Arctic shells which peopled our seas during the age of ice. Like the plants, they have been driven out by the migration of more temperate forms, and instead of now ranging from the shore-line down to the profoundest depths, they are confined to the latter parts of our seas, where they seem to be slowly but certainly dying out. The time may yet be distant, but it is probably not the less surely approaching, when the last of the Arctic forms, both of mountain-top and sea-bottom, will disappear, and when species of a more temperate character will spread over land and sea. And yet, such is the unceasing progress of terrestrial change, alike in organic and inorganic nature, that these newer forms will in all likelihood be themselves displaced by migrations from other parts of the globe, as the climate or the relative position of sea and land are changed, or as other mutations are brought about by those great geologic causes which, though seeming to operate at random, and wholly irrespective of either the animal or vegetable worlds, have yet been mysteriously linked with the grand onward march of life upon our globe.

CHAPTER VIII.

CAUSE OF THE LOCAL VARIETIES OF HIGHLAND SCENERY.

FROM what has been said in the foregoing pages, it appears that the larger elements of Highland Scenery-the grouping of the mountains, the excavation of the glens and straths, and the scoopingout of the lake-basins-are to be attributed to the action of the various powers of denudation—the sea, rains, springs, frosts, and moving ice. These features are everywhere to be seen, no matter what may be the nature of the rocks. We find the valleys cut out of gneiss, schist, slate, limestone, sandstone, quartz-rock or granite; but we feel sure there would still have been valleys and hills had the rocks been wholly different. The larger features of the scenery have probably been influenced rather by geological structure on the great scale, by anticlinal and synclinal axes, or lines of fault, or the boundaries of formations, than by mere diversities in the nature of the rocky masses that come to the surface.

But though the mineralogical changes in the Highland rocks may not have determined the framework

of the country, they have had much to do in lending character to its surface. There is not a glen or strath where their influence may not be seen. It is this influence, due to differences of weathering, which has given to Highland scenery its variety, and to each district or to each rock its own peculiarites of outline. There are glens and valleys and lakes, for instance, in the Isle of Skye, around the Trosachs, and among the Cairn Gorm mountains. They all can be traced to the action of those powers of waste already described, their formation has been carried on in accordance with the same laws. Yet the results are in each case very different. And why? Because in each of the districts the rocks are distinct, and yield after a fashion of their own to the ceaseless attacks of time. In the north of Skye the valleys wind among soft green terraced hills of igneous rocks, and almost recall some of the pastoral uplands of the southern counties. Around the Trosachs the glens and lakes have been cut out of tough, gnarled schist, which is worn away unequally into knobs and bosses and steep craggy declivities. Among the Cairn Gorms the savage caldron-like corries and precipices have been carved out of granite-a rock which, from its usual decomposing character and its abundant vertical joints, combines in its decay a grandeur of lofty cliff with a smoothness of mountain-top such as none of the other Highland rocks can boast.

These local peculiarities of scenery are brought out only during the course of a long process of subaërial waste, when the rains, rivers, and frosts have had ample leisure for their quiet work. They must have reached their highest development just before the glacial period began. The ice-sheets, however, did much to break off the sharpness of the angles everywhere, and to give to the whole country a much tamer aspect than it had worn before. Since that time the atmospheric agencies of erosion have been busy upon the ice-moulded surface. They have been re-asserting their old sway, and though the track of the ice still remains singularly fresh, it bears everywhere the proof that it is disappearing, and that in time the rains and frosts will restore to the outlines of our hills and mountains all the ruggedness which they possessed before they were swathed in the wintry folds of the ancient glaciers. In comparing and contrasting, therefore, the different forms of scenery to which the different geological formations give rise, it should not be forgotten that the distinctions between them are not so great as they were once, nor so marked as they will be again, when the ice-worn surfaces have faded away.

Perhaps the most interesting way of tracing this relation of the minor outlines of the landscape to the nature of the rocks, will be to take some of the more important rock-masses of the Highlands and compare

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their scenery with each other. Beginning with the oldest formation of the British Islands-the Fundamental or Laurentian gneiss, we find it stretching as a broken belt along the western coast of Sutherland and Ross, from Loch Inchard for forty miles or more to the south. Nothing can well be more impressive in its barrenness than the aspect of this great fringe of gneiss. You stand on one of its higher eminences and look over a dreary expanse of verdureless rock, grey, cold, and bare, protruding from the heather in endless rounded crags and knolls, and dotted over with tarns and lochans, which, by their stillness, heighten the loneliness and solitude of the scene. Acres of sombre peat-moss mark the site of former lakes, and their dinginess and desolation form no inconspicuous features in the landscape.1 Few contrasts of scenery in the Highlands, when once beheld, are likely to be better remembered than that between the cold grey hue and monotonous undulations of this ancient gneiss, and the colour and form of the sandstone mountains that rise along its inner margin. These heights are among the noblest in the

1 If the hypersthene rock of the Cuchullin Hills of Skye belongs to this ancient formation, it forms an illustrious exception to the general monotony of outline above noticed. (See Plate II.) Several years ago I stated my belief that this rock was of metamorphic origin (Trans. Roy. Soc. Edin. vol. xxii. p. 633, note), and more recent observations of Dr. Haughton tend to confirm the inference. (Geological Magazine for February, 1865.)