I. The structure of the Sidlaw and Ochil ranges, from the amorphous character of the rocks, furnishes little or no room for geological description. These nearly parallel chains of hills, separated only by an interval of from two to five miles, and forming the lower water-shed of the basin of the Tay, consist of the various members of the trap family usually denominated whinstone, and whose structure is very different, upon a glance, from that of the sandstones and other sedimentary deposits we have been considering. This class of rocks have all a tendency, in mineralogical phraseology, to a spathose structure, and discover at least the rudiments of crystallization: there is no lamination in their internal texture, and the lines of stratification which they sometimes exhibit are assumed, or impressed by the previously consolidated strata among which they have been injected. They are not lavas, which are sub-aërial products, nor are they aqueous formations, whose materials have been deposited in water. These rocks are the results of igneous fusion deep under the crust of the earth, poured over the bottom of the sea, and protruded into the diversified dome-shaped forms which they generally present.
Trap-rock consists of several varieties, as porphyry, clinkstone, compact feldspar, amygdaloid, greenstone, and basalt. These all pass into each other by insensible gradations, often forming one continuous mass, for the most part composed of the same ingredients, and have in consequence been regarded by geologists as belonging to one group, produced under similar circumstances, and elevated at intervals about the same period. The porphyritic structure prevails generally in both chains, and “porphyry has the peculiarity of being rarely found in any but the primary strata: it seems to be the whinstone of the Old World, or at least that which is of highest antiquity in the present.”[2] But not only are both ranges characterized by the same qualities and texture of rocks in hand-specimens, one hill answering to another; they also preserve the same general features of outline, and the same relations to the disrupted sandstones among which they have been injected. The highest point, for instance, in the eastern division of the Ochils, is Norman’s Law, attaining an elevation of nearly one thousand feet above the level of the sea: in its uprise the mass has brought along with it the lower beds of the gray sandstone, which flank its northern and eastern sides, within three hundred feet of the summit. To the north and west of Dundee the highest points of the Sidlaws are encompassed in like manner with their analogous beds of the gray rock. And so in every locality, whether along their base lines, or among the numerous ravines and valleys that intersect their cultivated slopes, the strata may be seen cropping out, bearing testimony to the convulsive movements to which they have been exposed, and the altered positions they have in consequence assumed.
A remarkable bed of conglomerate or tufaceous trap intersects the chains at different, but generally corresponding, points of elevation, varying from two to four hundred feet above the sea-level. On the Ochil side this bed crosses the chain of hills from Letham school-house to Lindores Loch, where, along the line of the Edinburgh and Northern Railway, the out-crop may be observed at various places—very interestingly on the western slope of Clatchart—and again appearing at intervals toward Abernethy, whence it is traceable through the glen. In the ravines of the Sidlaws, behind Rossie Priory, in the den of Pitroddy, on the face of Kinnoul and Moncrieffe Hills, and across the ridge intersected by the Perth tunnel, indications of the same tufaceous bed can be traced, consisting, for the most part, of quartz rock, schist, and rounded masses of the different varieties of the trap, mixed not unfrequently with bowlders and smaller pieces of the gray and red sandstones. This formation has, doubtless, been produced on the bottom of a troubled sea, where the crust has been exposed to violent action—much of it comminuted and broken into fragments, rolled and fashioned into nodules, large portions of it torn up, but retaining their continuity for a space—when the molten flood has poured from below, and diffusing itself through the mass, the whole, after successive eruptions, has been lifted to its present elevation.
II. The amygdaloidal portion of these hills forms an interesting feature, and prevails very widely in both chains. This rock has a conglomerated stratified appearance in some places; but generally the matrix is very compact, rather porphyritic, of a dark brown or greenish hue, and when exposed to weathering, the innumerable small cavities or vesicular tissue by which it is laminated are prominently exposed. These cavities are filled with zeolites, carbonate of lime, chalcedony, prebnite, and various other crystalline silicious deposits. The green hue is derived from the decomposition, on the exposed surfaces, of the imbedded substances. This rock forms the true habitat of the richest and most beautiful specimens of the agate and jasper family, of the purest Scottish pebble, and of large sparkling geodes of amethystine crystals. The agates of Kinnoul and Moncrieffe are prized by lapidaries, as they are admired by amateurs, and no mineralogist should fail to visit the romantic pass of Glen Farg—illustrated by the classic pens of Galt, Lauder, and Scott—adorned and stored, in every niche of its serpentine course, with calc-spars, analcime, chabasie, stilbite, heulandite, konilite, and the entire family of the zeolites, presenting often fasciculi of crystals several inches in length, thin as silken threads, and rivaling frost-work in the transparency and brilliancy of their texture. The mass of rock constituting Bein Hill, and intersected by the turnpike for miles, appears as a simple agglutination of nodules of the size and color of garden peas, and consisting principally of analcime, zeolite, and chalcedonic pebbles.
What account is given of these curious formations—of their color, structure, and qualities—all so different from those of the surrounding matrix? Assuming the igneous origin of the trap family of rocks, and against which there can scarcely exist the possibility of an argument, it is supposed that, when in the act of cooling, cavities would necessarily be produced in the heated molten mass by the expansive power of gases, and that upon their escape silicious and other deposits would be formed in the empty spaces. All the ingredients of the included crystals, of every genus, are plentifully diffused through nature, mixed up with the matter of every kind of rock; air and water are nowhere wanting, and substances sufficiently porous, for their transmission; a lamination or separation of coating, layer upon layer, is discernible in every agate; while the still partially existing hollows in some nodules, and the concentric nature of the bands of earthy matter which lines their surface, clearly demonstrate the deposition of the outer prior to that of the inner layer, and prove that at the very time when the crystallization had commenced, the cavities had assumed the form and shape which they now retain. Sometimes, too, the nodules have a compressed or flattened appearance; and the explanation in such cases is, that the cavities, if formed during the cooling of the beds, must have been altered in their shape by pressure either before the deposition of the silicious matter, or during the successive formation of the layers. Other, and indeed many, theories are broached, among which the most plausible is, that the cavities in which the agates are now found were caused by the “molecular aggregation of the silicious particles compelling the surrounding matter to yield in proportion to the attraction of these homogeneous particles.” The former explanation, however, is the most generally adopted, the most obvious in its conditions, and the most accordant with the existing processes of nature, the laws of heat, and the order of crystallization. The porosity and fibrous structure of agates, consisting of a congeries of minute radiating fibers at right angles to the rings or concentric layers, have also been established from microscopical examination, and hence the diversity of their colors, whether from vegetable matter or metallic oxides everywhere so abundant in the soils and crust of the globe.
The same law or mode of formation applies to crystallized minerals generally, and has continued to operate from primitive times to our own in their production. The sparkling topazes of Cairngorm and gigantic crystals of the Alps—the semi-opal of Iceland and the heliotrope of Kinnoul—the dazzling emeralds of Brazil and Ethiopia—the stupendous garnets of Fahlun—the delicately-colored fluors and calc-spars of Derbyshire and Cumberland—the gorgeous rubies and sapphires of India and Ceylon—the beautiful prismatic idocrase of Vesuvius and Etna—the splendid amethystine geodes of Oberstein, Siberia, and Spain, little grottoes lined with polished geometrical figures, all declare a common birth as they all nestle in rocks of a common origin. The diamond, the richest as it is the rarest of all, belongs to a totally different class of crystallized bodies, and owes its formation to the agency of entirely different causes.
III. The dykes or veins form another striking feature among the geological phenomena of these hills, and seem as if nature intended them for lacings or bands to give greater cohesion and stability to its parts. They consist of long narrow strips of rock, which have made their way through the previously consolidated strata, intersecting the planes of their several beds at nearly right angles, and constituting among themselves a system of parallel and vertical partitions in the rock. Once observed in any district, these dykes are of too marked a character not to excite inquiries as to their uses and mode of formation; and occurring, as they do, in every region and among all classes, from the oldest primary to the newest tertiary deposits, they are obviously designed for some great purpose in the plan of Creation.
Veins may be described as tabular masses that penetrate the earth’s crust to an unknown depth, and almost invariably consist of different materials from the rocks they traverse. They are supposed to have all been in a state of fusion, and either themselves to have produced rents and fissures in their pressure upward, or to have filled with their molten ingredients such as from other causes were already existing. When detected in sandstone or other stratified formations, they are readily distinguished, and acknowledged to be of foreign origin as well as of posterior date. The matter of them consists generally, among the secondary formations, of basalt or greenstone; more frequently of porphyry and feldspar among the older and crystalline rocks. In their passage through whinstone the sides of the veins are usually smoothed and polished as if by the action of another body rubbing against them; the sandstones and other sedimentary rocks are indurated, or discoloration may be traced for a considerable space inward from the walls of the vein. There is no mixing up of the materials of the dyke, nor any approach to incorporation with those of the including mass. After exposure on the surface to atmospheric influences, the basalt or greenstone splits up into large tabular blocks, which become extremely friable, and scale off in thin layers, leaving a central ball, which exfoliates in like manner, and gradually molders into dust. These dykes are very numerous in the Grampians, occurring everywhere, and diverging in every direction through the primary rocks. They traverse the lower district on the south of the range, five or six crossing the Tay, and running nearly parallel in a north-westerly course. They rise above the sandstone in various places of Strathearn, forming mural ridges, furrowed into broad jointed masses, or piled loosely above each other. The outgoing of some of these remarkable concretions can be traced into the German ocean. From St. Andrews westward, their line of bearing may be detected, both among the trap-hills and the sandstones which flank them, and like well-run stags, after debouching from the Ochills and Sidlaws, converging upon the forest of Glenartney. Doubling, winding, and dragging out and in among the passes from Crieff to Comrie, two of them may be descried on the steep face of Aberuchill, fairly scaling its lofty summits; and driving onward, may others be observed on the south of the Ruchle, to the far heights of Uam-Var.
The etymology of the term Ochil, would seem to be connected in some way with these geological phenomena. A tradition exists that, from time immemorial, the earthquakes of Comrie were cotemporaneous with subterranean movements or noises in the Ochil range, near Devon. The Gælic word ochain, or ochail, signifies, according to Armstrong, “moaning, wailing, howling;” and hence it is inferred that the name of the “Moaning Hills” may have been given to the range, from the sounds so frequently heard in the district. There can be no question as to the probability of a subterranean sympathy betwixt the two localities, through the instrumentality of these dykes, or otherwise; and, though the series of events referred to above belong to an anterior age—far remote, indeed, from the human and all its traditions—a plausible origin is thus given to the name, in connection with an analogous series of events that did happen within the human period.
IV. A vast historical interest, therefore, is to be attached to these hills, and their phenomena of veins, connected as they are with the first elevatory movements of the globe, and when form and outline were being given to its massive fullness. The hand of the Creator is clearly seen in raising them up from the depths below. Not a particle of the entire volume is in its original position, or that which it would of itself statically assume. God formed everything for use, while beauty and agreeableness of shape are inseparably combined. When viewed in the light of causation, it is not enough merely to say, and there to stop short, that we see in the outward face of nature the impress of power, wisdom, and goodness—that none of these things made themselves—that the rocks and mountains are an image of Jehovah’s greatness—the streams, plains, trees, corns, animals, the effect of His love and care. All this they unquestionably are, but they are more. Their arrangement and disposition, beside their mere existence, evince a continued superintendence—a purpose and a will to maintain an order and construction of elements which would otherwise separate and dissolve—a keeping together, and as one, each after their own kind, the inorganic and organic parts of creation. The philosophy, as well as theology, of these arrangements have been thus beautifully recorded: Thou coveredst the earth with the deep as with a garment, the waters stood above the mountains; at thy rebuke they fled, at the voice of thy thunder they hastened away into the place thou hast prepared for them. Thou hast set a bound that they may not pass over; that they turn not again to cover the earth.—It is demonstrable that, were all the rocks which compose our mountain ranges and dry land to be dissolved and carried into the sea, the waters of the globe are sufficient again to cover and conceal from view their vast and multiform materials, and to replace them in those depths whence they originally arose.