CHAPTER XLII

THE BASIC SILLS OF THE BASALT-PLATEAUX

We have now followed the distribution of the basalt-plateaux, the arrangement of their component materials which were erupted at the surface, and the character of the dyke-fissures and vents from which these materials were ejected. But there remains to be considered an extensive series of rocks which display some of the underground phenomena of the Tertiary volcanoes. The injection of many basaltic sheets had been clearly enforced by Macculloch. In 1871 I pointed out that at different horizons in the plateau-basalts, but especially at their base and among the stratified rocks underneath them, sheets of basalt and dolerite occur which, though lying parallel with the stratification of the volcanic series, are not truly bedded, but intrusive, and therefore younger than the rocks between which they lie.[309] The non-recognition of their true nature had led to their being regarded as proofs of volcanic intercalations in the Jurassic series of Scotland. There is, however, no trace of the true interstratification of a volcanic band in that series, every apparent example being due to the way in which intrusive sheets simulate the characters of contemporaneous flows.

[Footnote 309: _Quart. Jour. Geol. Soc._ xxvii. (1871), p. 296.]

If such sheets had been met with only at one or two localities, we might regard them as due to some mere local accident of structure in the overlying crust through which the erupted material had to make its way. But when we find them everywhere from the cliffs of Antrim to the far headlands of Skye and the Shiant Isles, and see them reappear among the Faroe Islands, it is obvious that, like those of Palæozoic time, they must be due to some general cause, and that they contain the record of a special period or phase in the building up of the Tertiary volcanic tablelands. I will first describe some typical examples of them from different districts, and then discuss their probable relations with the other portions of the plateaux.

i. ANTRIM

First to be examined, and now most familiar to geologists, are the remarkable sheets that underlie the plateau of Antrim, and project at various parts of the picturesque line of coast between Portrush and Fair Head. From the shore at Portrush, as I have already remarked, came the evidence that was supposed to prove basalt to be a rock of aqueous origin, inasmuch as shells were obtained there from what was believed to be basalt. The long controversy to which this supposed discovery gave rise is one of the most curious in the history of geology.[310] It continued even after the illustrious Playfair had shown that the pretended basalt was in reality highly indurated shale, and hence that, instead of furnishing proof of the aqueous formation of basalt, the Portrush sections only contributed another strong confirmation of the Huttonian theory, which claimed basalt to be a rock of igneous origin.

[Footnote 310: For an excellent summary of this controversy and an epitome of the descriptions of the Portrush section, see the _Report on the Geology of Londonderry_, etc. (_Mem. Geol. Survey_), by J. E. Portlock (1843), p. 37.]

It is now well known that the rock which yielded the fossils is a Liassic shale, that it is traversed by several sheets of eruptive rock, and that by contact-metamorphism it has been changed into a highly indurated substance, breaking with a splintery, conchoidal fracture, but still retaining its ammonites and other fossils. The eruptive material is a coarse, distinctly crystalline dolerite, in some parts of which the augite, penetrated by lath-shaped crystals of plagioclase, is remarkably fresh, while the olivine has begun to show the serpentinous change along its cracks.[311] This rock has been thrust between the bedding planes of the shales, but also breaks across them, and occurs in several sheets, though these may all be portions of one subterranean mass. Some of the sheets are only a few inches thick, and might at first be mistaken for sedimentary alternations in the shale. But their mode of weathering soon enables the observer readily to distinguish them. It is to be noticed that these thin layers of eruptive material assume a fine grain, and resemble the ordinary dykes of the district. This closeness of texture, as Griffith long ago pointed out,[312] is also to be noticed along the marginal portions of the thicker sheets where they lie upon or are covered by the shales. But away from the surfaces of contact, the rock assumes a coarser grain, insomuch that in its thickest mass it presents crystals measuring sometimes an inch in length, and then externally resembles a gabbro. A more curious structure is shown in one of these coarsely crystalline portions by the occurrence of a band a few inches broad which is strongly amygdaloidal, the cells, sometimes three inches or more in diameter, being filled with zeolites.[313] The general dip of the shales and of the intrusive sheets which have been injected between them is towards the east. From underneath them a thick mass of dolerite rises up to form the long promontory that here projects northwards from the coast-line, and is prolonged seawards in the chain of the Skerries.

[Footnote 311: Dr. F. Hatch, Explanation of Sheets 7 and 8, _Geol. Survey of Ireland_, p. 40.]

[Footnote 312: "Address to Geological Society of Dublin, 1835," p. 13, _Jour. Geol. Soc. Dublin_, vol. i. The varieties of the Portrush rock were described by the late Dr. Oldham, in Portlock's _Report on the Geology of Londonderry_, p. 150; see also the same work for Portlock's own remarks, p. 97.]

[Footnote 313: For a list of the minerals in this rock, see Oldham, _op. cit._ p. 151.]

An interesting feature of the Portrush sections is the clear way in which they exhibit the phenomena of "segregation-veins"--so characteristic of the thicker and more coarsely crystalline sills. These veins or seams here differ from the rest of the rock mainly in the much larger size and more definitely crystalline form of their component minerals. Though sharply defined, when looked at from a little distance, they are found on closer inspection to shade into the surrounding rock by a complete interlacing of crystals. On the shore, they can be seen to lie, on the whole, parallel with the bedding of the sheets in which they occur, but without rigidly following it, since they undulate and even ramify. A good section across their dip has been exposed in a quarry near the end of the promontory, and shows that they are considerably less regular than the plan of their outcrop on the shore would have led us to anticipate. The accompanying drawing (Fig. 314) represents the veins laid bare on a face of rock nine feet in length by five feet in height. It will be seen that while there is a general tendency to conform to the dip-slope, which is here from right to left, the seams or layers unite into a large rudely-bedded mass, which sends out processes at different angles. The peculiar aggregation of minerals which distinguishes such veins is perhaps best seen at Fair Head, and I reserve for the description of that locality what I have to say on the subject, only remarking with regard to the Portrush rock that the felspar shows a disposition to collect in the centre of the veins with the augite and the other dark minerals at the outer margins.

[Illustration: Fig. 314.--View of "Segregation-Veins" in a dolerite sill, Portrush, Antrim.]

The contact-metamorphism at this locality is of more historical interest in connection with the progress of geological theory than of scientific importance. It consists mainly in an intense induration of the argillaceous strata. These pass here from their usual condition of fissile, laminar, dull, dark shales into an exceedingly compact, black, flinty substance, which in its fracture, colour and hardness reminds one of Lydian stone. Yet the ammonites and other organic remains have not been destroyed. They are preserved in pyrites.

[Illustration: Fig. 315.--View of Fair Head, from the east, showing the main upper sill and a thinner sheet cropping out along the talus slope.]

Of all the examples of Tertiary sills in Britain few are more imposing than that of the noble range of precipices which form the promontory of Fair Head. Leaving out of account the minor masses of eruptive rock which occur underneath it, we find the main sheet to extend along the coast for nearly four miles, to rise to a height of 636 feet above the sea, and to attain a maximum thickness of 250 feet. This enormous bed dies out rapidly both to the east and west, and seems also to thin away inland. Seen from the north, it stands upon a talus of blocks as a sheer vertical wall, 250 feet high, and the rude prisms into which it is divided are continuous from top to bottom (Fig. 315). So regular is this prismatic structure, and so much does it recall the more minute columnar grouping of the bedded basalts, that at a little distance we can hardly realize the true scale of the structure. It is only when we stand at the base of the cliff or scramble down its one accessible gully, the "Grey Man's Path," that we appreciate how long and thick each of the prisms actually is (Fig. 316). It may here be remarked that this regular prismatic jointing is one of the distinguishing features of the large sills, and serves to mark them off from the bedded basalts, even when these have assumed a columnar structure. The prisms are much larger than the basalt-columns, and never display the irregular starch-like arrangement so common among the plateau-basalts.

[Illustration: Fig. 316.--View of Fair Head from the shore. (From a Photograph by Mr. R. Welch.)]

The rock composing this magnificent sheet is a coarsely crystalline, ophitic, olivine-dolerite.[314] The same diminution of the component crystals, which is so marked along the margins of the eruptive masses at Portrush, is strikingly exhibited at the borders of the Fair Head sill. For about 18 or 20 inches upward from the bottom, where the bed rests on the black, Carboniferous shales, the dolerite is dark and finely crystalline, weathering spheroidally in the usual manner. But immediately above that bottom layer of closer grain, the normal coarsely crystalline texture rapidly supervenes. A similar closeness of grain is observable at the surfaces of contact where the sheet splits up on its western border.

[Footnote 314: Professor Judd has described what he calls a "glomero-porphyritic structure" in this rock (_Quart. Journ. Geol. Soc._ xlii. (1886), p. 71).]

Nowhere, so far as I know, can the phenomena of "segregation-veins" be so instructively studied as along the abundant exposures of this great sheet. The veins are most conspicuous where the rock occurs in thickest mass. They vary up to three or four feet in thickness, and, as at Portrush and elsewhere, lie on the whole parallel to the upper and under surfaces of the sheet. An erroneous impression may be conveyed by the term "veins" applied to them. They are quite as much layers, parallel on the whole with the bedding of the sheet, yet not adhering rigidly to one plane, but passing across here and there from one horizon to another. That they are not due to any long subsequent protrusion of younger material through the main sheet is made manifest by the thorough interlocking of their component crystals with those of the body of the rock in which they lie. The material that fills these veins has obviously been introduced into them while there was still some freedom of movement among the crystals of the surrounding rock, which must thus have been still not quite consolidated and therefore intensely hot. Both crystallized slowly, and in so doing their component minerals dovetailed with each other. The constituents of the veins consist of an exceedingly coarse aggregate of crystals, or rather of crystalline lumps of the same minerals that constitute the general mass of the rock, the felspar and augite showing the ophitic intergrowth of the main rock, but on a far larger scale. Some of the pieces of augite measure two inches or more in diameter. The conditions under which these veins were produced must have differed in some essential respects from those that prevailed during the formation of the fine-grained, highly siliceous veins already described as occurring in some dykes and sills.

This great Fair Head sill lies upon Carboniferous strata, but that it is to be classed with the Tertiary volcanic series is, I think, demonstrated by its relations to the Chalk at its eastern end. It has there broken through that rock, and converted it for a short distance into a white, granular marble. But it is at the western side that the most interesting sections occur to show the truly intrusive nature of the mass. The rock there splits up into about a dozen sheets, which, keeping generally parallel with each other, have forced their way between and partly across the bedding planes of the Carboniferous shales (Fig. 317). In this way the huge, unbroken mass, 250 feet thick, subdivides itself and disappears in a few hundred yards, though it continues a little further inland, and approaches the shore again half a mile to the south-west. Further evidence of the intrusive nature of this rock may be observed along the base of the precipice, where at least one sheet 70 feet thick diverges from the main mass and runs eastwards between the Carboniferous shales (Fig. 315). At the contact with the eruptive rock the shales are everywhere much indurated.

[Illustration: Fig. 317.--Section at Farragandoo Cliff, west end of Fair Head, showing the rapid splitting up and dying out of an Intrusive Sheet.

_a_, Carboniferous sandstone; _b_, Carboniferous shale; _c_, intrusive sheet.]

ii. SKYE

All through the Inner Hebrides the base of the basalt-plateaux presents abundant examples of sills. The general parallelism of these intrusive sheets to the bedding of the Jurassic strata among which they lie has been above referred to as having given rise to the erroneous conclusion that in Skye and elsewhere the basalts are interstratified with Jurassic rocks, and are consequently of Jurassic age. It was Macculloch who first described and figured in detail the proofs of their intrusive nature. His well-known sections in plate xvii. of the illustrations to his work on the _Western Islands_ have been repeatedly copied, and have served as typical figures of intrusive igneous rocks.

Nowhere in North-Western Europe can the phenomena of sills be studied so fully and with such exuberance and variety of detail as in the island of Skye and its surrounding islets. On the western coast the greater subsidence of the basaltic plateau has for the most part submerged the platform of intrusive sheets, though wherever the base of the bedded lavas is brought up to the surface the accompanying sills are exposed to view. The east coast of the island has been classic ground for this part of volcanic geology since it supplied the materials for Macculloch's descriptions and diagrams. From the mouth of Loch Sligachan to Rudha Hunish, at the north end of Skye, a series of sills may be traced, sometimes crowning the cliffs as a columnar mural escarpment, sometimes burrowing in endless veins and threads through the Jurassic rocks. The horizontal distance to which this continuous band of sills extends in Skye is not far short of 30 miles. But it stretches beyond the limits of the island. It forms the group of islets which prolongs the geological structure and topographical features of Trotternish for 4 miles further to the north-west. It reappears 10 miles still further on in the Shiant Isles. Thus its total visible length is fully 40 miles, or if we include some outlying sills near the Point of Sleat, to be afterwards described, it extends over a distance of not less than 60 miles. From the last outlier in Skye to the sills of the Isle of Eigg is a distance of only 8 miles, thence to those of Ardnamurchan 17 miles, and to those of the south coast of Mull 25 miles. Thus this platform of intrusive sheets of the Inner Hebrides can be interruptedly followed for a space of not less than 110 miles.

[Illustration: Fig. 318.--View of the Trotternish Coast, showing the position of the band of Sills.

The dark band crowning the first slope above sea-level marks a conspicuous band of sills which towards the right descends to the beach and is prolonged seaward in the group of islands. The Storr Rock appears as a slanting obelisk of rock on the hill to the left.]

Though none of the sills in Skye itself attain the dimensions of the Fair Head sheet, they present a greater variety of rock and of geological structure than is to be found in Antrim. They are specially developed at the base of the thick, overlying, basalt-plateau--a platform on which such a prodigious quantity of eruptive material has been injected. Part of this material consists of basic rocks in the form of dykes, veins, or sills; part of it is included in the intermediate and acid groups, and comprises veins, sheets, and bosses of granitoid, felsitic, rhyolitic, trachytic, and pitchstone rocks. One of the peculiarities of the Skye sills is the occurrence among them of compound examples, where sheets of basic and acid material have been injected along the same general platform. These will be more specially referred to in