Chapter XX
VOLCANOES OF THE LOWER OLD RED SANDSTONE OF "LAKE CALEDONIA"--continued
The Southern Chain--The Pentland Volcano--The Biggar Centre--The Duneaton Centre--The Ayrshire Volcanoes.
We have now to note the leading features of the groups of volcanic rocks distributed along the southern line of vents already described. At least four different centres of eruption may be observed on that line. Their mutual limits are, on the whole, better seen than those of the northern line, for from the north-eastern to the south-western end of the volcanic belt the Old Red Sandstone and rocks of older date are almost continuously exposed at the surface. The encroaching areas of Carboniferous formations in Lanarkshire and Ayrshire interrupt but do not entirely conceal the volcanic tracts.
II. THE SOUTHERN CHAIN OF VOLCANOES IN "LAKE CALEDONIA"
5. The Pentland Volcano
Beginning at the north-east end of the line we first come upon the classic area of the Pentland Hills, for the study of which the geologist is prepared by the admirable description of Charles Maclaren,[363] and the earlier geognostical papers of Jameson.[364] The area mapped in detail is represented in Sheet 32 of the Geological Survey of Scotland, published in 1859, and described in the Memoir accompanying that sheet.
[Footnote 363: A Sketch of the Geology of Fife and the Lothians, 1839. The detailed descriptions in this work are accompanied with a map and two plates of sections. In the map all the volcanic rocks are represented by one colour. In the sections the bedding of the rocks is shown, and an indication is given of the succession of their chief varieties.]
[Footnote 364: See specially Mem. Wernerian Soc. vol. ii.; also MacKnight in vol. i. The account of the Pentland Hills by Hay Cunningham in vol. vii. (1838) is clear but brief.]
When in these early days I surveyed this ground I found it extremely difficult to understand. Being then myself but a beginner in geology, and the study of old volcanic rocks not having yet advanced much beyond its elementary stage, I failed to disentangle the puzzle. Not until after more than twenty years, largely spent in the investigation of volcanic rocks elsewhere, had I an opportunity of resurveying the ground and bringing to its renewed study a wider knowledge of the subject. A new edition of the map was issued in 1892, and I shall here embody in my summary the chief results obtained in the course of this revision.
The most obvious features in the Pentland area are the marked development of the volcanic rocks at the north end of the chain, their rapid diminution and disappearance towards the south-west, the abrupt truncation of the bedded masses by the line of craggy declivity which forms the northern termination of the hills, and lastly, the continuation of the volcanic series northward in a totally different form in the lower eminences of the Braid Hills.
The length of the whole volcanic tract is about eleven miles; its breadth at the widest northern part is four miles, but from that maximum it dwindles southwards and dies out in seven miles. Its western side is in large measure flanked by the unconformable overlap of the Upper Old Red Sandstone and Lower Carboniferous formations, though in some places the base of the volcanic series is seen. The eastern boundary is chiefly formed by a large fault which brings down the Carboniferous rocks against the volcanic ridge. At the northern end, this ridge plunges unconformably under the Upper Old Red Sandstone of the southern outskirts of Edinburgh.
The bedded aspect of the truncated end of the Pentland chain, as seen from the north, has been already alluded to (p. 281). The rocks dip to the south-east, hence the lower members of the series are to be found along the north-west side of the hills.
It will be noticed from the Geological Survey map that the volcanic rocks of the main body of the Pentland Hills are arranged in alternations of somewhat basic and more acid bands. The most basic sheets are some amygdaloidal diabases at the bottom of the whole series which make their appearance in Warklaw Hill (Fig. 86). The greater number of the dark lavas are varieties of andesite, sometimes tolerably compact, sometimes highly cellular and amygdaloidal. But interstratified with these are thick sheets of what used to be called "claystone," a term which here comprised decayed felsites (orthophyres), and also felsitic tuffs and breccias. The remarkably acid nature of some of these rocks has been already pointed out.
The total thickness of the volcanic series at the north end of the hills is about 7000 feet, but as neither the top nor the bottom is there visible, it may be considerably greater. At these maximum dimensions the rocks form the high scarped front of the Pentland Hills, which rises into so prominent a feature in the southern landscape of Edinburgh. A series of transverse sections across the chain from north to south will illustrate its structure and history. These I shall here describe, reserving for subsequent consideration the great vent of the Braid Hills.
A section taken through the north end of the chain, where the maximum depth of volcanic material is exposed, presents the arrangement represented in Fig. 86. It will be seen that the base of the series is here concealed by the unconformable overlap of the Lower Carboniferous rocks on the west side, while the top is cut off by the great fault which on the east side brings down the Midlothian Coal-field.
The Lower Carboniferous conglomerates (4) creep over the edge and up the slopes of the volcanic series of the Pentland Hills. They contain abundant pebbles of the lavas, and were evidently laid down along a shore from which the Pentland rocks rose steeply into land. Though the actual base of the lavas is not seen here, two miles further to the south highly-inclined Upper Silurian shales and mudstones are found emerging unconformably from under the volcanic pile, and similar strata probably underlie Warklaw Hill as indicated in the figure. The Upper Silurian strata pass up into a lower group of the Lower Old Red Sandstone, which has also been covered unconformably by the volcanic series. In these underlying deposits we have evidence of the pre-volcanic accumulations of the lake, which were broken up and tilted at the beginning of the volcanic eruptions.
The lowest lavas, consisting of well-marked beds of diabase (2), present their escarpments to the north-west and dip into the rising ground, as sketched in Fig. 87. Their characters have been already noticed in the general petrography of the Old Red Sandstone volcanic rocks. Dark solid compact portions of them pass rapidly into coarsely cellular slag, especially along the upper and under parts of the several sheets. No tuff has been noticed between these basic flows, but here and there thin lenticular layers of sandstone, lying in hollows of the lava-sheets, are connected with vertical or highly-inclined ramifying veins of similar material, with the plains of stratification passing across the breadth of the veins. These features are an exact reproduction of those above described in Forfarshire and Kincardineshire. The amygdales consist of chalcedony, crystallized quartz and calcite.
Torduff Hill, which rises to the east of Warklaw, consists of a mass of coarse volcanic breccia or agglomerate (n), markedly felsitic in its materials. It probably forms a neck marking a small volcanic vent, like some others at the north end of the chain to be afterwards referred to.
In the lower part of Capelaw Hill, the next eminence in an easterly direction, bedded andesites, with an intercalated band of sandstone and conglomerate (2_s_), appear and pass under rocks of so decomposing a kind that no good sections of them are to be found. The hill is covered with grass, but among the rubbish of the screes pieces of felsite-like rocks and breccias may be observed. Some of these blocks show an alternation of layers of felsitic breccia with a fine felsite-like material which may be a tuff. These rocks, conspicuous by the light colours of their screes, alternate further up with other dark andesitic lavas, and run south-westward for about five miles.
Beyond Capelaw Hill, upon a band of these pale rocks, comes a thick group of sheets of dark andesite, which form the main mass of Allermuir Hill. They are well seen from the south side and likewise from the north, dipping towards the south-east at angles of from 35° to 40°, and weathering along the crest of the hills into a succession of scars and slopes which show the bedded character of the lavas.
At Caerketton Hill another band of pale material forms the conspicuous craggy face so familiar in the aspect of the Pentland Hills as seen from Edinburgh. This band consists of pale felsitic breccia, and amorphous, compact, much-decayed rock, regarding which it is difficult to decide whether it should be considered as a fine felsitic tuff, or as a decomposed felsite. The band is better seen when traced southwards. The light colour of its screes makes it easily followed by the eye even from a distance along the hill-tops and declivities.
On the next hill to the south-west, known as Castlelaw Hill, this pale band of rock is exposed in a few crags and quarries, and its debris, protruding through the scanty herbage, slips down the slopes. On its north side the screes display the same felsitic breccias and compact, decayed felsitic rocks, occasionally showing a structure like the flow-structure of rhyolite. The breccia which projects in blocks from the summit of the hill has been quarried immediately below the crest on the south side, where it overlies a thin intercalated band of a dull, much-decomposed porphyry.
The breccias are composed almost entirely of thoroughly acid rock-fragments, as may be judged from the percentage of silica shown to occur in them. These fragments vary from the finest lapilli up to angular pieces several inches long. They not infrequently display a fine and extremely beautiful flow-structure. It is thus quite certain that there are acid breccias intercalated among the more basic lavas of the northern Pentlands, and that among the constituents of these breccias are fragments of felsite or perhaps even lithoid rhyolite.
We may therefore be prepared to find that actual outflows of felsitic lava accompanied the discharge of these highly-siliceous tuffs. Unfortunately the manner in which the rocks decay and conceal themselves under their own debris makes it difficult to separate the undoubtedly fragmental bands from those which may be true lavas. But an occasional opening, and here and there a scattered loose block, serve to indicate that the two groups of rock certainly do coexist in this pale band, which can be followed through the chain for upwards of six miles until it is cut off by the eastern boundary fault.
At the south-west end of Castlelaw Hill, where a quarry has been opened above the Kirk Burn, blocks of felsite may be observed showing flow-structure on a large scale. The bands of varied devitrification are sometimes a quarter of an inch broad, and weather out in lighter and darker tints. Some of them have retained their felsitic texture better than others, which have become more thoroughly kaolinized. That these are not deceptive layers of different texture in fine tuffs is made quite clear by some characteristic rhyolitic structures. The bands are not quite parallel, but, on the contrary, are developed lenticularly, and may be observed to be occasionally puckered, and to be even bent back and folded over as in ordinary rhyolites. There is no contortion to be observed among the stratified tuffs of the hills. This irregularity in the layers is obviously original, and can only be due to the flow of a moving lava.
On the east side of Castlelaw Hill, as shown in Fig. 86, dull reddish andesites overlie the pale belt of felsitic rocks. Their lower bands are marked by the presence of well-formed crystals of a dark green mica. Their central and higher portions consist of porphyrites of the prevalent type, both compact and vesicular. These lavas continue as far as any rock can be seen. Beyond the boundary fault, the Burdiehouse Limestone and oil-shales of the Lower Carboniferous series are met with, inclined at high angles against the hills. It is impossible to say how much of the volcanic series has here been removed from sight by the dislocation.
If now we move three miles further to the south-west and take a second section across the Pentland Hills, it will be found to expose the arrangement of rocks represented in Fig. 88. At the western end the Upper Old Red Sandstones (4) and Lower Carboniferous series (5) are seen lying unconformably on the upturned edges of the Upper Silurian shales (1). North Black Hill consists of a large intrusive sheet of pale felsite (F) that has broken through the Silurian strata and has in places thrust itself between them and the conglomerates of the Lower Old Red Sandstone which lie unconformably upon them. In the neighbouring Logan Burn, at the bottom of the Habbie's Howe Waterfall, the felsite can be seen injected into the conglomerate. The felsitic sill of North Black Hill runs for a mile and a half along the western base of the volcanic series, and has a breadth of about half a mile. It is the only important intrusive mass in the Pentland Hills.
To the south of the Silurian shales that lie against the southern flank of North Black Hill, pale felsitic tuffs (3) occur, which are a continuation of those already referred to as running southwards from Capelaw Hill. Above them a series of andesites (2), with intercalated bands of tuff, sandstone and conglomerate (2_s_), occupy the bottom of the Logan valley and part of the slopes on both sides. In the thickest band of tuffs, which is well-exposed along the road by the side of the Loganlee Reservoir, a group of well-bedded strata occurs from less than an inch to a foot or more in thickness. Generally they are pale in colour, and are made up of white felsitic detritus, but with a sprinkling of dull purplish-red fragments, and occasional larger rounded pieces of different andesites. Some of the rocks might be called felspathic sandstones. Other bands in the group are dark purplish-red in tint, and consist mainly of andesitic debris, with a dusting of white felsitic grains and fragments. There would thus seem to have been showers both of felsitic and of andesitic ashes and lapilli.
The dark lavas that overlie the tuffs are likewise well displayed along the same road-section. They vary rapidly from extremely compact homogeneous dark blue rocks, that weather with a greenish crust, to coarse, slaggy masses and amygdaloids.
These more basic lavas are a continuation of those of Allermuir Hill, and, as at that locality, they plunge here also under the same band of white tuffs, breccias and felsites (3), which has been referred to as stretching southward from Caerketton Crags. This band must here be at least 500 feet thick. It forms Scald Law (1898 feet) and the surrounding summits, and thus occupies the highest elevations in the Pentland chain. It dips beneath the uppermost group of andesites, which, as before, are here truncated by the eastern fault (f), the Calciferous Sandstones and Carboniferous Limestone series (6) being thrown against them.
A third section (Fig. 89), taken two miles still further south, shows a remarkable attenuation of the volcanic series, and the appearance of a thick group of conglomerates (2) lying conformably below that series, but resting on the upturned edges of the upper Silurian shales (1). The thick Allermuir porphyrites are here reduced to a few thin beds (3) intercalated among the conglomerates and sandstones, amidst which the whole volcanic series dies out southward. A detailed section of the rocks exposed on the western front of Braid Law shows the following succession:--
White felsitic rocks of Braid Law (4 in Fig. 89).
Coarse conglomerate passing down into sandstone. About 20 feet visible.
Dark andesite, 4 feet.
Parting of yellow felspathic grit, 8 or 10 inches.
Andesite, 10 feet.
Hard felspathic grit, 6 feet.
Dark green amygdaloidal andesite, 2 feet.
Yellow felspathic sandstone and grit, 2 feet.
Dark green amygdaloidal andesite, 6 feet.
Felspathic grit and red and brown sandstone, 4 feet.
Dark andesite, perhaps 6 or 8 feet.
Great conglomerate with alternating courses of sandstone, rapidly increasing in thickness southwards.
Above these dwindling representatives of the northern andesitic lavas comes the continuation of the white band of tuffs and breccias of Caerketton and Scald Law (4), which in turn dips under the highest group of andesites. The Carboniferous strata (5) are brought in by the fault (f). In little more than two miles beyond this line of section the volcanic series disappears, and the Old Red Sandstone for a brief space consists only of sedimentary deposits.
Besides the remarkable alternation of basic and acid ejections, there is a further notable feature in the geology of the Pentland Hills. This volcanic centre presents us with one of the most remarkable vents anywhere to be seen among the volcanic rocks of Britain. The full significance of this feature may best be perceived if we advance along the hills from their south-western end. As has now been made clear, the volcanic materials which begin about the line of the North Esk near Carlops rapidly augment in thickness until, in a distance of not more than seven miles, they attain a thickness of about 7000 feet, and then form the great scarped front of the hills that look over Edinburgh. But at the base of that wall their continuity abruptly ceases. The lower ground, which extends thence to the southern suburbs of Edinburgh, and includes the group of the Braid Hills, is occupied by another and more complex group of rocks in which the parallelism and persistence so marked in the Pentland chain entirely disappear.
This abrupt truncation of the bedded lavas and tuffs marks approximately the southern margin of a large vent from which at least some, if not most, of these rocks were probably ejected. The size of this vent cannot be precisely ascertained on account of the unconformable overspread of Lower Carboniferous strata. But that it must have been a large and important volcanic orifice may be inferred from the fact that the visible area of the materials that fill it up measures two miles from north-east to south-west, and a mile and a half from south-east to north-west, thus including a space of rather more than two square miles. Its original limits towards the north and south can be traced by help of the bedded lavas that partially surround it, but on the two other sides they are concealed by the younger formations. We shall probably not over-estimate the original area of the vent if we state it at about four square miles.
The materials that now fill this important orifice consist mainly of "claystones," like those of the Pentland series--dull rocks, meagre to the touch, varying in texture from the rough porous aspect of a sinter through stages of increasing firmness till they become almost felsitic, and ranging in colour from a dark purple-red, through shades of lilac and yellow, to nearly white, but often strikingly mottled. A more or less laminar structure is often to be observed among them, indicating a dip in various directions (but especially towards the north) and at considerable angles. Throughout this exceedingly fine-grained material, lines of small lapilli may occasionally be detected, also bands of breccia, consisting of broken-up tuff of the same character, and of fine "hornstone" and felsite, with delicate flow-structure. Exhibiting on the whole so little structure, this tract may be regarded as consisting largely of fine volcanic dust derived from the explosion of felsitic or orthophyric lavas. Some portions indeed are not improbably composed of decayed felsites, like those which present so many difficulties to the geologist who would try to trace their course among the other lavas and tuffs of the Pentland chain. Various veins, dykes and small bosses of felsite, andesite and even more basic material, such as fine dolerite, have been intruded into the general body of the mass.
On the outskirts of the main vent some subordinate necks may be observed (3, 3 in Fig. 90), perhaps, like Torduff Hill, already noticed (Fig. 86), marking lateral eruptions from the flanks of the great cone. Three of these occur in a line more than half a mile long, possibly indicating a fissure on the side of the old volcano, running in a south-westerly direction from the southern edge of the vent. The smallest of them measures about 500 feet in diameter; the largest is oblong in shape, its shorter diameter being about 500 feet, and its longer about 1000 feet. The materials that fill these lateral vents are coarse agglomerates, traversed by veins and irregular intrusions of a fine horny or flinty felsite.
From the acid character of most of the rocks that now fill the wide vent of the Braid Hills it may be inferred that at least the last eruptions from it consisted chiefly of acid tuffs and lavas. The upper portion of the volcanic series being everywhere concealed, there are no means left to verify this inference from an examination of the ejected material. It may be remarked, however, that the pale yellow sandstones which lie on the east side of the fault and are exposed in the Lyne Water above West Linton are in great measure composed of fine felsitic material.[365] They certainly belong to a higher horizon than the most southerly lavas of the Pentland Hills, and if they have not derived their volcanic detritus from the Biggar volcanic area, it may be assumed that they obtained it from the vent of the Braid Hills. In any case they show that after the lavas of the southern end of the Pentland Hills were buried, acid volcanic detritus continued to be abundantly distributed over this part of the floor of Lake Caledonia.
[Footnote 365: Explanation to Sheet 24 of the Geological Survey of Scotland, pp. 10, 12.]
6. The Biggar Centre[366]
[Footnote 366: This area is included in Sheets 23 and 24 of the Geological Survey of Scotland. It was mapped and described by myself. (Explanations of Sheets 23 and 24.) Various parts of it have been referred to by earlier writers, particularly Maclaren, _Geology of Fife_, etc., p. 176.]
Another distinct group of volcanoes had its centre about 25 miles south-westward from the Braid vent, and on the same line as those of the Pentland Hills. In no part of the basin can the isolation of the different volcanic clusters be so impressively observed as in the area to the south-west of these hills. On the one hand, the lavas and tuffs from the Braid vent die out, and on the other, as we follow the conglomerates south-westwards, a new volcanic series immediately makes its appearance.
The space between the last extremity of the Pentland lavas and the beginning of the Biggar series does not exceed some 500 yards. It will be remembered that the lower half of the Pentland volcanic series dies out long before it reaches the southern end of the hills, and that it is by lavas on the horizon of some of the dark andesites of Allermuir Hill that the volcanic band is finally prolonged to its extreme southern limit. The most northerly extension of the Biggar lavas lies somewhere on the same general platform. But whereas, at the north end of the Pentland chain, the volcanic sheets rest on the edges of the Upper Silurian shales, at the south end, several hundred feet of coarse conglomerate and sandstone intervene between the Silurian shales and the porphyrites. So rapidly does the bulk of these sedimentary formations increase that in the course of two miles they must be 3000 feet in thickness below the most northerly of the Biggar lavas just referred to. But after that point, when they cross the Lyne Water, they begin to be more and more interstratified with thin sheets of andesite. These lavas, the beginning of the Biggar series, soon number nine or ten distinct bands, and so quickly do they usurp the place of the sedimentary materials that in a distance of not more than twelve miles they form, where traversed by the river Clyde, the whole breadth of the visible tract of Old Red Sandstone, to the exclusion of the conglomerates.
Unfortunately, soon after the lavas make their appearance at the north end they are in great measure overlapped unconformably by the red sandstones at the base of the Carboniferous system, but where the Medwin Water has cut through this covering, they can be seen here and there underneath on their southerly course.
A section through the northern end of the Biggar series, where the successive lavas are dying out northwards among the conglomerates, shows the structure given in Fig. 91. The sedimentary strata consist largely of debris of andesite, and the lavas include dark red or purple andesites and also pale felsites, both having the same characters as those of the Pentland Hills.
In one important respect the volcanic series in the northern part of the Biggar area differs from that of the Pentland Hills, for whereas the uppermost parts of the latter are concealed by faults which bring down the Carboniferous strata against the base of the hills, the lavas at the north end of the Biggar district pass conformably under a thick group of Lower Old Red conglomerates and sandstones. We thus learn that here the volcanic eruptions ceased long before the close of the deposition of the Lower Old Red Sandstone. The overlying sedimentary series is disposed in a long synclinal trough, corresponding in direction with the general north-easterly strike of the volcanic rocks which reappear from under the sandstones and conglomerates along its south-eastern border, where they are abruptly truncated by the fault (f, Fig. 92), which brings them against the flanks of the Silurian Uplands. It is interesting to note that by this dislocation the lavas of the Lower Old Red Sandstone are placed almost in immediate contact with those of the Lower Silurian series, which appear here on the crests of numerous anticlinal folds that are obliquely cut off by the fault.
There is yet another feature of interest in the northern part of the Biggar volcanic centre. While the lowest visible lava is an olivine-diabase not unlike parts of the Warklaw group of the Pentland Hills, those which occur above it are partly andesites and partly orthoclase-felsites. The latter form, among the hills near Dolphinton, an important group which reaches its greatest development in the Black Mount (1689 feet). These rocks cover a breadth of more than a mile of ground, and probably attain a thickness of not less than 2000 feet. They so closely resemble in their general characters the corresponding rocks of the Pentland Hills that a brief description of them may suffice. As in that chain of hills, they are so prone to decomposition that they are in large part concealed under a covering of their own debris and of herbage, though their fragments form abundant screes, and numerous projecting knobs of rock suffice to show the main features of the lavas and their accompaniments.
The felsites weather into pale yellow and greyish "claystones," but where fresher sections can be procured they often show darker tints of lilac and purple. They are close-grained, sometimes flinty, generally porphyritic with scattered highly-kaolinized white felspars, but without quartz, often presenting beautiful flow-structure, and not infrequently showing a brecciated appearance, which in the usual weathered blocks is hardly to be distinguished from the breccia of interstratified tuffs.
A locality where some of these features may be satisfactorily examined is a dry ravine in the farm of Bank, on the south-east side of the Black Mount. Here the felsite possesses such a perfectly developed flow-structure as to split into slabs which, dipping S.E. at about 25°, might deceive the observer into the belief that it is a sedimentary rock. A fresh fracture shows the laminæ of flow, many of which are as thin as sheets of paper, to be lilac in colour, some of the more decomposed layers assuming tints of grey. The felspars and micas are arranged with their long axes parallel to the lines of flow. The rock is not vesicular, but it breaks up here and there into the brecciated condition just referred to. Below the sheet which displays the most perfect flow-structure, what is probably a true volcanic breccia makes its appearance. It consists of angular fragments of a similar lilac felsite, of all sizes up to pieces two or three inches in length, cemented in a matrix of the same material stained reddish-brown. In this breccia the stones show little or no flow-structure.
Above the group of felsites and felsitic breccias, grey andesites make their appearance, like some of those in the Pentland Hills. They are sometimes extraordinarily vesicular, the vesicles in the body of the rock being filled with calcite, agate, etc. Such lavas must have been originally sheets of rough slag. The elongated steam-vesicles have been partly filled up with micaceous sand and fine red mud that were washed into crannies of the lava in direct communication with the overlying water. It is evident that in the northern part of the Biggar centre the succession of volcanic events followed closely the order observable in the Pentland Hills, but on a feebler scale. We may suppose that the lower diabases and andesites are the equivalents of those of Warklaw and Allermuir, that the felsites and breccias were contemporaneous with those of Capelaw, Caerketton and Castlelaw, and that the last andesites made their appearance together with those which form the highest lavas of the Pentland chain.
A section across the southern end of the Biggar volcanic belt shows less diversity of structure (Fig. 92). The lavas (3) are there found to flatten out and to spread unconformably over the older part of the Lower Old Red Sandstone (2), which, as already stated, passes down into the Upper Silurian shales. A few intercalations of conglomerate, mainly made up of volcanic detritus, are here and there to be detected among these lavas. But the chocolate sandstones and conglomerates that lie unconformably below them contain no such detritus, for they belong to the pre-volcanic part of the history of Lake Caledonia, and were here locally upraised, perhaps as an accompaniment of the terrestrial disturbances that preceded or attended the first outburst of volcanic energy. Followed south-westwards, the stratigraphical break in the Lower Old Red Sandstone disappears, and, as will be shown in the account of the Duneaton centre, a continuous succession can there be traced from the Upper Silurian shales up into the volcanic series.
An interesting feature in this district is the felsitic boss of Quothquan already alluded to (p. 288) as rising up through the andesites, and possibly marking one of the vents of the district. It is one of a number of felsitic intrusions in this neighbourhood, of which the most important is Tinto.
A third section taken across Tinto, from Thankerton Moor on the north side to Lamington on the south, will serve further to illustrate the great unconformability in the Lower Old Red Sandstone of this district, and to show the relation of the largest felsitic intrusion to the surrounding rocks (Fig. 93). The conglomerates and sandstones that appear on the south slopes of Tinto lie near the base of the Old Red Sandstone, and if we could bore among the overlying andesites we should probably meet with the Upper Silurian shales among or conformably beneath the red passage-beds, as in the Lesmahagow district.
The andesitic lavas creep over the upturned denuded edges of these strata and sweep round the flanks of Tinto. This conspicuous hill reaches a height of 2335 feet above the sea, and consists of the felsitic rocks already described (p. 278). Seen from many points of view it rises as a graceful cone, distinguished from all the other eminences around it by the pinkish colour of its screes. In reality it forms a continuous ridge which runs in an east and west direction for about five and a half miles, with a breadth of about a mile. Some part at least, and possibly the whole of this oblong mass, is in the form of a sill or laccolite which dips towards the north. Conglomerates and sandstones plunge under it on the southern side, and similar sandstones overlie it on the north. If there be a neck in this mass, as one might infer from the shape of the hill, its precise limits are concealed. The rock does not break through the andesites, and may belong to an earlier period of eruptivity than the lavas immediately around it. There were other, though smaller, vents in the immediate neighbourhood. Besides the cone of Quothquan just referred to, another may be marked by the felsite boss which overlooks the village of Douglas, four miles to the south-west of the Tinto ridge, while a third rises into a low rounded hill close to the village of Symington.
The lavas spread out again to the south-west of Tinto in a group of hills, until they are interrupted by a fault which brings in the Douglas coal-field.[367] This dislocation abruptly terminates the Biggar volcanic band in a south-westerly direction, after extending for a length of 26 miles, with a breadth of sometimes as much as five miles.
[Footnote 367: See Explanation to Sheet 23 of the Geological Survey of Scotland (1873), p. 15. This ground was mapped and described by Mr. B. N. Peach.]
7. The Duneaton Centre
Among the high bleak muirlands on the confines of the three counties of Lanark, Ayr and Dumfries, traversed by the Duneaton Water, a distinct volcanic area may be traced.[368] Its boundaries, however, cannot be satisfactorily fixed. It is overspread with Carboniferous rocks both to the north-east and south-west, so that its rocks are only visible along a strip about seven miles long and two miles broad. On the north-western side its lower members are seen lying interstratified among the sandstones and conglomerates which thence pass down conformably into the Upper Silurian series (Fig. 94). But although we thus get below the volcanic series we meet with no vents or sills among the lower rocks. On the south-east side the highest lavas and tuffs are overlain by some 5000 feet of red sandstones and conglomerates (2, 3), which completely bury all traces of the volcanic history.
[Footnote 368: This area was mapped by Mr. B. N. Peach in Sheet 15 of the Geological Survey of Scotland, and is described by him in the accompanying Memoir.]
The volcanic series in this limited district reaches an estimated thickness of 4000 feet, built up of purple and green slaggy andesites, dark heavy diabases (melaphyres) and tuffs, with abundant interstratification of sandstone, especially towards the base. One of its chief features of interest is the manner in which it exhibits, better, perhaps, than can be found in any of the other volcanic areas, the frequent and rapid alternations of lavas and tuffs with sandstone and conglomerate. In this part of the region the volcanic discharges were obviously frequent and intermittent, while at the same time the transport and deposition of sediment were continuous. This sediment consisted largely, indeed, of volcanic detritus mixed with ordinary sand and silt. That these conditions of sedimentation were not wholly inimical to animal life is shown by the occasional occurrence of worm-burrows in the ashy sandstones.[369]
[Footnote 369: Memoir on Sheet 15 Geol. Surv. Scotland (1871), p. 22.]
The thick accumulation of sandstones and conglomerates above the main mass of lavas has been derived almost wholly from the waste of the volcanic rocks (3). Blocks of andesite, well rounded and often from six to twelve inches in diameter, may be seen in the remarkable band of coarse conglomerate which runs as a nearly continuous ridge from the Nith to the Clyde--a distance of more than twenty miles. Nothing impresses the geologist more, as he wanders over this district, than the evidence of the prodigious waste which the volcanic series underwent before it was finally buried. Some part of the detritus may have been supplied, indeed, by occasional discharges of fragmental matter, as has already been suggested in the case of the Ochil and Montrose conglomerates. But the nature of the pebbles in these masses of ancient shingle shows them to be not bombs, but pieces worn away from sheets of lava.
That the lavas underlie these piles of detritus and extend southwards even up to the very edge of the Silurian Uplands is shown by the rise of a number of successive beds from under the trough into which the conglomerate has been thrown. These lavas, however, are almost immediately cut off by the great boundary fault (f) which flanks the Silurian territory. That they are not met with now to the south-east of the dislocation, where they must once have lain, is an evidence of the great denudation which the district has undergone. Fig. 94, which gives a section across the broadest part of the area, from the edge of the Muirkirk coal-field to the Silurian uplands, shows the general structure of the ground.
No satisfactory evidence regarding the position of any of the vents of the period has been met with in this district. The rocks to the south of the boundary-fault are older than the Old Red Sandstone, and as they must have been for some distance overspread by the conglomerates, sandstones and volcanic series, we might hope to find somewhere among them traces of necks or bosses. The only mass of eruptive rock in that part of the district is the tract of Spango granite which has been already referred to in connection with the subject of the vents and granite protrusions of Old Red Sandstone time. This mass, about four miles long and two miles broad, rises through Silurian strata, and by means of the boundary fault is brought against the higher group of conglomerates and sandstones. The Silurian shales and sandstones around the granite have undergone contact-metamorphism, becoming highly micaceous and schistose. The ascent of this granite must have taken place between the upheaval and contortion of the Upper Silurian strata, and the deposition of the higher parts of the Lower Old Red Sandstone of this region. Its date might thus come within the limits of the volcanic period. But one must frankly own that there is no positive evidence to connect its production with the volcanic history.
8. The Ayrshire Group of Vents
The original limits of the volcanic districts in the remaining portion of the Old Red Sandstone area on the mainland of Scotland, from the valley of the Nith to the Firth of Clyde, can only be vaguely indicated.[370] There is a difficulty in ascertaining the south-western termination of the Duneaton area, and in deciding whether the lavas and tuffs of Corsincone in Nithsdale should be assigned to that district or be placed with those further to the south-west. Between Corsincone and the next visible volcanic rocks of the Lower Old Red Sandstone there intervenes a space of six miles, along which, owing to the effect of the great fault that flanks the north-western margin of the Southern Uplands, the Carboniferous Limestone and even the Coal-measures are brought against the Silurian formations, every intermediate series of rocks being there cut out. It may therefore be, on the whole, better to include all the volcanic rocks on the left side of the Nith as part of the Duneaton series. There will still remain a tract of five miles of blank intermediate ground before we enter upon the volcanic rocks of Ayrshire.
[Footnote 370: The mapping of the Old Red Sandstone volcanic areas of Ayrshire for the Geological Survey was thus distributed:--The district east of Dalmellington was surveyed by Mr. B. N. Peach, that between Dalmellington and Straiton by Prof. James Geikie, and all from the line of the Girvan Valley south of Straiton westward to the sea by myself. The ground is embraced in Sheets 8, 13 and 14 of the Map of Scotland, and is described in accompanying Explanations.]
Owing to complicated faults, extensive unconformable overlaps of the Carboniferous formations, and enormous denudation, the volcanic tracts of Old Red Sandstone age in Ayrshire have been reduced to mere scattered patches, the true relations of which are not always easily discoverable. One of these isolated areas flanks the Silurian Uplands as a belt from a mile to a mile and a half in breadth and about six miles long, but with its limits everywhere defined by faults. A second much more diversified district extends for about ten miles to the south-west of Dalmellington. It too forms a belt, averaging about four miles in breadth, but presenting a singularly complicated geological structure. Owing to faults, curvatures and denudation, the volcanic rocks have there been isolated into a number of detached portions, between some of which the older parts of the Old Red Sandstone, and even the Silurian rocks, have been laid bare, while between others the ground is overspread with Carboniferous strata. A third unbroken area forms the Brown Carrick Hills, south of the town of Ayr, and is of special interest from the fact that its rocks have been exposed along a range of sea-cliffs and of beach-sections for a distance of nearly four miles. Other detached tracts of volcanic rocks are displayed on the shore at Turnberry and Port Garrick, on the hills between Mochrum and the vale of the Girvan, and on the low ground between Dalrymple and Kirkmichael.
The isolation of these various outliers and separated districts is probably not entirely due to the effects of subsequent geological revolutions. More probably some of the areas were always independent of each other, and their igneous rocks were discharged from distinct volcanic centres. We may conjecture that one of these centres lay somewhere in the neighbourhood of New Cumnock, for the lavas between that town and Dalmellington appear to diminish in thickness and number as they are traced south-westward. Another vent, or more probably a group of vents, may have stood on the site of the present hills to the right and left of the Girvan Valley, south of the village of Straiton. A third probably rose somewhere between Dailly and Crosshill, and poured out the lavas of the ridges between Maybole and the Dailly coal-field. The important centre of eruption that produced the thick and extensive lavas of the Brown Carrick Hills may be concealed under these hills, or may have stood somewhere to the west of Maybole. Still another vent, perhaps now under the sea, appears to be indicated by the porphyrites of the coast-section between Turnberry and Culzean Bay.
Owing to the complicated structure of the ground, several important points in the history of the Old Red Sandstone of this region have not been established beyond dispute. In particular, the unconformability which undoubtedly exists in that system in the south-west of Ayrshire has not been traced far enough eastwards to determine whether it affects the volcanic belt east of Dalmellington, or whether the break took place before or after the eruption of that belt. West of Dalmellington it clearly separates a higher group of sandstones, conglomerates and volcanic rocks from everything older than themselves. The structure is similar to that in the Pentland Hills, a marked disturbance having taken place here as well as there after a considerable portion of the Lower Old Red Sandstone had been deposited. These earlier strata were upraised, and on their denuded ends another group of sandstones and conglomerates was laid down, followed by an extensive eruption of volcanic materials.
It is the upper unconformable series that requires to be considered here, as it includes all the volcanic rocks of the Old Red Sandstone lying to the west of the meridian of Dalmellington. The position of these rocks on their underlying conglomerates is admirably exposed among the hills between the valleys of the Doon and the Girvan, as well as on Bennan Hill to the south of Straiton. The andesites rise in a craggy escarpment crowning long green slopes that more or less conceal the conglomerates and sandstones below.
Along the coast-sections the structure of the volcanic rocks may be most advantageously studied. The shore from the Heads of Ayr to Culzean Castle affords a fine series of exposures, where every feature in the succession of the lavas may be observed. Still more instructive, perhaps, is the mile and a half of beach between Turnberry Bay and Douglaston, of which I shall here give a condensed account, for comparison with the coast-sections of Kincardineshire and Forfarshire already described.
The special feature of this part of the Ayrshire coast-line is the number of distinct andesite sheets which can be discriminated by means of the thin layers of sandstone and sandy tuff that intervene between them. In the short space of a mile and a half somewhere about thirty sheets can be recognized, each marking a separate outflow of lava. It was in this section that I first observed the sandstone-veinings which have been described in previous pages, and nowhere are they more clearly developed. Almost every successive stream of andesite has been more or less fissured in cooling, and its rents and irregular cavernous hollows have been filled with fine sand silted in from above. The connection may often be observed between these sandstone partitions or patches and the bed of the same material, which overspread the surface of the lava at the time that the fissures were being filled up.
The andesites of the Turnberry shore are of the usual dark purplish-red to green colours, more or less compact in the centre and vesicular towards the top and bottom. They display with great clearness the large empty spaces that were apt to be formed in such viscous slaggy lavas as they moved along the lake-bottom. These spaces, afterwards filled with fine sand, now appear as irregular enclosures of hard green sandstone embedded in the andesite. The example shown in Fig. 95 may be seen in one of the lavas at John o' Groat's Port.
From the arrangement of the veins of sandstone it is evident that irregularly divergent, but often more or less stellate, fissures opened in the lavas as they cooled. Sometimes, indeed, the molten rock appears to have broken up into a shattered mass of fragments, as must often have happened when lavas were poured over the lake-floor. What may be an instance of this effect is to be seen on the cliff under the walls of Turnberry Castle, whence the annexed sketch (Fig. 96) was taken. The lower andesite (a) is highly amygdaloidal towards the top, and is traversed in all directions with irregular veins and nests of sandstone which can be traced upward to the bed (b), consisting of sandstone, but so full of lumps or slags of amygdaloidal andesite that one is here and there puzzled whether to regard it as a sedimentary deposit, or as the upper layer of clinkers of a lava-stream strewn with sand. Above this fragmentary layer lies another bed of andesite (c) of a coarsely amygdaloidal structure, which encloses patches of the underlying sandstone. It passes upward, in a space of from four to six feet, into a mass of angular scoriaceous fragments (d) of all sizes up to blocks 18 inches in length cemented in a vein-stuff of calcite, chalcedony and quartz. This brecciated structure ascends for about 13 or 14 feet, and is then succeeded by a greenish compact andesite (e), which further north becomes amygdaloidal and much veined with sandstone, passing into a breccia of lava fragments and sandstone.
MAP OF THE VOLCANIC DISTRICTS OF THE LOWER OLD RED SANDSTONE OF "LAKE CALEDONIA" IN CENTRAL SCOTLAND & NORTH EAST IRELAND
The remarkable brecciated band (d) in this cliff, though 13 or 14 feet in the centre, immediately thins out on either side, until in the course of a few yards it completely disappears and allows the lavas c and e to come together, as shown in Fig. 97. We may suppose that this section reveals the structure of the terminal portion of a highly viscous lava which was shattered into fragments as it moved along under water.
No clear evidence of the sites of any of the volcanic vents has yet been detected in the Old Red Sandstone of Ayrshire. Possibly some of the numerous felsitic bosses to the south-west of Dalmellington may partly mark their positions. But the sills connected with the volcanic series are well exposed in the 12 miles of hilly ground between Dalmellington and Barr. Two groups of intrusive sheets may there be seen. The most numerous consist of pale or dark-pink felsite, often full of crystals of mica. They form prominent hills, such as Turgeny, Knockskae and Garleffin Fell. The second group comprises various diabase-sheets which have been intruded near the base of the red sandstones and conglomerates, over a distance of seven miles on the north side of the Stinchar Valley above Barr. They attain their greatest development on Jedburgh Hill, where they form a series of successive sills, the largest of which unite northwards into one thick mass and die out southward among the sandstones and conglomerates.
Chapter XXI
VOLCANOES OF THE LOWER OLD RED SANDSTONE OF THE CHEVIOT HILLS, LORNE, "LAKE ORCADIE" AND KILLARNEY
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.).
[Footnote 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.
[Footnote 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.
[Footnote 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]
[Footnote 374: Op. cit. p. 24.]
[Footnote 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]
[Footnote 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]
[Footnote 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.
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]
[Footnote 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.
[Footnote 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.
[Footnote 380: See "The Geology of Eastern Berwickshire" (Sheet 34), Mem. Geol. Surv. Scotland (1864), p. 29.]
[Footnote 381: These hills are chiefly represented in Sheet 25. But see "The Geology of East Lothian," Mem. Geol. Surv. Scotland (1866), p. 26.]
[Footnote 382: "Geology of East Lothian," Mem. Geol. Surv. Scotland, p. 15, and authorities there cited.]
"THE LAKE OF LORNE"
The basin of Lorne has not yet been carefully examined and described, though various writers have referred to different parts of it (Map I.). My own observations have been too few to enable me to give an adequate account of it. The volcanic sheets of this area form a notable feature in the scenery of the West Highlands, for they are the materials out of which the remarkable terraced hills have been carved, which stretch from Loch Melfort to Loch Creran (Fig. 99), and which reappear in picturesque outliers among the mountains traversed by Glen Coe. Between the ancient schists that form the foundation-rocks of this district and the base of the volcanic series, lies a group of sedimentary strata which in the western part of the district must be 500 feet thick. This group consists of exceedingly coarse breccias at the bottom, above which come massive conglomerates, volcanic grits or tuffs, fine sandstones and courses of shale. While the basement-breccias are composed mainly of detritus of the underlying schists, including blocks six feet long, they pass up into coarse conglomerates made up almost entirely of fragments of different lavas (andesites, diabases, etc.), and more than 100 feet thick, which often show little or no trace of stratification, but break up into large quadrangular blocks by means of joints which cut across the imbedded boulders. These volcanic conglomerates form some of the more conspicuous features of the coast to the south and north of Oban, and are well exposed in the Isle of Kerrera. They offer many points of resemblance to those of Lake Caledonia, but no certain proof has been noted that they belong to the Lower Old Red Sandstone. They have obviously been derived from lava-sheets that were exposed to strong breaker-action, which at the same time transported and rounded blocks of granite, schist and other crystalline rocks derived from the adjacent hills. During the intervals of quieter sedimentation indicated by the fine sandstones and shales, volcanic explosions continued, as may be seen by the occurrence of occasional large bombs which have fallen upon and pressed down the fine ashy silt that was gathering on the bottom.
It would seem from the characters of some of the strata in this sedimentary series that over the area of deposit portions of the shallower waters were occasionally laid bare to the sun and air. Among the conglomerates there lie certain bands of reddish sandy, ripple-marked, sun-cracked and rain-pitted shales and fine sandstones. Such accumulations, indicative of the ultimate exposure of fine sediment that silted up hollows in the great banks of coarse shingle, may be noticed at the south end of the Island of Kerrera, on at least two horizons which are separated from each other by thick masses of conglomerate and fine felspathic grit. We may infer, therefore, that the fine littoral mud, which gathered during pauses in the heaping up of the coarse gravel and shingle, was occasionally laid dry. Similar strata may be observed behind Oban, where the alternation of exceedingly fine sediment and granular ashy bands is well exhibited.
But the explosions that gave rise to the volcanic materials so largely represented in these lower conglomerates, were merely preliminary to those which led to the outflow of the great sheets of lava that now constitute so large a part of the hills of Lorne. In the few traverses which I have made across different parts of this district I have noted the general resemblance of the lavas to those of the Lower Old Red Sandstone of the Midland Valley of Scotland, their bedded character, and the occurrence of occasional layers of stratified material between them. The prominent features of these rocks, and their relation to the volcanic conglomerates below them, and to the underlying slates and schists are well displayed on Beinn Lora at the mouth of Loch Etive (Fig. 100). There the black slates of the district are unconformably covered by the coarse volcanic conglomerate, formed chiefly of blocks of andesite, cemented in a hard matrix of similar composition. About 150 or 200 feet of this material underlie the great escarpment of the lavas, which here rise in successive beds to the top of the hill, 1000 feet above its base.
On the south side of Loch Etive the base of the volcanic series, with its underlying conglomerate, may be followed westward to Oban and thence southward to Loch Feochan. The lavas cover most of the ground from the western shore eastwards to near Loch Awe. But this area is still very imperfectly known. The Geological Survey, however, has now advanced to this part of the country, so that we shall before long be in possession of more detailed information regarding the character and sequence of its volcanic history and the geological age of the eruptions.
Mr. H. Kynaston, who has begun the mapping of the eastern portion of the district, finds that there, as further west, the bottom of the volcanic series is generally a breccia or conglomerate. He has met with two leading types among the lavas, the more abundant being strongly vesicular, the other more compact. He has observed also numerous dykes and sills of intrusive porphyrite, trending in a general N.N.E. and S.S.W. direction, and pointing towards the great granite mass of Ben Cruachan.[383]
[Footnote 383: Ann. Report Geol. Surv. (1895), p. 29 of reprint.]
Mr. R. G. Symes has traced the volcanic series to the north and south of Oban. While visiting with him part of his ground, I was much struck with the evidence of an intrusive mass at the base of the volcanic series in the Sound of Kerrera. A prominent feature on the east side of the channel, known as Dun Uabairtich and 270 feet high, consists of andesite which appears to combine both a central boss and a sill. The rock breaks through the black slates and the overlying conglomerates and sandstones, and has wedged itself into the unconformable junction between the two formations. It is beautifully columnar on its sea-covered face, some of the columns being 120 feet or more in length, and gently curved.
"LAKE ORCADIE"
We now cross the whole breadth of the Scottish Highlands in order to peruse the records of another of the great detached water-basins of the Old Red Sandstone, which for the sake of brevity of reference I have named and described as "Lake Orcadie" (Map I.). This area has its southern limits along the base of the hills that enclose the wide Moray Firth. It spreads northward over the Orkney and much of the Shetland Islands, but its boundaries in that direction are lost under the sea. In the extensive sheet of water which spread over all that northern region the peculiar Caithness Flags, with their associated sandstones and conglomerates, were deposited to a total depth of 16,000 feet. A sigillaroid and lycopodiaceous vegetation flourished on the surrounding land, together with ferns, Psilophyton and conifers. The waters teemed with fishes of which many genera and species have now been described. The remains of these creatures lie crowded upon each other in the flagstones in such a manner as to indicate that from time to time vast quantities of fish were suddenly killed. Not impossibly, these destructions may have been connected with the volcanic activity which has now to be described.
In the year 1878 I called attention to the evidence for the existence of contemporaneous volcanic rocks in the Old Red Sandstone north of the range of the Grampians, and specially noted three localities where this evidence could be seen--Strathbogie, Buckie and Shetland.[384] Since that time Messrs. B. N. Peach and J. Horne have added a fourth centre in the Orkney Islands. At present, therefore, we are acquainted with the records of four separate groups of volcanic vents in Lake Orcadie.
[Footnote 384: Trans. Roy. Soc. Edin. xxviii. (1878).]
The southern margin of this water-basin appears to have indented the land with long fjord-like inlets. One of these now forms the vale of Strathbogie, which runs into the hills for a distance of fully 20 miles beyond what seems to have been the general trend of the coast-line. In this valley I found a bed of dark vesicular diabase intercalated among the red sandstones and high above the base of the formation, as exposed on the west side of the valley near Burn of Craig. On the east side a similar band has since been mapped for the Geological Survey by Mr. L. Hinxman who has traced it for some miles down the Strath.[385] This latter band, as shown in Fig. 101, lies not far above the bottom of the Old Red Sandstone of this district, and is thus probably distinct from the Craig outcrop. There would thus appear to be evidence of two separate outflows of basic lava in this fjord of the Old Red Sandstone period.
[Footnote 385: See Sheet 76 of the Geological Survey of Scotland.]
No vestige has here been found of any vent, nor is the lava accompanied with tuff. The eruptions took place some time after the earlier sediments of the basin were accumulated, but ceased before the thick mass of upper sandstones and shales was deposited. A section across the valley gives the structure represented in the accompanying diagram (Fig. 101).
Twenty-five miles further north a still smaller andesite band has been detected by Mr. J. Grant Wilson among the sandstones and conglomerates near Buckie.[386] It is a truly contemporaneous flow, for pebbles of it occur in the overlying strata. But again it forms only a solitary bed, and no trace of any accompanying tuff has been met with, nor of the vent from which it came. Both this vent and that of Strathbogie must have been situated near the southern coast-line of the lake.
[Footnote 386: See Sheet 95 of the Geological Survey of Scotland and Trans. Roy. Soc. Edin. vol. xxviii. (1878), p. 435.]
At a distance of some 90 miles northward from these Moray Firth vents another volcanic district lies in the very heart of the Orkney Islands.[387] The lavas which were there ejected occur at the south-eastern corner of the island of Shapinshay, where they are regularly bedded with the flagstones. They consist of dark green olivine-diabases with highly amygdaloidal and vesicular upper surfaces. Their thickness cannot be ascertained, as their base is not seen, but they have been cut by the sea into trenches which show them to exceed 30 feet in depth. The position of the vent from which they came has not been ascertained. Neither here nor in the Moray Firth area do any sills accompany the interbedded sheets, and in both cases the volcanic action would seem to have been of a feeble and short-lived character.
[Footnote 387: Messrs. B. N. Peach and J. Horne, Proc. Roy. Phys. Soc. Edin. vol. v. (1879), p. 80.]
Much more important were the volcanoes that broke out nearly 100 miles still further north, where the Mainland of Shetland now lies. I shall never forget the pleasure with which I first recognized the traces of these eruptions, and found near the most northerly limits of the British Isles proofs of volcanic activity in the Lower Old Red Sandstone. Since my observations were published,[388] Mr. Peach, who accompanied me in Shetland, has returned to the district, and, in concert with his colleague Mr. Horne, has extended our knowledge of the subject.[389] The chief vent or vents lay towards the west and north-west of the Mainland and North Mavine; others of a less active and persistent type were blown out some 25 miles to the east, where the islands of Bressay and Noss now stand. In the western district streams of slaggy andesite and diabase with showers of fine tuff and coarse agglomerate were ejected, until the total accumulation reached a thickness of not less than 500 feet. The volcanic eruptions took place contemporaneously with the deposition of the red sandstones, for the lavas and tuffs are intercalated in these strata. The lavas and volcanic conglomerates are traceable from the southern coast of Papa Stour across St. Magnus' Bay to the western headlands of Esha Ness, a distance of more than 14 miles. They have been cut by the Atlantic into a picturesque range of cliffs, which exhibit in some places, as at the singular sea-stalk of Doreholm, rough banks of andesitic lava with the conglomerate deposited against and over them, and in other places, as along the cliffs of Esha Ness, sheets of lava overlying the conglomerates.
[Footnote 388: Trans. Roy. Soc. Edin. vol. xxviii. (1878), p. 418.]
[Footnote 389: Ibid. vol. xxxii. (1884), p. 359.]
No trace of any vents has been found in the western and chief volcanic district, but in Noss Sound a group of small necks occurs, filled with a coarse agglomerate composed of pieces of sandstone, flagstone and shale. Messrs. Peach and Horne infer that these little orifices never discharged any streams of lava. More probably they were opened by explosions which only gave forth vapours and fragmentary discharges, such as a band of tuff which is intercalated among the flagstones in their neighbourhood.
But one of the most striking features of the volcanic phenomena of this remote region is the relative size and number of the sills and dykes which here as elsewhere mark the latest phases of subterranean activity. Messrs. Peach and Horne have shown us that three great sheets of acid rocks (granites and spherulitic felsites, to which reference has already been made, p. 292) have been injected among the sandstones and basic lavas, that abundant veins of granite, quartz-felsite and rhyolite radiate from these acid sills, and that the latest phase of igneous action in this region was the intrusion of a series of bosses and dykes of basic rocks (diabases) which traverse the sills.
The Killarney District
In the south of Ireland the Upper Silurian strata are followed upwards conformably by the great series of red sandstones and conglomerates known as the "Dingle Beds." Lithologically these rocks present the closest resemblance to the Lower Old Red Sandstone of Central Scotland. They occupy a similar stratigraphical position, and though they have not yielded any palæontological data for comparison, there can, I think, be no hesitation in classing them with the Scottish Lower Old Red Sandstone, and regarding them as having been deposited under similar geographical conditions. They offer one feature of special interest for the purpose of the present inquiry, since they contain a well-marked group of contemporaneous volcanic rocks, including nodular felsites, like those so characteristic of the Silurian period.
The area where this remote and isolated volcanic group is best developed forms a range of high rugged ground along the northern front of the hills that stretch eastward from the Lakes of Killarney. Their general distribution is shown on Sheets 184 and 185 of the Geological Survey of Ireland;[390] though I may again remark that petrography has made great strides during the thirty years and more that have passed since these maps and their accompanying Memoirs were published, and that, were the district to be surveyed now, probably a considerable tract of ground coloured as ash would be marked as felsite. At the same time the existence of both these rocks here cannot be gainsaid.
[Footnote 390: See the Memoir (by J. B. Jukes and G. V. Du Noyer) on Sheet 184, p. 15. Other volcanic rocks have been mapped at Valentia Harbour in the Dingle Beds, but these I have not had an opportunity of personally examining.]
The felsite was long ago brought into notice by Dr. Haughton, who published an analysis of it.[391] It is also referred to by Mr. Teall for its spherulitic structure.[392] Seen on the ground it appears as a pale greenish-grey close-grained rock, sometimes exhibiting flow-structure in a remarkably clear manner, the laminæ of devitrification following each other in wavy lines, sometimes twisted and delicately puckered or frilled, as in some schists. Portions of the rock are strongly nodular, the nodules varying in size from less than a pea to that of a hen's egg.
[Footnote 391: Trans. Roy. Irish Acad. vol. xxiii. (1859), p. 615.]
[Footnote 392: British Petrography, p. 349.]
The close resemblance of this rock to many of the Lower Silurian nodular felsites of Wales cannot but strike the geologist. It presents analogies also to the Upper Silurian felsites of Dingle. But its chief interest arises from the geological horizon on which it occurs. Lying in the so-called "Dingle-Beds," which may be regarded as the equivalents of the Lower Old Red Sandstone of England and Scotland, it is, so far as my observations go, the only example of such a nodular felsite of later date than the Silurian period. We recognize in it a survival, as it were, of the peculiar Silurian type of acid lava, the last preceding eruption of which took place not many miles to the west, in the Dingle promontory. But elsewhere this type does not appear to have survived the end of the Silurian period.
The detrital rocks accompanying the felsite, in the district east of Killarney, vary from such closed-grained felsitic material as cannot readily be distinguished from the felsite itself to unmistakable felsitic breccias. Even in the finest parts of them, occasional rounded quartz-pebbles may be detected, while here and there a reddish shaly band, or a layer of fine pebbly conglomerate with quartz-pebbles an inch in length, shows at once the bedding and the dip. Mr. W. W. Watts, who, with Mr. A. M'Henry of the Irish Staff of the Geological Survey, accompanied me over this ground, found that a microscopic examination of the slides which were prepared from the specimens we collected completely confirmed the conclusions reached from inspection of the rocks in the field.[393] He detected among the angular grains slightly damaged crystals of felspar, chiefly orthoclase. Many portions of these felspathic grits much resemble the detrital Cambrian rocks which in the Vale of Llanberis have been made out of the pale felsite of that locality.
[Footnote 393: Mr. Watts also examined the microscopic structure of the felsite of Benaun More. He found that the spherulites appear to have a micropegmatitic structure, owing to the intergrowth of quartz and felspar. In some parts of the rock the spherulites, from ·02 to ·01 inch in diameter, are surrounded by exceedingly minute green needles, possibly of hornblende, while inside some of them are small quartz-grains. Larger porphyritic felspars occur outside the spherulites, some being of plagioclase, but most of orthoclase. The spherulitic structure is not so well developed near the felspars. A few of the large nodules are hollow and lined with crystals, while some of them show a finely concentric lamination like the successive layers of an agate.]
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.]
XXIII. THE CARBONIFEROUS SYSTEM OF BRITAIN AND ITS VOLCANIC RECORDS
Geography and Scenery of the Carboniferous Period--Range of Volcanic Eruptions during that time--I. The Carboniferous Volcanoes of Scotland--Distribution, Arrangement and Local Characters of the Carboniferous System in Scotland--Sketch of the Work of previous Observers in this Subject.
Within the area of the British Isles, the geological record is comparatively full and continuous from the base of the Upper Old Red Sandstone to the top of the Coal-measures. We learn from it that the local basins of deposit in which the later portion of the Old Red Sandstone was accumulated sank steadily in a wide general subsidence, that allowed the clear sea of the Carboniferous Limestone ultimately to spread for some 700 miles from the west coast of Ireland into Westphalia. Over the centre of England this Carboniferous Mediterranean had a breadth of at least 150 miles, gradually shallowing northwards in the direction of land in Scotland and Northern Ireland. The gentle sinking of the floor of the basin continued until more than 6000 feet of sediment, chiefly composed of the remains of crinoids, corals and other marine organisms, had been piled up in the deeper parts. Traces of the southern margin of this sea, or at least of a long insular ridge that rose out of its waters, are to be seen in the protuberances of older rocks which appear at intervals from under the Coal-measures and later formations between the borders of Wales and the heart of Leicestershire, and of which the crags of Charnwood Forest are among the few peaks that still remain visible. To the south of this ridge, open sea extended far southward and westward over the site of the Mendip Hills and the uplands of South Wales.
The Carboniferous period, as chronicled by its sedimentary deposits, was a time of slow submergence and quiet sedimentation, terrestrial and marine conditions alternating along the margins of the sinking land, according as the rate of depression surpassed or fell short of that of the deposition of sediment. There is no trace of any general disturbance among the strata, such as would be marked by an important and widely extended unconformability. But many indications may be observed that the rate of subsidence did not continue uniform, if, indeed, the downward movement was not locally arrested, and even exchanged for a movement in the opposite direction. It is difficult, for instance, to believe the ancient ridge of the Midlands to have been so lofty that even the prolonged subsidence required for the accumulation of the whole Carboniferous system was insufficient to carry its highest crests below the level of the coal-jungles. More probably the depression reached its maximum along certain lines or bands running in a general north-easterly direction, the intervals between these lines sinking less, or possibly even undergoing some measure of uplift. One of the subsiding tracts, that of the wide lowlands of Central Scotland, was flanked on the south by a ridge which, while its north-eastern portion was buried under the Upper Old Red Sandstone and Lower Carboniferous rocks, remained above water towards the south-west, and does not appear to have been wholly submerged there even at the close of the Carboniferous period.
So abundant and varied are the sedimentary formations of Carboniferous time, and so fully have they preserved remains of the contemporary plants and animals, that it is not difficult to realise in some measure the general aspect of the scenery of the time, and the succession of changes which it underwent from the beginning to the end of the period. The land was green with a luxuriant if somewhat monotonous vegetation. Large pine trees flourished on the drier uplands. The lower grounds nourished dense groves of cycads or plants allied to them, which rose as slim trees twenty or thirty feet high, with long hard green leaves and catkins that grew into berries. The swamps and wetter lands bore a rank growth of various gigantic kinds of club-moss, equisetaceous reeds and ferns.
Nor was the hum of insect-life absent from these forests. Ancestral types of cockroaches, mayflies and beetles lived there. Scorpions swarmed along the margins of the shallow waters, for their remains, washed away with the decayed vegetation among which they harboured, are now found in abundance throughout many of the dark shales.
The waters were haunted by numerous kinds of fish quite distinct from those of the Old Red Sandstone. In the lagoons, shoals of small ganoids lived on the cyprids that peopled the bottom, and they were in turn preyed on by larger ganoids with massive armature of bone. Now and then a shark from the opener sea would find its way into these more inland waters. The highest types of animal life yet known to have existed at this time were various amphibians of the extinct order of Labyrinthodonts.
The open sea, too, teemed with life. Wide tracts of its floor supported a thick growth of crinoids whose jointed stems, piled over each other generation after generation, grew into masses of limestone many hundreds of feet in thickness. Corals of various kinds lived singly and in colonies, here and there even growing into reefs. Foraminifera, sponges, sea-urchins, brachiopods, gasteropods, lamellibranchs and cephalopods, in many genera and species, mingled their remains with the dead crinoids and corals to furnish materials for the wide and thick accumulation of Carboniferous Limestone.
Looking broadly at the history of the Carboniferous period, and bearing in mind the evidence of prolonged depression already referred to, we can recognize in it three great eras. During the first, the wide clear sea of the Carboniferous Limestone spread over the centre and south of Britain, interrupted here and there by islands that rose from long ridges whereby the sea-floor was divided into separate basins. Next came a time of lessened depression, when the sea-bottom was overspread with sand, mud and gravel, and was even in part silted up, as has been chronicled in the Millstone Grit. The third stage brings before us the jungles of the Coal-measures, when the former sea-floor became a series of shallow lagoons where, as in the mangrove-swamps of our own time, a terrestrial vegetation sprang up and mingled its remains with those of marine shells and fishes.
Such a state of balance among the geological forces as is indicated by the stratigraphy of the Carboniferous system would not prepare us for the discovery of the relics of any serious display of contemporary volcanic activity. And, indeed, throughout the Carboniferous rocks of Western Europe there is for the most part little trace of contemporaneous volcanic eruptions. Yet striking evidence exists that, along the western borders of the continental area, in France as well as over much of Britain, which had for so many previous geological ages been the theatre of subterranean activity, the older half of Carboniferous time witnessed an abundant, though less stupendous and prolonged, renewal of volcanic energy.
From the very commencement of the Carboniferous period to the epoch when the Coal-measures began to be accumulated, the area of the British Isles continued to be a scene of active volcanism. In the course of that prolonged interval of geological time the vents shifted their positions, and gradually grew less energetic, but there does not appear to have been any protracted section of the interval when the subterranean activity became everywhere entirely quiescent.
The geologist who traces, from older to younger formations, the progress of some persistent operation of nature, observes the evidence gradually to increase in amount and clearness as it is furnished by successively later parts of the record. He finds that the older rocks have generally been so dislocated and folded, and are often so widely covered by younger formations, that the evidence which they no doubt actually contain may be difficult to decipher, or may be altogether concealed from view. In following, for instance, the progress of volcanic action, he is impressed, as he passes from the older to the younger Palæozoic chronicles, by the striking contrast between the fulness and legibility of the Carboniferous records and the comparative meagreness and obscurity of those of the earlier periods. The Carboniferous rocks have undergone far less disturbance than the Cambrian and Silurian formations; while over wide tracts, where their volcanic chapters are fullest and most interesting, they lie at the surface, and can thus be subjected to the closest scrutiny. Hence the remains of the volcanic phenomena of the later Palæozoic periods present a curiously modern aspect, when contrasted with the fragmentary and antique look of those of older date.
The history of volcanic action during the Carboniferous period in Britain is almost wholly comprised in the records of the earlier half of that period, that is, during the long interval represented by the Carboniferous Limestone series and the Millstone Grit. It was chiefly in the northern part of the region that volcanic activity manifested itself. In Scotland there is the chronicle of a long succession of eruptions across the district of the central and southern counties, from the very beginning of Carboniferous time down to the epoch when the Coal-measures began to be accumulated. In England, on the other hand, the traces of Carboniferous volcanoes are confined within a limited range in the Carboniferous Limestone, while in Ireland they appear to be likewise restricted to the same lower division of the system. During the whole of the vast interval represented by the Coal-measures volcanic energy, so far as at present known, was entirely dormant over the region of the British Isles.
These general statements will be more clearly grasped from the accompanying table, which shows the various sections into which the Carboniferous system of Britain has been divided, and also, by black vertical lines, the range of volcanic intercalations in each of the three kingdoms.
+-----------------------------------------+----------+-----------+----------+ | | England. | Scotland. | Ireland. | +-----------------------------------------+----------+-----------+----------+ |Coal-measures. | | | | | { Upper Red Sandstones with | | | | | { Spirorbis-limestone. | | | | | { Middle or chief coal-bearing | | | | | { measures. | | | | | { Gannister group. | | | | | | | | | |Millstone Grit. | | || | | | } Grits, flagstones and shales with | | || | | | } thin coals. | | || | | | | | || | | |Carboniferous Limestone. | | || | | | { Yoredale group of shales and grits | | || | | | { with limestones. | | || | | | { Thick (Scaur or Main) Limestone | || | || | | | { of England, with sandstones | || | || | || | | { and coals in Scotland. | || | || | || | | { Lower Limestone Shale (Calciferous | | || | | | { Sandstones of Scotland). | | || | | +-----------------------------------------+----------+-----------+----------+
Such being the general range in time of the Carboniferous volcanic phenomena, it may be convenient, in this preliminary survey, to take note of the general distribution of the volcanic districts over the British Isles, as in this way we may best realise the extent and grouping of the eruptions, which will then be considered in further detail (see Map I.).
Not only were the Carboniferous volcanoes most abundant and persistent in Scotland, but they attained there a variety and development which give their remains an altogether exceptional interest in the study of volcanic geology. They were distributed over the wide central valley, from the south of Cantyre to beyond the mouth of the estuary of the Forth. On the southern side of the Silurian Uplands, they were likewise numerous and active. There is thus no considerable tract of Lower Carboniferous rocks in Scotland which does not furnish its evidence of contemporaneous volcanic action.
Although some portions of the Scottish Carboniferous igneous rocks run for a short distance into England, it is remarkable that, when these at last die out southwards, no other relics of contemporaneous volcanic energy take their place. Along the Pennine chain, from the Border into the heart of England, though natural sections are abundant, no trace of included volcanic rocks appears until we reach Derbyshire. The whole of that wide interval of 150 miles, so far as the present evidence goes, remained during Carboniferous time entirely free from any volcanic eruption. But from the picturesque country of the Peak southwards, the sea-floor of the Carboniferous Limestone, in what is now the heart of England, was dotted with vents whence the sheets of "toadstone" were ejected, which have so long been a familiar feature in English geology. Beyond this limited volcanic district the Carboniferous formations of the south-west of England remain, on the whole, devoid of contemporaneous volcanic intercalations, traces of Carboniferous volcanic action having been recognized only in West Somerset and Devonshire. In the Mendip district and in the ridges of limestone near Weston-super-Mare bands of cellular lava and tuff have been observed. To the west of Dartmoor, Brent Tor and some of the surrounding igneous masses may mark the positions of eruptive vents during an early part of the Carboniferous period.
At the south end of the Isle of Man relics remain of a group of vents among the Carboniferous limestones. Passing across to Ireland, where these limestones attain so great a thickness and cover so large a proportion of the surface of the island, we search in vain for any continuation of the abundant and varied volcanic phenomena of Central Scotland. So far as observation has yet gone, only two widely separated areas of Carboniferous volcanic rocks are known to occur in Ireland.[399] One of these shows that a little group of vents probably rose from the floor of the Carboniferous Limestone sea, near Philipstown, in King's County. The other lies far to the west in the Golden Vale of Limerick, where a more important series of vents poured out successive streams of lava with showers of ashes, from an early part of the Carboniferous period up to about the beginning of the time of the Coal-measures.
[Footnote 399: The supposed Carboniferous volcanic rocks of Bearhaven on the coast of Cork are noticed on p. 49, vol. ii.]
The total area within which the volcanic eruptions of Carboniferous time took place was thus less than that over which the volcanoes of the Lower Old Red Sandstone were distributed, yet they were scattered across the larger part of the site of the British Isles. From the vents of Fife to those of Limerick is a distance of above 300 miles; from the latter eastward to those of Devonshire is an interval of 250 miles; while the space between the Devonshire volcanoes and those of Fife is about 400 miles. In this triangular space volcanic action manifested itself at each of the apices, to a slight extent along the centre of the eastern side, but with much the greatest vigour throughout the northern part of the area.
Since the volcanic phenomena of Carboniferous time are exhibited on a much more extensive scale in Scotland than in any other region of the world yet studied, it will be desirable to describe that area in considerable detail. The other tracts in Britain where volcanic rocks of the same age occur need not be so fully treated, except where they help to a better comprehension of the general volcanic history.
* * * * *
It is in the southern half of Scotland that the Carboniferous system is developed (Map IV.). A line drawn from Machrihanish Bay, near the Mull of Cantyre, north-eastward across Arran and Bute to the south end of Loch Lomond, and thence eastward by Bridge of Allan, Kinross and Cupar to St. Andrews Bay, forms the northern limit of this system. South of that line Carboniferous volcanic intercalations are to be met with in nearly every county across into the borders of Northumberland.
That we may follow intelligently the remarkably varied volcanic history of this region, it is desirable to begin by taking note of the nature and sequence of the sedimentary formations among which the volcanic rocks are intercalated, for these serve to bring before us the general conditions of the geography of the period. The subjoined table exhibits the subdivisions into which the Carboniferous system in Scotland has been grouped:--
Coal-measures. { Upper Red Sandstone group, nearly devoid of coal-seams. { { Coal-bearing, white, yellow and grey sandstones, dark shales and { ironstones (Upper Coal series).
Millstone Grit. { Thick white and reddish sandstones and grits.
Carboniferous Limestone series. { Sandstones, shales, fireclays, coal-seams, ironstones and three seams { of marine limestone, of which the uppermost is known as the { Castlecary seam, the second as the Calmy or Arden, and the lowest { as the Index (Lower Coal series). { { Bands of marine limestone intercalated among sandstones, shales and { some coal-seams. A thick band of limestone lying at or near the { bottom of the group, traceable all over Central Scotland, is known { as the Hurlet or Main Limestone. Some higher and thinner seams are { called Hosie's (see Fig. 155).
Calciferous Sandstones.[400] { In the basin of the Firth of Forth, below the Hurlet Limestone, comes { a varied series of white and yellow sandstones, black shales { (oil-shales), cyprid shales and limestones (Burdiehouse), and { occasional coal-seams (Houston), having a total depth of about 3000 { feet. This local group abounds in fossil plants, entomostraca and { ganoid fishes. It passes down into the Cement-stone group, which, { however, is feebly developed in this district, unless it is partly { represented by the sandstones, shales, limestones and coals just { mentioned. { Cement-stone group consisting of red, blue and green marls and { shales, red and grey sandstones, and thin bands of cement-stone: { fossils scarce. { Reddish and grey sandstones and shales, with occasional plant- { remains, passing down into the deep red (sometimes yellow) { sandstones, red marls and cornstones of the Upper Old Red Sandstone.
[Footnote 400: The Calciferous Sandstones are the stratigraphical equivalents of the Limestone Shale and lower portion of the Carboniferous Limestone of England.]
From this table the gradual geographical evolution of the Carboniferous period in Scotland may be gleaned. We observe that at the beginning, the conditions under which the Old Red Sandstone had been accumulated still in part continued. The great lacustrine basins of the Lower Old Red Sandstone had indeed been effaced, and their sites were occupied by comparatively shallow areas of fresh or brackish water in which the Upper Old Red Sandstone was laid down. Their conglomerates and sandstones had been uplifted and fractured. Their vast ranges of volcanic material, after being deeply buried under sediment, had been once more laid bare, and extended as ridges of land, separating the pools and lagoons which they supplied with sand and silt. This singular topography had not been entirely effaced at the beginning of the Carboniferous period, for we find that many of the ridges which bounded the basins of the Upper Old Red Sandstone remained as land until they sank beneath the waters in which the earliest Carboniferous strata accumulated. Thus, while no trace of an unconformability has yet been detected at the top of the Upper Old Red Sandstone, there is often a strong overlap of the succeeding deposits. At the south end of the Pentland Hills, for example, the Upper Old Red Sandstone attains a thickness of 1000 feet, but only three miles further south it entirely disappears, together with all the overlying mass of Calciferous Sandstones, and the Carboniferous Limestone then rests directly on the Lower Old Red Sandstone. Again, at the north end of the same chain the upper division of the Old Red Sandstone dies out against the lower, which is eventually overlapped by the Calciferous Sandstones.
The change from the physical conditions of the Scottish Old Red Sandstone to those of the Carboniferous system was no doubt gradual and slow. The peculiar red sandy sediment continued to be laid down in basins that were apparently being gradually widened by access of water from the open sea. Yet it would seem that in Scotland these basins still for a long time continued saline or, from some other cause, unfavourable to life; for the red, blue and green shales or marls, and occasional impure limestones or cement-stones and gypseous layers, which were deposited in them, are in general unfossiliferous, though drifted plants from the neighbouring land are here and there common enough. The sediments of these early Carboniferous waters are met with all over the southern half of Scotland, but in very unequal development, and constitute what is known as the "Cement-stone Group."
It was while these strata were in course of deposition that the earliest Carboniferous volcanoes broke into eruption. In some localities a thickness of several hundred feet of the Cement-stone group underlies the lowest lavas. In other places the lavas occur in and rest on the Upper Old Red Sandstone and have the Cement-stone group wholly above them; while in yet other districts the volcanic rocks seem entirely to take the place of that group. So vigorous was the earliest display of volcanic action in Carboniferous times that from the borders of Northumberland to the uplands of Galloway, and from the slopes of the Lammermuirs to Stirlingshire and thence across the estuary of the Clyde to Cantyre, innumerable vents were opened and large bodies of lava and ashes were ejected.
The Cement-stone group, save where succeeded by volcanic intercalations, passes up conformably into the lowest crinoidal limestones of the Carboniferous Limestone series. In the basin of the Firth of Forth, however, the cement-stones, feebly represented there, are overlain by a remarkable assemblage of white sandstones, black carbonaceous shales, or "oil-shales," cyprid limestones, occasional marine limestones and thin seams of coal, the whole having a thickness of more than 3000 feet. These strata, unlike the typical Cement-stone group, abound in fossils both vegetable and animal. They prove that, over the area of the Forth, the insalubrious basins wherein the red and green sediments of the Cement-stone group were laid down, gave place to opener and clearer water with occasional access of the sea. The peculiar lagoon-conditions which favoured the formation of coal were thus developed in Central Scotland earlier than elsewhere in Britain. We shall see in later pages that these conditions were accompanied by a fresh outbreak of volcanic activity, in a phase less vigorous but more enduring and extensive than that of the first Carboniferous eruptions.
The Carboniferous Limestone sea over the site of the southern half of Scotland appears never to have reached the depth which it attained in England and Ireland. To the north of it lay the land from which large quantities of sand and mud were carried into it, as shown by the deep accumulations of sandstone and shale, which far surpass in thickness the few comparatively thin marine limestones intercalated in them. There is thus a striking contrast between the thick masses of limestone in central and south-western England and their dwindled representatives in the north. Another marked difference between the Scottish and English developments of this formation is to be noticed in the abundant proof that the comparatively shallow waters of the northern basin were plentifully dotted over with active volcanoes. The eruptions were especially vigorous and prolonged in the basin of the Firth of Forth. They continued at intervals, even after the peculiar geographical conditions of the Carboniferous Limestone had ceased. But they had died out by the time of the beginning of the Coal-measures.
Owing to the number and variety of the natural sections, the Carboniferous volcanic rocks of Scotland have been the subject of numerous observations and descriptions, from the early days of geology down to the present time. The mere enumeration of the titles of the various publications regarding them would make a long list. These rocks formed the subject of some of Hutton's early observations, and furnished him with facts from which he established the igneous origin of "whinstone."[401] They supplied Playfair with numerous apt illustrations in support of Hutton's views, and he seems to have made himself thoroughly familiar with them.[402] In the hands of Sir James Hall they became the groundwork of those remarkable experiments on the fusion of whinstone which may be said to have laid the foundation of experimental geology.[403] In the controversies of the Neptunian and Plutonian schools these rocks were frequently appealed to by each side in confirmation of its dogmas. The appointment in 1804 of Jameson to the Chair of Natural History in the Edinburgh University gave increased impetus to the study of the igneous rocks of Scotland. Though he did not himself publish much regarding them, we know that he was constantly in the habit of conducting his class to the hills, ravines and quarries around Edinburgh, and that the views which he taught were imbibed and extended by his pupils.[404] Among the early writers the names of Allan,[405] Townson,[406] Lord Greenock,[407] and Ami Boué,[408] deserve especial mention.
[Footnote 401: Hutton's Theory of the Earth, vol. i. p. 155 et seq.]
[Footnote 402: Playfair's Illustrations of the Huttonian Theory, § 255 et seq.]
[Footnote 403: Trans. Roy. Soc. Edin. (1805), vol. v. p. 43.]
[Footnote 404: Mem. Wern. Soc. ii. 178, 618; iii. 25; Edin. Phil. Journ. i. 138, 352; xv. 386.]
[Footnote 405: Trans. Roy. Soc. Edin. (1811), vi. p. 405.]
[Footnote 406: _Tracts and Observations in Natural History and Physiology_, 8vo, Lond. 1799.]
[Footnote 407: Trans. Roy. Soc. Edin. (1833), xiii. pp. 39, 107.]
[Footnote 408: Essai géologique sur l'Écosse. Paris; no date, probably 1820.]
The first broad general sketch of the Carboniferous igneous rocks of a large district of the country was that given by Hay Cunningham in his valuable essay on the geology of the Lothians.[409] He separated them into two series, the Felspathic, including "porphyry" and "clinkstone," and the Augitic or Trap rocks. To these he added "Trap-tufa," which he considered to be identical in origin with modern volcanic tuff. It was the eruptive character of the igneous rocks on which he specially dwelt, showing by numerous sections the effects which the protrusion of the molten masses have had upon the surrounding rocks. He did not attempt to separate the intrusive from the interstratified sheets, nor to form a chronological arrangement of the whole.
[Footnote 409: Mem. Wern. Soc. vii. p. 1. Published separately, 1838.]
Still more important was the sketch given by Maclaren, in his classic Geology of Fife and the Lothians.[410] This author clearly recognized that many of the igneous rocks were thrown out contemporaneously with the strata among which they now lie. He constantly sought for analogies among modern volcanic phenomena, and presented the Carboniferous igneous rocks of the Lothians not as so many petrographical varieties, but as monuments of different phases of volcanic action previous to the formation of the Coal-measures. His detailed descriptions of Arthur Seat and the rocks immediately around Edinburgh, which alone the work was originally intended to embrace, may be cited as models of exact and luminous research. The portions referring to the rest of the basin of the Forth did not profess to be more than a mere sketch of the subject.
[Footnote 410: Small 8vo, Edin. 1838, first partly published as articles in the Scotsman newspaper. A second edition, which was little more than a reprint of the first, appeared in 1866.]
Various papers of more local interest, to some of which allusion will be made in the sequel, appeared during the next quarter of a century. But no systematic study of the volcanic phenomena of any part of Scotland was resumed until the extension in 1854 of the Geological Survey to the north of the Tweed by A. C. Ramsay. The volcanic rocks of the Lothians and Fife were mapped by Mr. H. H. Howell and myself. The maps of that district began to be published in the year 1859, and the Memoirs two years later. In 1861, in a chronological grouping of the whole of the volcanic phenomena of Scotland, I gave an outline of the Carboniferous eruptions.[411] By degrees the detailed mapping of the Geological Survey was pushed across the whole of the rest of the south of Scotland, and the Carboniferous volcanic rocks of each area were then for the first time carefully traced and assigned to their various stratigraphical horizons. In the following pages reference will be given to the more important features of the Survey maps and Memoirs. In the year 1879, availing myself of the large amount of information which my own traverses and the work of the Survey had enabled me to acquire, I published a Memoir on the geology and petrography of the volcanic rocks of the basin of the Firth of Forth;[412] and lastly, in my Presidential Address to the Geological Society in 1892, I gave a summary of all that had then been ascertained on the subject of the volcanic rocks of Carboniferous time in the British Isles.[413]
[Footnote 411: Trans. Roy. Soc. Edin. vol. xxii.]
[Footnote 412: Ibid. vol. xxix. (1879), p. 437.]
[Footnote 413: Quart. Journ. Geol. Soc. xlviii. (1892), p. 104. This summary, with additional details and illustrations, is embodied in the text.]
Two well-marked types of volcanic accumulations are recognizable in the British Isles, which may be conveniently termed Plateaux and Puys.
1. Plateaux.--In this type, the volcanic materials were discharged over wide tracts of country, so that they now form broad tablelands or ranges of hills, reaching sometimes an extent of many hundreds of square miles and a thickness of more than 1000 feet. Plateaux of this character occur within the British area only in Scotland, where they are the predominant phase of volcanic intercalations in the Carboniferous system.
It is noteworthy that the Carboniferous plateaux appeared during a well-marked interval of geological time. The earliest examples of them date from the close of the Upper Old Red Sandstone. They were all in vigorous eruption during the time of the Calciferous Sandstones, but in no case did they survive into that of the Hurlet and later limestones. They are thus eminently characteristic of the earliest portion of the Carboniferous period.
2. Puys.--In this type, the ejections were often confined to the discharge of a small amount of fragmentary materials from a single solitary vent, and even where the vents were more numerous and the outpourings of lava and showers of ash more copious, the ejected material usually covered only a small area round the centres of eruption. Occasionally streams of basic lava and accumulations of tuff were piled up into long ridges. Volcanoes of this character were specially abundant in the basin of the Firth of Forth, and more sparingly in Ayrshire and Roxburghshire. They form the persistent type throughout the rest of the British Isles.
The Puys also occupy a well-defined stratigraphical position. They did not begin until some of the volcanic plateaux had become extinct. From the top of the Cement-stone group up into the Carboniferous Limestone series, their lavas and tuffs are met with on many platforms, but none occur above that series save in Ayrshire, where some of the eruptions appear to have been as late as about the beginning of the Coal-measures.
Arranged in tabular form the stratigraphical and geographical distribution of the two great volcanic types of the Carboniferous system in Scotland will be more easily followed. I have therefore drawn up the accompanying scheme:--
Location Key: A. Ayrshire and Renfrewshire. B. Stirlingshire. C. West Lothian. D. Midlothian. E. East Lothian. F. Fife. G. Berwick & Roxburghshire.
+--------------------------------------------------++--------------------+ | Plateau-type. || Puy-type. | +-----------------------------------+--+--+--+--+--++--+--+--+--+--+--+--+ | |A.|B.|D.|E.|G.||A.|B.|C.|D.|E.|F.|G.| |Coal Measures |..|..|..|..|..||..|..|..|..|..|..|..| | | | | | | ||‖ | | | | | | | |Millstone Grit |..|..|..|..|..||‖ |..|..|..|..|..|..| | | | | | | ||‖ | | | | | | | |Carboniferous Limestone Series. | | | | | ||‖ | | | | | | | | { Castlecary Limestone |..|..|..|..|..||‖ |..|..|..|..|..|..| | { | | | | | ||‖ | | | | | | | | { Calmy " |..|..|..|..|..||‖ |..|..|..|..|..|..| | { | | | | | ||‖ | |‖ | | |‖ | | | { Index " |..|..|..|..|..||‖ |..|‖ |..|..|‖ |..| | { | | | | | || | |‖ | | |‖ |‖ | | { Hurlet " |..|..|..|..|..||..|..|‖ |..|..|‖ |‖ | | |‖ |‖ | |‖ | || | |‖ | | |‖ |‖ | |Calciferous Sandstone Series. |‖ |‖ | |‖ | || | |‖ | | |‖ |‖ | | { Burdiehouse Limestone |‖ |‖ | |‖ | || | |‖ | | |‖ |‖ | | { and Oil-shale Group |‖ |‖ |..|‖ |..||..|..|‖ |..|..|‖ |‖ | | { |‖ | |‖ |‖ | || | |‖ |‖ | |‖ |‖ | | { Cement-stone Group |..|..|..|..|‖ ||..|..|‖ |‖ |..|‖ |‖ | | { | | | | |‖ || | | | | | | | | { Red Sandstones passing down |..|..|..|..|‖ ||..|..|..|..|..|..|..| | { into Upper Old Red Sandstone|..|..|..|..|‖ ||..|..|..|..|..|..|..| +-----------------------------------+--+--+--+--+--++--+--+--+--+--+--+--+
XXIV. CARBONIFEROUS VOLCANIC PLATEAUX OF SCOTLAND
I. The Plateau-type restricted to Scotland--i. Distribution in the Different Areas of Eruption--ii. Nature of the Materials erupted.
In the division of the Plateaux I group all the more copious eruptions during the Carboniferous period, when the fragmentary materials generally formed but a small part of the discharges, but when the lavas were poured out so abundantly and frequently as to form lava-fields sometimes more than 2000 square miles in area, and to build up piles of volcanic material sometimes upwards of 3000 feet in thickness. As already remarked, this phase of volcanic action, especially characteristic of the earlier part of the Carboniferous period across the south of Scotland, but not found elsewhere in the same system in Britain, preceded the type of the Puys. Its eruptions extended from about the close of the Old Red Sandstone period through that section of Carboniferous time which was marked by the deposition of the Calciferous Sandstones, but they entirely ceased before the accumulation of the Main or Hurlet Limestone, at the base of the Carboniferous Limestone Series of Scotland. Its stratigraphical limits, however, are not everywhere the same. In the eastern part of the region, the lavas appear to be intercalated with, and certainly lie directly upon, the Upper Old Red Sandstone containing scales of Bothriolepis and other characteristic fishes, and they are covered by the Cement-stone group of the Calciferous Sandstones. In the western district a considerable thickness of Carboniferous strata sometimes underlies the volcanic sheets. On the other hand, the type of the Puys, although it appeared in Fife, Linlithgowshire and Midlothian during the time of the Calciferous Sandstones, attained its chief development during that of the Carboniferous Limestone, and did not finally die out in Ayrshire until the beginning of the deposition of the Coal-measures.
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.
ii. NATURE OF THE MATERIALS ERUPTED
The volcanic materials characteristic of the plateau-type of eruptions consist mainly of lavas in successive sheets, but include also various tuffs in frequent thin courses, and less commonly in thick local accumulations. The lavas are chiefly andesites in the altered condition of porphyrites. They vary a good deal in the relative proportions of silica. Some of them are decidedly basic and take the form of dolerites and olivine-basalts. With these rocks are occasionally associated "ultra-basic" varieties, where the felspar almost disappears and the material consists mainly of ferro-magnesian minerals. The more basic rocks are generally found towards the bottom of the volcanic series, where they appear as the oldest flows. In the Garleton Hills lavas of a much more acid nature are met with--true sanidine-trachytes, which overlie the porphyrites and basalts of the earlier eruptions.
No adequate investigation has yet been made of the chemical and microscopic characters of these various rocks, regarded as a great volcanic series belonging to a definite geological age, though many of the individual rocks and the petrography of different districts have been more or less fully described. I cannot here enter into much detail on the subject, but must content myself with such a summary as will convey some idea of the general composition and structure of this very interesting volcanic series.
(a) Augite-olivine Rocks (Picrites and Limburgites).--Towards the bottom of the plateaux there are found here and there sheets of "ultra-basic" material, some of which appear to be bedded with the other rocks and to have flowed out as surface-lavas, though it may be impossible to prove that they are not sills. Thus at Whitelaw Hill, on the south side of the Garleton Hills, a dark heavy rock is found to contain hardly any felspar, but to be made up mainly of olivine and augite. Dr. Hatch has published a description and drawing of this rock, together with the following analysis by Mr. Player:[421]--
Silica 40·2 Titanic oxide 2·9 Alumina 12·8 Ferric oxide 4·0 Ferrous oxide 10·4 Lime 10·4 Magnesia 11·9 Potash 0·8 Soda 2·7 Loss by ignition 3·4 ---- Spec. grav. 3·03. 99·5
[Footnote 421: Trans. Roy. Soc. Edin. vol. xxxvii. (1893), p. 116.]
(b) Dolerites and Basalts.[422]--These rocks are found both as interstratified lavas and as intrusive masses. In the former condition they take a conspicuous place among the sheets of the plateaux, but especially in the lower parts of the series. They are dark, often black, usually more or less porphyritic, with large felspars, frequently also large crystals of augite or olivine, and may be described as porphyritic olivine-dolerites and olivine-basalts, more rarely as olivine-free dolerites and basalts. Their groundmass consists of short laths or microlites of felspar (probably labradorite) and granules or small crystals of augite and magnetite, with sometimes a little fibrous brown mica. The large porphyritic felspars are striped (probably labradorite), the augites are frequently chloritized, and the olivines are generally more or less serpentinized. But in some cases all these minerals are as fresh as in a recent basalt. The rocks are sometimes beautifully columnar, as at Arthur Seat.
[Footnote 422: A general classification of the whole series of Scottish Carboniferous dolerites and basalts, including both the plateau and puy examples, will be given in the account of the rocks of the puys in Chapter XXVI. (p. 418).]
Of these basic lavas conspicuous examples may be seen at Arthur Seat, Calton Hill and Craiglockhart Hill. The eastern part of Arthur Seat, known as Whinny Hill, furnishes examples of olivine-dolerites of the Jedburgh type (p. 418). The beautiful basalt of Craiglockhart with its large porphyritic olivines and augites has afforded a distinct type of Carboniferous basalt (p. 418). The same type occurs on the Calton Hill in the cliff below the gaol. Similar basic lavas are especially abundant and remarkable in the Clyde plateau near Campbeltown in Argyllshire, and at the south end of Bute and in the Cumbraes, where they are associated with an interesting series of dykes and sills. But even where, as in the Garleton Hills, the lavas are for the most part somewhat acid in composition, those first poured out, which form the lowest band, include some typical olivine-basalts, of which a characteristic example occurs at Kippie Law at the base of the Garleton plateau (p. 418). It has been described by Dr. Hatch as exhibiting under the microscope porphyritic crystals of felspar and olivine lying in a groundmass composed of lath-shaped felspars, granular olivine and magnetite, and microlitic augite. The olivine, originally the most abundant constituent, has been converted into a fibrous aggregate of serpentine. All the minerals are more or less idiomorphic, but especially the augite, which crowds the groundmass in delicately-shaped prisms, most of which are terminated at both ends by faces of the hemi-pyramid. The analysis of this rock is given in the accompanying table of analyses of Garleton basalts. The Kippie Law type of basalt was recognized by Dr. Hatch among the Geological Survey collections from other districts, as in the intrusive bosses of Neides Law and Bonchester near Jedburgh, and from the Campsie plateau a mile and a half north of Lennoxtown.[423]
[Footnote 423: Trans. Roy. Soc. Edin. vol. xxxvii. (1893), pp. 117-119.]
At Hailes Castle, in the Garleton plateau, the lower basic lavas include another olivine-basalt somewhat more felspathic than that just described, and at Markle quarry the rock is still more felspathic and contains the olivine only in small sporadic grains. The composition of these basic rocks of the Garleton plateau is shown in the subjoined table of analyses by Mr. J. S. Grant Wilson:--
+------------+--------------+----------------+----------------+ | | Kippie Law, | Hailes Castle, | Markle Quarry, | | | specific | specific | specific | | | gravity 2·8 | gravity 2·76 | gravity 2·7 | +------------+--------------+----------------+----------------+ |SiO_{2} | 46·01 | 49·07 | 49·54 | |Al_{2}O_{3} | 19·19 | 19·43 | 22·23 | |Fe_{2}O_{3} | 5·91 | 10·58 | 9·55 | |FeO | 6·75 | 2·35 | 1·12 | |MnO | 0·19 | 0·32 | 0·08 | |CaO | 8·68 | 7·87 | 7·19 | |MgO | 6·81 | 4·36 | 2·80 | |K_{2}O | 1·20 | 0·98 | 1·81 | |Na_{2}O | 3·27 | 3·31 | 4·56 | |H_{2}O | 3·07 | 2·26 | 2·42 | |Total | 101·08 | 100·53 | 101·30 | +------------+--------------+----------------+----------------+
Olivine-dolerites are more especially developed in the district around Jedburgh, where they form some of the most prominent bosses, such as Dunian and Black Law. They show a sub-ophitic groundmass, with inconspicuous porphyritic crystals, among which those of olivine are more prominent than the felspars (p. 418).
(c) Andesites (Porphyrites).--These are the most abundant lavas of the plateaux. They occur in every district, and usually form the main constituents of the pile of volcanic material. They vary in colour from a pale pinkish grey, through many shades of red, purple, brown and yellow, to sometimes a dark green or nearly black rock. Their texture ranges from almost semi-vitreous, through different degrees of compactness, to open, cellular, slaggy masses. Generally through their base porphyritic felspars are abundantly disseminated, sometimes in large, flat, tabular forms, like those of the Lower Old Red Sandstone already referred to. The amygdaloidal kernels consist of calcite, zeolites, chalcedony or quartz. It is from the amygdaloids on either side of the Clyde that the fine examples of zeolites have been chiefly obtained for which the south of Scotland has long been famed. Occasionally, as at the south end of Bute, the andesitic lavas display a marked columnar structure.
Under the microscope these rocks present the usual fine felted aggregate of felspar microlites, with granules or crystals of magnetite and sometimes pyroxene. The porphyritic felspars, often large and well defined, generally contain inclusions of the groundmass. Occasionally some of the large porphyritic constituents are augite, or pseudomorphs after that mineral. The alteration of the rocks has oxidized some of the iron-ore and given rise to the prevalent purplish and reddish tints.
(d) Trachytes.--Some of the most remarkable lavas to be found in any of the plateaux are those which constitute a large part of the Garleton Hills. They overlie the lower andesite and basalt platform, which surrounds them as a narrow belt, while they occupy the central and much the largest part of the area. They have been included among the porphyrites, but are pale rocks, generally with a yellowish crust, presenting when quite fresh a grey, compact, felsitic base with large porphyritic crystals of unstriped felspar.
A number of specimens selected as illustrative of the different varieties have been analyzed and the results are stated in the subjoined table.[424] The specific gravity of the rocks is about 2·6.
[Footnote 424: The first two analyses are by Mr. J. S. Grant Wilson, the last two by Mr. A. Dick jun., and that from Hopetoun Monument by Mr. G. Barrow. Trans. Roy. Soc. Edin. vol. xxxvii. p. 122.]
+------------+--------+--------+----------+-------------+----------+ | | Pepper | Kae | Hopetoun | Phantassie | Bangley | | | Craig | Heughs | Monument | | Quarry | +------------+--------+--------+----------+-------------+----------+ | SiO_{2} | 62·61 | 61·35 | 62·50 | 59·50 | 58·50 | | Al_{2}O_{3}| 18·17 | 16·88 | 18·51 | 18·25 | 21·12 | | Fe_{2}O_{3}| 0·32 | 0·41 | } 4·39 | 4·81 | 4·68 | | FeO | 4·25 | 5·01 | } | 2·34 | ... | | MnO | 0·21 | 0·26 | ... | ... | ... | | CaO | 2·58 | 2·39 | 2·00 | 2·10 | 3·70 | | MgO | 0·74 | 0·44 | 0·61 | 0·70 | 0·93 | | K_{2}O | 4·02 | 6·12 | 6·31 | 6·30 | 5·84 | | Na_{2}O | 6·49 | 5·26 | 3·44 | 5·03 | 3·90 | | H_{2}O | 0·80 | 1·70 | 2·10 | 1·60 | 2·00 | | Total | 100·19 | 99·82 | 99·86 | 100·63 | 100·67 | +------------+--------+--------+----------+-------------+----------+
The microscopic characters of these rocks, as worked out by Dr. Hatch, show them to be well-marked and wonderfully fresh sanidine-trachytes. Some of them are porphyritic, with large crystals of perfectly unaltered sanidine, sometimes also oligoclase. Small but well-formed crystals of yellowish-green augite, in addition to the porphyritic felspars, are imbedded in a fine groundmass composed chiefly of microlites of sanidine, but with granules of augite and magnetite plentifully interspersed, and occasionally prisms of apatite. There is a group in which the porphyritic felspars are scarce or absent. In these there is little or no ferro-magnesian constituent. Other trachytes, rather less basic than the augite-bearing varieties here referred to, occur as bosses in the Garleton Hills district, and are referred to in the following section (e).[425]
[Footnote 425: For fuller petrographical details consult Dr. Hatch's paper above cited.]
(e) Rocks of the Necks.--In the necks connected with the plateaux other types of massive rock are to be found. Among these perhaps the most frequent are trachytes, grey to pink in colour, but apt to weather yellow, exceedingly compact, sparingly porphyritic, and with a peculiar platy structure and waxy lustre. Rocks of this character also appear as sills and dykes. Other varieties that occur in similar positions are more basic in composition, including dark, coarse, granular diabases. In the Jedburgh district the most frequent rocks are beautiful varieties of olivine-dolerite and olivine-basalt, which form most of the prominent hills of the neighbourhood. These bosses are sometimes associated with agglomerates as at Rubers Law.
In the Garleton Hills district, some of the necks present another petrographical type which directly connects them with the remarkable lavas of the higher part of that plateau. Thus the rock of Traprain Law was ascertained by Dr. Hatch to be a true phonolite. In its general platy structure and sonorous ring under the hammer it reminds one of typical phonolites. Under the microscope the rock is found to consist mainly of small lath-shaped crystals of sanidine arranged in a marked minute flow-structure, but with few porphyritic crystals. It contains small crystals and ophitic patches of a light green soda-augite, with practically no magnesia in it. A small quantity of iron-ore and isolated granules of apatite are also present, together with patches of nepheline which, though generally decomposed and replaced with zeolitic products, occasionally display six- and four-sided crystal-contours. An analysis of the Traprain phonolite by Mr. Player is subjoined:--[426]
Silica 56·8 Titanic acid 0·5 Alumina 19·7 Ferric oxide 2·2 Ferrous oxide 3·5 Manganous oxide 0·2 Lime 2·2 Magnesia 0·4 Soda 4·3 Potash 7·1 Loss by ignition 2·5 ---- Spec. grav. 2·588 99·4
[Footnote 426: Trans. Roy. Soc. Edin. vol. xxxvii. p. 125.]
The neck of North Berwick Law was found by Dr. Hatch to be a trachyte, showing a plexus of lath-shaped sanidines that diminish in size to minute microlites, but with no porphyritic or ferro-magnesian constituent. The Bass Rock, though its geological relations are concealed by the sea, is in all probability another neck of this district. It is likewise a mass of trachyte, composed almost entirely of lath-shaped crystals of sanidine, with no ferro-magnesian constituent, but a good deal of iron ore. It shows none of the large porphyritic felspars so characteristic of the Garleton Hills lavas, but it closely resembles the non-porphyritic varieties, particularly the lavas of Score Hill, Pencraig, Lock Pit Hill, and Craigie Hill.[427]
[Footnote 427: The composition of the rocks of North Berwick Law and the Bass closely resembles that of the trachytic lavas of the plateau. For analyses, see Dr. Hatch's Paper, ibid. pp. 123, 124.]
(f) Tuffs.--The fragmentary ejections of the plateaux vary in texture from the finest-grained tuffs to coarse agglomerates.[428] As they have been derived from the explosion of andesite-lavas, they consist mainly of the debris of these rocks. They are often deep red in colour, as for example those of Dunbar, but are most frequently greenish. They have a granular texture, due to the small lapilli of various porphyrites imbedded in a fine dust of the same material. Grains of quartz, frequently to be detected even in the finer tuffs, may either have been ejected from the volcanic vents, or may have been grains of sand in the ordinary sediment of the sea-bottom. Both at the base and at the top of the plateau-series, the tuffs are interstratified with and blend into sandstones and shales, so that specimens may be collected showing a gradual passage from volcanic into non-volcanic detritus. In many of the tuffs of the necks fragments of sandstone and other stratified rocks occur, representing the strata through which the vents were drilled. In the tuffs of the Eaglesham district pieces of grey and pink granite have been met with which, if they are portions of an old granite mass below, must have come from a great depth.[429] In the coarser tuffs and agglomerates a larger variety of lava-form rocks is to be found than can be seen among the bedded lavas of the Plateaux. They include felsites and quartz-porphyries, and more rarely basic lavas (diabases, etc.).
[Footnote 428: For accounts of these rocks, see Explanation of Sheet 33 Geol. Surv. Scot. p. 32; Sheet 22, pp. 11-14; Sheet 31, pp. 14-17.]
[Footnote 429: Explanation of Sheet 22 Geol. Surv. Scot. p. 12.]