "that a sea habitually infested with melting ice, which would chill and freshen the water, might render the same uninhabitable by marine mollusca." But then, on the other hand, it is equally a fact that half a million of seals have been killed in a single season on the meadow-ice a little to the north of Newfoundland, and that many millions of cod, besides other fish, are captured yearly on the shores of that island, though grooved and furrowed by ice-floes almost every spring. Of the seal family it is specially recorded by naturalists, that many of the species "are from choice inhabitants of the margins of the frozen seas towards both poles; and, of course, in localities in which many such animals live, some must occasionally die." And though the grinding process would certainly have disjointed, and might probably have worn down and partially mutilated, the bones of the amphibious carnivora of the boulder period, it seems not in the least probable, judging from the fragments of loose-grained sandstone and soft shale which it has spared, that it would have wholly destroyed them. So it happened, however, that from North Berwick to the Ord Hill of Caithness, I had never found in the boulder-clay the slightest trace of an organism that could be held to belong to itself; and as it seems natural to build on negative evidence, if very extensive, considerably more than mere negative evidence, whatever the circumstances, will carry, I became somewhat skeptical regarding the very existence of boulder-fossils,—a skepticism which the worse than doubtful character of several supposed discoveries in the deposit served considerably to strengthen. The clay forms, when cut by a water-course, or assailed on the coast by some unusually high tide, a perpendicular precipice, which in the course of years slopes into a talus; and as it exhibits in most instances no marks of stratification, the clay of the talus—a mere re-formation of fragments detached by the frosts and rains from the exposed frontage—can rarely be distinguished from that of the original deposit. Now, in these consolidated slopes it is not unusual to find remains, animal and vegetable, of no very remote antiquity. I have seen a human skull dug out of the reclining base of a clay-bank once a precipice, fully six feet from under the surface. It might have been deemed the skull of some long-lived contemporary of Enoch,—one of the accursed race, mayhap,
"Who sinned and died before the avenging flood."
But, alas! the laborer dug a little further, and struck his pickaxe against an old rybat that lay deeper still. There could be no mistaking the character of the champfered edge, that still bore the marks of the tool, nor that of the square perforation for the lock-bolt; and a rising theory, that would have referred the boulder-clay to a period in which the polar ice, set loose by the waters of the Noachian deluge, came floating southwards over the foundered land, straightway stumbled against it, and fell. Both rybat and skull had come from an ancient burying-ground, that occupies a projecting angle of the table-land above. I must now state, however, that my skepticism has thoroughly given way; and that, slowly yielding to the force of positive evidence, I have become as assured a believer in the comminuted recent shells of the boulder-clay as in the belemnites of the Oölite and Lias, or the ganoid ichthyolites of the Old Red Sandstone.
I had marked, when at Wick, on several occasions, a thick boulder-clay deposit occupying the southern side of the harbor, and forming an elevated platform, on which the higher parts of Pulteneytown are built; but I had noted little else regarding it than that it bears the average dark-gray color of the flagstones of the district, and that some of the granitic boulders which protrude from its top and sides are of vast size. On my last visit, however, rather more than two years ago, when sauntering along its base, after a very wet morning, awaiting the Orkney steamer, I was surprised to find, where a small slip had taken place during the rain, that it was mottled over with minute fragments of shells. These I examined, and found, so far as, in their extremely broken condition, I dared determine the point, that they belonged in such large proportion to one species,—the Cyprina islandica of Dr. Fleming,—that I could detect among them only a single fragment of any other shell,—the pillar, apparently, of a large specimen of Purpura lapillus. Both shells belong to that class of old existences,—long descended, without the pride of ancient descent,—which link on the extinct to the recent scenes of being. Cyprina islandica and Purpura lapillus not only exist as living molluscs in the British seas, but they occur also as crag-shells, side by side with the dead races that have no place in the present fauna. At this time, however, I could but think of them simply in their character as recent molluscs; and as it seemed quite startling enough to find them in a deposit which I had once deemed representative of a period of death, and still continued to regard as obstinately unfossiliferous, I next set myself to determine whether it really was the boulder-clay in which they occurred. Almost the first pebble which I disengaged from the mass, however, settled the point, by furnishing the evidence on which for several years past I have been accustomed to settle it;—it bore in the line of its longer axis, on a polished surface, the freshly-marked grooves and scratchings of the iceberg era. Still, however, I had my doubts, not regarding the deposit, but the shells. Might they not belong merely to the talus of this bank of boulder-clay?—a re-formation, in all probability, not more ancient than the elevation of the most recent of the old coast lines,—perhaps greatly less so. Meeting with an intelligent citizen of Wick, Mr. John Cleghorn, I requested him to keep a vigilant eye on the shells, and to ascertain for me, when opportunity offered, whether they occurred deep in the deposit, or were restricted to merely the base of its exposed front. On my return from Orkney, he kindly brought me a small collection of fragments, exclusively, so far as I could judge, of Cyprina islandica, picked up in fresh sections of the clay; at the same time expressing his belief that they really belonged to the deposit as such, and were not accidental introductions into it from the adjacent shore. And at this point for nearly two years the matter rested, when my attention was again called to it by finding, in the publication of Mr. Keith Johnston's admirable Geological Map of the British Islands, edited by Professor Edward Forbes, that other eyes than mine had detected shells in the boulder-clay of Caithness. "Cliffs of Pleistocene," says the Professor, in one of his notes attached to the map, "occur at Wick, containing boreal shells, especially Astarte borealis."
I had seen the boulder-clay characteristically developed in the neighborhood of Thurso; but, during a rather hurried visit, had lacked time to examine it. The omission mattered the less, however, as my friend Mr. Robert Dick is resident in the locality; and there are few men who examine more carefully or more perseveringly than he, or who can enjoy with higher relish the sweets of scientific research. I wrote him regarding Professor Forbes's decision on the boulder-clay of Wick and its shells; urging him to ascertain whether the boulder-clay of Thurso had not its shells also. And almost by return of post I received from him, in reply, a little packet of comminuted shells, dug out of a deposit of the boulder-clay, laid open by the river Thorsa, a full mile from the sea, and from eighty to a hundred feet over its level. He had detected minute fragments of shell in the clay about a twelvemonth before; but a skepticism somewhat similar to my own, added to the dread of being deceived by mere surface shells, recently derived from the shore in the character of shell-sand, or of the edible species carried inland for food, and then transferred from the ash-pit to the fields, had not only prevented him from following up the discovery, but even from thinking of it as such. But he eagerly followed it up now, by visiting every bank of the boulder-clay in his locality within twenty miles of Thurso, and found them all charged, from top to bottom, with comminuted shells, however great their distance from the sea, or their elevation over it. The fragments lie thick along the course of the Thorsa, where the encroaching stream is scooping out the clay for the first time since its deposition, and laying bare the scratched and furrowed pebbles. They occur, too, in the depths of solitary ravines far amid the moors, and underlie heath, and moss, and vegetable mould, on the exposed hill-sides. The farm-house of Dalemore, twelve miles from Thurso as the crow flies, and rather more than thirteen miles from Wick, occupies, as nearly as may be, the centre of the county; and yet there, as on the sea-shore, the boulder-clay is charged with its fragments of marine shells. Though so barren elsewhere on the east coast of Scotland, the clay is everywhere in Caithness a shell-bearing deposit; and no sooner had Mr. Dick determined the fact for himself, at the expense of many a fatiguing journey, and many an hour's hard digging, than he found that it had been ascertained long before, though, from the very inadequate style in which it had been recorded, science had in scarce any degree benefited by the discovery. In 1802 the late Sir John Sinclair, distinguished for his enlightened zeal in developing the agricultural resources of the country, and for originating its statistics, employed a mineralogical surveyor to explore the underground treasures of the district; and the surveyor's journal he had printed under the title of "Minutes and Observations drawn up in the course of a Mineralogical Survey of the County of Caithness, ann. 1802, by John Busby, Edinburgh." Now, in this journal there are frequent references made to the occurrence of marine shells in the blue clay. Mr. Dick has copied for me the two following entries,—for the work itself I have never seen:—"1802, Sept. 7th.—Surveyed down the river [Thorsa] to Geize; found blue clay-marl, intermixed with marine shells in great abundance." "Sept. 12th.—Set off this morning for Dalemore. Bored for shell-marl in the 'grass-park;' found it in one of the quagmires, but to no great extent. Bored for shell-marl in the 'house-park.' Surveyed by the side of the river, and found blue clay-marl in great plenty, intermixed with marine shells, such as those found at Geize. This place is supposed to be about twenty miles from the sea; and is one instance, among many in Caithness, of the ocean's covering the inland country at some former period of time."
The state of keeping in which the boulder-shells of Caithness occur is exactly what, on the iceberg theory, might be premised. The ponderous ice-rafts that went grating over the deep-sea bottom, grinding down its rocks into clay, and deeply furrowing its pebbles, must have borne heavily on its comparatively fragile shells. If rocks and pebbles did not escape, the shells must have fared but hardly. And very hardly they have fared: the rather unpleasant casualty of being crushed to death must have been a greatly more common one in those days than in even the present age of railways and machinery. The reader, by passing half a bushel of the common shells of our shores through a barley-mill, as a preliminary operation in the process, and by next subjecting the broken fragments thus obtained to the attritive influence of the waves on some storm-beaten beach for a twelvemonth or two, as a finishing operation, may produce, when he pleases, exactly such a water-worn shelly debris as mottles the blue boulder-clays of Caithness. The proportion borne by the fragments of one species of shell to that of all the others is very extraordinary. The Cyprina islandica is still by no means a rare mollusc on our Scottish shores, and may, on an exposed coast, after a storm, be picked up by dozens, attached to the roots of the deep-sea tangle. It is greatly less abundant, however, than such shells as Purpura lapillus, Mytilus edule, Cardium edule, Littorina littorea, and several others; whereas in the boulder-clay it is, in the proportion of at least ten to one, more abundant than all the others put together. The great strength of the shell, however, may have in part led to this result; as I find that its stronger and massier portions,—those of the umbo and hinge-joint,—are exceedingly numerous in proportion to its slimmer and weaker fragments. "The Cyprina islandica," says Dr. Fleming, in his "British Animals," "is the largest British bivalve shell, measuring sometimes thirteen inches in circumference, and, exclusively of the animal, weighing upwards of nine ounces." Now, in a collection of fragments of Cyprina sent me by Mr. Dick, disinterred from the boulder-clay in various localities in the neighborhood of Thurso, and weighing in all about four ounces, I have detected the broken remains of no fewer than sixteen hinge joints. And on the same principle through which the stronger fragments of Cyprina were preserved in so much larger proportion than the weaker ones, may Cyprina itself have been preserved in much larger proportion than its more fragile neighbors. Occasionally, however,—escaped, as if by accident,—characteristic fragments are found of shells by no means very strong,—such as Mytilus, Tellina, and Astarte. Among the univalves I can distinguish Dentalium entale, Purpura lapillus, Turritella terebra, and Littorina littorea, all existing shells, but all common also to at least the later deposits of the Crag. And among the bivalves Mr. Dick enumerates,—besides the prevailing Cyprina islandica,—Venus casina, Cardium edule, Cardium echinatum, Mytilus edule, Astarte danmoniensis (sulcata), and Astarte compressa, with a Mactra, Artemis, and Tellina.[15] All the determined species here, with the exception of Mytilus edule, have, with many others, been found by the Rev. Mr. Cumming in the boulder-clays of the Isle of Man; and all of them are living shells at the present day on our Scottish coasts. It seems scarce possible to fix the age of a deposit so broken in its organisms, on the principle that would first seek to determine its per centage of extinct shells as the data on which to found. One has to search sedulously and long ere a fragment turns up sufficiently entire for the purpose of specific identification, even when it belongs to a well-known living shell; and did the clay contain some six or eight per cent. of the extinct in a similarly broken condition (and there is no evidence that it contains a single per cent. of extinct shells), I know not how, in the circumstances, the fact could ever be determined. A lifetime might be devoted to the task of fixing their real proportion, and yet be devoted to it in vain. All that at present can be said is, that, judging from what appears, the boulder-clays of Caithness, and with them the boulder-clays of Scotland generally, and of the Isle of Man,—for they are all palpably connected with the same iceberg phenomena, and occur along the same zone in reference to the sea-level,—were formed during the existing geological epoch.
These details may appear tediously minute; but let the reader mark how very much they involve. The occurrence of recent shells largely diffused throughout the boulder-clays of Caithness, at all heights and distances from the sea at which the clay itself occurs, and not only connected with the iceberg phenomena by the closest juxtaposition, but also testifying distinctly to its agency by the extremely comminuted state in which we find them, tell us, not only according to old John Busby, "that the ocean covered the inland country at some former period of time," but that it covered it to a great height at a time geologically recent, when our seas were inhabited by exactly the same mollusca as inhabit them now, and so far as yet appears, by none others. I have not yet detected the boulder-clay at more than from six to eight hundred feet over the level of the sea; but the travelled boulders I have often found at more than a thousand feet over it; and Dr. John Fleming, the correctness of whose observations few men acquainted with the character of his researches or of his mind will be disposed to challenge, has informed me that he has detected the dressed and polished surfaces at least four hundred feet higher. There occurs a greenstone boulder, of from twelve to fourteen tons weight, says Mr. M'Laren, in his "Geology of Fife and the Lothians," on the south side of Black Hill (one of the Pentland range), at about fourteen hundred feet over the sea. Now fourteen or fifteen hundred feet, taken as the extreme height of the dressings, though they are said to occur greatly higher, would serve to submerge in the iceberg ocean almost the whole agricultural region of Scotland. The common hazel (Corylus avellana) ceases to grow in the latitude of the Grampians, at from one thousand two hundred to one thousand five hundred feet over the sea level; the common bracken (Pteris aquilina) at about the same height; and corn is never successfully cultivated at a greater altitude. Where the hazel and bracken cease to grow, it is in vain to attempt growing corn.[16] In the period of the boulder-clay, then, when the existing shells of our coasts lived in those inland sounds and friths of the country that now exist as broad plains or fertile valleys, the sub-aërial superficies of Scotland was restricted to what are now its barren and mossy regions, and formed, instead of one continuous land, merely three detached groups of islands,—the small Cheviot and Hartfell group,—the greatly larger Grampian and Ben Nevis group,—and a group intermediate in size, extending from Mealfourvonny, on the northern shores of Loch Ness, to the Maiden Paps of Caithness.