becomes obvious, and subsequent weathering of the hills has produced some great scree slopes.
South-west of Skye there is another fairly large island of nearly 30,000 acres, called Rum. Its highest hills do not go beyond 2,600 feet, but in beauty of line they are not less than the Cuillins; and when we come to examine their geology, we find that nearly a third of the island, including these fine hills, is composed of gabbro.
Another mass of gabbro and allied rocks occurs over a hundred miles farther west to form the group of islands known as St. Kilda. They are unique in British scenery and in British natural history. Here, the Atlantic has not allowed the accumulation of the fragments which result from weathering and make the scree slopes seen in the Cuillins. Instead, the gabbro stands up out of the sea in naked pinnacles. The highest point of the largest island, Hirta, is 1,396.8 feet above sea level, from just below which is the highest sheer sea-cliff in Britain. (This is disputed by those who say the Kame of Foula, 1,220 feet, is the highest.) Even so, the island being three miles or so across, the height of this cliff is not so striking as the smaller island of Boreray on the north side of the group, which being roughly triangular and less than a mile across rises to 1,245 feet. Stac Lee and Stac an Armin are mere rocks in the sea, a furlong or less across at their foot, but they rise to 544 and 627 feet and make a difficult climb for a good man (Plates XXIb and XXIX). It is on these stacks of gabbro in the western ocean that the largest gannet colonies in the world are found.
Finally, the 70-foot stack of Rockall (Plate XXII), 184 miles west of St. Kilda, is of a rock allied to gabbro though more acid in composition. The natural history of that rock, in so far as it is known, would not fill a volume. No birds breed on it regularly though guillemots do from time to time. Lichens there may be but no other vegetation. Gannets and other sea birds are often found resting there in summer. Near it is Leonidas or Hazelwood Rock, usually awash. Farther away Helen’s reef, about a mile long comes to the surface only at spring tides in one place, so the geological nature of this submerged land is unknown to us. The naturalist, of all men, cannot resist the temptation sometimes to dream of what Rockall would have been had it thrust itself just another hundred feet through the waves and been able to withstand the wave action which has reduced it to its present proportions. We should have had a long low island with a hump and a few smaller eminences. What sort of a place would it have been botanically? What haven would it have made for nesting birds from the sea, and would the Atlantic seal have been an altogether more numerous species than it is, by having a North Atlantic sanctuary which would usually have been unapproachable during the autumn breeding season? James Fisher tells me that on an exceptionally calm day the Leonidas Rock may dry out and would provide the only possible hauling-out place for the seal; but in fact, the Atlantic seal has been seen only once in the vicinity.
I have very roughly indicated the geological character of the Highlands in so far as it affects the topography; there remains, however, the phenomenon of glaciation as the most tremendous carver of scenery from the matrix of rocks of one kind or another. I mentioned the effect of glaciation on the Cuillins, but that was a minor area of ice action compared with the Highlands as a whole. Glaciologists now appear to be agreed that there were four glacial periods in which the Highlands were involved, though it is improbable that the whole region was covered by the ice each time. There were certainly ice caps on the North-West Highlands and on the Grampians.
From these two or more central ice caps there are well-defined radial courses which have gouged out or deepened many of the glens and polished the summits of some of the lower hills. The country of the Hebridean gneiss in Sutherland and north-west Ross has been heavily scored by glaciers flowing westwards from the direction of the higher Torridonian sandstone hills inland and in the melting of the ice many boulders of this dark red rock have come to rest on the grey gneiss hills. Such erratic blocks are often well-worn and rounded: the stalker in this ground is sometimes tricked into putting his glass on them if he does not know it well, thinking such boulders are deer, seen from a mile away. And in the Torridonian area we see some fine hanging corries facing eastwards at about 1,750 feet, with a large area of boulder moraine below the lip of the corries. Where the boulders, long carried in the ice, have been open to the weather, only the deeper scorings and the rounding are obvious; but I have had occasion to move some of these boulders from their bed of fine glacial sand, their tops being in the way of my plough. The lower faces of such rocks, which had lain in their beds since the melting ice had lost momentum to carry them farther, were flat, scratched in some places but carrying quite a high polish on other parts of the sole. The coarser qualities of Torridonian sandstone do not take a smooth surface easily, but these surfaces gave the finger-tips a palpable sensation of polish.
The glaciers have not only carved and gouged the countryside but have had a profound effect on the subsequent natural history. The rock surfaces, shorn and left bare, have often remained bare or have gathered only a superficial layer of peat on which there grows but a poor vegetation. Again, some of the gougings have made saucer-like depressions which will not drain completely and either become lochs or peat-filled bogs with their own flora. The moraines are exceptionally well drained and the herbage grows in the rock detritus or on a very thin layer of peat. The best heather in the West Highlands—where heather does not normally grow well—is on the moraines.
The Spey Valley, and perhaps the Dee Valley also, is probably the best coniferous-tree-growing area in Scotland, a point to which return will be made later in the book: what is emphasized here is the comparative dryness of the ground of all this region, caused by the immense quantity of glacial drift (Plate 1) which drains readily and has only a thin layer of peat above it. The glacial drift of the Highlands is sandy and gravelly, not the boulder clay which is found in the north of England and in parts of the Central Plain of Scotland; for which fact we can be heartily glad, for to have boulder clay beneath our feet in a country of high rainfall would be unendurable.
CLIMATE
The relation of geology and scenery we have taken for granted; that of geology, climate and vegetation we take almost for granted, and because of that we are too apt, perhaps, to generalize. Our old school geography books informed us that the climate of the British Isles was mild and humid and that the laving waters of the Gulf Stream (which we now call the Atlantic Drift) kept our insular climate an equable one. Though it is generally true that the south-eastern side of Britain is drier than the north-western, it is for the naturalist to inquire more deeply into such generalizations, for he knows that altitude, position and slope in relation to sun, nearness to sea and so on have considerable local effects on climate. At least, he is finding out that he must know local climates quite well if he is to be a good naturalist—for example, Loch Tay is 20 miles long; from east to west there is an increase of an inch per year of rainfall for every mile you go. We should rightly judge, therefore, that the more detailed natural history of Killin would show considerable differences from that of Aberfeldy on the scoe of rainfall alone. When I lived at Dundonnell at the head of Little Loch Broom I found the rainfall to be about 72 inches a year. Seven miles to the south in Strath na Sheallag beyond the Torridonian cones of An Teallach (Plate IIIb), the rainfall in my gauge there measured 100 inches. Seven miles north of Dundonnell lies Ullapool, which receives an average of 48 inches. Both snow and frost are more severe in Strath na Sheallag than in Ullapool. The differences in the vegetational complex of these two areas were marked, and so were those of the animal groupings, though of course it would be wrong to put it all down to the climate. If climates can alter so markedly within a few miles—and that is the rule rather than the exception in the Highlands—so can they alter within a few yards. The ecologist, the man who studies organisms in relation to their environment, is now giving more attention to what are called micro-climates. That gully in the north corrie of Ben Nevis which was mentioned previously as never getting the sun, and where the snow sometimes remains the year round, is an example of a distinct micro-climatic region. What is its annual mean temperature and its extremes? What does its relative-humidity chart look like? How much light gets in there? We do not know. And coming to its living things, what plants are found there? Perhaps the fauna would include a few spiders, some of which creatures have a habit of living in unlikely places on mountains. Again,
