at Roman Fell and in the spectacular amphitheatre of High Cup Nick, where it is eighty feet thick.
Dykes are often on a much smaller scale, but when found among resistant rocks they often give rise to striking gullies and cols. Perhaps the best-known mountain structure of this type is Mickledore, the great gap separating Scafell from Scafell Pike.
The larger intrusions of igneous rock are very often great bosses of granite which may be many miles across. Classical examples are those in Galloway, which give the mountains of Criffel and of Cairnsmore of Fleet. To this type of structure belong the summit of Crib Goch and also Penmaen Mawr in Wales, the latter familiar to every one who drives along the coastal road. Generally similar is the huge granite mass of Dartmoor. In all these cases the granite boss is harder than the surrounding country rock and so has been left more elevated than the areas around. Where the surrounding rocks are hard, however, granite bosses may contribute no noteworthy structure to a mountain region, and this is the case in the Lake District, for example, where the Shap, Eskdale or Ennerdale granites are all relatively inconspicuous among the hard slates into which they were intruded.
Along the western seaboard of Scotland granite intrusions occur among other traces of volcanic or plutonic activity. The Western Isles and many of their mountains include the remains of vast flows of basaltic lavas which formerly stretched from Antrim, through Staffa, Mull and Arran to Skye, and, indeed, as far north as the Faeroe Islands and Iceland. Geologically, these lava beds are of Tertiary Age and very much more recent than the tuffs of the Lake District and Wales. Even to-day the beds lie nearly horizontal, and though they form the well-known columns of Staffa and the Giant’s Causeway and are often exposed in sea-cliffs (those of Eigg and of Portree Harbour, for example), they do not as a whole contribute much to our mountain scenery. Nevertheless, the familiar view of the mountains of Mull, Sgurr Dearg and its neighbours seen from Oban, consists almost wholly of rocks of this type, forced upward by later volcanic action in Central Mull. Still farther north, in Skye, the Storr Rocks (see Pl. IV) and the Quirang are, moreover, both composed of these Tertiary lavas overlying softer Jurassic shales, and the whole of the coastal scenery is dominated by them.
Much more important scenically were the great subsequent upwellings of molten igneous matter in this area, which are associated with the noble mountain scenery of Skye, Rhum and Arran. In Skye, the principal contrast is between the Black Coolin and the Red Hills. The crags of the former are composed mainly of a hard and basic rock called gabbro, with a coarsely crystalline structure that delights the climber’s heart. The Red Hills, in contrast, are granite and this has weathered far more rapidly and uniformly to give mountains of smooth and rounded aspect. The contrast, known to every visitor to Skye, is extremely well shown in the fine photograph (Pl. 1) of Blaven and Ruadh Stac, the former of gabbro and the latter of granite. The gabbro is intersected by igneous “dykes” which, running mainly north-west and south-east, serve to accentuate the differences, for these are more easily eroded than gabbro and so tend to form the gullies in the great gabbro ridges. Pl. VII gives an excellent impression of the distant aspects of the rock and the ridges.
Somewhat similar contrasts are to be seen in Rhum, where the outstanding peaks of Hallival and Askival are composed of ultra-basic and coarsely crystalline rocks of an unusual type. Their craggy outlines contrast noticeably with the grassy and rounded appearance of the hills farther west, such as Fionchre and Bloodstone Hill, both mainly built of more easily weathered basalt. A similar contrast is seen between the peaks of igneous rock and the gentle moorland contours of the Torridonian sandstones in the northern part of the island, which form a foreground as seen from Skye. In northern Arran, too, there were great intrusions of igneous rocks. The granite of Goatfell stands out boldly, as seen from Brodick Bay, against a foreground of softer sandstones.
The igneous geology of these western mountains is extremely complex and cannot adequately be discussed here except where it plays a part in determining the characteristic features of a mountain mass. But a few words may perhaps be spared for Ben Nevis (4406 ft.) which, as the highest mountain in Britain, deserves at least a passing mention. Ben Nevis represents a central plug of rock, surrounded by two cylinders of intrusive granite, that is presumably by two cylindrical faults, filled up from below by molten rock. The cap of the mountain core consists of ancient lavas (Old Red Sandstone Period) overlying Dalradian schists, and it is supposed that this central core of rock must have sunk considerably into the molten rock now represented by the granite cylinders. Going east from Ben Nevis, Carn Mor Dearg lies on the inner cylinder of granite and Aonach Mor (3,999 ft.) on the outer cylinder. From the north-west, both types of granite can be distinguished on the route from Fort William to the summit of Ben Nevis.
A similar complex system centres round Glen Etive, with the Buchailles of Etive representing a cap of rhyolites and tuffs on a core surrounded by cylinders of granite. Ben Cruachan lies wholly on one of the granite intrusions and so too does the greater part of the Moor of Rannoch.
From the point of view of their influence on the animal and plant life, a highly important property of the volcanic and igneous rocks is whether or not they are rich in basic substances like lime, potash and magnesia.
The geological classification expresses these features inversely in terms of the amount of the non-basic material, silica, which is present, as shown in the following table:
Table 1 SILICA CONTENT OF IGNEOUS ROCKS
Biologically, the basic and ultra-basic rocks provide habitats which are generally more interesting largely because they yield richer soil. The favourable feature of a high base content is, it is true, often partly counteracted by the hardness of the rocks and an accompanying resistance to weathering and erosion, as in the examples already given from Skye and Rhum. But many British basalts are not only basic but they also weather especially easily to yield a comparatively rich soil. The Ordovician tufls are often intermediate in character and may include much andesitic material. In contrast, most British granites contain on an average over 70 per cent of silica and they yield soils which may consist of little but sand and which, as a result, are correspondingly infertile. The biologist thus soon learns to regard granite areas as a distinctive upland type, just as they are geologically and scenically. On the other hand, he has learnt to approach areas dominated by basic or ultra-basic rocks with a certain amount of optimism. Their more varied vegetation and fauna runs parallel with the higher base-status of the soils and rocks, and the latter, indeed, often contain large amounts of bases such as potash, magnesia and iron oxides instead of the lime that prevails in many sedimentary rocks. By analogy with other parts of the world, it is probable that the presence of certain plants and animals on the basic and ultra-basic rocks is associated with these peculiarities of chemical composition of the latter.
The great variety of rock type and of rock arrangement which runs through the Western Islands is less apparent on the Scottish mainland. There the mountain masses of the Grampians are mainly composed of hard and ancient rocks, so greatly contorted by subsequent earth movement that their arrangement is often obscure and it is consequently less easy to describe in broad general terms their relation to mountain structure. They are geologically, for the most part, schists or gneisses (which are, respectively, metamorphosed and distorted shales or sandstones and grits) or finely crystalline igneous rocks. But the simple principles which have been stressed above are generally applicable when the structure of any individual mountain or upland area is considered. Without considering these in detail, it may be noted that the Grampians include three main areas of differing structural type, which have biological interest. Towards the south and west there is an area in which mica-schists predominate. This is a rock which weathers easily, yielding an open and uniform soil. It is marked by a group of characteristic and somewhat lumpy, grass-covered mountains lying roughly along a curved line between Ben Lawers, Ben Doireann and Ben Alder,
