VI. SKETCH OF THE GEOLOGY AND MINERALOGY OF THE SHIANT ISLANDS. BY PROFESSOR M. FORSTER HEDDLE, M.D. (President of the Mineralogical Society of Great Britain). Read 27th January, 1880. DURING the Tertiary epoch of the earth's formation, and probably in Miocene times, its internal fires found out a line of weakness or thinness of the crust, perchance an open rent; or if they did not find it out, they made it of themselves. This line extended in not very devious course, from Antrim, in Ireland, to Yan Mayen. This was a good long line, and afforded abundant space for the expending of their fury, and the belching forth of their smoke, and fire, and ashes. They exhibited local partialities, however. Concentrating their energies, they overflowed some 1200 square miles in the north of Ireland, made a whole island and a volcanic cone, probably of 14,000 feet in height, at Mull,-a hideous tumbled waste at Ardnamurchan, a grand crater, with an island of dependencies, in Skye, a smaller one in Rum,-little more than a cliff-lined chaldron at St. Kilda,-a sprinkling of terraced islands at Faröe,— a whole country at Iceland, and a cone of matchless beauty at the northern limits of their dwindling fires. There are great gaps in the uniformity of this disposition of the centres of energy; but there may have been many others of which no, or almost no relic now remains; for, after the fire, came the water-gravers, in the form of scalping rains, and rutting rivers, and gouging glaciers, and battering and undermining waves. Of the great Mull volcano only some 3000 feet remain; of that of Skye, little more; of that of Ardnamurchan only enough to vouch for its former existence. The waves work their will in the very throat of that of St. Kilda, and have cut huge lanes through the once continuous plateau of the Faröes. Aside of the smoke-associated lapilli and ashes of a volcano, which may be spattered anywhere, its lava-flows are divided by chemical requirements into well-marked classes; of these one is characterised by an excess of those ingredients which we term acid, while another is deficient in the normal amount of that ingredient, and is termed basic. Of these two classes of rocks the former is much the lighter, and, in the seething and boiling vortex of the volcanic heat, floats upon the surface of the latter. When, therefore, an eruptive-throe disgorges the liquid contents, the superior rock, being closer to the vent, is the first to be thrown out, while some residual portion at least of the latter must remain, or be thrown back by gravitation into the volcanic throat, to form a solid plug to its central and vertical shaft, upon the expiry and the abstraction of the heat. Such a central plug must be the last portion of a volcanic cone to be assailed by assaults from without : and, as it is not only the densest, but the hardest of the materials of the structure (from the prolonged continuance of its vitrification), it frequently long remains to vouch for the former existence of those. other rocks which had performed for it the function of a mould. The throats of former volcanoes may therefore present themselves to us either by a more or less circular vertical and highly vitrified shaft of dense and heavy rock, or by a somewhat more ample circlet of rocks which have moulded themselves against the retaining cliff-slopes of a crater. Such a crater-cast the Shiant Islands seem to be. They may represent the first of a line of orifices between Skye and the Faröes. Tachylite is the most perfectly vitrified form of a basic igneous rock; basalt is the next. The Shiants consist of basalt. But they also contain rocks other than those of volcanic origin. Some small portions of the land over whose planes the volcanic cone was thrust, remain, but they remain only because held in the grasp, and preserved from total ruin and removal by the durable material which now sheathes and almost envelopes them. These rocks consist of Liassic limestones and shales. They, however, repose in flat sheets upon igneous rock, though they are also overflowed by it. That igneous rock, therefore, formed the sea-bottom. upon which the silts and limestones of the Lias found a resting place; and as in the West of Scotland we find the Lias nowhere but in association with these igneous rocks, we may safely assign to them the function of having fitted the waters, in depth and perchance in temperature, to the necessities of a Liassic period and life. The Liassic land, moreover, may never have been dry land until during the last days of the volcanic agency; there is, at least, strong evidence that it was not so at the time of its overflow by the molten sheets. Dry land soon comes to be covered with soil, and clothed with vegetables; when this is overflowed by molten matter, the earth is burnt into a red brick-like clay, termed plynthite. Many successive beds of this are to be seen in Skye, but no trace of it appears between the Liassic schists and the basaltic overflow in the Shiants. Still, the form of the land leads to the conclusion that a sub-aerial crater had flung the light of its lurid fires -hailing St. Kilda from afar, Morvern, and the distant Mull-across the Laurentian fringe of the Long Isle-lighting, like a mid-ocean Pharos, a narrower and river-like Minch, and flashing ruddy from the cliffs of Torridon, or with snowy glimmer from the quartzite peaks of Ross. The Shiants consist of three islands: two, Eilan an Tigh and Garabli Eilan, which are connected by a low spit, lie in a north and south line; the third, Eilan Whirry, lies north.cast of these, and landlocks a deep-water cove, which may represent the former volcanic throat. The material of basalt, obeying the usual law, contracts during cooling, and if it be contact-bound at its extremities, it tears itself into rents and fissures in contracting; if the sheet of molten basalt be uniform in thickness, and if it be overlaid by rock of such thickness that its cooling must be a slow and regular process, these rents pass with singular regularity and persistence from top to bottom of the stratum, giving it an appearance and a structure similar to a multitude of pillars: to this structure the term basaltic has hence been applied. The southern island, though basaltic in structure, especially on its eastern face, does not exhibit any marked regularity in that structure; but it is from that regularity, or rather the surface-roughness consequent thereto, that the larger or northern island derives its name. Few things are rougher than an Elizabethan collar; and a circular Elizabethan collar held in a vertical position well represents the rock surface of this island. So uniformly is it girt with the plications of lofty pillars, that there are but two spots at which, in a circuit of perhaps two miles, it can be scaled. The feathered inhabitant, which rejoices in the happily-bestowed cognomen of "Tammy Cheeky," indulges very persistently in the gymnastic exercise of skipping from one fractured pillar-summit to another; it is only by assuming the function, with not a little of the concomitant assurance of the aforesaid bird, that the summit can be reached at one of these spots. Though falling far short of Staffa, and especially of the north-east coast of Skye, in the diversified features of basaltic scenery, this island very much surpasses these, and indeed all the basaltic scenery of the west, in the altitude of its pillared cliffs, and especially in the gloomy vista produced by their regularly extended sweep. In this last feature there is, indeed, no locality in Scotland which even approaches in effect the grandly-curved colonnade which extends for more than half-a-mile along the north frontlet of this island. The total height of the island is 523 feet, and the cliff edge is but 24 feet below the summit; from this grand altitude the shafts drop, sometimes with but a single joint, nearly plumb into the deep green water. As the eastern horn of the crescentic cliff projects boldly northward, it cuts off the rays even of the morning light, and throws both cliff-face and its pediment of waters into deep shade-with the occasional exception of a bastion-like cluster of grouped pillars, or perchance a chance-lit single shaft, which glints grandly from out the gloom,-five hundred feet of a streak of light set in the midst of darkness. When a boat is rowed into the deep shades of the western horn there is discovered a close-set range of incurved columns which surpass in chasteness of flexure and regularity anything of the kind which is to be seen in Staffa, and almost equal the clusters at the caves of Duntulm in Skye. Fully to appreciate basaltic scenery all its varieties should be seen, and the stupendous scale of this, enables one the more to appreciate even the model-like minuteness and formal regularity of the over-lauded Staffa. The vast difference in their dimensions must be apparent when it is remembered that the pillars of Fingal's Cave are 18 feet in altitude, while those of Garabh Eilan are 499! The Liassic rocks of this island first appear in a skerry-like ridge of half-submerged rocks on the north-east shore; rising with a gentle dip out of the waters they ascend into a low cliff, and cut through a projecting tongue. They consist of a lower bed of indurated clay slate, and an upper one which is converted almost into porcelain jasper. The locality is one of much mineralogical interest, as that at which Dr. Patrick Neill first discovered Wavellite in Scotland; and, with the exception of a single specimen found by the writer in Glencoe, it is still the only Scottish locality for the mineral. It occurs in general in dead-white flattened spheres of radiating crystals; these spheres are over half-an-inch in diameter. Occasionally it is lustrous and transparent. The flattened spheres line cracks in the rocks; this frequently has assumed a globular structure which is due to its being permeated with radiating and spherical groups of crystals of the mineral. Analcime in weathered specimens was also found; as also a crystal or two of indifferent Stilbite. Massive Mesolite also occurs, both here and filling the jointings of the basaltic pillars. Where this schist is seen on the south side of the prolonged eastern promontory of Garabh Eilan, it exhibits singular oval cavities lined with a coating a quarter of an inch in thickness of a material which has been set aside for analysis. These cavities have much more the appearance of being due to the former presence of organisms than of being druses. The writer having been unable to land upon Eilan Whirry, can only say of it that there is evidently a greater amount of the sedimentary rock there than occurs on Garabh Eilan, and that it seems in certain parts to be much dislocated and fractured. Of the Dolerite, which forms the pillars of these islands, it has to be noted that it frequently shows the same radiated structure which has been stated to occur in the schist. THE GLACIATION OF THE ISLAND. There may be said to be at present three theories as to the glaciation of the north of Britain. First, that which maintains that a great sheet of ice, of 2500 or 3000 feet in thickness, swept across the North Sea VOL. III. F |