massive conglomerate are partly shown in Fig. 38. That this gorge has been entirely excavated since the Ice Age cannot be doubted. Immediately above it the valley expands and is filled with boulder-clay, which, burying the old channel, has driven the stream to cut a new one through the barrier of conglomerate.

The ravines of the Beauly, Nairn, and Findhorn are on a larger scale. But more striking still are the gorges that have been dug out of the Old Red Sandstone along the southern margin of the Highlands in Perthshire and Forfarshire. That of the Ericht, above Blairgowrie, reminds one of the cañons of the far west of America. It has been eroded in the conglomerate, and averages about 150 yards in width, and 150 to 180 feet in depth. As will be observed in the woodcut (Fig. 39), the parallel lines of joint, along which the rock splits, enable the ravine to maintain the verticality and parallelism of its walls. Farther east the gorges of the Isla and the Esk likewise furnish excellent illustrations of the peculiar aptitude with which the Old Red Sandstone lends itself to picturesqueness of detail in landscape. Among the crystalline schists of the Highlands, the cañon or steep-walled type of river-gorge is much less frequent, obviously because, as a rule, there is much less regularity of structure, and especially of joints, among these rocks. But where the schists are not much crumpled, and are traversed by regular parallel joints, they supply examples of deep, narrow ravines. Perhaps the best example in the Highlands of this feature is to be seen in the course of the River Broom, which falls into the head of Loch Broom. The flaggy schists which there dip in regular beds at low angles towards the south are cut by well-defined parallel joints, and the little river has excavated in them a profound chasm

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that terminates at its upper end in a waterfall, across the top of which the strata can be traced in unbroken sequence from side to side.

In a previous chapter allusion was made to the frequent association of gorges and wide alluvial plains above them; but some further account of this feature may here be given. Where a valley contracts and its stream flows through a narrow rocky defile, there is commonly an expansion of the valley above the ravine. This association is usually explained by a difference in the relative hardness of the rocks eroded by the stream. The harder masses resist more and are therefore less rapidly worn down. Where they cross a valley, therefore, they retard the excavation, while the softer rocks lying farther up are more rapidly wasted. But as the stream cannot lower its bed below the level of the barrier of hard rock, it attacks the side of the valley and thus widens it out, while at the same time it is sawing down the rocky barrier and making a ravine. In districts of comparatively simple geological structure, where, for instance, soft slates alternate with courses of hard sandstone, the truth of this explanation can be readily seen. But it is not so easy to apply it in many Highland valleys where there is no very evident reason in any apparent difference in the rocks to account for the alternate expansion and contraction. Instances of this common feature will readily occur to any one who is familiar with Highland scenery. As an easily accessible locality, I have already cited Glen Falloch, above the head of Loch Lomond, in which between that lake and the watershed some four or five examples may be seen in the course of some six or seven miles. Other excellent examples may be noticed in Glen Spean, above the Bridge of Roy. One of these is conspicuous just above the great gorge at Achluachrach (field

of rushes), whence a broad alluvial plain stretches for almost two miles to near Inverlair, where it gives place to another gorge in the hard massive schists. Above this second ravine is another alluvial plain with lateral terraces. A large moraine has been thrown across the valley, which has turned the stream aside and compelled it to saw out a narrow gorge in the schists. It is probable that such meadow-like expansions were lakes in the glaciated regions, and that these have gradually been silted up. Reference will be made in a later chapter to the occurrence of glenlakes above gorges in many parts of the Highlands.

Among the high grounds, where disintegration proceeds apace, the gradual narrowing of ridges into sharp, narrow, knife-edged crests and the lowering of these into cols or passes can be admirably studied. Where two glens begin opposite to each other on the same ridge, their corries are gradually cut back until only a sharp crest separates them. This crest, attacked on each front and along the summit, is lowered with comparative rapidity, until in the end merely a low col, pass, or balloch, may separate the heads of the two glens. The various stages in this kind of demolition are best seen where the underlying rock is of granite or some similar material which possesses considerable toughness, while at the same time it is apt to be split and splintered by means of its numerous transverse joints. The district around Ben Nevis furnishes good illustrations. The narrow crest of granite to which reference was made earlier in this chapter, forms a kind of shattered partition wall between two glens running northward and one running southward. This intervening and lessening partition is doomed in the end to be wholly removed, and then one long glen will run along the east side of Ben Nevis, with perhaps a low, scarcely perceptible watershed

The cone

FIG. 40.-View from the Ridge of Suidhe Fhearghas (Fergus Seat), Arran, to show the formation of corries and ballochs. in the centre is Cir Mhòr (Big Comb), that to the right Caisteal Abhail. In the far distance across the ridge of Cantyre are seen the quartzite cones of Isla and Jura.