i. DISTRIBUTION OF THE PLATEAUX

Notwithstanding the effects of many powerful faults and extensive denudation, the general position of the Plateaux and their independence of each other can still be traced. They are entirely confined, as I have said, to the southern half of Scotland (see Map IV.). In noting their situations we are once more brought face to face with the remarkable fact, so strikingly manifested in the geological history of Britain, that volcanic action has been apt to recur again and again in or near to the same areas. The Carboniferous volcanic plateaux were poured out from vents, some of which not impossibly rose among the extinct vents of the Old Red Sandstone. Another fact, to which also I have already alluded as partially recognizable in the records of Old Red Sandstone volcanism, now becomes increasingly evident--the tendency of volcanic vents to be opened along lines of valley rather than over tracts of hill. The vents that supplied the materials of the largest of the Carboniferous volcanic plateaux broke forth, like the Old Red Sandstone volcanoes, along the broad Midland Valley of Scotland, between the ridge of the Highlands on the north and that of the Southern Uplands on the south. Others appeared in the long hollow between the southern side of these uplands, and the Cheviot Hills and hills of the Lake District. It is not a question of the rise of volcanic vents merely along lines of fault, but over broad tracts of low ground rather than on the surrounding or neighbouring heights. It can easily be shown that this distribution is not the result of better preservation in the valleys and greater denudation from the higher grounds, for, as has been already remarked in regard to the volcanoes of the Old Red Sandstone, these higher grounds are singularly free from traces of necks which, had any vents ever existed there, would certainly have remained as memorials of them. The following summary of the position and extent of the Plateaux will afford some idea of their general characters:--

1. The Clyde Plateau.--The chief plateau rises into one of the most conspicuous features in the scenery of Central Scotland. Beginning at Stirling, it forms the tableland of the Fintry, Kilsyth, Campsie and Kilpatrick Hills, stretching westwards to the Clyde near Dumbarton. It rises again on the south side of that river, sweeping southwards into the hilly moorlands which range from Greenock to Ardrossan, and spreading eastwards along the high watershed between Renfrewshire, Ayrshire, and Lanarkshire to Galston and Strathavon. But it is not confined to the mainland, for its prolongation can be traced down the broad expanse of the Firth of Clyde by the islands of Cumbrae to the southern end of Bute, and thence by the east of Arran to Campbeltown in Cantyre. Its visible remnants thus extend for more than 100 miles from north-east to south-west, with a width of some thirty-five miles in the broadest part. We shall probably not exaggerate if we estimate the original extent of this great volcanic area as not less than between 2000 and 3000 square miles.

It is in this tract that the phenomena of the plateaux are most admirably displayed. Ranges of lofty escarpments reveal the succession of the several eruptions, and the lower ground in front of these escarpments presents to us, as the result of stupendous denudation, many of the vents from which the materials of the plateau were ejected, while in the western portion of the area admirable coast-sections lay bare to view the minutest details of structure.[414]

[Footnote 414: This plateau is represented in Sheets 12, 21, 29, 30, 31 and 39 of the Geological Survey, and is described in the accompanying Memoirs as far as published. The eastern part of the Campsie Hills was surveyed by Mr. B. N. Peach, the western part by Mr. R. L. Jack, who also mapped the rest of the plateau to the Clyde, and a portion of the high ground of Renfrewshire and Ayrshire; the rest of the area, south to Ardrossan, was surveyed by myself. The tract from Stewarton to Strathavon was surveyed by Mr. James Geikie, the Cumbraes and Bute by Mr. W. Gunn, and southern Cantyre by Mr. R. G. Symes. The Campsie Hills have been partly described by Mr. John Young in the first volume of the _Transactions of the Glasgow Geological Society_. The occurrence of plants in the tuffs of the east coast of Arran was discovered by Mr. E. Wunsch. The Campbeltown igneous rocks were described by J. Nicol, _Quart. Journ. Geol. Soc._ viii. (1852), p. 406. See also J. Bryce's _Arran and Clydesdale_.]

It will be seen from the map (No. IV.), that the Clyde plateau extends in a general north-east and south-west direction. It is inclined on the whole towards the east, where, when not interrupted by faults, its highest lavas and tuffs may be seen to pass under the Carboniferous Limestone series. Its greatest elevations are thus towards its escarpment, which, commencing above the plains of the Forth a little to the west of Stirling, extends as a striking feature to the Clyde above Dumbarton. On the south side of the great estuary the escarpment again stretches in a noble range of terraced slopes for many miles into Ayrshire. It is well developed in the Little Cumbrae Island (Fig. 107), and in the south of Bute, where its successive platforms of lava mount in terraces and green slopes above the Firth. Even as far as the southern coast of Cantyre the characteristic plateau scenery reappears in the outliers which there cap the hills and descend the slopes (Fig. 108).

While the escarpment side of this plateau is comparatively unfaulted, so that the order of succession of the lavas and their superposition in the sedimentary rocks can be distinctly seen, the eastern or dip side is almost everywhere dislocated. Innumerable local ruptures have taken place, allowing the limestone series to subside, and giving to the margin of the volcanic area a remarkably notched appearance. To the effects of this faulting may be attributed the way in which the plateau has been separated into detached blocks with intervening younger strata. Thus a complex series of dislocations brings in a long strip of Carboniferous Limestone which extends from Johnston to Ardrossan, while another series lets in the limestone that runs from Barrhead to near Dalry. In each of these instances, the continuity of the volcanic plateau is interrupted. To the same cause we owe the occasional reappearance of a portion of the plateau beyond the limits of the main mass, as for instance in the detached area which occurs in the valley of the Garnock above Kilwinning.

Denudation has likewise come into play, not only in reducing the area of the plateau, but in isolating portions of it into outliers, with or without the assistance of faults. The site of the Cumbraes and Bute was no doubt at one time covered with a continuous sheet of volcanic material, and there appears to be no reason for refusing to believe that this sheet formed part of that which caps the opposite uplands of Ayrshire. From the southern end of Bute it is only about seven miles across to the shore of Arran near Corrie, where the lavas and tuffs reappear. They are so poorly represented there, however, that we are evidently not far from the limit of the plateau in that direction. So vast has been the denudation of the region that it is now impossible to determine whether the volcanic ejections of Campbeltown, which occupy the same geological platform as those of Arran, Bute and Ayrshire, were also actually continuous with them. But as the distance between the denuded fragments of the volcanic series in Arran and in Cantyre is only about 20 miles it is not improbable that this continuity existed, and thus that the volcanic accumulations reached at least as far as the southern end of Argyllshire, where they now slip under the sea.

2. The East Lothian or Garleton Plateau.--Some 50 miles to the east of the Clyde volcanic district, and entirely independent of it, lies the plateau of the Garleton Hills in East Lothian, which, as its limits towards the east and north have been reduced by denudation, and towards the west are hidden under the Carboniferous Limestone series of Haddington, covers now an area of not more than about 60 square miles.[415] That the eruptions from this area did not extend far to the north is shown by the absence of all trace of them among the Lower Carboniferous rocks of Fife. A relic of them occurs above Borthwick, in Midlothian, about twelve miles to the south-west of the nearest margin of the plateau. The area over which the lavas and tuffs were discharged may not have exceeded 150 square miles. Small though this plateau is, it possesses much interest from the remarkable variety of petrographical character in its lavas, from the size and composition of its necks, and from the picturesque coast-line where its details have been admirably dissected by the waves. In many respects it stands by itself as an exception to the general type of the other plateaux.

[Footnote 415: This plateau is represented in Sheets 33 and 41 of the Geological Survey of Scotland, and is described in the Explanation to accompany Sheet 33.]

From its proximity to Edinburgh this volcanic area has been often studied and described. The memoirs of Hay Cunningham and Maclaren gave the fullest account of it until its structure was mapped by the Geological Survey. Its scenery differs from that of the other plateaux chiefly in the absence of the terraced contour which in them is so characteristic. The peculiar lavas of the Garleton Hills form irregularly-uneven ground, rising to not more than 600 feet above the sea. They slope gradually down to the coast, where a succession of fine sections of the volcanic series has been laid bare for a distance of altogether about ten miles. Nowhere, indeed, can the phenomena of the plateau-tuffs and their association with the Carboniferous strata be so well studied as along the coast-line from North Berwick to Dunbar. Among the necks of this plateau distinguished for their size, conspicuous prominence and component materials, the most important are those that form the conical eminences of North Berwick Law (Fig. 109), Traprain Law (Fig. 133), and the Bass Rock (Fig. 110).

3. The Midlothian Plateau.--On the same general stratigraphical horizon as the other volcanic plateaux, a narrow band of lavas and tuffs can be followed from the eastern outskirts of the city of Edinburgh into Lanarkshire, a distance of about 23 miles. It is not continuously visible, often disappearing altogether, and varying much in thickness and composition. This volcanic tract, which may be conveniently termed the Midlothian Plateau, is the smallest and most fragmentary of all the series. Its most easterly outliers form Arthur Seat and Calton Hill at Edinburgh.[416] Three miles to the south-west a third detached portion is known as Craiglockhart Hill. After another interval of ten miles, the largest remaining fragment forms the prominent ridge of Corston Hill (Fig. 111), whence a discontinuous narrow strip may be traced nearly as far as the River Clyde.

[Footnote 416: I formerly classed these eminences with the Puys, but I am now of opinion that they ought rather to be regarded as fragments of a long and somewhat narrow plateau. Their basic lavas and overlying sheets of porphyrite repeat the usual sequence of the plateaux, which is not met with among the Puys. But, as will be pointed out in the sequel, Arthur Seat in long subsequent time became again the site of a volcanic vent.]

The well-known Arthur Seat and Calton Hill have been fully described by Maclaren, and have been the subject of numerous observations by other geologists.[417] They have been likewise mapped in detail on a large scale by the Geological Survey, and have been described in the Survey Memoirs. The rest of the plateau to the south-west is much less familiar.

[Footnote 417: Maclaren's _Geology of Fife and the Lothians_, 1839, pp. 1-67; and Hay Cunningham, _Mem. Wer. Soc._ vii. pp. 51-62. The plateau is represented in Sheets 24 and 32 of the Geological Survey, and Arthur Seat and Calton Hill will be found on Sheet 2 of the Geological Survey map of Edinburghshire on the scale of 6 inches to a mile.]

In Fig. 112 the great escarpment which descends from the right towards the centre is the sill of Salisbury Crags. The long dark crag (Long Row) rising between the two valleys is the lowest of the interstratified lavas. The slope that rises above it has been cut out of well-bedded tuffs, on which lie the basalts and andesites in successive sheets that form all the eastern or left side of the hill. The rocks around the summit belong to a much later period of volcanic eruption, and are referred to in Chapter xxxi.

The rocks of this plateau are comparatively limited in thickness, and have a much more restricted vertical range than those of other districts. At Arthur Seat and Corston Hill they begin above the cement-stones and cease in a low part of the great group of white sandstones and dark shales which form the upper half of the Calciferous Sandstones of Midlothian. They do not ascend as high as the Burdiehouse Limestone, which to the west of Corston Hill is seen to come on above them. One of their most remarkable features is the manner in which they diminish to a single thin bed and then die out altogether, reappearing again in a similar attenuated form on the same horizon. This impersistence is well seen in the south-western part of the area, between Buteland, in the parish of Currie, and Crosswood, in the parish of Mid-Calder. The lowest more basic band may there be traced at intervals for many miles without the overlying andesitic group. Yet that andesites followed the basalts, as in other plateaux, is well shown by large remnants of these less basic lavas left in Arthur Seat and Calton Hill. On the extreme southern margin of the area also a thin band of porphyrite with a group of overlying tuffs is seen above the red sandstones near Dunsyre.[418] The eruptions over the site of this plateau seem to have been much more local and limited than in the other plateaux. They appear to have gathered chiefly around two centres of activity, one of which lay about the position of Edinburgh, the other in the neighbourhood of Corston Hill. It is worthy of remark that this tract of volcanic material flanks the much older range of lavas and tuffs of the Pentland Hills and wraps round the south-western end of this range, thus furnishing another illustration of the renewal of volcanic activity in the same region during successive geological periods.

[Footnote 418: _Explanation, Geol. Surv. Scotland_, Sheet 24, p. 13 (1869).]

4. The Berwickshire Plateau.--Another and entirely disconnected area occurs in the broad plain or Merse of the lower portion of the valley of the Tweed.[419] The northern limit of its volcanic tuff occurs in the River Whitadder above Duns, whence the erupted materials rapidly widen and thicken towards the south-west by Stitchell and Kelso, until they die out against the flanks of the Cheviot Hills. The eastern extension of the area is lost beneath the Cement-stone group which covers the Merse down to the sea. Its western boundary must once have reached far beyond its present limits, for the low Silurian ground in that direction is dotted over with scattered vents to a distance of ten miles or more from the present outcrop of the bedded lavas, extensive denudation having cleared away the erupted materials and exposed the volcanic pipes over many square miles of country. Among the more prominent of these old vents are the Eildon Hills, Minto Crags and Rubers Law, as well as many other eminences familiar in Border story.

[Footnote 419: This plateau is shown on Sheets 17, 25, 26 and 33 of the Geological Survey Map of Scotland. It was chiefly mapped by Prof. James Geikie and Mr. B. N. Peach.]

The bedded volcanic rocks of this area form a marked feature in the topography and geology of the district. They rise above the plain of the Merse as a band of undulating hills, of which the eminence crowned by Hume Castle, about 600 feet above the sea, is the most conspicuous height. In the geological structure of this part of Scotland they are mainly interposed between the Upper Old Red Sandstone and the base of the Carboniferous system, which they thus serve to divide from each other. But their lowest sheets appear to be in some places intercalated in the Old Red Sandstone, so that their eruption probably began before the beginning of the Carboniferous period. They form a band that curves round the end of the great Carboniferous trough at Kelso and skirts the northern edge of the andesites of the Lower Old Red Sandstone in the Cheviot Hills.

5. The Solway Plateau.--The last plateau, that of the Solway basin, though its present visible eastern limits approach those reached by the lavas from the Berwickshire area, was quite distinct, and had its chief vents at some distance towards the south-west.[420] On the north-western flanks of the Cheviot Hills, the Upper Old Red Sandstone is overlain by the lowest Carboniferous strata, without the intercalation of any volcanic zone, so that there must have been some intermediate ground that escaped being flooded with lava from the vents of the Merse on the one hand, and of the Solway on the other. The Solway lavas form a much thinner group than those of Berwickshire. From the wild moorland between the sources of the Liddell and the Rule Water, they run in a narrow and much-faulted band south-westward across Eskdale and the foot of Annandale, and are traceable in occasional patches on the farther side of the Nith along the southern flanks of Criffel, even as far as Torrorie on the coast of Kirkcudbright--a total distance of about 45 miles. It is probable that this long outcrop presents merely the northern edge of a volcanic platform which is mainly buried under the Carboniferous rocks of the Solway basin. Yet it exhibits many of the chief characters of the other plateaux, and even occasionally rivals them in the dignity of the escarpments which mark its progress through the lonely uplands between the head of Liddesdale and the Ewes Water (Figs. 113, 142).

[Footnote 420: For a delineation of the distribution and structure of this plateau see Sheets 5, 6, 10, 11 and 17 of the Geological Survey of Scotland. In the upper part of Liddesdale, Ewesdale and Tarras it was mapped by Mr. B. N. Peach; in lower Liddesdale and Eskdale by Mr. R. L. Jack and Mr. J. S. Grant Wilson; from Langholm to the Annan by Mr. H. Skae; and in Kirkcudbright by Mr. John Horne.]

The plateaux of the Merse and the Solway illustrate in a striking manner the distribution of the volcanic eruptions along valleys and low plains. The vents from which the lavas and tuffs proceeded are chiefly to be found on the lower grounds, though these bedded volcanic rocks rise to a height of 1712 feet (the Pikes) to the west of the Cheviot Hills. Between the Silurian uplands of Selkirkshire and Berwickshire on the north and the ridge of the Cheviot Hills on the south, the broad plain was dotted with volcanic vents and flooded with lava, while to the south-west the corresponding hollow between the uplands of Dumfries and Galloway on the one side, and those of Cumberland on the other, was similarly overspread. The significance of these facts will be more apparent when the grouping of the vents has been described. We shall then also be better able to realize the validity of the inference that the present plateaux are mere fragments of what they originally were, wide areas having been removed from the one side of them by denudation, and having been concealed on the other under later portions of the Carboniferous system.

The same two plateaux likewise supply further illustrations of the outflow of similar volcanic materials in the same locality at widely separated intervals of time. They may be traced up to and round the margin of the great pile of andesites of Lower Old Red Sandstone age forming the Cheviot Hills.