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.