of Ben Nevis is not of the granite rock of which the main mass of the hill is composed. The summit is the top of a gigantic cylinder of tertiary basalt. Presumably this volcano originally spread the basalt over the whole of the hill, but all except the protected summit has been eroded away.
It is worth noting that the two highest mountainous groups in Scotland are formed of granite—Ben Nevis, and the Cairngorm region, 4,296 feet, east of the Spey; and east again, there is Lochnagar, on the eastern side of the Highland area, also granite and reaching 3,760 feet. The Cairngorms are of much greater extent than the Ben Nevis massif and, partly because of their considerable alpine plateaux at about the 4,000-foot contour, have a special place in Highland natural history. Topographically, the Cairngorms viewed from afar may seem as uninteresting as Ben Nevis, but the physical beauty of these hills is for intimate observation in the magnificent corries and on the high plateaux. It is possible to walk a pony on to these high ridges and plateaux without any trouble, the ground being good all the way.
There are two more striking geological phenomena which will be less obvious to the casual observer than those of the Highland Boundary Fault and the Great Glen. First, that definite line of tectonic tumult known as the Moine Overthrust which reaches from the south-west corner of Skye north-north-eastwards to the eastern shore of Loch Eriboll on the north coast. Here the older rocks of the Moine schists are thrust westwards over younger rocks along a hundred-mile line. Through the whole length, like a sandwich filling between archaean gneiss with the overlying Torridonian sandstone on the west side, and the Moine schists on the east, are beds of Cambrian age including an extremely hard and shiny metamorphosed quartzite which makes a barren strip of country. This rock is so hard and shiny that even the peat finds it difficult to keep a hold on slopes of any considerable inclination. There are many patches of acres of bare rock (Plate 3a) to be found on the passage of the quartzite from Eriboll to Skye, and it may be best described in the climber’s graphic phrase of “boiler-plating.”
Here and there along the line of this uninteresting sandwich filling there appears a geological titbit in the shape of an outcrop of limestone such as the famous Durness limestone of Cambrian age. The effect of this is to enliven the natural history and change the scenery of this vast geological sandwich. Instead of bareness and blackness of peat we get greenness and soil. It is this limestone which makes the Assynt district a place which no naturalist should ignore from the geological, entomological, conchological or botanical points of view. The effect is, perhaps, still more marked around Durness. Even the bird life has its particular interest in this area and so has the world of loch and river. A little farther south than Assynt, in a black area of bog to the east of Suilven and Canisp, there rises an island of limestone a few hundred acres in extent. The climber on these hills in spring or autumn will experience pleasure and something of a shock to see the townships of Elphin and Cnockan on their geological emerald. If he goes down to these thriving villages he will notice that the sheep are larger, the cattle better-looking, and there will be Highland pony mares and foals such as he will see nowhere else so commonly till he reaches the machairs of South Uist. The roadsides are like the verges of an English lane. All this is part of the paradox of the Highlands and of Highland life. They are full of surprises and facts which do not seem to fit in. No sooner does the theorizing type of mind construct a hypothesis which looks neat than some disconcerting fact will create paradox.
So much for the Moine Thrust and its consequences; the second striking geological phenomenon is the mixture in the West Highlands of old rocks and new volcanic ones. From Cape Wrath to Applecross the West Coast is a wild jumble of the ancient Hebridean gneiss and that very old and barren sandstone known as Torridonian. These two formations make for scenery which is exceptionally wild in quite different ways. Then at the foot of Loch Linnhe is the Isle of Mull and, to the north of it, some similar tertiary volcanic rocks in Morvern and Ardnamurchan. Such volcanic rock is a dull grey in colour and amorphous in texture, except where there occur amygdaloid pockets of crystals of much beauty, but it makes some distinctive scenery. These tertiary rocks as we see them in Mull are the remains of immense beds of lava, possibly 50 million years of age as against the 1,500 million years or so of the gneiss farther north. The lava erodes into terrace-like formations which correspond to the actual flows, and on which terraces the soil is found to be brown and rich—a real soil without peat—and the grass grows thick in summer. The country of the tertiary terraces is cattle country and turns up again in the western part of Skye. Sometimes the terraced denudation gives way to a natural castellated architecture such as the Castle Rock of the Treshnish Isles (Plate Va), and further still to towers and spires like the Quirang and the Old Man of Storr in Skye (Plate II). This latter rock is like a natural Tower of Pisa and visible from many miles away. Sometimes, again, the tertiary basalt has solidified in a peculiar way to make those giant hexagonal columns much visited at Staffa, the small island between Iona and the Treshnish Isles. Such columns may also be found elsewhere, as in Mull, Canna and Oidhsgeir, on a much smaller scale, but probably the most impressive examples in Scotland are those of the northern face of Garbh Eilean of the Shiant Isles. Though the steamer from Kyle of Lochalsh to Stornoway passes within four miles of the Shiants, these islands are rarely visited, and the tremendous columnar architecture remains almost unknown. The Shiants are the northern outposts of Scotland’s youngest rocks; only four miles away to the west is the east coast of the Hebrides, composed of her oldest rock, the archaean gneiss. Geology is a hard subject to learn, but it helps one to understand scenery, and with a little knowledge it gives great sense of wonderment at the immensity of the movements of the earth’s crust.
The Moine Thrust has been mentioned as one of the major geological features of the Highlands. The Moine schists which occur to the east of the Thrust are the most extensive group of rocks found in the Highlands as a whole. They are sometimes called the Undifferentiated Eastern Schist, and words such as gneissose and schistose crop up in a detailed geological description; but whatever the names, the group of rocks reaches in a broad, roughly parallel-sided band, fifty miles wide, from the north coast of Scotland to the foot of the Great Glen where it comes up short against the tertiary basalt of Mull and western Morven. South of the Great Glen, similar schists and gneisses, the Dalriadan, underlie a large part of the Central Highlands and reach far into Aberdeenshire. Many of the high tops of the Highlands are on this formation—Ben Lawers 3,984 feet, Craig Meagaidh 3,700, Mam Soul 3,862 and Carn Eige 3,877 feet. The schists form a great plateau on the western side of the Spey opposite the Cairngorms. This schist plateau has the Gaelic name of Monaliadh —the grey mountains—and the Cairngorms across the valley are called the Monaruadh—the red mountains. The grey is most obvious when it comes up against either the granite as on the east side of the Highlands, or against the reddish-purple Torridonian, as on the west. Though schists and gneisses are intimately associated and both very ancient, the schists break down more easily, and as they contain a fair quantity of alumina they often make good soil, and produce a different vegetational complex from the adjoining Torridonian, for example. The presence of overmuch peat, of course, may entirely cut out the influence of the slowly-disintegrating rock.
FIG 1.—Geology of the Highlands The exposures of Tertiary and Cretaceous are not large enough to show on this scale
An igneous rock known as gabbro is of infrequent occurrence in the Highlands, but its appearances and the consequences of glacial action on the gabbro constitute some of the most spectacular scenery not only in the Highlands but in the whole world. The more important masses of gabbro are closely associated with the tertiary lava and are in fact intrusive masses of molten rock which solidified underground but have since been exposed by denudation. The small example of gabbro scenery in the shape of Ardnamurchan Point, the most westerly point of the mainland of Great Britain, is wild, but not high enough to be considered grand, but this rock in Skye is the stuff of the Cuillin hills (Plate IIIa) which rise to 3,309 feet. Gabbro is hard and knobbly, which makes it safe for the experienced climber, and by the coincidence of the Cuillin range being the centre of an ice cap in glacial times, the glaciers working outwards to the perimeter have carved the great corries and left the sharp ridges in which we delight to-day. With the retreat of the glaciers the shape
