CHAPTER XLI

THE ERUPTIVE VENTS OF THE BASALT-PLATEAUX

Vents filled with Basalt or other Lava-form Rock--Vents filled with Agglomerate

It is one of the most interesting points in the Tertiary volcanic history that, in spite of the enormous geological revolutions that have passed since they became extinct, the sites of many scattered vents can still be recognized. A far greater number must lie buried under the basalts, and of others the positions are concealed by the sea, which now covers so large an area of the old lava-fields. Nevertheless, partly within the area of the plateaux, but still more on the surrounding tracts from which the basalts have been removed by denudation, the traces of unmistakable vents of discharge may be recognized amid the general wreck.

In Britain and the Faroe Isles, it is chiefly along the coast-line that the process of denudation has revealed the volcanic vents of Tertiary time. The interior of the country is often loaded with peat, covered with herbage, or strewn with glacial detritus: and even where indications of the vents are to be detected, it is not always possible to ascertain their true limits and connections. But where the structure of the plateaux has been laid bare along ranges of rocky precipice, the vents have sometimes been so admirably dissected by the sea that every feature of their arrangements can be satisfactorily determined.

As the actual physical connexion of these volcanic orifices with the plateaux has been in most cases removed by denudation, we can usually only by inference place them in what was probably their true relation to the plateau-eruptions. Those which project from the surface of the plateaux must, of course, be younger than the basalts through which they rise; how much younger we cannot tell. They may possibly be later than any of the plateau-sheets; they may even belong to a subsequent and waning condition of volcanic action. On the other hand, the vents which can now be traced outside of the present limits of the edges of the plateaux may, like those just mentioned, be younger than the basalt-sheets, or, on the contrary, they may be records of a period of eruptivity anterior to the emission of any of the rocks of the plateaux, and may have been deeply buried under a mass of basalt-beds subsequently removed. Positive demonstration is, from the nature of the case, impossible in these instances. But examples will be cited from the Western Isles and from Faroe, where the vents can be proved to belong to the time of the plateau-eruptions, for they are seen to have broken through some of the basalt-sheets and to have been buried under others. With this clear evidence of relationship in some cases, there need be little hesitation in believing that in other instances where no such positive connexion can be found, but where the vents are obviously such as the general structure of the plateaux would have led us to expect, they may be confidently regarded as part of the phenomena of the plateau-eruptions.

Sometimes the vents can be linked with lines of fissures or dykes. This is especially the case where they are small in size. More usually, however, no such relation can be demonstrated. It will be remembered that among the modern Icelandic eruptions, some eruptive vents, like the later cinder-cones of Laki, are ranged in a linear direction along the great fissure, while others, of an older series in the same district, almost engulphed amidst the more recent lavas, are clustered irregularly in groups. A similar diversity of arrangement has been observed among the volcanic cones of the Velay in Central France.

Considering as a whole the volcanic necks or eruptive vents which rise from the older rocks around the Tertiary basalt-plateaux, and sometimes even from the surface of these plateaux themselves, we may conveniently follow the same classification as was adopted in dealing with those of Palæozoic age, and, according to the nature of the material that now fills them, arrange them in two series: (1) Those occupied by some form of crystalline eruptive rock, and (2) those filled with volcanic agglomerate.

i. VENTS FILLED WITH DOLERITE, BASALT, ETC.

These, as the composition of the plateaux would lead us to anticipate, are numerous. They perhaps attain their most conspicuous development in Antrim, either on the tableland or among the underlying rocks round its edges. The finest example in that district is undoubtedly furnished by the lofty eminence called Slemish, which rises above the surrounding basalt-terrace, to a height of 1437 feet above the sea (Fig. 294). It is elliptical in ground-plan, measuring some 4000 feet in length by 1000 in breadth. Seen from the north, it appears as a nearly perfect cone. The material of which it consists is a coarsely crystalline olivine-dolerite, presenting under the microscope a nearly holocrystalline aggregate, in which the lath-shaped felspars penetrate the augite, with abundant fresh olivine, and wedge-shaped patches of interstitial matter. The rock is massive and amorphous, except that it is divided by parallel joints into large quadrangular blocks like a granitic rock, and wholly different from the character of the surrounding basalts. The latter, which possess the ordinary characters of the rocks of the plateaux, can be followed to within 80 yards of this neck, which rises steeply from them, but their actual junction with it is concealed under the depth of talus.

At the nearest point to which the two rocks are traceable, the basalts appear somewhat indurated, break with a peculiar splintery fracture, and weather with a white crust. These characters are still better shown on abundant fragments which may be picked up among the debris further up the slope. There can be no doubt, I think, that a ring of flinty basalt, differing considerably in texture from the usual aspect of that rock in the district, surrounds the neck. The meaning of this ring will be more clearly seen from the description of another example in Mull. About four miles to the north-east of Slemish, a smaller and less conspicuous neck rises out of the plateau-basalts. The rock of which it consists is less coarsely crystalline than that of Slemish, but its relations to the surrounding volcanic rocks are obviously the same. On the west side of Belfast Lough a boss of similar rock, about 1200 feet in diameter, rises at the very edge of the basalt escarpment into the eminence known as Carnmony Hill (Fig. 295). On its northern side it presents along its wall a mass of interposed volcanic agglomerate.[295] On visiting with Mr. M'Henry the quarry opened on the eastern face of this vent, I was much struck with the remarkable cellular structure of some parts of the dolerite. Many of the vesicles are lined with a thin pellicle of black glass, and the same substance occurs in minute patches in the body of the rock. A thin slice exhibiting this structure was found by Mr. Watts to possess the following characters:--"The rock is an ophitic dolerite consisting of plagioclase, augite, and iron ores, without olivine, enclosing one or two patches of finer basalt. The vesicles in the latter, and certain angular spaces between the crystals of the former, have been wholly or partially filled with brown glass, the outer part of which has been converted into radiating crystals of a brown mineral." The occurrence of patches of glass which seem to have been squeezed into vesicles or cracks in the body of a dolerite or andesite has been noticed in some of the Tertiary dykes. But in the present case the glass occurs as a mere coating on the walls of the larger spheroidal vesicles, the interior of which generally remains empty.

[Footnote 295: This neck was recognised by Du Noyer in 1868 as "one of the great pipes or feeders of the basaltic flows." See Prof. Hull, Explanation of Sheets 21, 28 and 29, _Geol. Survey of Ireland_ (1876), p. 30.]

Of the other doleritic necks scattered over the surface of the Antrim plateau, I will refer to only one which occurs on the hillslopes between Glenarm and Larne. It forms a prominence known as the Scawt Hill, and consists of a boss of basalt, which, in rising through a vent in the plateau-sheets, has carried up with it and converted into marble a large mass of chalk which is now exposed along its eastern wall (Fig. 296).

As examples of similar necks which have been exposed by denudation outside the present limits of the same plateau, I may allude to those which rise through the Cretaceous and other Secondary strata on the northern coast near Ballintoy. One of the most striking of these may be seen at Bendoo, where a plug of basalt, measuring about 1400 feet in one diameter and 800 feet in another, rises through the Chalk, and alters it around the line of contact (Fig. 297). Another remarkably picturesque example is to be seen near Cushendall, where a prominent doleritic cone rises out of the platform of Old Red Sandstone, some distance to the north of the present edge of the volcanic escarpment (Fig. 298).

The greater coarseness of grain of the material filling these pipes, compared with that of the sheets in the terraces, is only what the very different conditions of cooling and consolidation would lead us to expect. There is no essential difference of composition between the two rocks. Where the erupted material has been poured out at the surface, it has assumed a finely crystalline texture, while, where it has slowly solidified within a volcanic pipe at some depth beneath the surface, and where consequently its component crystals have had more time for development, the resulting structure is much more largely crystalline, with a more or less complete development of the ophitic structure.

In the island of Mull, another instance of the same kind of vent has been observed and described by Professor Judd.[296] It rises in the conspicuous hill, 'S Airde Beinne (Sarta Beinn), about two miles south-west from Tobermory, and consists of a coarsely crystalline dolerite, which becomes finer in grain towards the outer margin (Fig. 299). No bedding, or structure of any kind beyond jointing, is perceptible in it. Examined in thin sections under the microscope, this rock is found to be another typical ophitic dolerite, consisting of lath-shaped felspars embedded in augite, with here and there wedge-shaped portions of interstitial matter and grains of olivine. Dr. Hatch found the felspars to contain spherical inclusions of devitrified glass, filled with black granules and trichites, and he observed that, under a high power, the interstitial matter is seen to consist mainly of a greenish-brown isotropic substance, in which are inclosed small crystals of augite, skeleton-forms and microlites of felspar, sometimes in stellate aggregates, as well as club-shaped, cruciform, arrow-headed and often crested microlites of magnetite.

[Footnote 296: _Quart. Jour. Geol. Soc._, xxx. (1874), p. 264.]

Towering prominently above the flat basalt sheets, this neck has an oval form, measuring about half a mile in length by a quarter of a mile in breadth. Its central portion, however, instead of rising into a rugged hill-top, as is usually the case, sinks into a deep hollow, which is filled with water, and reminds one of a true crater-lake (Figs. 299, 300). The middle of the neck is thus concealed from view, and we can only examine the hard prominent ring of dolerite that surrounds the tarn. The material occupying the hollow may be softer than that of the ring, and may have been scooped out by denudation. What we now see may not be the original surface, but may have been exposed after the removal of possibly hundreds of feet of overlying material. On the other hand, it is conceivable that the hollow is really a crater-lake which was filled up with detritus and may have been overspread with basalt, since removed. It may be suggestively compared with the crater-hollows revealed by denudation on the cliffs of Stromö and Portree Harbour, which will be described in a later part of this chapter. Possibly some more easily removable agglomerate, representing an eruption later than that of the dolerite, may occupy the centre of the volcanic pipe.

One of the most interesting features of this vent is to be found in its relation to the surrounding basalts. The marginal parts of the rock along the line of contact are much finer in grain than the rest, and have obviously cooled more rapidly. The contrast between them and the ordinary dolerite nearer the centre, however, cannot be properly understood, except in thin sections under the microscope. Dr. Hatch, to whom I submitted my specimens, observed that, in place of the structure above described, the marginal parts show an absence of the ophitic grouping except in small isolated patches. Instead of occurring in large grains or plates enveloping the felspars, the augite is found in numerous small roundish grains, together with grains of magnetite, in equal abundance and of similar size. The felspars are speckled over with opaque particles; olivine has not been detected.

For miles around the vent, the plateau-rocks are of the usual type--black, compact, sometimes amygdaloidal, alternating with more coarsely crystalline decomposing bands, the separation between different sheets being often marked by the ordinary red ferruginous partings. But around the margin of the neck, they have undergone a remarkable metamorphism. The portions of them which adhere to the outer wall of the neck have lost their distinct bedding, and have been, as it were, welded together into an indurated compact, black to dull-grey rock, so shattery and jointed that fresh hand-specimens, three or four inches in length, are not easily obtainable. Especially marked is one set of joints which, running approximately parallel, cause the rock to split into plates or slabs. These joints are sometimes curved. Yet, in spite of the alteration from its normal character, the basalt retains in places some of its more usual external features, such, for instance, as its amygdaloidal structure, the amygdales consisting of calcite, finely acicular mesotype, and other minerals.

Examined under the microscope, this altered basalt presents "a confused aggregate of colourless microlites (felspar?) and innumerable minute granules of magnetite, these two constituents being very unequally distributed. Sometimes the colourless portions preponderate, in other places the opaque granules are heaped together in black patches, which may possibly mark the position of fused augites."[297]

[Footnote 297: Notes by Dr. Hatch.]

In the zone of contact-metamorphism around some of the volcanic pipes in the plateaux, we see changes analogous to, but less developed than, those which have been superinduced on so large a scale round the great eruptive bosses of gabbro, granophyre, etc., that have broken up the terraced basalts along the west coast of Scotland. I shall accordingly return to this subject in connection with phenomena presented by these younger rocks (p. 386).