Mountains and Moorlands. W. Pearsall H.

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, which possess a well-recognised biological type.

      The chief contrast in the Grampians is, however, between the eastern and western halves of the country. The former, exemplified particularly by the Cairngorms, is mainly a high though deeply dissected plateau, which constitutes the greatest continuous area of high ground in the British Isles. The Cairngorms are evidently in the early stages of a new erosion cycle, and their typical outlines, already discussed in connection with Pl. 5, contrast remarkably with those of Dartmoor, for example, also a granite mass, but one characterised by land-forms indicating far advanced weathering and erosion (see Pl. XXXI).

      In the western part of the Highlands, erosion and dissection have proceeded far more effectively, so that more often the mountains are partly isolated peaks or broken ridges. The change has undoubtedly been hastened not only by greater precipitation and glacial erosion in these areas, but also by the presence of numerous faults, running roughly from north-east to south-west, which have offered full play to eroding influences and have given us a series of loch-filled valleys. The most notable of these fault-lines is that of the Great Glen. Nevertheless, in spite of the much greater amount of erosion, the general level of the summits among the western mountains is very uniform and is indicative of that of the original plateau from which they must have been derived.

      The Scottish Highlands illustrate very well a point that was emphasised a long time ago by the late Professor J. E. Marr. In general, as upland surfaces recover from disturbances, they will tend to develop systems of gentle slopes and to approach, as Dartmoor is doing, characteristic forms of “subdued relief.” Among the upper levels of our British mountain regions it is possible to see a large proportion of land forms which are predominantly those of subdued relief. This implies that these forms must be of great age, for on account of the great hardness of the rocks, it must have taken an enormous time for the outlines to have “softened” in such an extreme manner. From arguments such as these, it may be assumed that the general form of our mountain regions is often ancient, and this usually applies particularly to the positions of the main summits and the river valleys. Superimposed on these ancient features we have also features which are the result of comparatively recent agencies. Foremost among these are the effects