These features, which are repeated on cliff after cliff, may be considered typical for all the plateaux. Another characteristic point, well displayed here, is the intervening red parting between the successive beds. If the occurrence and thickness of this layer could be assumed as an indication of the relative lapse of time between the different flows of lava, it would furnish us with a rude kind of chronometer for estimating the proportionate duration of the intervals between the eruptions. It is to be noticed on the top both of the compact prismatic and of the earthy amygdaloidal sheets; but it is more frequent and generally thicker on the latter than on the former, which may only mean that the surfaces of the cellular lavas were more prone to subærial decay than those of the compact varieties. Nevertheless, I am disposed to attach some value to it, as an index of time. In the present instance, for example, it seems to me probable that the lavas in the lower half of Macleod's Maiden, where the red layers are very prominent, were poured out at longer intervals than those that form the upper half. The remarkable banded arrangement of the vesicles in one of the cellular lavas of this sea-stack has been already referred to ([p. 191]).
Another characteristic plateau-feature is admirably displayed in Skye—the flatness of the basalts and the continuity of their level terraces (though not of individual sheets) from cliff to cliff and hillside to hillside. This feature may be followed with almost tiresome monotony over the whole of the island, north of a line drawn from Loch Brittle to Loch Sligachan. Throughout that wide region, the regularity of the basalt-plateau is unbroken, except by minor protrusions of eruptive rock, which, as far as I have noticed, do not seriously affect the topography. But south of the line just indicated, the plateau undergoes the same remarkable change as in Rum, Ardnamurchan and Mull. Portions of it which have survived indicate with sufficient clearness that it once spread southwards and eastwards over the mountainous district, and even farther south into the low parts of the island. Its removal from that tract has been of the utmost value to geological research, for some of the subterranean aspects of volcanism have thereby been revealed, which would otherwise have remained buried under the thick cover of basalt. Denudation has likewise cut deeply into the eruptive bosses, and has carved out of them the groups of the Red Hills and the Cuillins, to whose picturesque forms Skye owes so much of its charm.
In this, as in each of the other plateaux, there is no trace of any thickening of the basalts towards a supposed central vent of eruption. The nearly level sheets may be followed up to the very edge of the great mountainous tract of eruptive rocks, retaining all the way their usual characters; they do not become thicker there either collectively or individually, nor are they more abundantly interstratified with tuffs or volcanic conglomerates. On the contrary, their very base is exposed around the mountain ground, and the thickest interstratifications of fragmentary materials are found at a distance from that area. So far as regards the structure of the remaining part of the plateau, the eruption of the gabbros and granitoid rocks might apparently have taken place as well anywhere further north.
v. THE FAROE ISLANDS[265]
[265] For references to the recent geological literature connected with these islands see the footnote ante, [p. 191].
Though these islands lie beyond the limits of the region embraced by the present work, I wish to cite them for the singular confirmation and extension they afford to observations made among British Tertiary volcanic rocks. Over a united extent of coast-cliffs which may be roughly estimated at about 500 English miles, the nearly level sheets of basalts, with their occasional tuffs, conglomerates, leaf-beds and coals, can be followed with singular clearness. Although the Faroe Islands have been so frequently visited and so often described that their general structure is sufficiently well known, they present in their details such a mass of new material for the illustration of volcanic action that they deserve a far more minute and patient survey than they have yet received. They cannot be adequately mapped and understood by the traveller who merely sails round them. They must be laboriously explored, island by island and cliff by cliff.
While I cannot pretend to more than a mere general acquaintance with their structure, I have learnt by experience that one may sail near their precipices and yet miss some essential features of their volcanic structure. In the summer of the year 1894 I passed close to the noble range of precipices on the west side of Stromö, at the mouth of the Vaagöfjord, and sketched the sill which forms so striking a part of the geology of that district (Figs. [312], [328] and [329]). But I failed to observe a much more remarkable and interesting feature at the base of the same sea-cliffs. The following summer, probably under better conditions of light, I was fortunate enough to detect with my field-glass, from the deck of the yacht, what looked like a mass of agglomerate, and found on closer examination the interesting group of volcanic vents described in Chapter xli. The magnificent precipices of Faroe, which in Myling Head reach a height of 2260 feet, present a series of natural sections altogether without a rival in the rest of Europe. They are less concealed with verdure than those of Mull and Skye, and therefore display their geological details with even greater clearness than can be found either in Scotland or in Ireland. I would especially refer to the bare precipitous sides of the long narrow islands of Kalsö and Kunö, as admirable sections wherein the characters of the plateau-basalts are revealed as in a series of gigantic diagrams. The scarcity of vegetation, and the steepness of the declivities which prevents the abundant accumulation of screes of detritus, enable the observer to trace individual beds of basalt with the eye for several miles. Thus on the west side of Kunö, one conspicuous dark sheet in the lower part of the section can be followed from opposite Mygledahl in Kalsö to the southern end of the island. There is one concealed space at the mouth of the corrie behind Kunö village, but the same, or at least a similar band of rock at the same level, emerges from the detritus on the further side, and may possibly run into the opposite promontory of Bodö. It extends in Kunö for at least six geographical miles.
Fig. 288.—Dying out of Lava-beds, east side of Sandö, Faroe Isles.
These vast escarpments of naked rock show, with even greater clearness than the precipices of the Inner Hebrides, how frequently the basalts die out, now in one direction now in another. The two sides of the Kalsöfjord exhibit many examples of this structure, and some striking instances of it are to be seen on the west side of Haraldsfjord. In these cliffs, which must be about 2000 feet high, upwards of forty distinct flows can sometimes be traced from the sea-level to the crest. The average thickness of each bed is thus somewhat less than 50 feet. Such vast escarpments, with wide semicircular corries scooped out of their sides, such serrated crests and dark rifts in the precipices, such deep fjords winding through nearly horizontal basalts, of which the parallel sheets can be followed by the eye from island to island, fill the mind with a vivid conception at once of the enormous scale of the volcanic eruptions and of the stupendous denudation which this portion of North-Western Europe has undergone since Tertiary time.