THE CHEVIOT AND BERWICKSHIRE DISTRICT
In the south-east of Scotland, and extending thence into the north of England, the remains of several distinct volcanic centres of the Lower Old Red Sandstone may still be recognized. Of these the largest and most interesting forms the mass of the Cheviot Hills; a second has been partially dissected by the sea along the coast south from St. Abb's Head; while possibly relics of others may survive in detached bosses of eruptive rock which rise through the Silurian formations of Berwickshire. The water-basin in which these volcanic groups were active was named by me "Lake Cheviot,"[371] to distinguish it from the other basins of the same geological period ([Map I.]).
[371] Trans. Roy. Soc. Edin. xxviii. (1878), p. 354.
The volcanic rocks of the Cheviot Hills, though their limits have been reduced by faults, unconformable overlap of younger formations and severe denudation, still cover about 230 square miles of ground, and rise to a height of 2676 feet above the sea. As they have been mapped in detail by the Geological Survey, both on the English and the Scottish sides of the Border, their structure is now known.[372] No good horizontal section, however, has yet been constructed to show this structure—a deficiency which, it is hoped, may before long be supplied.
[372] The Geology of the Cheviot Hills is comprised in Sheets 108 N.E., 109 N.W., and 110 S.W. of the Geological Survey of England and Wales, and in Sheets 17, 18 and 26 of the Geological Survey of Scotland. For descriptive accounts the Memoirs to some of these Sheets may be consulted, particularly "Geology of the Cheviot Hills" (English side), by C. T. Clough (Mem. Geol. Surv. 1888); "Geology of Otterburn and Elsdon," by H. Miller and C. T. Clough (Mem. Geol. Surv. 1887); "Geology of Part of Northumberland between Wooler and Coldstream," by W. Gunn and C. T. Clough, with Petrographical Notes by W. W. Watts (Mem. Geol. Surv. 1895). Other descriptions have been published by Professor James Geikie, Good Words, vol. xvii. (1876), reprinted in Fragments of Earth-lore (1893), and by Prof. Lebour, Outlines of the Geology of Northumberland, 2nd edit. 1886. For the petrography of the rocks consult Mr. J. J. H. Teall, Geol. Mag. 1883, pp. 100, 145, 252, 344; 1884, p. 226; 1885, p. 106; Proc. Geol. Assoc. ix. (1886) p. 575; and his British Petrography, 1888; Dr. J. Petersen, Mikroskopische und chemische Untersuchungen am Enstatit-porphyrit aus den Cheviot Hills, Inaugural Dissertation, Kiel, 1884.
This volcanic pile, consisting mainly of bedded andesites which rest unconformably on the upturned edges of Wenlock shales and grits, presents a most typical display of the lavas of the Lower Old Red Sandstone. These rocks range from vitreous or resinous pitchstone-like varieties to coarsely porphyritic forms, on the one hand, and to highly vesicular and amygdaloidal kinds, on the other. Analyses of some of these rocks, and an account of their petrography, have already been given.
The lavas are often separated by thin partings of tuff, and their upper surfaces show the fissured character with sandstone infillings, so characteristic among the lavas of "Lake Caledonia."[373] Tuffs form a very subordinate part of the whole volcanic series. One of the most important bands is a thick mass at the base of the series, lying immediately on the highly inclined Silurian shales. The fragments are generally of a fine-grained purple mica-andesite, often two or three feet and sometimes at least five feet long. For a few feet near the bottom of this mass of tuff, pieces of Silurian shale an inch in length may be noticed. Mr. Clough remarks that distinct bedding is not usual among the tuffs. Though no doubt most of the fragmental materials really lie intercalated between successive lava-streams, yet some of the isolated patches of coarse volcanic breccia may mark the sites of eruptive vents. One such probable neck has been mapped on the Scottish side between Cocklawfoot, at the head of the Bowmont Water, and King's Seat, while others may perhaps occur among the detached patches that have been observed on the Northumbrian side. No thick conglomerates or sandstones have been noticed in the Cheviot District. The volcanic eruptions appear to have usually succeeded each other without the spread of any notable amount of ordinary detritus over the floor of the water-basin. It is difficult to estimate the total thickness of volcanic material here piled up, but it probably amounts to several thousand feet. The top of the series is not visible, having been partly removed by denudation and partly buried under the Carboniferous formations.
[373] Clough, Geology of the Cheviot Hills, p. 15.
It will thus be seen that the Cheviot area stands apart from the other volcanic districts of the Lower Old Red Sandstone in the great relative thickness of its accumulated lavas, the comparative thinness of its tuffs, and the absence of the thick intercalations of coarse conglomerate so abundantly developed among the volcanic series all over Central Scotland. But there is yet another characteristic in which this area is pre-eminently conspicuous. In the heart of the andesites lies a core of augite-granitite, around which these rocks are traversed with dykes.
This interesting granitic boss rises into the highest summit of the whole Cheviot range, and covers an area of rather more than 20 square miles. While its petrographical characters have been described by Mr. Teall, its boundary has been mapped by Mr. Clough, who found the line difficult to trace, owing partly to the prevalent covering of peat, and partly to the jagged and irregular junction caused by the protrusion of dykes from and into the boss. He obtained evidence that the granite has broken through the bedded andesites, and that it is in turn traversed by dykes composed of a material indistinguishable from that of some of the flows. He therefore considered that it is essentially of the same age as the rest of the volcanic series, and "not improbably the deep-seated source of it."[374] Mr. Teall also, from a chemical and microscopical examination of the rocks, drew a similar conclusion.[375]
[374] Op. cit. p. 24.
[375] Geol. Mag. 1885, p. 106.
The andesites around the granite have undergone contact-metamorphism, but the nature and extent of the change have not yet been studied. There occur around the granite many dykes of felsite and quartz-felsite, to the petrographical character of which reference has already been made. But the most abundant and remarkable dykes of the district are those of a reddish mica-porphyrite, of which Mr. Clough has mapped no fewer than forty, besides those in the granitic area. He has called attention to the significant manner in which all the dykes of the district tend to point in a general way to the great core of granite, as if that were the nucleus from which they had radiated.[376]
[376] Op. cit. pp. 26-28.
The central granite of the Cheviot Hills, with its peripheral dykes, has no accompanying agglomerates nor any decided proof that it ever communicated with the surface. When, however, we consider its petrographical and chemical constitution, its position as a core among the bedded lavas, and the intimate way in which it is linked with these rocks by the network of dykes, we are, I think, justified in accepting the inference that it belongs to the volcanic series. It possesses some curious and interesting features in common with the great granophyre bosses of Tertiary age in the Inner Hebrides. Like these it has no visible accompaniment of superficial discharges. Yet it may have ascended by means of some central vent or group of vents which, offering to it a weak part of the crust, allowed it to communicate with the surface and give rise to the outflow of lavas and fragmental ejections. In any case, it affords us a most interesting and instructive insight into one of the deeper-seated ducts of a volcanic region, and the relation of a volcanic focus to the ascent of the granitic magma.
About twenty miles to the north of the Cheviot Hills, and separated from them by the Carboniferous and Upper Old Red Sandstones which spread across the broad plain of the Merse, a group of volcanic rocks has been laid open in a singularly instructive manner along the coast of Berwickshire, between the village of Eyemouth and the promontory of St. Abb's Head. Not only the actual vents, but the lavas and tuffs connected with them, have there been admirably dissected by the forces of denudation.
That this volcanic area was quite distinct from that of the Cheviot Hills may be inferred from its coarse agglomerates, and from the fact that when the rocks are followed inland in a south-westerly direction, that is, towards the Cheviot area, they are found to diminish in thickness and to disappear among the ordinary sediments. For the same reason we may regard the area as independent of any vents which may have risen further west about Cockburn Law and the Dirrington Laws. Unfortunately, however, only a small part of the area comes into view, the rest of it lying beneath the waters of the North Sea.[377]
[377] This area lies in Sheet 34 Geological Survey of Scotland, and was described by myself in the Memoir to accompany that Sheet ("Geology of Eastern Berwickshire," 1864, p. 20). More recently the shore between St. Abb's Head and Coldingham has been re-mapped by Professor James Geikie who has also studied the microscopic character of the rocks, Proc. Roy. Soc. Edin. xiv. (1887).
Of the several vents dissected along this coast-line, one may be seen at Eyemouth, filled with a very coarse tumultuous agglomerate of andesite fragments embedded in a compact felspathic matrix, through which are scattered broken crystals of felspar, and imperfect tabular crystals of black mica. Another of similar character is exposed for more than a mile and a half along the shore at Coldingham. It contains blocks, sometimes more than a yard in diameter, of different varieties of andesite, and, as at Eyemouth, is much invaded by veins and bosses of intrusive andesite.
Fig. 98.—Section across the volcanic area of St. Abb's Head (after Prof. J. Geikie).
11. Silurian formations; 2. Lower Old Red Conglomerate and Sandstone; 3 3. Sheets of andesitic lava; 4. Volcanic tuffs, largely composed of scoriæ in the higher parts; 5. Volcanic agglomerate of neck on shore; 6. Intrusive andesites. f, Fault.
To the north of Coldingham, a series of bedded volcanic rocks which form the picturesque headland of St. Abb's Head, are, according to the estimate of Professor James Geikie, about 1000 to 1200 feet thick, but neither their bottom nor their top is seen. The same observer found them to consist of three groups of andesite sheets separated and overlain by bedded tuffs. The lowest lavas have their base concealed under the sea, and are covered by a thick band of coarse agglomeratic tuff, above which lies the second group of andesites, about 250 feet thick. An intercalation of various tuffs from 40 to 50 feet thick then succeeds, followed by the third lava-group, 250 or 300 feet in depth. The highest member of the series is a mass of bedded tuffs some 400 feet thick.
The andesites lie in beds varying from about 15 to about 50 feet or more in thickness. They are fine-grained, purplish-blue, or greyish-blue, often reddish rocks, of the usual type. Generally rather close-grained, they are not as a rule very porphyritic, but often highly scoriaceous and amygdaloidal, especially towards the top and bottom of each bed. The more slaggy portions are sometimes so filled in with fine tuff that the rock might be mistaken for one of fragmental origin.
The bedded tuffs are usually well stratified deposits. The most important band of them is that which forms the highest member of the volcanic series. It consists of successive beds that vary from fine red mudstones up to volcanic breccias with blocks one foot or more in diameter. The materials have been derived from the explosion of andesitic lavas. Most of the lapilli are vesicular or amygdaloidal, and many of them have evidently come from vitreous scoriaceous lavas. Professor Geikie remarks that "from their highly vesicular character, they might well have floated in water at the time of their ejection—they are in short mere cinders." He could detect no trace of ordinary sediment in the matrix, the whole material being thoroughly volcanic in origin.
The lavas, tuffs and agglomerates have been abundantly invaded by intrusive rocks, chiefly andesites.[378]
[378] See Prof. J. Geikie, op. cit.
The agglomerates of this Berwickshire coast extend for a short way inland from the Coldingham and Eyemouth vents, but the fragmental material soon becomes finer and more water-rolled, and assumes a distinctly stratified structure, as it is gradually and increasingly interleaved with layers of ordinary sediment. Hence in passing towards the south-west, away from the coast-line, we are obviously receding from the vents of eruption and entering into the usual non-volcanic deposits of the time. That these deposits belong to the Lower Old Red Sandstone was first ascertained during the progress of the Geological Survey in this district by the discovery of abundant plant-remains in the form of linear grass-like strips, and also pieces of Pterygotus in some of the green shales interstratified among fine tuffs and ashy sandstones.[379] Before the volcanic detritus disappears from the strata as they are followed in a south-westerly direction, the whole series is unconformably overlain by the Upper Old Red Sandstone. The lower division of the formation is not again seen until it rises from under the southern margin of the plain of the Merse into the Cheviot Hills.
[379] "Geology of Eastern Berwickshire," Mem. Geol. Surv. Scotland (1864), pp. 26, 27, 57.
About ten miles to the south-west of the large Coldingham neck the great boss of Cockburn Law and Stoneshiel Hill rises out of the Silurian rocks.[380] Five miles still further in the same direction the group of the beautiful cones of Dirrington ([Fig. 70]) overlooks the wide Merse of Berwickshire,[381] and six miles to the north of these hills, in the very heart of Lammermuir, lies the solitary boss of the Priestlaw granite.[382] To these protrusions of igneous material reference has already been made as possible volcanic vents connected with the eruptions of the Lower Old Red Sandstone. As regards their age they must certainly be younger than the Llandovery rocks which they disrupt, and older than the Upper Old Red Sandstone, of which the conglomerates, largely made from their debris, lie on them unconformably. It seems therefore probable that these great bosses may form a part of the volcanic history of the Lower Old Red Sandstone period. But no positive proof has yet been obtained that any one of them was the site of an eruptive vent, and no trace has been detected around them of any lavas or tuffs which might have proceeded from them.
[380] See "The Geology of Eastern Berwickshire" (Sheet 34), Mem. Geol. Surv. Scotland (1864), p. 29.
[381] These hills are chiefly represented in Sheet 25. But see "The Geology of East Lothian," Mem. Geol. Surv. Scotland (1866), p. 26.
[382] "Geology of East Lothian," Mem. Geol. Surv. Scotland, p. 15, and authorities there cited.