I was struck, during my explorations at this time, as I had been often before, by the style of grouping, if I may so speak, which obtains among the Lower Old Red fossils. In no deposit with which I am acquainted, however rich in remains, have all its ichthyolites been found lying together. The collector finds some one or two species very numerous; some two or three considerably less so, but not unfrequent; some one or two more, perhaps, exceedingly rare; and a few, though abundant in other localities, that never occur at all. In the Cromarty beds, for instance, I never found a Holoptychius, and a Dipterus only once; the Diplopterus is rare; the Glyptolepis not common; the Cheirolepis and Pterichthys more so, but not very abundant; the Cheiracanthus and Diplacanthus, on the other hand, are numerous; and the Osteolepis and Coccosteus more numerous still. But in other deposits of the same formation, though a similar style of grouping obtains, the proportions are reversed with regard to species and genera: the fish rare in one locality abound in another. In Banniskirk, for instance, the Dipterus is exceedingly common, while the Osteolepis and Coccosteus are rare, and the Cheiracanthus and Cheirolepis seem altogether awanting. Again, in the Morayshire deposits, the Glyptolepis is abundant, and noble specimens of the Lower Old Red Holoptychius—of which more anon—are to be found in the neighborhood of Thurso, associated with remains of the Diplopterus, Coccosteus, Dipterus, and Osteolepis. The fact may be deemed of some little interest by the geologist, and may serve to inculcate caution, by showing that it is not always safe to determine regarding the place or age of subordinate formations from the per centage of certain fossils which they may be found to contain, or from the fact that they should want some certain organisms of the system to which they belong, and possess others. These differences may and do exist in contemporary deposits; and I had a striking example, on this occasion, of their dependence on a simple law of instinct, which is as active in producing the same kind of phenomena now as it seems to have been in the earlier days of the Old Red Sandstone. The Cromarty and Moray Friths, mottled with fishing boats (for the bustle of the herring fishers had just begun), stretched out before me. A few hundred yards from the shore there was a yawl lying at anchor, with an old fisherman and a few boys angling from the stern for sillocks (the young of the coal-fish) and for small rock-cod. A few miles higher up, where the Cromarty Frith expands into a wide landlocked basin, with shallow sandy shores, there was a second yawl engaged in fishing for flounders and small skate,—for such are the kinds of fish that frequent the flat shallows of the basin. A turbot-net lay drying in the sun: it served to remind me that some six or eight miles away, in an opposite direction, there is a deep-sea bank, on which turbot, halibut, and large skate are found. Numerous boats were stretching down the Moray Frith, bound for the banks of a more distant locality, frequented at this early stage of the herring fishing by shoals of herrings, with their attendant dog-fish and cod; and I knew that in yet another deep-sea range there lie haddock and whiting banks. Almost every variety of existing fish in the two friths has its own peculiar habitat; and were they to be destroyed by some sudden catastrophe, and preserved by some geologic process, on the banks and shoals which they frequent, there would occur exactly the same phenomena of grouping in the fossiliferous contemporaneous deposits which they would thus constitute, as we find exhibited by the deposits of the Lower Old Red Sandstone.
The remains of Holoptychius occur, I have said, in the neighborhood of Thurso. I must now add, that very singular remains they are,—full of interest to the naturalist, and, in great part at least, new to Geology. My readers, votaries of the stony science, must be acquainted with the masterly paper of Mr. Sedgwick and Sir R. Murchison "On the Old Red Sandstone of Caithness and the North of Scotland generally," which forms part of the second volume (second series) of the "Transactions of the Geological Society," and with the description which it furnishes, among many others, of the rocks in the neighborhood of Thurso. Calcareo-bituminous flags, grits, and shales, of which the paving flagstones of Caithness may be regarded as the general type, occur on the shores, in reefs, crags, and precipices; here stretching along the coast in the form of flat, uneven bulwarks: there rising over it in steep walls; yonder leaning to the surf, stratum against stratum, like flights of stairs thrown down from their slant position to the level; in some places severed by faults; in others cast about in every possible direction, as if broken and contorted by a thousand antagonist movements; but in their general bearing rising towards the east, until the whole calcareo-bituminous schists of which this important member of the system is composed disappear under the red sandstones of Dunnet Head. Such, in effect, is the general description of Mr. Sedgwick and Sir R. Murchison, of the rocks in the neighborhood of Thurso. It indicates further, that in at least three localities in the range there occur in the grits and shales, scales and impressions of fish. And such was the ascertained geology of the deposit when taken up last year by an ingenious tradesman of Thurso, Mr. Robert Dick, whose patient explorations, concentrated mainly on the fossil remains of this deposit, bid fair to add to our knowledge of the ichthyology of the Old Red Sandstone. Let us accompany Mr. Dick in one of his exploratory rambles. The various organisms which he disinterred I shall describe from specimens before me, which I owe to his kindness,—the localities in which he found them, from a minute and interesting description, for which I am indebted to his pen.
Leaving behind us the town at the bottom of its deep bay, we set out to explore a bluff-headed parallelogramical promontory, bounded by Thurso Bay on the one hand, and Murkle Bay on the other, and which presents to the open sea, in the space that stretches between, an undulating line of iron-bound coast, exposed to the roll of the northern ocean. We pass two stations in which the hard Caithness flagstones so well known in commerce are jointed by saws wrought by machinery. As is common in the Old Red Sandstone, in which scarce any stratum solid enough to be of value to the workmen, whether for building or paving, contains good specimens, we find but little to detain us in the dark coherent beds from which the flags are quarried. Here and there a few glittering scales occur; here and there a few coprolitic patches; here and there the faint impression of a fucoid; but no organism sufficiently entire to be transferred to the bag. As we proceed outwards, however, and the fitful breeze comes laden with the keen freshness of the open sea, we find among the hard dark strata in the immediate neighborhood of Thurso Castle, a paler-colored bed of fine-grained semi-calcareous stone, charged with remains in a state of coherency and keeping better fitted to repay the labor of the specimen-collector. The inclosing matrix is comparatively soft: when employed in the neighboring fences as a building stone, we see it resolved by the skyey influences into well-nigh its original mud; whereas the organisms which it contains are composed of a hard, scarce destructible substance,—bone steeped in bitumen; and the enamel on their outer surfaces is still as glossy and bright as the japan on a papier-maché tray fresh from the hands of the workman. Their deep black, too, contrasts strongly with the pale hue of the stone. They consist chiefly of scales, spines, dermal plates, snouts, skull-caps, and vegetable impressions. A little farther on, in a thick bed interposed between two faults, the same kind of remains occur in the same abundance, largely mingled with scales and teeth of Holoptychius, tuberculated plates, and coprolitic blotches; and further on still, in a rubbly flagstone, near where a little stream comes trotting merrily from the uplands to the sea, there occur skull-plates,—at least one of which has been disinterred entire,—large and massy as the helmets of ancient warriors. We have now reached the outer point of the promontory, where the seaward wave, as it comes rolling unbroken from the Pole, crosses, in nearing the shore, the eastward sweep of the great Gulf-stream, and then casts itself headlong on the rocks. The view has been extending with almost every step we have taken, and it has now expanded into a wide and noble prospect of ocean and bay, island and main, bold surf-skirted headlands, and green retiring hollows. Yonder, on the one hand, are the Orkneys, rising dim and blue over the foam-mottled currents of the Pentland Frith; and yonder, on the other, the far-stretching promontory of Holborn Head, with the line of coast that sweeps along the opposite side of the bay; here sinking in abrupt flagstone precipices direct into the tide; there receding in grassy banks formed of a dark blue diluvium. The fields and dwellings of living men mingle in the landscape with old episcopal ruins and ancient burying-grounds; and yonder, well-nigh in the opening of the Frith, gleams ruddy to the sun,—a true blood-colored blush, when all around is azure or pale,—the tall Red Sandstone precipices of Dunnet Head. It has been suggested that the planet Mars may owe its red color to the extensive development of some such formation as the Old Red Sandstone of our own planet: the existing formation in Mars may, at the present time, it is said, be a Red Sandstone formation. It seems much more probable, however, that the red flush which characterizes the whole of that planet,—its oceans as certainly as its continents,—should be rather owing to some widely-diffused peculiarity of the surrounding atmosphere, than to aught peculiar in the varied surface of land and water which that atmosphere surrounds; but certainly the extensive existence of such a red system might produce the effect. If the rocks and soils of Dunnet Head formed average specimens of those of our globe generally, we could look across the heavens at Mars with a disk vastly more rubicund and fiery than his own. The earth, as seen from the moon, would seem such a planet bathed in blood as the moon at its rising frequently appears from the earth.
We have rounded the promontory. The beds exposed along the coast to the lashings of the surf are of various texture and character,—here tough, bituminous, and dark; there of a pale hue, and so hard that they ring to the hammer like plates of cast iron; yonder soft, unctuous, and green,—a kind of chloritic sandstone. And these very various powers of resistance and degrees of hardness we find indicated by the rough irregularities of the surface. The softer parts retire in long trench-like hollows,—the harder stand out in sharp irregular ridges. Fossils abound: the bituminous beds glitter bright with glossy quadrangular scales, that look like sheets of black mica inclosed in granite. We find jaws, teeth, tubercled plates, skull-caps, spines, and fucoids,—"tombs among which to contemplate," says Mr. Dick, "of which Hervey never dreamed." The condition of complete keeping in which we discover some of these remains, even when exposed to the incessant dash of the surf, seems truly wonderful. We see scales of Holoptychius standing up in bold relief from the hard cherty rock that has worn from around them, with all the tubercles and wavy ridges of their sculpture entire. This state of keeping seems to be wholly owing to the curious chemical change that has taken place in their substance. Ere the skeleton of the Bruce, disinterred entire after the lapse of five centuries, was recommitted to the tomb, there were such measures taken to secure its preservation, that were it to be again disinterred even after as many centuries more had passed, it might be found retaining unbroken its gigantic proportions. There was molten pitch poured over the bones in a state of sufficient fluidity to permeate all their pores, and fill up the central hollows, and which, soon hardening around them, formed a bituminous matrix, in which they may lie unchanged for more than a thousand years. Now, exactly such was the process of keeping to which nature resorted with these skeletons of the Old Red Sandstone. The animal matter with which they were charged had been converted into a hard black bitumen. Like the bones of the Bruce, they are bones steeped in pitch; and so thoroughly is every pore and hollow still occupied, that, when cast into the fire, they flamed like torches. In one of the beds at which we have now arrived Mr. Dick found the occipital plates of a Holoptychius of gigantic proportions. The frontal plates measured full sixteen inches across, and from the nape of the neck to a little above the place of the eyes, full eighteen; while a single plate belonging to the lower part of the head measures thirteen and a half inches by seven and a half. I have remarked, in my little work on the Old Red Sandstone,—founding on a large amount of negative evidence, that a mediocrity of size and bulk seems to have obtained among the fish of the Lower Old Red, though in at least the Upper formation, a considerable increase in both took place. A single piece of positive evidence, however, outweighs whole volumes of a merely negative kind. From the entire plate now in my possession, which is identical with one figured in Mr. Noble of St. Madoes' specimen, and from the huge fragments of the upper plates now before me, some of which are full five-eighth parts of an inch in thickness, I am prepared to demonstrate that this Holoptychius of the Lower Old Red must have been at least thrice the size of the Holoptychius Nobilissimus of Clashbennie.
Still we pass on, though with no difficulty, over the rough contorted crags, worn by the surf into deep ruts and uneven ridges, gnarled protuberances, and crater-like hollows. The fossiliferous beds are still very numerous, and largely charged with remains. We see dermal bones, spines, scales, and jaws, projecting in high relief from the sea-worn surface of the ledges below, and from the weatherworn faces of the precipices above; for an uneven wall of crags some thirty or forty feet high, now runs along the shore. We have reached what seems a large mole, that sloping downwards athwart the beach from the precipices, like a huge boat-pier, runs far into the surf. We find it composed of a siliceous bed, so intensely compact and hard, that it has preserved its proportions entire, while every other rock has worn from around it. For century after century have the storms of the fierce north-west sent their long ocean-nursed waves to dash against it in foam; for century after century have the never-ceasing currents of the Pentland chafed against its steep sides, or eddied over its rough crest; and yet still does it remain unwasted and unworn,—its abrupt wall retaining all its former steepness, and every angular jutting all the original sharpness of edge. As we advance the scenery becomes wilder and more broken: here an irregular wall of rock projects from the crags towards the sea; there a dock-like hollow, in which the water gleams green, intrudes from the sea upon the crags; we pass a deep lime-encrusted cave, with which tradition associates some wild legends, and which, from the supposed resemblance of the hanging stalactites to the entrails of a large animal wounded in the chase, bears the name of Pudding-Gno; and then, turning an angle of the coast, we enter a solitary bay, that presents at its upper extremity a flat expanse of sand. Our walk is still over sepulchres charged with the remains of the long-departed. Scales of Holoptychius abound, scattered like coin over the surface of the ledges. It would seem—to borrow from Mr. Dick—as if some old lord of the treasury, who flourished in the days of the coal-money currency, had taken a squandering fit at Sanday Bay, and tossed the dingy contents of his treasure-chest by shovelfuls upon the rocks. Mr. Dick found in this locality some of his finest specimens, one of which—the inner side of the skull-cap of a Holoptychius, with every plate occupying its proper place, and the large angular holes through which the eyes looked out still entire—I trust to be able by and by to present to the public in a good engraving. There occur jaws, plates, scales spines,—the remains of fucoids, too, of great size and in vast abundance. Mr. Dick has disinterred from among the rocks of Sanday Bay flattened carbonaceous stems four inches in diameter. We are still within an hour's walk of Thurso; but in that brief hour how many marvels have we witnessed!—how vast an amount of the vital mechanisms of a perished creation have we not passed over! Our walk has been along ranges of sepulchres, greatly more wonderful than those of Thebes or Petræa, and mayhap a thousand times more ancient. There is no lack of life along the shores of the solitary little bay. The shriek of the sparrow-hawk mingles from the cliffs with the hoarse deep croak of the raven; the cormorant on some wave-encircled ledge, hangs out his dark wing to the breeze; the spotted diver, plying his vocation on the shallows beyond, dives and then appears, and dives and appears again, and we see the silver glitter of scales from his beak; and far away in the offing the sunlight falls on a scull of seagulls, that flutter upwards, downwards, and athwart, now in the air, thick as midges over some forest-brook in an evening of midsummer.
But we again pass onwards, amid a wild ruinous scene of abrupt faults, detached fragments of rocks, and reversed strata: again the ledges assume their ordinary position and aspect, and we rise from lower to higher and still higher beds in the formation,—for such, as I have already remarked, is the general arrangement from west to east, along the northern coast of Caithness, of the Old Red Sandstone. The great Conglomerate base of the formation we find largely developed at Port Skerry, just where the western boundary line of the county divides it from the county of Sutherland; its thick upper coping of sandstone we see forming the tall cliffs of Dunnet Head; and the greater part of the space between, nearly twenty miles as the crow flies, is occupied chiefly by the shales, grits, and flagstones, which we have found charged so abundantly with the strangely-organized ichthyolites of the second stage of vertebrate existence. In the twenty intervening miles there are many breaks and faults, and so there may be, of course, recurrences of the same strata, and re-appearances of the same beds; but, after making large allowance for partial foldings and repetitions, we must regard the development of this formation, with which the twenty miles are occupied, as truly enormous. And yet it is but one of three that occur in a single system. We reach the long flat bay of Dunnet, and cross its waste of sands. The incoherent coils of the sand-worm lie thick on the surface; and here a swarm of buzzing flies, disturbed by the foot, rises in a cloud from some tuft of tangled sea-weed; and here myriads of gray crustaceous sand-hoppers dart sidelong in the little pools, or vault from the drier ridges a few inches into the air. Were the trilobites of the Silurian system,—at one period, as their remains testify, more than equally abundant,—creatures of similar habits? We have at length arrived at the tall sandstone precipices of Dunnet, with their broad decaying fronts of red and yellow; but in vain may we ply hammer and chisel among them: not a scale, not a plate, not even the stain of an imperfect fucoid appears. We have reached the upper boundary of the Lower Old Red formation, and find it bordered by a desert devoid of all trace of life. Some of the characteristic types of the formation re-appear in the upper deposits; but though there is a reproduction of the original works in their more characteristic passages, if I may so speak, many of the readings are diverse, and the editions are all new.
It is one of the circumstances of peculiar interest with which Geology at its present stage is invested, that there is no man of energy and observation who may not rationally indulge in the hope of extending its limits by adding to its facts. Mr. Dick, an intelligent tradesman of Thurso, agreeably occupies his hours of leisure, for a few months, in detaching from the rocks in his neighborhood their organic remains; and thus succeeds in adding to the existing knowledge of palæozoic life, by disinterring ichthyolites which even Agassiz himself would delight to figure and describe. Several of the specimens in my possession, which I owe to the kindness of Mr. Dick, are so decidedly unique, that they would be regarded as strangers in the completest geological museums extant. It is a not uncurious fact, that when the Thurso tradesman was pursuing his labors of exploration among rocks beside the Pentland Frith, a man of similar character was pursuing exactly similar labors, with nearly similar results, among rocks of nearly the same era, that bound, on the coast of Cornwall, the British Channel. When the one was hammering in "Ready-money Cove," the other, at the opposite end of the island, was disturbing the echoes of "Pudding-Gno;" and scales, plates, spines, and occipital fragments of palæozoic fishes rewarded the labors of both. In an article on the scientific meeting at York, which appeared in "Chambers' Journal" in the November of last year, the reading public were introduced to a singularly meritorious naturalist, Mr. Charles Peach,[9] a private in the mounted guard (preventive service), stationed on the southern coast of Cornwall, who has made several interesting discoveries on the outer confines of the animal kingdom, that have added considerably to the list of our British zoöphites and echinodermata. The article, a finely-toned one, redolent of that pleasing sympathy which Mr. Robert Chambers has ever evinced with struggling merit, referred chiefly to Mr. Peach's labors as a naturalist; but he is also well known in the geological field.