CHAPTER IV.
THE DEVONIAN SYSTEM, OR OLD RED SANDSTONE.
A geologist requires not, like the tourist, to be told of the various conflicting roads that run among the mountains, in what precise course he is to wend his way. He will follow his own pathways, roads of nature’s forming, guided by the strike and lie of the rocks rather than by the beaten tracks of every-day life. But come whither he will—through Glentilt, Glenericht, Glenbeg, and the Spittal, Glenisla, and Clova,—or along the Dee, the heights of Glentanner, and penetrating to the sources of the Esks—sure we are, when he reaches by any of those passes the frontiers of the Grampians, he will pause and gaze wistfully, thoughtfully, admiringly, ere he descends, upon the magnificent prospect that stretches before him, unrivaled by any on the terraqueous globe. The Gran-pen, celticé, the shelvy or precipitous summit, Romanized into Grampius, has its own inner charms, peaceful rock-girt valleys where princes dwell, and happy as Rasselas ever trod.—And escaped from these, what an outer world beneath, fertile, abundant, replete with everything that can charm the eye or interest the student. Looming in the far distance, the Lammermuirs, of silurian origin, can just be descried as a dark-blue line on the verge of the horizon; the Ochils and Lomonds, of carboniferous age, repose like islets on the pendant sky; while, in the foreground of the picture, there is the most charming variety of woodland, meadow, farmstead, town, and mansion, all as I now gaze upon them in their autumn coloring, invested with a Claud-like mellowness that speaks with a moral yet romantic sympathy to the heart. The round tower of Brechin, the moldering walls of Edzell, the frowning battlements of Glammis, the worn-out and now verdant ramparts of Dunsinane, have each their crowds of visitants, and are all within the compass of a single day’s journey.
The eye of the geologist is in search of another object as it wanders over that lovely scene: Kinnordie, the birth-place of Sir Charles Lyell, must ever be classic ground in the history of our science. It rests on the old red sandstone, and furnishes some of the most valuable illustrations in Sir Charles’s early sketches. What influences, may we here ask, gave being and shape to the ingenious and splendid generalizations of this accomplished geologist? Is it too much to assume that the philosopher, as well as the poet, is all his life-long captive to first impressions, that the scenes of his boyhood claim “a local habitation” for many of his future speculations, and that his most matured trains of thinking have been dependent upon casual circumstances? Born and educated in the shadow of the Grampians, who can doubt that the spirit within was early stirred to lofty views as he gazed upon their elevated forms, and wondered how their peaks rose so high in air, and were thus lifted above the valleys? May it not be presumed, though the philosopher himself may have no recollection of the matter, that his speculations regarding the alternate elevation and depression of land and sea had its germ in some such happy moment of mountain inspiration? Byron owned the influence in all its power, when, in the rocky defiles and dark pine forests of Lochnagar, he had early communings with spiritual beings, the wreathe-forms and kelpies of the streams; and in visions imparted amidst the wilds of the Dee, prepared his mind for the daring flights of the Alps. The geologist had here all the materials of after-thought, which in his various essays and works he has so skillfully expanded—from his explorations of Bakie-loch with its alluvions, peat, marl, shells, and horns, in which he had the type of some of his Alpine tertiaries—the old canoe and ripple-mark here too, the representatives of their far-sundered ages and onward to his bold speculations on the elevatory hypothesis, of which the Grampians, as well as Sidlaws, supplied him with ample illustrations.
The descent from the mountains upon the series of rocks that occupy the plains, is one not merely of space, but likewise of time. A geological epoch has vanished, and a new order of things has been called into existence. This implies a change in the animal as well as in the mineral kingdom. The change may not have been sudden, but it has been thorough and pervading, accompanied by circumstances that show a general shift in the sea-bottom, and causes that have been nearly uniform in their operation over the surface of the globe. The shift in the sea-bottom is detected in the elevation of the silurian group of rocks, which have been lifted from a horizontal into a highly-inclined position: in some instances they are nearly vertical; and in most cases where the igneous rocks occur, they are bent and twisted, greatly altered and disrupted, by the process of upheaval to which they have been subjected.
Geology notes in this an epoch or age of organic existence. The superjacent series of rocks are seen lying unconformably upon the silurians, that is, the older series had been consolidated and upheaved, and a period of intervening time had elapsed before the deposition of the newer. The fossils imbedded are likewise distinct and peculiar—one and the same over the superficial area of the globe—and thus we learn to mark the great and interesting cosmical changes which had already begun to be effected. We are now among the Old Red Sandstone, or Devonian system of rocks, so denominated from their great development in that district of the sister kingdom.
As contrasted with the former system, the rocks of this period indicate considerable disturbance in the waters of the ocean, currents and agitations widely prevailing, and perhaps also deeper seas. The crust of the earth was still rising, and the mountains becoming higher, and these effects would necessarily follow. A superior order of animals were introduced. The fishes, which begin to appear in the upper beds of the silurian group, are now increased both in numbers and in variety of structures. The invertebrata were the prevailing types of the former age. The old red sandstone is pre-eminently characterized by the vertebrata, when, completely adapted to the element to be inhabited, mailed and plated over with thick horny scales, huge bony heads, fins and tails of corresponding strength and size; the Sauroid family appear upon the stage, capable all of buffeting the waves and fulfilling their destiny amid the greatest commotions. The fish of this early period are generally well preserved, even better than those of the tertiary age, in consequence of their osseous scales being harder than the bones, and which, from their interlocked arrangement, have contributed to preserve the general form of the body when the inner skeleton has disappeared and every other part and organ have been destroyed.
The old red sandstone formation is very extensively distributed in the northern counties, forming a great belt round the coast from Caithness-shire to Aberdeenshire, and consisting of three well-marked divisions, the lower, middle, and upper series of beds. The strata flank the northern walls of the Grampians and their out-liers, traversing the great central or Caledonian valley for a hundred miles, and training round the western coast by Oban, the shores of Mull and Morven. They are of great thickness in many places; and in some of the beds, as at Cromarty, Lethen-bar, and Gamrie, contain nearly all the fossils peculiar to the formation.
The order of Ganoid fishes, which afterward fulfill so distinguished a part in the kingdom of nature, is wholly absent from the silurian group, while, in the Devonian, nearly thirty genera, and considerably above sixty species, have been described and named. The scales of these creatures would appear to have been richly ornamented, enameled, and shining, and hence the term Ganoid applied to the order. In the northern districts, beyond Ben Mac-Dhui, the following genera, with several species belonging to each, have been found, namely, coccosteus, cheiracanthus, cheirolepis, dipterus, diplopterus, diplocanthus, glyptolepis, osteolepis, pterichthys. The principal localities of these fossils are—the Dipple on the Spey, Tynet Burn in Banffshire, Seat-Craig near Elgin, Altyre on the Findhorn, Clune, and Lethen-bar in Nairnshire, Gamrie, Cromarty, and various places in Sutherland and Caithness. Shetland is chiefly composed of the old red sandstone, which yields abundantly the fossils peculiar to the deposit. The formation extends through the Orkney islands, inexhaustibly fertile in organic remains, and among which have been found plates and fragments of the Asterolepis, the largest of all the genera belonging to the period: the head and jaws, at least, appear to have been of enormous dimensions, and portions of the inner skeleton must have been bony, contrary to the general cartilaginous structure of the class. The Placoids of the subjacent rocks have many resemblances to the cestracions, centrinæ, and spinaxes of our present seas, their scales being set like plates at irregular distances over the body. The Ganoids, on the other hand, whose scales were continuous, and enveloped the entire animal, have no affinities to any living types.
Specimens of vegetable organisms are very common in some of the flagstones of Orkney, resembling, in some instances, the Lycopodiaceæ, or club-mosses, so abundant in the carboniferous strata: and branching fucoid plants, of which portions have been found from two to three feet in length, and of nearly the same diameter of stem throughout. But in tracing the course of creation in this department of her works, the most important fact to relate is, the discovery of a coniferous lignite, imbedded in the old red sandstone of Cromarty. This interesting relic was obtained from these beds, several years ago, by Mr. Miller; and, though still of that remote age an instantia solitaria of its kind, like the foot-print of Robinson Crusoe, it is the sure token of a race that inhabited the island, and harbinger of a luxuriant flora then waving along the shores of the boundless waters. These northern localities, on the mainland, as well as in the islands, are also remarkable for their shell-beds in this deposit, while very few of such organisms have yet been detected in any of the Scottish rocks of the system to the south of the Grampians. The relics are confined to one species of shell, resembling in general appearance the form of the Cyclas, and are found in various quarries in the district.
What a revolution in letters, knowledge, and civilization since the days of the Romans! This, their Ultima Thule! and a science in the very rocks of which they never even dreamed. Proud they were of their fabled origin from the twin boys suckled by the wolves. Here are the spoils of ages long anterior to their myths of remotest genealogy—families of creatures that had fulfilled their destiny—buried in the sand, and upheaved into lofty mountains, while the Seven Hills of their proud city slept beneath the waves.