CHAPTER XXII

VOLCANOES OF THE UPPER OLD RED SANDSTONE--THE SOUTH-WEST OF IRELAND, THE NORTH OF SCOTLAND

In the northern half of Britain, where the Old Red Sandstone is so well displayed, the two great divisions into which this series of sedimentary deposits is there divisible are separated from each other by a strongly marked unconformability. The interval of time represented by this break must have been of long duration, for it witnessed the effacement of the old water-basins, the folding, fracture, and elevation of their thick sedimentary and volcanic accumulations, and the removal by denudation of, in some places, several thousand feet of these rocks. The Upper Old Red Sandstone, consisting so largely as it does of red sandstones and conglomerates, indicates the return or persistence of geographical conditions not unlike those that marked the deposition of the lower subdivision. But in one important respect its history differs greatly from that which I have sketched for the older part of the system. Though the Upper Old Red Sandstone is well developed across the southern districts of Scotland from the Ochil to the Cheviot Hills, and appears in scattered areas over so much of England and Wales, no trace has ever been there detected in it of any contemporaneously erupted volcanic rocks. The topographical changes which preceded its deposition must have involved no inconsiderable amount of subterranean disturbance, yet the volcanic energy, which had died out so completely long before the close of the time of the Lower Old Red Sandstone, does not appear to have been rekindled until the beginning of the Carboniferous period.

Two widely separated tracts in the British Isles have yielded traces of contemporaneous volcanic rocks in the Upper Old Red Sandstone. One of these lies in the south-west of Ireland, the other in the far north of Scotland.

THE SOUTH-WEST OF IRELAND

The Irish locality is situated a few miles to the south of the town of Limerick, where the Carboniferous Limestone has been thrown into long folds, and where, along the anticlines, strips of the underlying red sandstones have been brought up to the surface. Two such ridges of Upper Old Red Sandstone bear, each on its crest, a small but interesting relic of volcanic activity[394] (Map I.).

[Footnote 394: See Sheet 153 of the Geological Survey of Ireland, and Explanation to that Sheet (1861), by Messrs. G. H. Kinahan and J. O'Kelly. The account of the ground above given is from notes which I made during a personal visit.]

The more northerly ridge rises in the conical eminence of Knockfeerina to a height of 949 feet above the sea. Even from a distance the resemblance of this hill (Fig. 102) to many of the Carboniferous necks of Scotland at once attracts the eye of the geologist. The resemblance is found to hold still more closely when the internal structure of the ground is examined. The cone consists mainly of a coarse agglomerate, with blocks generally somewhat rounded and varying in size up to two feet in length. The most prominent of these, on the lower eastern slopes, are pieces of a fine flinty felsite weathering white, but there also occur fragments of grit and baked shale. The matrix is dull-green in colour, and among its ingredients are abundant small lapilli of a finely vesicular andesite or diabase. These more basic ingredients increase in number towards the top of the eminence, where much of the agglomerate is almost wholly made up of them. No marked dip is observable over most of the hill, the rock appearing as a tumultuous agglomerate, though here and there, particularly near the top and on the south side, a rude bedding may be detected dipping outwards. On the west side the agglomerate is flanked with yellow sandstone baked into quartzite, so that the line of junction there between the two rocks not improbably gives us the actual wall of the vent. The induration of the surrounding sandstones is a familiar feature among the Carboniferous vents. Some intrusive dark flinty rock traverses the agglomerate near the top on the north side.

Retiring eastwards from the cone, the observer finds evidence of the intercalation of tuff among the surrounding Upper Old Red Sandstone. At the east end of the village of Knockfeerina a red nodular tuff, with rounded pieces of andesite, grit and sandstone, sometimes 12 inches long, is seen to dip under yellow, grey and red sandstones and shales, while other shales and sandstones underlie this tuff and crop out between it and the agglomerate. There is thus evidence of the intercalation of volcanic tuff in the Upper Old Red Sandstone of this district. And there seems no reason to doubt that the tuff was ejected from the adjoining vent of Knockfeerina.

On the next ridge of Old Red Sandstone, which runs parallel to that of Knockfeerina at a distance of little more than a mile to the south, another mass of volcanic material rises into a prominence at Ballinleeny. On the north side it consists of agglomerate like that just described, and is flanked by sandstone baked into quartzite. Here again we probably see the edge of a volcanic funnel. There may possibly be more than one vent in this area. But well-bedded tuffs can be observed to dip away from the centre and to pass under sandstones and shales which are full of fine ashy material. Gradations can be traced from the tuff into ordinary sediment. In this instance, therefore, there is additional proof of contemporaneous volcanic action in the Upper Old Red Sandstone. There can be no uncertainty as to the horizon of the strata in which these records have been preserved, for they dip conformably under the shales and limestones at the base of the Carboniferous Limestone series. They are said to have yielded the characteristic fern _Palæopteris_ of Kiltorcan.[395]

[Footnote 395: There may be some other examples of Upper Old Red Sandstone volcanic rocks in Ireland which I have not yet been able personally to examine. On the maps of the Geological Survey (Sheet 198, and Explanation, pp. 8, 17) contemporaneous rocks of this age are marked as occurring at Cod's Head and Dursey Island, on the south side of the mouth of the Kenmare estuary.]

THE NORTH OF SCOTLAND

The only district in England or Scotland wherein traces of volcanic action during the time of the Upper Old Red Sandstone have been observed lies far to the north among the Orkney Islands, near the centre of the scattered outliers which I have united as parts of the deposits of "Lake Orcadie"[396] (Map I.). The thick group of yellow and red sandstones which form most of the high island of Hoy, and which, there can be little doubt, are correctly referred to the Upper Old Red Sandstone, rest with a marked unconformability on the edges of the Caithness flagstones (Fig. 103). At the base of these pale sandstones, and regularly interstratified with them, lies a band of lavas and tuffs which can be traced from the base of the rounded hills to the edge of the cliffs at the Cam, along the face of which it runs as a conspicuous feature, gradually sloping to a lower level, till it reaches the sea. At the Cam of Hoy it is about 200 feet thick, and consists of three or more sheets of andesite. The upper 50 or 60 feet show a strongly slaggy structure, the central portion is rudely columnar, and the lower part presents a kind of horizontal jointing or bedding. There can be no question that this rock is not a sill but a group of contemporaneous lava-flows. Beneath it, and lying across the edges of the flagstones below, there is a zone of dull-red, fine-grained tuff, banded with seams of hard red and yellow sandstone. This tuff zone dies out to the eastward of the Cam.

[Footnote 396: First noticed in _Geol. Mag._ February 1878; and _Trans. Roy. Soc. Edin._ xxviii. (1878), p. 411.]

A few miles south of the Cam the volcanic zone appears again as the platform on which the picturesque natural obelisk of the Old Man of Hoy stands. Here the lava runs out as a promontory from the base of the cliff, and on this projection the Old Man has been left isolated from the main precipice. The cliffs of Hoy are traversed by numerous small faults which have shifted the volcanic zone. But on the great face of rock behind the Old Man there appears to be a second volcanic zone lying several hundred feet above that just described. It is probably this upper zone which emerges from under the hills a mile and a half to the south at Black Ness in the bay of Rackwick. A good section is there visible, which is represented in Fig. 104. The ordinary red and yellow sandstones (_a_) appear from under the volcanic rocks at this locality, and stretch southwards to the most southerly headland of Hoy. The lowest volcanic band in the section is one of red sandy well-bedded tuffs (_b_). Some of the layers are coarse and almost agglomeratic, while others are fine marly and sandy, with dispersed bombs, blocks and lapilli of diabase and andesite. Hard ribs of a sandy tufaceous material also occur. These fragmental deposits are immediately overlain by a dark-blue rudely prismatic diabase with slaggy top (_c_). It is about 150 feet thick at its thickest part, but rapidly thins away in a westerly direction. It passes under a zone of red tuff (_d_) like that below, and above this highest member of the volcanic group comes the great overlying pile of yellow and reddish sandstone of Hoy. Followed westward for a short distance, the whole volcanic zone is found to die out and the sandstones below and above it then come together.

The interest of this little volcanic centre in Hoy is heightened by the fact that the progress of denudation has revealed some of the vents belonging to it. On the low ground to the east of the Cam, and immediately to the north of the volcanic escarpment, the flagstones which there emerge from under the base of the unconformable upper sandstones are pierced by three volcanic necks which we may with little hesitation recognize as marking the sites of vents from which this series of lavas and tuffs was discharged (Fig. 105). The largest of them forms a conspicuous hill about 450 feet high, the smallest is only a few yards in diameter, and rises from the surface of a flagstone ridge. They are filled with a coarse, dull-green, volcanic agglomerate, made up of fragments of the lavas with pieces of hardened yellow sandstone and flagstone. Around the chief vent the flagstones through which it has been opened have been greatly hardened and blistered. The most easterly vent, which has been laid bare on the beach at Bring, due east of Hoy Hill, can be seen to pierce the flagstones, which, although their general dip is westerly at from 10° to 15°, yet at their junction with the agglomerate are bent in towards the neck, and are otherwise much jumbled and disturbed.

On the northern coast of Caithness I have described a remarkable volcanic vent about 300 feet in diameter, which rises through the uppermost group of the Caithness flagstones. It is filled with a coarse agglomerate consisting of a dull-greenish diabase paste crowded with blocks of diabase, sometimes three feet in diameter, and others of red sandstone, flagstone, limestone, gneiss and lumps of black cleavable augite (Fig. 106).[397] The sandstones around it present the usual disrupted, indurated and jointed character, and are traversed by a small diabase dyke close to the western margin of the neck. Another similar neck has since been found by the officers of the Geological Survey on the same coast. That these volcanic orifices were active about the same time with those in the opposite island of Hoy may be legitimately inferred.

[Footnote 397: See _Trans. Roy. Soc. Edin._ xxviii. (1878), p. 405; also p. 482 of the same volume for an account of the cleavable augite.]

These northern volcanoes made their appearance in a district where during the preceding Lower Old Red Sandstone period there had been several widely separated groups of active volcanic vents. So far as the fragmentary nature of the geological evidence permits an opinion to be formed, they seem to have broken out at the beginning, or at least at an early stage, of the deposition of the Upper Old Red Sandstone, and to have become entirely extinct after the lavas of Hoy were poured forth. No higher platform of volcanic materials has been met with in that region. With these brief and limited Orcadian explosions the long record of Old Red Sandstone volcanic activity in the area of the British Isles comes to an end.[398]

[Footnote 398: There appear to be traces of volcanic eruptions contemporaneous with the Upper Old Red Sandstone of Berwickshire, but as they merely formed a prelude to the great volcanic activity of Carboniferous time, they are included in the account of the Carboniferous plateau of Berwickshire in Chapters xxiv. and xxv.]