Banded lavas possessing the characters now described are of frequent occurrence among the Inner Hebrides. Many striking examples of them may be seen along the west coast of Skye. Still more abundant in Faroe, they form one of the most conspicuous features in the geology of that group of islands. Along the whole of its western seaboard, on island after island, they are particularly prominent in the lower parts of the precipices, while the upper parts consist largely of amorphous or prismatic sheets. So much do they resemble stratified rocks that it was not until I had landed at various points that I could satisfy myself that they are really banded lavas.[226]
[226] For recent contributions to the Geology of the Faroe Islands, see Prof. James Geikie, Trans. Roy. Soc. Edin. vol. xxx. (1880), p. 217, where the banding of the basalts is noticed; Prof. A. Helland, Dansk. Geografisk. Tidskr. (1881); R. Bréon, Notes pour servir à l'étude de la Géologie de l'Islande et des Isles Faeroe (1884); Mr. J. Lomas, Proc. Geol. Soc. Liverpool, vol. vii. (1895), p. 292. Various writers have treated of the petrography of Faroe, particularly A. Osann, Neues Jahrb. (1884), vol. i. p. 45, and M. Bréon in the volume here cited.
5th. Ordinary flow-structure, save in these banded lavas, is rather rare among the plateaux. It may, however, be occasionally observed, where there is no distinct banding. On a weathered surface it appears in fine, widely parallel streaks, which are sometimes wavy, puckered and broken up, as in rhyolites and felsites, while the porphyritic felspars are arranged with their long axes in the direction of flow. A good example of these characters may be seen on the summit of the Dùn Can—the remarkable truncated cone which forms the highest point on the Island of Raasay. The rock is a black olivine-basalt, partly amygdaloidal, with zeolites filling up the cavities, and its flow-lines are prominent on the weathered faces where they lie parallel to the general bedding of the lavas. Another illustration may be observed in the basalt already cited from Loch-na-Mna, in the island of Eigg, where the rock presents in places a remarkable streaky structure which, though hardly visible on a fresh fracture, reveals itself on a weathered face in thin nearly parallel ribs coincident in direction with the upper and under surfaces of the mass.
Great variety is to be found in the thickness of different sheets of lava in the plateaux. Some of them are not more than 6 or 8 feet; others reach to 80 or 100 feet, and sometimes, though rarely, to even greater dimensions. In Antrim, the average thickness of the flows is probably from 15 to 20 feet.[227] In the fine coast-sections at the Giant's Causeway, however, some bands may be seen far in excess of that measurement. The bed that forms the Causeway, for instance, is about 60 or 70 feet thick, and seems to become even thicker further east. Along the great escarpment, 700 feet high, which rises from the shores of Gribon, on the west coast of Mull, there are twenty separate beds, which give an average of 35 feet for the thickness of each flow. On the great range of sea-precipices along the west coast of Skye, which present the most stupendous section of the basalts anywhere to be seen within the limits of the British Islands, the average thickness of the beds can be conveniently measured. At the Talisker cliffs some of the flows are not more than 6 or 8 feet; others are 30 or 40 feet. The chief precipice, 957 feet high ([Fig. 286]), contains at least 18 or 20 separate lava-sheets, which thus average of from 47 to 53 feet in thickness. In the cliffs that form the seaward margin of the tableland of Macleod's Tables ([Fig. 283]) fourteen successive beds of basalt can be counted in a vertical section of 400 feet, which is equal to an average thickness of about 28 feet. But some of the basalts are only about 6 feet thick, while others are 50 or 60. The Hoe of Duirinish, 759 feet high, is composed of about sixteen distinct beds, which thus have a mean thickness of 46 feet. The average thickness of the successive flows on Dunvegan Head, which is 1000 feet high and contains at least twenty-five separate sheets, is about 40 feet. Still further north, the cliffs, 800 feet high, comprise sixteen successive flows, which have thus an average of 50 feet each. Among the Faroe Islands the average thickness of the basalt-sheets seems to be nearly the same as in Britain. Thus in the magnificent ranges of precipices of Kalsö, Kunö and Borö, forty or more sheets may be counted in the vast walls of rock some 2000 feet high, giving a mean of about 50 feet.
[227] See Explanation of Sheet 20, Geol. Survey, Ireland, p. 11.
Each bed appears, on a cursory inspection, to retain its average thickness, and to be continuous for a long distance. But I believe that this persistence is in great measure deceptive. We can seldom follow the same bed with absolutely unbroken continuity for more than a mile or two. Even in the most favourable conditions, such as are afforded by a bare sea-cliff on which every sheet can be seen, there occur small faults, gullies where the rocks are for the time concealed, slopes of debris, and other failures of continuity; while the rocks are generally so like each other, that on the further side of any such interruption, it is not always possible to make sure that we are still tracing the same bed of basalt which we may have been previously following. On the other hand, a careful examination of one of these great natural sections will usually supply us with proofs that, while the bedded character may continue well marked, the individual sheets die out, and are replaced by others of similar character. Cases may not infrequently be observed where the basalt of one sheet abruptly wedges out, and is replaced by that of another. Where both are of the same variety of rock, it requires close inspection to make out the difference between them; but where one is a green, dull, earthy, amorphous amygdaloid, and the other is a compact, black, prismatic basalt, the contrast between the two beds can be recognized from a distance ([Fig. 261]). In the basaltic cliffs of the west coast of Skye, the really lenticular character of the flows can be well seen. I may especially cite the great headland south of Talisker Bay, already referred to, where, in the pile of nearly horizontal sheets, two beds may be seen to die out, one towards the north, the other towards the south. Further north, in the cliff of the Hoe of Duirinish, a similar structure presents itself. Along the coast-cliffs of Mull, Morven and Canna the same fact is clearly displayed. Thus on the west side of the Sound of Mull the slopes above Fishnish Bay show a group of basalts, which die out southward, and are overlapped by a younger group that has been poured over their ends. Such sections are best seen in the evening, when the grass-covered lavas show their successive sheets by their respective shadows, their individuality being lost in the full light of day. A more striking example occurs beyond the west end of Glen More in Mull, where one series of basalts has been tilted up, probably during some volcanic episode, and has had a younger series banked up against its edges.
Fig. 261.—Termination of Basalt-beds, Carsaig, Mull.
In Antrim also, remarkable evidence is presented of the rapid attenuation not of single beds only, but of a whole series of basalts. Thus, at Ballycastle, the group of lavas known as the Lower Basalts, which underlie the well-known horizon of iron-ore, are at least 350 feet thick. But, as we trace them westwards, bed after bed thins out until, a little to the west of Ballintoy, in a distance of only about 6 miles, the whole depth of the group has diminished to somewhere about 40 feet. A decrease of more than 300 feet in six miles or 50 feet per mile points to considerable inequalities in the accumulation of the lavas. If the next series of flows came from another vent and accumulated against such a gentle slope, it would be marked by a slight unconformability. Structures of this kind are much rarer than we should expect them to be, considering the great extent to which the plateaux have been dissected and laid open in cliff-sections.
The basalt-plateau of the Faroe Islands exhibits with remarkable clearness the lenticular character of the basalt-sheets, and a number of examples will be cited in the description of that region to be given in Chapter xxxix. In these northern climes vegetation spreads less widely over rock and slope than it does in the milder air of the Inner Hebrides. Hence the escarpments sweep in precipices of almost bare rock from the level of the sea up to the serrated crests of the islands, some 2000 feet in height. Each individual bed of basalt can thus be followed continuously along the fjords, and its variation or disappearance can be readily observed. Coasting along these vast natural sections, we readily perceive that, as among the Western Isles, the successive sheets of basalt have proceeded from no one common centre of eruption. They die out now towards one quarter, now towards another, yet everywhere retain the universal regularity and gentle inclinations of the whole volcanic series.