The geologist, in penetrating these primeval wilds, has but little choice left him as to the comforts of his pathway. Arrived at the top of Glen-lui, the two diverging passes, right and left, are equally desolate, savage, and grand. He may make his selection as the feeling of the moment prompts, but he will not be able to congratulate himself as the traveler in a different field—
Hic locus est, partes ubi se via fundit in ambas:
Dextera, quæ ditis magni sub mænia tendit;
Hac iter Elysium nobis: ut læva malorum
Exercet pænas, et ad impia Tartara mittit.
No “fiends,” indeed, as Dryden renders it, are here, unless the belated traveler may allow his fancy to shape these gnarled withered stumps of the old forest, as it well may, into grisly living forms; or the red deer breaking from their coverts, and gazing in wild amazement from the crags, startle him from his propriety. Still Loch Avon, black as pitch, and imbosomed in horrid rocks, is not an unfitting emblem of the Tartarean lake.
Pursuing his route to Strathspey, either through the desolate openings of Ben-Avon, or by the wild passes of Brae-Riach and Cairn-gorm, the geologist again drops down among the gneiss, schists, limestone, and quartz. These types of rock line the trough of the Spey, on both sides, as far as the granite district of Ericht and Laggan, presenting the usual phenomena of granitic and feldspathic dykes, and in some places, as at Loch-an-Eilan, remarkable twistings and flexures in the mica-schist around this eagle-haunted lake. Glen Tilt, on the south-west, is distinguished by a singular display of granitic veins, appearing to radiate from a common center—the well-known phenomena which the philosophers of the Hutton and Playfair school pressed so keenly and successfully into the service of their theory. The gneiss is generally to be observed in the form of low ridges, interstratified with quartz-rock, and approaching in mineral qualities to the mica-slate.
The bearing of all these stratified rocks is, on the main, sufficiently indicated by the outline of the Grampian range. The quartz, mica, and chlorite slates, are nearly continuous along the chain, traversing in a S. W. by N. E. direction the breadth of the island, from sea to sea. The line of strike, however, is often interrupted, either by the eruptive veins above mentioned, or by the upheaval of the central axis, which, as it rose with greater violence, or was parted into higher and unequal ridges, would necessarily occasion corresponding changes in the lie and direction of their coverings. This principle in geological dynamics has been satisfactorily established by Mr. Hopkins of Cambridge, who has shown, that in the production of any great line of elevatory disturbance, whether affecting straight, curvilinear, or ellipsoidal masses, the strata would frequently be broken by fissures at various angles to the chief line of strain or elevation. Hence these interminable glens, transverse straths, cul-de-sacs, and countless depressions, forming tarns and lochs, all inosculating into each other, and which give such variety and grandeur to this alpine region. The pent up ebullient matter beneath the crust would thereby force its way to the surface—now in the form of veins—now in long narrow ridges—and in other quarters assuming the contour of broad mountain domes. The dip, in like manner, corresponding to these partial strikes, as well as great axis of the chain, is often various—as at the Linn of Dee, and along the braes of Corry Mulzie, the beds being almost horizontal, while generally they are so highly inclined as to be nearly vertical.
There are also numerous examples where the crystalline strata dip inward toward the granite ridges, and in this manner form an acute angle with the base, instead of being infolded over and welded to them. The only admissible explanation in these instances of the dip is, that the ends of the strata adjacent to the eruptive masses have sunk into depressions occasioned by the evolution of igneous matter, while their upper edges have been tilted backward. Hence the schists often rise into independent elevated crests all along the chain, and even where no granite appears at the surface. The rocks in Glen-Beg and Glen-Clunie afford examples of this kind, where, as in Cairn-na-well, and the other mountains here, they are highly inclined, and plunge in the direction of the principal range. Geology, viewed in this light, becomes an auxiliary to physical geography, explains many anomalous appearances on the earth’s surface, and successfully accounts for all the flexures, breaks, undulations, and inequalities, that constitute such marked features in the primary strata.
Until very recently, the doctrine maintained was, that nearly all the inequalities on the earth’s surface were produced by the erosive and denuding effects of water; that not merely the small lateral valleys and branches of rivers, but likewise all their main trunks, were caused by the slow and gradual working of the stream, cutting the most solid and massive rocks in the same way and almost with the same instrument by which the lapidary divides a block of marble or granite. Nay, with such a ready agent, acting through incalculably remote and indefinite periods of time, the conclusion was arrived at, that “on our continents there is no spot on which a river may not formerly have run.” A sounder philosophy, and one far more accordant with the facts, is now beginning to prevail, namely, that nearly all transverse gorges, by which rivers escape across ridges from one water basin to another, are nothing more than ancient apertures in the crust of the earth, which have resulted from the former disruption and denudation of the rocks: and that rivers, properly so called, have never cut sections through chains, but simply flow in chasms prepared for them.