GEOLOGY OF SCOTLAND.PART I.
CHAPTER I.INTRODUCTORY.
Geology is that branch of science which comprehends the knowledge of all that relates to the form, structure, mineral and fossil constituents, of the earth. The Scottish Grampians, it is generally admitted, form part of the lowest sections of its crust, to which the researches of geologists extend. We must go to other countries for any coeval, and, to North America for any older, competing land: and still, there, the rocks are of the same mineral qualities and arrangement. The Ben-Mac-Dhui group form the highest and most prominent masses in the whole range of these crystalline mountains.
When I first stood on the broad flat top of Ben-Mac-Dhui, I had no thought or purpose of ever recording its geological history. The excursion was undertaken simply for recreation, and a delightful one it proved. I longed to plunge into the deep recesses of the old forest, and to see the trees which nature had strewed with careless hand, ere, perhaps, Caledonia was tenanted by the human family. I looked down from its rugged sides, as I ascended, with awe and wonder—snatched a little alpine as I drewbreath for the next spring—chipped a piece of granite as I obtained a footing over a yawning chasm, or breasted along by jagged precipitous defile,—and when, having fairly scaled the summit, I gazed out upon the world beneath, the feeling which for a moment flitted across my mind was one of no merely vain complacency, that I was then the most elevated subject of all the twenty-six or twenty-seven millions inhabiting the British Islands—and the lowest, too, in their stony regions! The mountains of the earth serve to inspire some of the loftiest sentiments that can fill the breast of its intelligent inhabitants. Imbosomed in their deep solitudes, man feels his own littleness, and is forced to inquire who made these wonders, and who sustains them? We are all the better, morally speaking, for leaving occasionally our daily-trodden haunts, where we see only human things, and hear only of the triumphs of human craft, the excitement of human passions, the littleness and vanity of even the noblest human daring. There is an image of Jehovah’s greatness impressed upon the outward face of nature, which for a time will awaken and sustain the most salutary reflections, breathing, as it were, a new life into the soul of the wayfarer. A man escapes from himself, forgetting the burden of a thousand petty cares, and rising above his sensual condition, when he looks upon the physical world in these its grander features and secluded scenes, which irresistibly speak to the inner sense of divinity, wisdom, and omnipotence.
The philosophy of the mountains, in the classic ages of Greece and Rome, inclined but little to any analysis of their grosser materials of earth and stone. The poetic and ideal were exclusively associated with their structures and form. The dii majores dwelt upon, and thundered from, their lofty summits. The clouds hovered in peaceful majesty over their council of sage or fierce debate. The elements were the ready ministers of their will; and Oreads, Dryads, and Naiads, peopling all the hills, forests, and streams, were the creations of that principle of the inner man, which has always searched for the spiritual behind and beyond the tangible attributes of Nature. Hence, too, the gnomes of the caverns, the spirits of the mists, the fairies of the glens, the kelpies of the torrents, were all the embodiment of forms, which fancy, in her later superstitions, has cast around the mountain landscape,with the witchery at once of the terrible and beautiful. The charm that spell-bound the human mind for ages, is not dissolved when, with ruder intent, we traverse these rocky solitudes, listening to the echo of our obedient hammer, learning the secrets of the universe amidst the voices of the everlasting hills, and seeing the wonders of the material world throwing light on the wonders of the spiritual.
We are reminded, among the mountains, of one of the first and loveliest of all material things, the creation of light. Loving them for their own sakes, as well as for the legends of the old world with which they are everywhere inscribed, the geologist takes to the hills with the first faint fresh streak of dawn. Emerging with earliest day from the somber shades of the forest which, like night, invests the prospects with its own sadness and gloom, speedily a scene of joy and activity bursts upon the sight. The light comes upon you like a real tangible thing. You see it glinting and breaking on the lofty ridge, then nearing down along the brown slope of the mountain, here projecting in long bright lines through the trees, and there—delicious, golden morn! first-born of Nature’s children, harbinger of life and gladness. How beautiful are thy first footsteps upon the heath-clad mountains! What a brood of gloomy thoughts thou dispellest, chasing them before thee, like yonder envious mists rising lazily from the plains, valleys, and streams, which they would fain hide from the eyes that now revel amid their exuberant loveliness. These lofty peaks are worthy altars for the beacon-fires of the orb of day, after he has finished his journey through the nations; and comes back to us, over the floating splendor of the sea, in the eastern heavens. And see! he hath lit a hundred on these splintered summits, which blaze now as they blazed centuries ago, and diminish not!
The view from this remarkable group of mountains—the most remarkable by far in the island—differs much from any other with which I am acquainted. The impressions at first are all very confused, and some time is required to resolve into distinct pictures the wondrous panorama before you. We have stood upon Skiddaw, where everything is clear, distinct, and palpable in distance and form; on Ben Lomond, where the far-stretch of perspective over lakes, rivers, and plains, is like a first lesson inpainting; on Ben Lawers, where the eye sweeps rapidly over well known, familiar objects, spots of wood, glen, and mansion; on Ben Nevis, where you fancy yourself in mid-air, every object is so separate and apart, and so disposed the whole you are looking on, that the view is alldownwardupon the picture. But here, these dark giant masses crowd as it were against you. There is a struggle for the post of elevation. You are highest, no doubt of that; but so jealous all are these proud somber peaks, that every one seems to overlook, though yet actually beneath, the broad ample table-head of the center of the group. Sometimes one is tempted to leap across the narrow dells of separation, and at once master the geology of the district, so near seems every hill-top as almost to be touched. But as you approach their several positions, expanding valleys, deep fathomless chasms, and the channels of noble rivers, bar farther approach, and attest the wide, independent domains of each. They are monarchs every one of them—Brae-Riach, Cairn-Toul, Cairn-Gorm, Ben-Avon, Ben-y-Bourd—each holds his own regal court, over tarn, lake, and stream; torrents, cataracts, and all the appurtenances of the boldest mountain scenery.
After one has time to gather up his thoughts and perceptions, the scene resolves itself, still indeed as of one whole, but of distinct component parts. In the far distance you attempt in vain to number the peaks that everywhere rise against the sky line; but more closely around, five or six summits are seen to spring from a single root; a common circumference marks out the limits of the group; and, by no unreasonable liberty with the imagination, you easily replace the old materials into the vacant interstices, before the water had begun its work of abrasion, or the earthquake coming to its assistance shivered their solid rounded forms into these hideous, precipitous gorges and chasms. The great hills here stand, every one of them, upward of four thousand feet above the level of the sea; and when entire, one aggregated whole, as possibly they originally were, the center mass may have towered thousands more into the overhanging firmament. The scene is utterly unmatched, as it cannot be described, by any other in Great Britain: and make your ascent when you may, there are sights and objects to be met with at every step, in every salientdell, that will cause you evermore to rejoice you commenced your travels among the Aiguilles of Ben-Muich-Dhui.
It is in the great mountain groups that the true key is to be found to the science of geology, as well as all those collateral circumstances which impart so much charm to it as a healthful and invigorating exercise to mind and body. Here, amidst these piled-up masses, we are furnished with the lowest ascertained sections of the earth’s crust, from which we can at once study the nature of its rocky divisions, and the laws which prevail in the order of their superposition. When the world was in its primeval state of chaos, without form and void, we are warranted to assume that the mountains as yet had no place on its surface, but subsequently arose out of the bosom of the deep; and lifted up, as they emerged above the waters, the rocky strata already enveloping the globe. These strata are still to be seen folded round the central masses, disrupted and torn like a garment too tight for the body, and displaying through innumerable cracks and fissures the inclosed rocks. This fact lies at the foundation of all geological inquiries, gives to the subject all its pretensions as a science, and before proceeding on our “Course” a word of explanation will be in place.
The first condition of the earth, of which we have any historical notice, is that which is represented in Genesis, where, after the initial declaration that God was creator of all things, we are told of a period when the whole of its materials were as yet unarranged, “and darkness was upon the face of the deep.” The Divine Spirit moved upon the surface of the shapeless mass, when the various elements of air, earth, and water gradually assumed their respective positions. The form which the earth had impressed upon it, as philosophy has demonstrated, was that of a spherical body, flattened at the poles, a figure resembling as nearly as possible that of an orange. There is reason to believe, therefore, that every part of the solid mass of earth is symmetrically arranged, and that every individual particle occupies the position which Divine wisdom has assigned it.
Rocks, let the reader be assured, have not been indiscriminately heaped together. Everything here, amidst all the apparent confusion which surrounds us, is in the most perfect order, followingone uniform law of superposition. When God fixed the foundations of the earth, stretched his compass “upon the face of the deep,” and laid “the beams of his chambers in the waters,” he completed the mighty edifice agreeably to the plan which he had determined upon “from the beginning:” the different portions of the building rise one above another in regular succession; and the work, so far as we can survey the interior, displays the severalcoursesinto which the materials have been thrown. These constitute what geologists call the strata of the earth, layers of varying thickness, such as our slates, sandstones, and limestones exhibit, and which nearly envelope the circumference of the globe. The order in which the strata are disposed is uniform from below upward, and this order is never inverted. From the blue slates of the Grampians to the Chalk cliffs at Dover, there is a regular succession of intermediate rocks, piled one upon another like the mason-work of our houses; and while to many there appears nothing but confusion, to the scientific eye every portion of the series, although the same ingredients enter into several classes of rocks, is as well defined and as easily recognized, as the two members at the extreme points are by the common observer.
But beside the stratified rocks, there is another class of rocks equally extensive, and which occupy an important place in the economy of nature. These are the granites and whinstones of which the highest mountain ranges are usually composed. There are many subordinate varieties belonging to both classes, which are characterized by slight shades of texture and composition and distinguished by different names. One thing is common to the members of each group. They are not disposed in layers, and exhibit no lines of stratification, except in the granite rarely, throughout the entire mountain chain. These rocks occupy no fixed place in the order of superposition, but seem to be intruded in the most irregular manner among the stratified rocks, separating one bed from another, filling up fissures and rents; and binding and interlacing the various deposits more closely and firmly together. They are often composed of the fragments of other rocks, agglutinated into a compound mass by a base of clay. Remarkable changes are also produced upon all the strata where they come in contact with granite and whinstone—chalk beingconverted into crystalline limestone—limestone into chert—clay and sandstone into a substance as hard and compact as flint—and coal is deprived of its bitumen or the quality which renders it so useful as a combustible body.
From these, and other appearances, geologists have been led to the conclusion, that these rocks are of later origin than those which are stratified, that they have been injected among them in a state of fusion; and by the expansive force of internal heat, that they have burst through the stony crust of the earth, and elevated and disrupted the strata which compose it. They are, if we may use the expression, theleverswhich the Almighty has employed in bringing up the lower deposits to the surface, in laying open the interior chambers, and in producing all that infinite variety in our earthly habitation which ministers to the comfort and well-being of man. Much seeming confusion and disturbance mark everywhere the course of these rocks, similar, though upon a more extensive scale, to the disorders attendant upon the irruption of a modern volcano; but throughout the whole there reigns such a harmony of purpose, that the conclusion is irresistible, these operations could only have taken place by Divine permission, and are in accordance with the Divine plan, controlling the most refractory agencies of nature, and causing them to contribute to the general good.
These eruptive rocks have been produced under the sea, at a period, many of them, when the waters and the dry land were not as yet separated from each other. They are therefore termed sub-aqueous products, and are, in consequence of the pressure to which they have been subjected, hard, compact, and heavy. They differ in this respect from the products of modern volcanoes, which are light and porous, as being formed under the simple pressure of atmosphere, and are denominated sub-aerial. The most prevailing ingredient both in ancient and modern lavas is feldspar: this, combined with hornblende, quartz, and augite, characterizes the whole of the two families of the trap and granitic rocks; and completely establishes their claim to be regarded as originating in submarine volcanoes. Geology is thus in its first step, and initial principles, in perfect accordance with the scripture record; and, in walking over the varied fields of creation, we shalltread all the firmer, and enjoy our recreations all the more, that we find the word and works of God illustrative of each other, revelation never contradicted, and science bearing enlightened testimony to the wonderful truth—that the hills melted like wax before the Lord.
Two reasons, therefore, are to be assigned for the starting point of our investigations, and the route fixed upon in following them out. This center group of mountains comprises the first or lowest phenomena connected with the science of geology: here the earliest lessons are inscribed; and here, developed on a great scale, we are presented with the axis of elevation which has given character and outline to the whole surrounding district. Ben-Mac-Dhui is the most prominent type of our primary mountains, and has been mainly instrumental in lifting up a large portion of the Grampian range. Looking abroad from its summit, over all that varied landscape of plain and valley, and further than the eye can reach, summoning in imagination before us the successive strata as they recede in the far distance, a diagram which would faithfully represent the order of the rocks and their relation and proximity to the granite, would be quite correct in making Ben-Mac-Dhui a pyramidal basis, and the other formations as steps to the apex of the pyramid.
This lofty chain of primary rocks on the one hand, and the Alpine region of Switzerland on the other, may likewise be regarded as constituting the barriers or edges of one great basin, within which are inclosed members of almost every rock formation, fossiliferous as well as non-fossiliferous, existing anywhere on the face of the earth. Along the line of tour indicated, you pass over every intermediate deposit, from below upward; and have laid before you, for inspection, specimens of all that is interesting and curious in the science. Betwixt the two points, selected as our termini, lie strata upon strata, organic bed upon bed, not piled up in one colossal mass, but drawn out and slipped over the edges of one another, and so arranged and disposed at successive intervals as most happily to suit the convenience and successive stages of the journey. This is one of the most remarkable facts in descriptive geology, whereby we learn that a depth of nearly ten miles of solid rock can be duly examined, every particle andfossil of it, not by perforationdownwardto the bottom, but by the natural inclination of the beds, and their several outcrops rising to the surface like the inverted tiles on a roof. In consequence of this persistent arrangement, objects, both new and strange, will at every step meet the view. There the whole system of geology, page after page, is spread out before you. Every day opens up a new chapter geographically, as well as mineralogically divided. And when you have gained the summit of Mount Blanc, you can leisurely, in the mind’s eye, look back over the wholeCourse of Creation.
It is a reproach, I am aware, sometimes cast upon geological researches, that the portion of the earth’s surface exposed to view is as nothing compared with the entire mass, and that another portion, by far the largest segment, is concealed by the ocean, and its own debris. In addition to these disadvantages, it may now be objected that the line of description indicated narrows the field of research still farther, and that a few disconnected materials only are all that can therefrom be extracted. It may be answered,—“That the earth is constructed with such a degree of uniformity, that a tract of no very large extent may afford instances, in all the leading facts, that we can ever observe in the mineral kingdom. The variety of geological appearances which a traveler meets with, is not at all in proportion to the extent of country he traverses; and if he take in a portion of land sufficient to include primitive and secondary strata, together with mountains, rivers, and plains, and unstratified bodies, in veins and in masses, though it be not a very large part of the earth’s surface, he may find examples of all the most important facts in the history of fossils.”[1]We shall, however, along with our lineal descriptions of the mineral kingdom, notice the occurrence, position, and fossil contents of the strata as represented in other parts of the world.