Chapter XXV
GEOLOGICAL STRUCTURE OF THE CARBONIFEROUS VOLCANIC PLATEAUX OF SCOTLAND
1. Bedded Lavas and Tuffs; Upper Limits and Original Areas and Slopes of the Plateaux; 2. Vents; Necks of Agglomerate and Tuff; Necks of Massive Rock; Composite Necks; 3. Dykes and Sills; 4. Close of the Plateau-eruptions.
The structure of the various plateaux presents a general similarity, with many local variations. Each plateau is built up entirely, or almost entirely, of sheets of volcanic material, the intercalations of ordinary sedimentary layers being, for the most part, few and unimportant, and usually occurring either towards the base or the top of the volcanic series, though at a few localities interstratifications of shale and sandstone, marking pauses in the eruptions, occur throughout that series. The vents of eruption are in some instances still to be recognized on the plateaux themselves. More usually they occur on the lower ground flanking the volcanic escarpments, where they have been laid bare by denudation. Dykes, though seldom abundant, are associated with the plateaux, while the sills which may mark the latest manifestations of volcanic energy, though not developed on so large a scale as among the Cambrian and Silurian volcanoes, can nevertheless be distinctly recognized.
It is a question of some interest to determine the geological date of the commencement of the plateau-eruptions by fixing the precise stratigraphical horizon on which the base of the volcanic series rests. I have already referred to the fact that this base does not always lie on the same platform among the Lower Carboniferous formations. In Berwickshire, as above mentioned, the earliest eruptions appear to have taken place before the close of the Upper Old Red Sandstone period. These are the earliest of the whole series. In Cantyre, the lowest lavas and tuffs come directly upon the sandstones, marls and cornstones of the Upper Old Red Sandstone. In Stirlingshire, Renfrewshire and Ayrshire several hundred feet of the Cement-stone group are sometimes interposed between the bottom of the volcanic rocks and the top of the Old Red Sandstone. This divergence doubtless indicates that the eruptions began earlier in some districts than in others. But there were also probably unequal terrestrial movements preceding, and perhaps accompanying, the volcanic outbursts. In the case of the Clyde plateau, for example, if we examine its base in the neighbourhood of Fintry, we find that it lies upon some 500 feet of Carboniferous white sandstone, red and green marls and cement-stones, which rest on the Upper Old Red Sandstone. Yet only eight miles to the eastward, this considerable mass of strata disappears, and the bottom of the lavas comes down upon the red sandstones. Five miles still further in the same direction the volcanic masses likewise die out, and then the Carboniferous Limestone series is found at Abbey Craig to lie, with scarcely any representative of the Cement-stone group, on the Upper Old Red Sandstone (Fig. 114). Again, to the south-west of Fintry, the zone of cement-stones below the volcanic series continues to vary considerably in thickness and sometimes almost to disappear, while in Ayrshire the lavas lie immediately on the red sandstones.
These irregularities, not improbably indicative of inequalities of subsidence and of deposition, may have been connected with the subterranean disturbances which culminated in the abundant outbreak of volcanic action. But though the volcanic rocks of the plateaux may be traced overlapping the underlying strata, no evidence has anywhere been detected of an unconformability between them and the Lower Carboniferous or Upper Old Red Sandstone series.
1. BEDDED LAVAS AND TUFFS
The successive sheets of lava in a plateau usually form thin and widespread beds which are only occasionally separated by intercalations of tuff or of red marl. In this, as well as in other respects, they present much resemblance to the lavas of the Tertiary plateaux of Antrim and the Inner Hebrides. They are generally marked off from each other by the slaggy upper and under portions of the successive flows, and this structure gives a distinctly bedded aspect to the escarpments, as in the Campsie and Largs Hills, or still more conspicuously in Little Cumbrae (Fig. 107) and the southern end of Bute. Considerable diversity of structure may be noticed among these sheets. Some present a compact jointed centre passing up and down into the slaggy material just referred to; others have assumed a vesicular character throughout, the vesicles being often elongated in the direction of flow. Where, as usually occurs, the vesicular is replaced by the amygdaloidal structure, some of the rocks have long been famous for the minerals found in their cavities. The beautiful zeolites of the Kilpatrick and Renfrewshire Hills, for example, may be found in every large mineralogical collection in the country. Well-developed columnar structure occasionally appears among the lavas of the plateaux, but chiefly, so far as I have observed, in the lower or more basic group, as in the basalts along the east side of the Dry Dam at Arthur Seat.
In each plateau the lavas may be observed to thicken in one direction, or more usually towards more than one, and this increase no doubt indicates in which quarters the chief centres of discharge lay. Thus in the Clyde plateau, several areas of maximum development may be detected. In the Kilpatrick Hills the total thickness of lavas and tuffs exceeds 3000 feet (Fig. 120). Above Largs it is more than 1500 feet, rapidly thinning away towards the south. The continuation of the plateau far to the north-east in the Campsie Fells reveals a thickness of about 1000 feet of lavas at Kilsyth, which become thicker further west, but eastward rapidly diminish in collective bulk, until in about twelve or thirteen miles they disappear altogether, and then, as already remarked, the Calciferous Sandstone series closes up without any volcanic intercalation.
In the Solway plateau, the lavas attain a maximum development about Birrenswark, whence they diminish in bulk towards the north-east and south-west. The Berwickshire plateau reaches its thickest mass about Stitchill, whence it rapidly thins away towards the north-east, until at a distance of some twelve miles it disappears altogether, the last trace of it in that direction being a band of tuff which dies out in the Calciferous Sandstones to the north of Duns.
In the Midlothian Plateau, the development of the volcanic series is more irregular than in any of the others. As already remarked, there appear to have been at least two chief centres of discharge in this region, one at Edinburgh and one some fourteen miles to the south-west. At the former, the volcanic materials attain in Arthur Seat and Calton Hill a thickness of about 1100 feet. In Craiglockhart Hill, three miles distant, they are still about 600 feet thick. But beyond that eminence they cease to be traceable for about eight miles, either because they entirely die out, or because their dwindling outcrops are concealed under superficial deposits. As we approach the south-western centre of eruption around Corston Hill a new volcanic group begins and soon increases in bulk.
A distinguishing feature of the plateaux is found in the difference between the lavas that were first erupted and those which followed them. The earlier eruptions, as above remarked, were generally basic, sometimes highly so. Thus at Arthur Seat the thick series of lavas which form the eastern part of the hill have at their base several sheets of columnar basalt, over which come the andesites that make up the main mass of the erupted material. In the Calton Hill the same sequence may be observed. Underneath the andesites of Campbeltown comes a well-marked and persistent band of olivine-dolerite. Still more basic are some portions of the earliest lavas of the Garleton plateau where, as already stated, rocks present themselves composed mainly of olivine and augite.
It is worthy of notice that where the lavas of a plateau diminish greatly in thickness or become impersistent, the lowest basic group may continue while the overlying andesites disappear. This feature has been already mentioned as well seen in the Midlothian plateau. The thick group of andesites in Arthur Seat and Calton Hill is not to be found in the next volcanic eminence, Craiglockhart Hill; but the basalts with their underlying tuffs continue. In the south-western tract from Harper Rig to Hare Law in Lanarkshire, the thin lava-band, which can be found only at intervals along the line of outcrop of the volcanic series for about nine miles, is a dolerite often highly slaggy in structure. Again, at Corrie in Arran, the lavas which appear upon the shore, apparently at the extreme western limits of the Clyde plateau, are basic rocks.
But whether or not the lowest and more basic lavas appear in any plateau, the main mass of the molten material erupted has usually consisted of varieties of andesite. The successive discharges of these intermediate lavas have flowed out in sheets, some of which must have been little more than heaps of clinkers and scoriæ, while others were more fluid and rolled along with a ropy or slaggy surface. Occasionally the upper part of an andesite shows the reddened and decomposed character that suggests some degree of disintegration or weathering before the next lava-stream buried it. The intervals between successive outflows of these lavas are not, as a rule, defined by any marked breaks or by the intercalation of other material. In general, the plateaux are mainly built up of successive sheets of lava which have followed each other at intervals sufficiently short to prevent the accumulation of much detritus between them. Thus the Campsie Hills have the upper 600 feet of their mass formed of admirably-well-defined sheets of andesite, separated sometimes by thin partings of tuff, but more usually only by the slaggy vesicular surfaces between successive flows.
Where the lavas consisted of trachytes they were apt to assume more irregular forms. Of this tendency the rocks of the Garleton Hills supply an excellent example. As already stated, their lumpy character gives to these hills an outline which offers strong contrast to the ordinary symmetrical terraced contours of the andesitic plateaux.
Although tuffs play, on the whole, a comparatively unimportant part among the constituents of the plateaux, they attain in a few localities an exceptionally great development, and even where they occur only as thin partings between the successive lava-flows, they are always interesting memorials of the volcanic activity of a district. In many portions of the plateaux, the lowest members of the volcanic series are tuffs and agglomerates, showing that the eruptions often began with the discharge of fragmentary materials. Thus in the Midlothian plateau at Arthur Seat, though the lowest interbedded volcanic sheet is a dolerite, it is immediately followed by a series of bedded tuffs, before the main mass of the lavas of that hill make their appearance. At Craiglockhart Hill, three miles distant (Fig. 115), this lowest lava is absent, and a group of tuffs about 300 feet thick rests immediately on the red Carboniferous sandstones and shales, and is overlain by sheets of columnar basalt. The scoriaceous bottom of the latter rock may here and there be seen to have cut out parts of the tuff as it rolled over the still unconsolidated material. In the same district, a few miles further to the south-west, some interesting sections of the Midlothian plateau are laid bare in the streams which descend from the western slopes of the Pentland Hills. I may cite, in particular, those exposed in the course of the Linnhouse Water. At the railway viaduct near the foot of Corston Hill, a good section is displayed of the Cement-stone group--thick reddish, purplish, and greenish-blue marly shales or clays, with thin ribs and bands of cement-stone and grey compact cyprid-limestone, as well as lenticular seams and thicker beds of grey shaly sandstone, sometimes full of ripple-marks and sun-cracks. These strata, which exactly reproduce the typical lithological characters of the Cement-stone group of Stirlingshire (Ballagan Beds), Ayrshire and Berwickshire, are surmounted by a group of reddish, yellow and brown sandstones, sometimes pebbly and containing a band of conglomerate. Among the stones in this band, pieces of the radiolarian cherts of the Lower Silurian series of the Southern Uplands are conspicuous, likewise pieces of andesite which may have come from the neighbouring Pentland Hills.
Above these strata lie the lavas of Corston Hill. These are highly vesicular in some parts, and include bands of tuff which are well exposed further down the same stream, immediately above the railway bridge near the Mid-Calder oilworks (Fig. 116). There the lavas, though much decomposed, show a highly vesicular structure with a rugged upper surface, in the hollows and over the prominences of which fine flaky and sandy tuffs have been deposited, while thin seams of vesicular lava are intercalated among these strata.
The upper part of the same plateau, as exposed in the course of the Murieston Water, contains evidence that the last eruptions consisted of tuff. The highly slaggy lava (1 in Fig. 117) is there surmounted by a thick mass of grey and greenish-white well-bedded granular tuff (2) including occasional lumps of the basic lava, and passing up into black shale (3). But that the volcanic eruptions continued during the accumulation of the shale is proved by the intercalation of thin partings and thicker layers of tuff in the black sediment. A short way higher up the Burdiehouse Limestone comes in.
The great lava-escarpment of the Kilpatrick Hills rests on a continuous band of tuff which is thickest towards the west, near the group of vents above Dumbarton, while it thins away eastward and disappears in Strathblane, the lavas then forming the base of the volcanic series. But perhaps the most remarkable group of basal tuffs is that which underlies the lavas of the Garleton plateau, to which further reference will be immediately made.
Extensive accumulations of tuff form in one or two localities a large proportion of the thickness of the whole volcanic series of a plateau. Thus in the north-eastern part of Ayrshire, between Eaglesham and the valley of the Irvine, the lavas die out for a space and give place to tuffs. During the discharge of the fragmentary materials over that ground no lava seems to have flowed out for a long period. Ordinary sediment, however, mingled with the volcanic detritus, and there were even pauses in the eruptions when layers of ironstone were deposited, together with thin impure limestone that inclosed shells of Productus giganteus.[430]
[Footnote 430: Explanation of Sheet 22 Geol. Surv. Scotland, p. 12.]
In some of the plateaux, particularly within the older part of the volcanic series, intercalations of ordinary sediment among the tuffs and lavas show that eruptions occurred only occasionally, and that during the long intervals between them the deposition of sand and mud went on as before. Thus the lower 400 feet of the Campsie Fells are built up of slaggy andesites and thick beds of fine-grained stratified tuff, with bands of red, green and grey clays and cement-stone and a zone of white sandstone. The Calton Hill at Edinburgh (Fig. 118) affords an excellent illustration of the interstratification both of tuffs and ordinary sediments among the successive outflows of lava. In the total thickness of about 1100 feet of volcanic material in this hill, at least eight intervals in the discharge of the lavas are marked by the intercalation of as many bands of nodular tuff, together with seams of shale and sandstone more or less charged with volcanic detritus. The highest lava is immediately covered by the white sandstones and black shales of the Calciferous Sandstone series.
The tuffs, as might be expected, are coarsest in texture and thickest in mass where they approach most nearly to some of the vents of eruption, and, on the other hand, become finer as they recede from these. As a rule, they are distinctly stratified, and consist of layers varying in the size of their component lapilli. Here and there, near the centres of discharge, the bedding becomes hardly traceable or disappears, and the fragmentary materials take the form of agglomerate.
In the admirable range of coast-cliffs which extend from North Berwick to Dunbar, we learn that above the red sandstones at the base of the Carboniferous system, a thick pile of volcanic ashes was accumulated by numerous discharges from vents in the immediate neighbourhood. Some of the explosions were so vigorous that blocks of different lavas, sometimes a yard or more in length, were thrown out and heaped up in irregular mounds and hollows. Others discharged exceedingly fine dust, and between these two extremes every degree of coarseness of material may be recognized.
As an illustration of the remarkable alternation of coarse and fine materials, according to the varying intensity of the volcanic paroxysm, Fig. 119 is here introduced. It represents a portion of the tuff-cliffs east of Tantallon Castle, and shows at the bottom fine well-stratified tuff, over which a shower of large blocks of lava has fallen. Fine detritus is seen to cover the deposits of this shower, and successive discharges of large stones may be noticed higher up on more or less well-defined horizons.
The space over which this pyroclastic material can now be traced, large though it is, does not represent the whole of the original area included within the range of the discharges of ash and stones, for much has been removed by denudation. During pauses of various length between the eruptions, waves and currents washed down the heaps of volcanic material and distributed ordinary sediment over the bottom of the water. Hence, abundantly interstratified in some parts of the tuff, seams of sandstone, blue and green shale, cement-stone and limestone occur. One thick band of limestone may be traced from near Tynningham House to Whittinghame, a distance of about four miles; another patch appears near Rockville House; and a third at Rhodes, near North Berwick. No fossils have been noticed in these limestones. The calcareous matter, together sometimes with silica, appears to have been supplied, at least in part, by springs, which may have been connected with the volcanic phenomena of the district. The North Berwick limestone, in particular, has the peculiar carious wavy structure with minute mamillated interstices so common among sinters. It contains grains of pyrites, flakes of white kaolin, which probably represent decayed prisms or tufts of natrolite, and cavities lined with dog-tooth spar. Some portions give out a strongly fœtid odour when freshly broken.
After the tuffs of the Garleton plateau had accumulated to a depth of perhaps 200 feet or more, lavas began to be poured out. First came basic outflows (olivine-basalts with picrites) and andesites (porphyrites), which form a thin but continuous sheet all over the area. These were succeeded by the series of trachytes which distinguish this area. Although the observer remarks the absence there of the usual terraced arrangement, yet from some points of view, particularly from the westward, a succession of low escarpments and longer dip-slopes can be detected among the trachytes of the Garleton Hills, while there can be no doubt that, in spite of their irregular lumpy contours, these lavas lie as a great cake above the lower platform of more basic flows (Fig. 10). There is evidence that during the emission of the trachytes occasional eruptions of andesite took place. Not the least striking and interesting feature of this plateau is the size and distribution of its necks, to which reference will be made in the sequel.
The latest eruption in the Garleton area had ceased and the cones and lava sheets had probably been buried under sediment before the commencement of the deposition of the Hurlet or thick Main Limestone of the Carboniferous Limestone series which lies immediately to the west of the plateau.
The tuffs of the plateaux are seldom fossiliferous, probably for the same reason that fossils are scarce in the Cement-stone group which the plateau volcanic rocks overspread and with which they are interstratified. Occasional stems and other fragments of vegetation occur in the plateau-tuffs, as in those of North Berwick, where I have found a decayed coniferous trunk three feet in length. The green tuff at the base of the volcanic group of Arthur Seat contains abundant macerated plant-remains, together with scales of Rhizodus and other fishes. In some places the plants are represented by trunks or roots, which appear to remain in their positions of growth. A remarkable instance of this nature occurs in some bands of tuff in the volcanic group of the east coast of the Isle of Arran, first brought to notice by Mr. E. Wunsch,[431] and of which the plants have been so fully investigated by Professor Williamson.[432]
[Footnote 431: Trans. Geol. Soc. Glasgow, vol. ii. (1867) p. 97.]
[Footnote 432: Phil. Trans. 1871-1883.]
Plant-remains also occasionally occur in the stratified layers intercalated among the lavas and tuffs of the plateaux. Some of the best examples of their occurrence are to be found in the shales and tuffs interstratified among the enormous pile of volcanic material near Bowling. Not only does abundant vegetable debris occur distributed through the detrital strata in the volcanic series at that locality, but it is even aggregated into thin seams of coal which have been examined and described by various observers.[433] It may be remarked that the plant remains thus found intercalated in the volcanic series, especially when they have been entombed in tuff, have often had their internal structure admirably preserved, the organic tissues having been delicately replaced by calcite or other petrifying medium. The remarkably perfect structure of some of these plants has been demonstrated by Professor Williamson, especially in the case of the Arran deposit just referred to. Mr. John Young has also found the structure well preserved among the Sigillariæ and Stigmariæ that occur in the stratified intercalations between the lavas near Bowling.
[Footnote 433: See in particular J. Young, Trans. Geol. Soc. Glasgow, vol. iv. (1874) p. 123.]
Upper Limits and Original Areas and Slopes of the Plateaux.--Where the highest members of the volcanic series can be seen passing conformably under the overlying Carboniferous strata they are frequently found to be mainly composed of fine tuffs, the last feeble efforts of the plateau-volcanoes having consisted in the discharge of showers of ashes. These materials were mingled with a gradually increasing proportion of ordinary mechanical sediment, which finally overspread and buried the volcanic tracts of ground, as these slowly sank in the general subsidence of the region. The characteristic corals, crinoids and shells of the Carboniferous Limestone begin to appear in these ashy sediments. There is thus an insensible passage from volcanic detritus into fossiliferous shales and limestones. Examples of this gradation may be seen in many natural sections along the flanks of the Ayrshire plateau from above Kilbirnie to Strathavon.
It is still possible to fix in some quarters the limits beyond which neither the lavas nor the tuffs extended, and thus partially to map out the original areas of the plateaux. For example, in certain directions the Carboniferous formations can be followed continuously downward below the Main Limestone, without the intervention of any volcanic material, or with only a slight intermixture of fine volcanic lapilli, such as might have been carried by a strong wind from some neighbouring active vents. By this kind of evidence and by the proved thinning-out of the materials of the plateau, we can demonstrate that in the north of Ayrshire the southern limits of the great volcanic bank did not pass beyond a line drawn from near Ardrossan to Galston. We can show, too, that the lavas of the Campsie Fells ended off about a mile beyond Stirling before they reached the line of the Ochil heights, and that the coulées which flowed from the Solway vents did not quite join with those from the Berwickshire volcanoes.
Moreover, evidence enough remains to enable us to form a tolerably clear conception of the original average slopes of the surface of some of the plateaux. Thus in the great escarpment above Largs and the high ground eastward to Kilbirnie the volcanic series, as already stated, must be at least 1500 feet thick. This thick mass of lavas and tuffs thins away southwards and probably disappears a short distance south from Ardrossan in a space of about ten miles (Fig. 121). The original southward slope of the plateau would thus appear to have been about 1 in 35. Again, the northward slope of the same plateau may be estimated from observations in the Campsie Fells. We have seen that above Kilsyth the total depth of the volcanic sheets is about 1000 feet, while to the westward it is much thicker. From the top of the Meikle Bin (1870 feet) above Kilsyth north-eastwards to Causewayhead, where the whole volcanic series has died out, is a distance of 12 miles, so that the slope of the surface of erupted materials on this side was about 1 in 63 (Fig. 122).
Judging from the sections exposed along the faces of the escarpments, we may infer that the volcanic sheets had a tolerably uniform surface which sloped gently away from the chief vents, but with local inequalities according to the irregularities of the lava-streams that were heaped up round the vents and flowed outward in different directions and to various distances from them. At the beginning, these flat volcanic domes were certainly subaqueous. While they were being formed, continuous subsidence appears to have been in progress. But the great thickness of the volcanic accumulations, as in the Kilpatrick and Renfrewshire areas, and the paucity of ordinary sedimentary strata among them, make it not improbable that at least their higher parts rose above the water. Where this was the case there may have been considerable degradation of the lava-banks before these were reduced or were by subsidence submerged beneath the water-level. Evidence of this waste is probably to be recognized in the bands of conglomerate, occasionally of considerable thickness, which, particularly in some parts of Ayrshire, intervene between the top of the volcanic group and the Hurlet Limestone. As I shall have occasion to point out further on, there seems to be some amount of evidence in favour of the view that a considerable interval of time elapsed between the close of the plateau-eruptions and the date of that widespread depression which led to the deposition of the Hurlet Limestone over the whole of Central Scotland. If such an interval did occur it would include a prolonged abrasion of any projecting parts of the plateaux, and the production and deposition of volcanic conglomerate.
2. VENTS
We have now to consider the external forms, internal contents and distribution of the vents from which the material of the plateaux was discharged. In the Carboniferous system these interesting relics of former volcanoes are far more distinctly defined and better preserved than in older geological formations. Moreover, in Scotland, they are laid bare to greater advantage, both inland and along the sea-coast, and may indeed be studied there as typical illustrations of this kind of geological structure.
In external form the necks connected both with the plateaux and the puys generally rise from the surrounding ground as isolated, rounded, conical or dome-shaped prominences, their details of contour depending mainly upon the materials of which they consist. When these materials are of agglomerate, tuff or other readily disintegrated rock, the surface of the domes is generally smooth and grass-covered. Where, on the other hand, they consist wholly or in part of dolerite, basalt, diabase, andesite, trachyte or other crystalline rock, they present more irregular rocky outlines. Illustrations of some of those varying forms are given in Figs. 23 and 123. In rare instances the vent is marked at the surface not by a hill but by a hollow, as in the great neck in the heart of the Campsie Fells (Fig. 128).
As regards their ground-plan, which affords a cross-section of the original volcanic funnel, the plateau-vents present considerable variety. The simplest cases are those in which the form is approximately circular or somewhat elliptical. Here the outline corresponds to the cross-section of a single and normal orifice. Some examples of this simple type are given in Fig. 124, which represents a group of vents on the edge of the Clyde plateau near Strathblane. The two larger necks here shown are the same which appear in the view in Fig. 123.[434] Where two vents have been successively opened close to each other, or where the same vent has shifted its position, the ground-plan may be greatly modified. In some instances the double funnel can be distinctly traced. Thus in the conspicuous Knock Hill above Largs in Ayrshire (Fig. 125, B) there are two conjoined necks, and such appears to be also the structure shown by the ground-plan of the neck of Barwood Hill and Raven's Craig, east of Dumbarton (Fig. 125, A).[435] But more complex forms occur which point to a still larger number of coalescing necks. A group of hills to the east of Dumbarton gives the ground-plan shown in C, Fig. 125, where traces may be detected of three separate vents. Still more irregular are long narrow dyke-like masses of tuff or agglomerate which have probably risen along lines of fissure (Fig. 22, No. 1). The most striking example of these, however, occur in association with the puys and will be described in later pages.
[Footnote 434: The illustrations in Figs. 124 and 125 are taken from the field-maps of the Geological Survey on the scale of 6 inches to a mile. The ground represented in Fig. 124 was mapped by Mr. R. L. Jack.]
[Footnote 435: These ground-plans are likewise taken from the field-maps of the Geological Survey. A and C were mapped by Mr. Jack, B by myself. The shaded parts are intrusive andesites and dolerites; the dark bars in A and C being dolerite dykes of much later date than the necks. The dotted portions mark tuff and agglomerate.]
Connected with their ground-plan is the relative size of the plateau-vents. On the whole they are larger than those of the puy series. The simple circular or elliptical type presents the smallest necks, some of them not exceeding 100 feet in diameter. The more complex forms are generally also of larger dimensions. By much the largest vent or connected group of vents is that which lies among the uplands of Misty Law in the heart of the Renfrewshire part of the Clyde plateau, where a connected mass of tuff and agglomerate now occupies a space of about 4 miles in length by 2½ miles in breadth (Fig. 129). It has not been found possible, however, to trace the boundaries of the separate vents of this tract, nor to distinguish the material of the necks from that which surrounds them. Another large mass which from its shape may be conjectured to represent more than one vent is the great tract north of Melrose, which measures 8800 by 4200 feet.[436]
[Footnote 436: The following measurements are, like those in the text, taken from the field-maps of the Geological Survey. Carewood Rig, on the borders of Roxburghshire and Dumfriesshire, 7000 × 2400 feet; the great vent in the middle of the Campsie Fells, 5200 × 2600; Black Law, between Bedrule and Jedburgh, 3400 × 1600; Dumgoyn, Strathblane, 2300 × 1300; Rubers Law, 1500 × 1000; Minto Hill (south), 2300 × 1650; Minto Hill (north), 1500 × 1100; Doughnot Hill, Kilpatrick range, 1000 × 700; four of the smallest agglomerate vents along the northern escarpment of the Clyde plateau between Strathblane and Fintry, 500 × 450, 450 × 400, 250 × 100, 200 × 200; Pike Law, Arkleton, Tarras Water, 500 × 500; Harwood, Stonedge, 5 miles S.E. from Hawick, 500 × 300; Arkleton Burn, Dumfriesshire, 400 × 100; Dalbate Burn, 250 × 120.]
The distribution of the necks can best be understood from the maps of the Geological Survey, where they have been carefully indicated. As might have been expected, they are not found outside the original limits within which it may be reasonably inferred that the lavas and tuffs were erupted. They occur most abundantly and attain their largest size in and around the districts where the plateaux are most extensively developed. No doubt a large number of them are concealed under these plateaux. A few appear at the surface among the lavas and tuffs, but by far the largest number now visible have been revealed by denudation, the escarpments having been cut back so as to lay bare the underlying rocks through which the necks rise. Thus, along the flanks of the great escarpment that extends from near Stirling by Fintry and Strathblane to Dumbarton, more than two dozen of agglomerate necks may be counted in a distance of about sixteen miles, while if the necks of lava-form material are included, the number of vents must be about fifty. Nowhere in Scotland do such necks form a more conspicuous feature in the scenery as well as the geology than they do between Fintry and Strathblane, where, standing out as bold isolated hills in front of the escarpments, their conical and rounded outlines present a striking contrast to the terraced escarpments behind them. I would especially refer again to the two remarkable cones of Dumfoyn and Dumgoyn above Strathblane (Figs. 123, 124, 127). Along the west front of the hills between Gourock and Ardrossan seventeen agglomerate-vents occur in a distance of sixteen miles. In Roxburghshire a group of large agglomerate-necks is dotted over the Silurian country around Melrose and Selkirk[437] (see Fig. 130).
[Footnote 437: In this region and farther southward, besides the plateau-eruptions, a later group of puys is to be seen, and it is difficult to discriminate between the necks belonging to the two groups. Those which lie to the east are probably connected with the plateaux, those to the west with the puys. The latter are referred to on p. 475.]
From the evidence of these necks it is plain that the volcanic materials of the plateaux must in each case have been supplied not from great central orifices, but from abundant vents standing sometimes singly, with intervening spaces of several miles, often in groups of four or five within a single square mile.
In the interior of the country, it is seldom possible to examine the actual junction of necks with the rocks through which they rise, the boundary-line being usually obscured by debris or herbage. On the coast, the vents of the plateaux have not been bared by the sea so fully as in the case of the much younger series of the east of Fife to be described in later pages. But where the East Lothian plateau touches the shore, the waves have laid bare a number of its minor vents, which have thus been dissected in ground-plan on the beach. As an illustration of these vents an example is given in Fig. 126, from the shore east of Dunbar. Here the sandstones, which are inclined in an easterly direction at 20° to 25°, are pierced by an irregular mass of tuff. It is observable that in this instance long tongue-like projections of the sandstones protrude into the neck; more frequently the material of a neck sends veins or dykes into the surrounding walls. A volcanic chimney would seem to have been often much shattered and fissured in the course of the volcanic explosions, and the fragmentary material has fallen or been injected into the rents thus caused. As a rule, the rocks immediately around the Carboniferous necks are more or less indurated, as in this instance from the Dunbar shore.
The materials which have filled up the vents connected with the plateau-eruptions generally consist of (a) agglomerates or tuffs, but occasionally of (b) some kind of lava, and frequently (c) of both these kinds of rock combined.
(a) Necks of Agglomerate or Tuff.--These materials vary greatly in the nature and relative proportions of their constituents. Usually the included blocks and lapilli are pieces of andesite, diabase, basalt or other lava, like the rocks of the plateaux. But with these occur also fragments probably detached from the sides of the funnels through which the explosions took place, such as pieces of greywacke, sandstone, limestone and shale. Considerable induration may be observed among these non-volcanic ingredients. In some cases, as in that of the occurrence of pieces of granite referred to on p. 382, the stones have probably been brought up from some considerable depth. In others it is easy to see that the blocks have slipped down from some higher group of strata now removed from the surrounding surface by denudation. Some striking illustrations of this feature will be cited from necks of the puy-series in the south of Roxburghshire (p. 476).
The lava blocks in the tuffs and agglomerates are usually rounded or subangular. Pear-shaped blocks, or flattened discs, or hollow spherical balls are hardly ever to be observed, though I have noticed a few examples in the tuffs of Dunbar. A frequent character of the blocks is that of roughly rounded, highly amygdaloidal pieces of lava, the cellular structure being specially developed in the interior, and the cells on the outside being often much drawn out round the circumference of the mass. Such blocks were probably torn from the cavernous, partially consolidated, or at least rather viscous, top of a lava column. Most of the stones, however, suggest that they were produced by the explosion of already solidified lava, and were somewhat rounded by attrition in their ascent and descent. The vents filled with such materials must have been the scene of prolonged and intermittent activity; successive paroxysms resulting in the clearing out of the hardened lava column in the throat of the volcano, and in the rise of fresh lava, with abundant ejection of dust and lapilli.
Necks formed entirely of agglomerate are abundant among the vents connected with the plateaux. As examples of them I may refer to the series already mentioned as fronting the escarpment of the Clyde plateau from Fintry to Largs. Another interesting group rises through the Silurian and Old Red Sandstone rocks to the west of the escarpment of the Berwickshire plateau, that near Melrose forming one of the largest in Scotland.
Illustrations of the varying structure of these vents are given in the accompanying figures. In Fig. 127, a section is drawn through the two necks Dumgoyn and Dumfoyn, which have already been shown in outline and in ground-plan. The relation of these two vents to the neighbouring plateau to the right can here be seen. Fig. 128 gives a section taken through the great vent of the Campsie Hills, with the minor adjacent necks of Dungoil, Bin Bairn, and the Meikle Bin.
The diagram in Fig. 129 is meant to convey in a general way what appears to be the structure of the central vent of the Renfrewshire plateaux, to be afterwards referred to. But, as already mentioned, the limits of the various rocks are too much obscured to allow an accurate delineation to be given of their areas and relations to each other. The Berwickshire plateau supplies abundant interesting examples of tuff necks which rise through the Old Red Sandstone many miles distant from the edge of the lavas. This structure is shown in Fig. 130.
Indications may occasionally be observed of an agglomerate vent having been first occupied by one kind of material and then, after being in great measure cleared out by explosions, having been subsequently filled up with another. As an example of this structure I may cite again the double neck of the Knock Hill a little to the north of Largs, of which the outline is shown in Fig. 23, and the ground-plan in Fig. 125, B. This hill rises from the red sandstone slopes that front the great Ayrshire plateau and forms a conspicuous cone the top of which is rather more than 700 feet above the sea. Its summit commands a remarkably extensive and interesting panorama of the scenery of the Clyde, but to the geologist perhaps the most striking feature in the landscape is the range of terraced hills behind, mounting up into the great vents of the Renfrewshire uplands. On these declivities the successive lava-streams that have built up the plateau can be seen piled over each other for a thickness of more than 1000 feet, and presenting their escarpments as parallel lines of brown crag with green slopes between.
The Knock has had its upper part artificially dressed, for lines of trench have been cut out of its rocks by some early race that converted the summit of the hill into a strongly intrenched camp. From the apex of the cone the ground falls rapidly westward into a hollow, beyond which rises a lower rounded ridge of similar materials. It is possible that this western ridge may really form part of the main hill, but the grass-covered ground does not afford sufficient exposures of the rocks to settle this point. From the contours of the surface, it may be inferred that there are two closely adjacent vents, and that the western and lower eminence is the older of the two. This hill or ridge consists partly of a coarse agglomerate, and partly of veins and irregular protrusions of a dark, compact, slightly cellular lava. The stones in the fragmental rock are different olivine-basalts, or other basic lavas, and sandstones. The paste is rough, loose and granular. The sandstone fragments are much indurated and sometimes bleached.
The Knock itself is formed mainly of a remarkably coarse and strikingly volcanic agglomerate. Round the outside, and particularly on the south-east, the rock is finer in texture, compact, and gravelly, or like a mudstone, with few or no imbedded blocks, dull-green to red in colour, and breaking with a clean fracture which shows angular lapilli of various basalts or diabases. At the southern end of the neck, where the surrounding red sandstone can be seen within a few feet of the tuff, the latter is bright red in colour, and contains much debris of red sandstone and marl. Possibly this finer tuff, which is traceable as an irregular band round the outside of the neck, may mark an older infilling of the vent than the agglomerate of the centre; but there is no sharp line to be drawn between the two, though a hollow can sometimes be traced on the surface where they join.
The agglomerate of this locality is one of the most characteristic among the plateau-necks of the Clyde region. Its blocks sometimes measure from two to three feet in diameter. They consist almost wholly of a dark crystalline porphyritic olivine-basalt. These blocks are subangular in form, often with clean-fractured surfaces. Though occasionally slightly cellular, they are never slaggy so far as I could see, nor are any true scoriæ to be noticed among them. The blocks suggest that they were derived from the disruption of an already solidified mass of lava. The agglomerate is entirely without any trace of stratification.
Through this tumultuous accumulation of volcanic debris some irregular veins of olivine-basalt, sometimes glassy in structure, have been injected, and reach nearly to the summit of the hill. This intrusive material resembles generally some of the dark intrusive masses in the Dumbartonshire necks. Like these, it exhibits a tendency to assume a more or less distinctly columnar structure, its columns having the same characteristic wavy sides and irregular curvature. The intrusive rocks in the two eminences of the Knock may be paralleled among the stones in the agglomerate. The neck on its north-eastern side rises steeply from the red sandstones which it pierces, but which, although they are much jointed and broken, are not sensibly indurated. Unfortunately the actual junction of the igneous and sedimentary rocks is concealed under herbage.
As a rule, the fragmental materials of the plateau-necks are quite unstratified. Their included blocks, distributed irregularly through the mass, have evidently undergone little or no assortment after they fell back into the vents. Occasionally, however, a more or less distinct bedding of the agglomerate or tuff may be observed, the layers having a tendency to dip inward into the centre. One of the most conspicuous examples of this structure is to be found in the hill of Dumbuck, to the east of Dumbarton. This neck, which forms so prominent a feature in the landscape, presents a precipitous face towards the south, and allows the disposition of its component materials to be there seen. The agglomerate consists of a succession of rudely stratified beds of coarser and finer detritus, which on both sides are inclined towards the centre, where a plug of fine-grained olivine-basalt has risen and spread out into a columnar sheet above (Fig. 131). In general form this basalt resembles such intrusions as that of Largo Law, to be afterwards described (Fig. 226), where what may have been the hollow or bottom of the crater is filled with basalt.
(b) Necks of Andesite, Trachyte, Dolerite, Diabase, or other massive Rock.--When the vents have been filled by the uprise of some molten rock, it is generally, as we have seen, of a more acid character than the ordinary lavas of the plateaux. Frequently it consists of some variety of trachyte or andesite, commonly of a dull yellow or grey tint and waxy lustre. Good examples may be seen among the remarkable group of necks on either side of the valley north of the village of Strathblane and in those above Bowling. The three great necks in East Lothian, already alluded to,--Traprain Law (Figs. 132, 133), North Berwick Law (Fig. 109), and the Bass Rock (Fig. 110)--are masses of phonolite and trachyte, obviously related to the trachytes of the adjacent plateau. A smaller but very perfect instance of a vent similarly filled is to be seen in the same neighbourhood on the shore to the east of North Berwick Law.[438]
[Footnote 438: See "Geology of East Lothian," _Geological Survey Memoir_, p. 40.]
Examples occur where the funnels of eruption have been finally sealed up by the rise of more basic material, and this has happened even in a district where most of the lava-form necks consist of trachyte or some other intermediate lava. Thus, in the Campsie Fells, several such bosses appear, of which the most conspicuous forms the hill of Dungoil (1396 feet, Fig. 128). Further west, among the Kilpatrick Hills, bosses of this kind are still more numerous. The group of bosses near Ancrum and Jedburgh is mainly made up of olivine-dolerites and olivine-basalts (Fig. 130). This more basic composition of itself suggests that these bosses may be connected rather with the puy- than with the plateau-eruptions.
(c) Necks of Composite Character.--In not a few examples, the vents have been filled with agglomerate which has been pierced by a plug or veins of lava-form material. Many illustrations of this composite structure may be observed along the west front of the great escarpments from Fintry to Ardrossan (see Figs. 124, 125, 127 and 128). In that region the intruded rock is often a dull yellowish or grey trachytic or andesitic material. Olivine-basalt is the chief rock intruded in the vents in the Dumbarton district. Among the Roxburghshire vents, where the injected material is commonly olivine-basalt or dolerite, it occasionally happens, as in Rubers Law, that the uprise of the lava has almost entirely cleared out or concealed the agglomerate, and in some of the bosses, where no agglomerate is now to be seen, the basalt may have taken its place (Fig. 130).
The largest and most interesting vents connected with this type of Carboniferous volcano, are those which occur within the limits of the plateaux, where they are still surrounded with lavas and tuffs that probably came out of them. Of these by far the most extensive and remarkable lies among the high moorlands of Renfrewshire between Largs and Lochwinnoch, where the ground rises to more than 1700 feet above the sea (see Fig. 129). This area, as already remarked, is unfortunately much obscured with drift and peat, so that the limits of its rocks cannot be so satisfactorily traced as might be desired. I think it probable that several successive vents have here been opened close to each other, but their erupted ashes probably cannot be distinguished. Over a space measuring about four miles in length by two and a half in breadth, the rocks exposed at the surface are fine tuffs, breccias and coarse agglomerates, largely made up of trachytic, andesitic or felsitic material, and pierced by innumerable protrusions of various andesitic, trachytic or felsitic rocks in bosses and veins, as well as also by dykes of a more basic kind, such as dolerites and basalts. Some of the tuffs present a curiously indurated condition; and they are frequently much decayed at the surface.[439] Another large mass of tuff and agglomerate lies a little to the south-west of the main area.
[Footnote 439: This tract of ground was mapped for the Geological Survey by Mr. R. L. Jack, now in charge of the Geological Survey of Queensland. See Sheet 31, Geological Survey of Scotland.]
After the explosions ceased, by which the vents were opened and the cones of debris were heaped up, heated vapours would in many cases, as in modern volcanoes, continue for a long while to ascend in the funnels. The experiments of Daubrée on the effects of water and vapour upon silicates under great pressure and at a low red heat, have shown how great may be the lithological changes thereby superinduced. It is improbable that where a mass of tuff and lava, lying deep within a volcanic vent, was thoroughly permeated with constantly ascending heated vapours, it should escape some kind of change. I am inclined to attribute to this cause the frequent conversion of the sandstones round the walls of the vents into quartzite. The most remarkable example of metamorphism within a vent which I have observed among the plateaux, occurs in the heart of the Campsie Fells, where, instead of forming a prominence, the neck is marked by a great hollow, measuring about a mile in length and half a mile in breadth (Fig. 128).[440] It is occupied mainly by a coarse tumultuous agglomerate, like that of other necks in the same district, but with a matrix rather more indurated, and assuming in certain parts a crystalline texture, so as to be at first sight hardly distinguishable from some of the surrounding andesites. Even in this altered condition, however, its included fragments may be recognized, particularly blocks of sandstone which have been hardened into quartzite. Numerous small veins of pink and yellow trachyte traverse the agglomerate, and are found also cutting the bedded andesites that encircle it.
[Footnote 440: See Explanation to Sheet 31, Geological Survey of Scotland, par. 21 (1878).]
3. DYKES AND SILLS
Intrusive masses both in the form of dykes and of sills are of frequent occurrence in connection with the Carboniferous volcanic plateaux. From the variety of their component materials it may be inferred that these rocks belong to different ages of intrusion.
Dykes.--The great majority of the Dykes consist of trachyte or of andesite, resembling in lithological characters the material of the necks and doubtless connected with its uprise. There occur also dykes of diabase, basalt or dolerite. Some of the latter, especially those which run for many miles, cutting every rock in the districts in which they occur, and crossing large faults without deviation, are certainly long posterior to the plateau volcanic period. Whether the small inconstant dykes of more basic composition, found in the same districts with the trachytes, are to be looked upon as part of the volcanic phenomena of the plateaux, is a question to which at present no definite answer can be given. I shall have occasion to show that in the next volcanic period the lavas that flowed from the puys are more basic than most of those of the plateaux, and that they are associated with more basic dykes and sills. In Roxburghshire, where it is so difficult to distinguish between the denuded vents of the two periods, the dark heavy olivine-basalts and dolerites of the bosses may possibly belong rather to the later than to the earlier volcanic episode. And if that be their true age, the dykes of similar material may be connected with them. At the same time it must be remembered that the earliest eruptions of the plateaux were markedly basic, that many vents in the plateaux are pierced by basic intrusions, and that basic dykes may have been associated with the uprise of the same magma.
The dykes occur in considerable numbers and in two distinct positions, though these may be closely related to each other: 1st, among the rocks outside and beneath the plateau-lavas, or cutting these lavas; and 2nd, in and around the vents.
1. Among the rocks which emerge from under the Carboniferous volcanic plateaux, dykes are sometimes to be observed in considerable numbers. They may be compared to the far more extensive series connected with the Tertiary basalt-plateaux, like which they may have had a close relation to the actual building up of the successive sheets of andesite, trachyte and basalt that were erupted at the surface. They are particularly well developed in the Clyde plateau, where by extensive denudation they have been admirably exposed. I would especially refer to those that traverse the tract of red sandstones which underlie the volcanic series along the flanks of the great escarpments from Fintry to Strathblane and Dumbarton, and between Gourock and Ardrossan. These dykes have been dissected by the sea along both sides of the estuary of the Clyde and in the islands of Cumbrae. In these islands and in Bute they have recently been mapped in great detail for the Geological Survey by my colleague, Mr. W. Gunn, who has supplied me with notes of his observations on the subject, from which the following summary is compiled.
"There are at least four distinct groups of intrusive rocks in the Greater Cumbrae. The oldest of these is trachytic in character, and occurs both as dykes and sheets, which run generally in the same E.N.E. direction. The rock is usually pinkish in colour, sometimes grey or purplish. A specimen from the dyke of the Hawk's Nest, north of Farland Point, analyzed by Mr. Teall, was found to contain 11 per cent of alkalies, principally potash, while the percentages of lime and iron were very low. Sometimes these rocks are fine in grain with only a few porphyritic orthoclase crystals, though numerous small crystals of this mineral are found with the aid of the microscope. These red trachyte dykes are almost confined to the Upper Old Red Sandstone, rarely entering the overlying white Calciferous Sandstones, and never invading the plateau-lavas. They are therefore probably of early Carboniferous age.
"The next group follows the same general direction, but clearly traverses the trachytes, and must therefore be of later date. The dykes of this group are the most numerous of the whole, the greater part of the island being intersected by them. In the north-east corner about 40 of them may be counted in half a mile of coast-line, some being of large size. All of them which can be clearly made out are porphyritic olivine-basalts of the type of the Lion's Haunch at Arthur's Seat. They are generally grey in colour and finer at the edges than in the centre, which is often coarsely porphyritic and amygdaloidal. Olivine seems always characteristic, but has often been replaced by hæmatite or calcite. In Bute a good many dykes have been mapped to the north of Kilchattan Bay resembling this basalt series of Cumbrae, and running in the same direction. But they appear to be all porphyritic andesites. The second group of dykes, though it cuts the first and is thus proved to be later in date, is nevertheless confined within the same stratigraphical limits. It may thus belong nearly to the same period of intrusion.
"The dykes of the third group are dolerites without olivine, and follow on the whole an east and west direction. They cut both of the two foregoing sets of dykes, and likewise the lavas of the plateau. They must thus belong to a far later period of intrusion. They may be connected with other dykes and sills on the mainland, which traverse the Coal-measures, and would thus be not older than late Carboniferous or Permian time.
"The fourth group of dykes intersects all the others, and is probably of Tertiary age. The prevalent direction of these dykes in the Cumbraes is N.N.W." The Tertiary dykes are more fully described in Chapters xxxiv. and xxxv.
The great group of tuffs which underlies the lavas of the East Lothian plateau is traversed by numerous dykes and sills, of which many good examples may be seen in the coast-cliffs of North Berwick. Among these rocks are beautiful olivine-basalts with singularly fresh olivine, as on the shore at North Berwick. Some of them are still more basic, as in the case of a limburgite intrusion at the Gin Head, Tantallon Castle.
2. In the necks, dykes are sometimes abundant, and they may be observed occasionally to traverse the surrounding lavas. They consist of similar materials to those found outside the plateaux. Some of the larger necks are intersected by a network of dykes and veins. The great vent or group of vents among the uplands of Renfrewshire, already described (Fig. 129), furnishes some admirable examples of this characteristic volcanic feature. An illustration from that locality forms the subject of Fig. 134. The agglomerate which fills the large hollow among the Campsie Hills may be quoted as another illustration (Fig. 128). Further instances will be found in some of the sections given in preceding pages (see Figs. 124, 125, 127). The general aspect of a dyke in the volcanic series is shown in Fig. 135.
The Sills associated with the plateau-type of Carboniferous volcanic action form a less prominent feature than they do among the earlier Palæozoic formations or in the puy-type which succeeded them. They consist in general of short lenticular sheets of andesite or trachyte, like the necks and dykes in proximity to which they commonly appear. The best area for the study of them is the ground which stretches out from the base of the great escarpments of the Campsie, Kilpatrick and Ayrshire Hills (Fig. 136), where, among the agglomerate-vents and abundant dykes, intrusive sheets have likewise been injected between the bedding-planes of the red sandstones. But these sheets are of comparatively trifling dimensions. Very few of them reach a mile in length, the great majority falling far short of that size. In the Cumbraes and in Bute, Mr. Gunn has observed that the trachytic, olivine-basalt and dolerite dykes are apt to pass into intrusive sheets. That the sills, as well as the dykes and bosses of the same material, are not of older date than the lavas of the plateaux is proved by the manner in which they pierce these lavas, especially towards the bottom of the series. The general absence of basic sills, when we consider how thick a mass of these rocks has sometimes been poured out in the plateaux, is not a little remarkable. Only in the basin of the Firth of Forth do we encounter thick basic sills near the plateaux, such, for instance, as Salisbury Crags at Edinburgh. But it is doubtful whether they ought not rather to be classed with the sills of the puys, to be afterwards described.
4. Close of the Plateau-eruptions
The relative geological date when the eruptions of each plateau ceased can fortunately be determined with much more precision than the time of their beginning. The Hurlet Limestone, so well known as the lowest thick calcareous seam in the Carboniferous Limestone series, of which it is generally taken as the base, can be identified over the whole of Central Scotland, and thus forms an excellent stratigraphical horizon, from which the upward termination of the volcanic sheets underneath it can be measured.
When the volcanic episode of the plateau-eruptions came to an end, such banks or cones as rose above the level of the shallow sea which then overspread Central Scotland were brought beneath the water, as I have already remarked, either by prolonged denudation or more probably in large part by the continued subsidence of the region. The downward movement may possibly for a time have been accelerated, especially in some districts. Thus the Hurlet Limestone, though usually not more than five or six feet thick, increases locally to a much greater thickness. At Petersfield, near Bathgate, for example, it is between 70 and 80 feet in depth, while at Beith, in North Ayrshire, it increases to 100 feet (Fig. 137), which is the thickest mass of Carboniferous Limestone known to exist in Scotland. At both of these localities the limestone lies upon a series of volcanic rocks, and we may perhaps infer that the subsidence advanced there somewhat more rapidly or to a greater extent, so as to form hollows in which the limestone could gather to an abnormal depth. The water would appear to have become for a time tolerably free from mechanical sediment. The limestone is hence comparatively pure, and is extensively quarried all over the country for industrial purposes. It is a crinoidal rock, abounding in many species of corals, brachiopods, lamellibranchs, and gasteropods, with trilobites, cephalopods, and fishes.
A variable thickness of strata intervenes between the top of the volcanic series and the Main Limestone. Sometimes these deposits consist in large measure of a mixture of ordinary sandy and muddy material with the washed-down tuff of the cones, and probably with volcanic dust and lapilli thrown out by the latest eruptions. Thus along the flank of the hills from Barrhead to Strathavon, yellow and green ashy sandstones, grits and conglomerates are succeeded by ordinary sandstones, black shales and ironstones, while here and there true volcanic tuff and conglomerate make their appearance.[441] Further west, in the Kilbirnie district, the limestone lies directly on the tuffs that rest upon the andesites (Figs. 137, 138).
[Footnote 441: Explanation of Sheet 22, Geol. Surv. Scotland, p. 12.]
But perhaps the most striking contrast between adjacent localities in regard to the distance between the limestone and the top of the volcanic series is to be observed along the southern front of the Campsie Fells. In spite of the abundant faults which have there so broken up the regular sequence of the rocks, we can see that at Banton and Burnhead the limestone lies almost immediately on the volcanic series (Fig. 139). But a little to the westward, sandstones, conglomerates, shales and thin limestones begin to intervene between the volcanic series and the Hurlet Limestone and swell out so rapidly that on Craigmaddie Muir and South Hill of Campsie, only some five miles off, they must form a total thickness of not less than from 600 to 800 feet of ordinary non-volcanic deposits, chiefly thick pebbly sandstones (Fig. 140). Such local variations not improbably serve to indicate hollows on the flanks of the plateaux that were filled up with detritus before the depression and clearing of the water that led to the deposition of the Hurlet Limestone.
I have already remarked that the eruptions of the plateau period lasted longer in the western than in the eastern parts of the region. In the Garleton district, where the peculiar viscous trachytic lavas probably gave rise to a more uneven surface or more prominent cones than was usual among the andesitic plateaux, the eruptions ceased some time before the deposition of the Hurlet Limestone. As the area sank, the successive zones of the Calciferous Sandstones crept over the flanks of the trachytes, until at last they had completely buried these rocks before the limestone spread over the area (Fig. 141). In consequence, probably, of the uneven surface of this plateau, there is here a strong overlap of the higher part of the Calciferous Sandstones. On the west side of the volcanic area there can hardly be more than some 200 feet of strata between the top of the trachytic series and the limestone, while on the south side there must be greatly more than that thickness. This structure probably indicates that the Garleton volcanoes became extinct after having piled up a mass of tuffs and lavas to such a height that its summits were not submerged until the area had subsided 800 or 1000 feet in the waters, over the floor of which the Calciferous Sandstones were laid down. Hence, in spite of the proximity of the lavas to the limestone, there may have been a vast interval of time between their respective epochs, as has been already suggested with regard to other plateaux. This subject will be again referred to in discussing the relative chronology of the plateaux and puys.
In the Berwickshire and Solway districts, the extinction of the plateau-vents appears to have taken place at a still earlier part of the Carboniferous period, for there the andesites, while they rest on the Upper Old Red Sandstone, are covered with at least the higher group of the Calciferous Sandstones (Fig. 142). The equivalent of the Hurlet Limestone of Central Scotland must lie many hundred feet above them.
The submergence of the plateaux, and their entombment under the thick Carboniferous Limestone series, did not mark the close of volcanic activity in Central Scotland during Carboniferous time. The plateau-type of eruption ceased and was not repeated, but a new type arose, to which I would now call the reader's attention.
XXVI. THE CARBONIFEROUS PUYS OF SCOTLAND
i. General Character and Distribution of the Puys; ii. Nature of the Materials Erupted--Lavas Ejected at the Surface--Intrusive Sheets--Necks and Dykes--Tuffs.
i. GENERAL CHARACTER AND DISTRIBUTION
After the beginning of the Carboniferous Limestone period, when eruptions of the plateau-type had generally ceased, volcanic activity showed itself over the area of the British Isles in a different guise both as regards the nature of its products and the manner and scale of their discharge. Instead of widely extended lava-sheets and tuffs, piled above each other sometimes to a thickness of many hundred feet, and stretching over hundreds of square miles, we have now to study the records of another phase of volcanism, where scattered groups and rows of Puys, or small volcanic cones, threw out in most instances merely tuffs, and these often only in trifling quantity, though here and there their vents also poured forth lavas and gradually piled up volcanic ridges which, in a few cases, almost rivalled some of the plateaux. The evidence for these less vigorous manifestations of volcanic activity is furnished (1) by layers of tuff and sheets of basaltic-lavas intercalated among the strata that were being deposited at the time of the eruptions, (2) by necks of tuff, agglomerate, or different lava-form rocks that mark the positions of the orifices of discharge, and (3) by sills, bosses, and dykes that indicate the subterranean efforts of the volcanoes. The comparatively small thickness of the accumulations usually formed by these vents, their extremely local character, the numerous distinct horizons on which they appear, and the intimate way in which they mingle and alternate with the ordinary Carboniferous strata are features which at once arrest the attention of the geologist, presenting, as they do, so striking a contrast to those of the plateaux.
From the clear intercalation of these volcanic materials on successive platforms of the Carboniferous system, the limits of geological time within which they were erupted can be fixed with considerable precision. It may be said that, in a broad sense, they coincided with the period of the Carboniferous Limestone, and certainly it was during the deposition of that formation that the eruptions which produced them reached their greatest vigour and widest extent. Here and there in Scotland evidence may be found that the phase of the Puys began during that earlier section of Carboniferous time recorded by the Calciferous Sandstones. This is markedly the case in Liddesdale and the neighbouring territory. Over the western part of Midlothian also, the eastern portion of West Lothian, and the southern margin of Fife, abundant traces occur of puy-eruptions during the deposition of the Calciferous Sandstones. Elsewhere in Central Scotland there is no evidence of the vents having been opened until after the deposition of the Hurlet Limestone, which, as we have seen, may conveniently be taken as the base of the Scottish Carboniferous Limestone series. The volcanoes remained active in West Lothian until near the close of the time represented by that series; but in Ayrshire they continued in eruption until the beginning of the accumulation of the Coal-measures. These western examples of the puy-type are, so far as I am aware, the latest known in Britain.
Whether or not the earliest puy-eruptions began before the latest plateau-lavas and tuffs were accumulated is a question that cannot be readily answered. It will be remembered that in the basin of the Firth of Forth a thickness of more than 3000 feet of sedimentary strata, including the Burdiehouse Limestone and numerous oil-shales as well as thin coal-seams, lies above the red and green marls, shales, sandstones and cement-stones of the Calciferous Sandstone series. This remarkable assemblage of strata is absent in the western parts of the country, where the top of the Clyde volcanic plateau is almost immediately overlain by the Hurlet Limestone. If we were to judge of the sequence of events merely from the stratigraphy, as expressed in such sections as Figs. 137, 138, 139 and 140, we might naturally infer that as no trace of any break occurs at the top of the Clyde plateau, the tuffs shading upward there into the limestone series, no important pause in sedimentation took place, but that the last volcanic eruptions were soon succeeded by the conditions that led to the deposition of the widespread encrinite-limestones. If this inference were well founded it would follow that while the plateau-eruptions in the west lasted till the time of the Hurlet Limestone, those in the east ceased long before that time and were succeeded by the puys of Fife and the Lothians. There would thus be an overlap of the two phases of volcanic action.
I am inclined to believe, however, that in spite of the superposition of the Hurlet Limestone almost immediately upon the volcanic rocks of the Clyde plateau, and the absence of any trace of a break in the process of sedimentation, a long interval nevertheless elapsed between the last eruptions and the deposit of that limestone. The Campsie section (Fig. 140) shows us how rapidly a thick mass of strata can come in along that horizon. The volcanic ridges may have remained partly unsubmerged for such time as was required for the subsidence of the Forth basin and the deposit of the thick Calciferous Sandstone series there, and their summits may only have finally sunk under the sea not long before the Hurlet Limestone grew as a continuous floor of calcareous material over the whole area of central Scotland. In these circumstances, the puy-eruptions of that basin would be long subsequent to the eruptions of the Clyde plateau, as they certainly were to those of the plateaux of Midlothian and the Garleton Hills.
In tracing the geographical distribution of the puy-eruptions we are first impressed with the force of the evidence for their extremely local and restricted character (Map IV.). Thus in the area of the basin of the Firth of Forth, which may be regarded as the typical region in Britain for the study of this form of Carboniferous volcano, traces of them are abundant to the west of the line of the Pentland Hills. To the east of that line, however, not a vestige of puy-eruptions, save a few sills of uncertain relationship, can be detected, though the same series of stratigraphical horizons is well developed on both sides of the Lothian coal-field. Again, to the westward of the Forth basin over the area of Stirlingshire, Lanarkshire and Renfrewshire lying to the north of the great volcanic plateau, no record of puy-eruptions has been noticed. Immediately to the south of that plateau, however, these eruptions were numerous in the north of Ayrshire. Yet the rest of the Carboniferous area in that large county has supplied no relics of these eruptions save at one locality--the Heads of Ayr. Lastly, while no trace of any younger display of volcanic activity occurs in the Merse of Berwickshire, east of the plateau series of that district, the ground immediately to the west abounds in puys, and contains likewise extensive sheets of tuff and beds of basic lavas connected with these vents.
Another fact which at once attracts notice in Scotland is the way in which the puy-vents have generally avoided the areas of the plateaux, though they sometimes approach them closely. As a rule, it is possible to distinguish the tuffs and agglomerates which have filled up these vents from those that mark the sites of the eruptive orifices of the plateaux. There are, no doubt, some instances, as in Liddesdale, where puys have appeared on the sites of the older lavas, but these are exceptional collocations.[442] On the other hand, many examples may be found where puys have risen in the interspace between the limits of the eruptions of two plateau-areas. Thus the tract between the Clyde plateau-eruptions on the west and those of the Garleton Hills on the east was dotted over with puys. Again, the southern margin of the Clyde plateau in Ayrshire, from Dalry to Galston is flanked with puys and long sheets of their lavas and tuffs.[443]
[Footnote 442: A means of definitely placing some of these vents in the series of puy-eruptions is stated further on, at p. 476.]
[Footnote 443: Reference may again be made here to the remarkable similarity between the Scottish Carboniferous puy-vents and those of older Tertiary time in the Swabian Alps so fully described by Professor Branco in the work already cited p. 46. Denudation in that region has bared the cones and exposed the structure of the necks which, down to even minute details, repeat the phenomena of Carboniferous and Permian time in Scotland.]
ii. NATURE OF THE MATERIALS ERUPTED
A. The Lava-form Rocks
We have now to consider the nature of the materials erupted by the volcanic activity of the puys. The geologist who passes from the study of the plateau lavas to those of the puys at once remarks the prevalent more basic character of the latter. The great majority of them are basalts, generally olivine-bearing, in the various types embraced in the table on the following page. The olivine-free dolerites are generally found as intrusive bosses, sills and dykes. Such more acid rocks as andesites occur only rarely, and still more seldom are quartziferous masses met with in some of the bosses.
Dolerites and Basalts.--The great majority of the lava-form rocks connected with the puys are basic in composition, and belong to the family of the Dolerites and Basalts. They graduate, on the one hand, into ultra-basic rocks such as limburgite and picrite, and on the other, into compounds that approach andesites or trachytes in composition. A large series of specimens from Central Scotland was studied a few years ago by Dr. Hatch, who proposed a petrographical classification of the rocks, and arranged them in a number of types which he named after localities where they are well developed.[444] More recently the rocks have again been subjected to microscopic investigation by my colleague Mr. Watts, who, confirming generally Dr. Hatch's discriminations, has made some modifications of them. He has furnished me with a revised classification (p. 418), based on purely petrographical considerations. The doleritic and basaltic series may be grouped into two divisions, one with, and the other without, olivine, and each division may be further separated into a dolerite group, which presents an ophitic or sub-ophitic structure, and a basalt-group in which the groundmass is made up of felspar and granular augite, and possesses the "intersertal structure" of Rosenbusch, or consists of idiomorphic augite embedded in felspar substance. The term "sub-ophitic" is employed by Mr. Watts "to imply that the augite grains are neither very large nor very continuous, optically, and that they rarely contain entire felspar-crystals imbedded in them, merely the ends of a group of these crystals as a rule penetrating into them."
[Footnote 444: This classification was given in my Presidential Address to the Geological Society, 1892, Quart. Journ. Geol. Soc. vol. xlviii. p. 129. See Report of Geological Survey for 1896.]
Transitional forms occur between many of the following types by the increase or diminution in the relative proportions of the constituents. Thus it is not easy to draw a line between 2_b_ and 2_c_; the latter again shades into 2_d_ and 2_e_ by the decrease of the felspar.
Mr. Watts has further observed that the rocks containing no olivine offer greater difficulties in classification than those in which that mineral is present. "The very distinction," he remarks, "between dolerites and basalts is less marked, the types are much less sharply distinguished, and decomposition and masking of the structure are more common. While using the term Dolerite for such rocks as have a sub-ophitic structure, I have extended it to those rocks in which evidence exists that a great part of the crystallization took place under intratelluric conditions. Although not quite holocrystalline, the crystals of felspar, augite and magnetite are large and the structure coarse-grained, while the groundmass is confined to comparatively small interstitial patches. In these rocks there is usually no one dominant porphyritic ingredient."
I. The Olivine-bearing Series
1. Olivine-Dolerites
1_a_. Porphyritic elements inconspicuous, olivine being } the principal, and felspar of secondary importance; } Jedburgh Type. groundmass sub-ophitic. }
1_b_. Strongly porphyritic; felspar-phenocrysts large; } Kilsyth Type. olivine smaller; groundmass sub-ophitic. }
1_c_. Porphyritic olivine, but not felspar; groundmass } Gallaston Type. sub-ophitic. }
2. Olivine-Basalts
2_a_. Porphyritic olivine, augite and felspar; groundmass} Lion's Haunch of felspar-laths imbedded in granules of augite. } Type. } (See Fig. 207.)
2_b_. Porphyritic olivine and augite; groundmass of } felspar-laths imbedded in granules of augite. More } Craiglockhart rarely the groundmass is made of idiomorphic augite } Type. imbedded in felspar-substance. }
2_c_. Porphyritic olivine abundant, augite much less } common, and felspar very rare or absent; groundmass } Dalmeny Type. with granular or idiomorphic augite (one of the most } common types). }
2_d_. Porphyritic olivine more common than augite; } groundmass of granules of augite set amongst lath-like } Picrite Type. felspars which are much fewer in number than in 2_c_. }
2_e_. Porphyritic olivine more common than augite; } groundmass of idiomorphic augite imbedded in } Limburgite felspathic material which is not abundant. } Type.
2_f_. Porphyritic olivine and felspar, without augite; } Kippie Law Type. groundmass of granular or idiomorphic augite, with } (For analysis lath-shaped felspars. } see p. 379).
II. The Non-olivine-bearing Series
3. Olivine-free Dolerites
Felspar, augite, magnetite in coarse-grained aggregate usually ophitic or sub-ophitic; groundmass not plentiful.
3_a_. Groundmass absent Ophitic Type.
3_b_. Groundmass micropegmatitic Ratho Type.
3_c_. Groundmass an unstriated felspar (not orthoclase) } and occasionally some interstitial altered glass or a } Burntisland little quartz. } Sill Type.
4. Doleritic Basalts
Felspar, augite and magnetite in coarse-grained aggregate; groundmass rather more plentiful and often in large patches.
4_a_. Felspar and augite, related sub-ophitically where } together, but augite showing crystalline contours in } Bowden contact with the groundmass; some interstitial quartz } Hill Type. and unstriated felspar. }
5. Basalts
Finer-grained rocks, generally with a porphyritic ingredient and much scattered interstitial matter in the groundmass.
5_a_. Porphyritic felspar, and occasionally a little } augite; groundmass of granular augite, felspar needles } Binny Craig and magnetite with some interstitial matter. } Type.
5_b_. Porphyritic felspars not conspicuous and small; the } rock mainly made up of a mesh of fine felspar-laths set } Tholeiite amongst granular augite, magnetite and base. } Type.
5_c_. Similar to the last but even finer-grained, and } Crypto- with the base in a cryptocrystalline condition. } crystalline } Type.
Taking first the superficial lavas, I know of only one locality where picrite occurs in such a position that it may be included among the surface outflows. This is the quarry near Blackburn, to the east of Bathgate, where I originally observed it.[445] The rock occurs there on the line of the basalt-flows from the Bathgate Hills, and I mapped it as one of them before the microscope revealed the remarkable composition of the mass. I still believe it to be a lava like the "leckstone" described on p. 443, though the other known examples of this rock in the basin of the Firth of Forth are intrusive sheets. The rock locally known as "leckstone" or "lakestone" has long been quarried for the purpose of constructing the soles of bakers' ovens, as it stands a considerable temperature without cracking. Its microscopic structure is now well known. As exposed in Blackburn quarry, an interesting difference is observable between the lower and upper parts of the sheet. The lower portion is a picrite, with abundant serpentinized olivine, large crystals of augite, and a considerable amount of ores. The upper portion, on the other hand, has plagioclase as its most abundant definite mineral, with a minor quantity of minute prisms of augite and of iron-ores, and scattered crystals of olivine. Here, within the compass of a few yards and in one continuous mass of rock, we have a transition from a variety of olivine-basalt into a picrite.
[Footnote 445: Trans. Roy. Soc. Edin. vol. xxix. (1879) p. 506.]
The great majority of the puy lavas belong to the olivine-bearing series. A few of them are dolerites, but most are true basalts of the Dalmeny type, of which typical examples may be seen at the Kirkton quarries, Bathgate, and in the coast section between Pettycur and Kinghorn. Occasionally they present transitions towards picrite, as in the sheet overlying the lowest limestone at Kirkton, and in the lowest lava of King Alexander's Crag, Burntisland. These puy lavas exhibit considerable variety of structure as seen in the field. Some are solid, compact, black rocks, not infrequently columnar and weathering into spheroidal exfoliating forms. Others are somewhat granular in texture, acquiring green and brown tints by weathering, often showing amygdaloidal kernels, and even passing into well-marked amygdaloids. Many of them exhibit a slaggy structure at their upper and under surfaces (Figs. 153, 170, 171). These external differences are an index to the corresponding variations in composition and microscopic structure enumerated in the foregoing tabular arrangement.
As a rule, the basic rocks which occur intrusively in connection with the puys, especially where they form a considerable mass, have assumed a much more coarsely crystalline texture than those of similar composition which have been poured out at the surface. They are generally dolerites rather than basalts. But with this obvious distinction, the two groups have so much in common, that the geologist who passes from the study of the subterranean phenomena of the Plateaux to that of the corresponding phenomena of the Puys is at once impressed with the close relationship between the material which, in the case of the puys, has consolidated above ground, and that which has been injected below. There is no such contrast between them, for example, as that between the basic and intermediate lavas of the plateaux and the more acid intrusions associated with them.
By far the largest number of the basic sills, bosses and dykes associated with the puys are somewhat coarsely crystalline dolerites without olivine. They include, however, olivine-dolerites and basalts, and even some extremely basic compounds. Of these last, a typical example is supplied by the now well-known picrite of Inchcolm, in the Firth of Forth, which occurs as an intrusive sheet among the Lower Carboniferous Sandstones.[446] In recent years one or two other picrite-sills have been observed in the same district. An interesting example has been described from a railway cutting between Edinburgh and Cramond where the rock invades and alters shales. More detailed reference to it will be made in the account of the sills connected with the puys. Another instance of the occurrence of this rock is in a railway cutting immediately to the west of Burntisland where it has been intruded among the Calciferous Sandstones below the Burdiehouse Limestone.
[Footnote 446: Trans. Roy. Soc. Edin. vol. xxix. (1879) p. 506. Teall, British Petrography, p. 94.]
Rocks approaching limburgite occur among the sills and bosses which pierce the Carboniferous Limestone series of Fife between Cowdenbeath and Inverkeithing. One of these is found at Pitandrew, near Fordel Castle. Dr. Hatch observed that it consists of "numerous porphyritic crystals of olivine, with a few grains of augite and an occasional small lath-shaped crystal of felspar imbedded in a groundmass which is composed principally of idiomorphic augite microlites, small crystals of a brown mica, granules of magnetite and prisms of apatite. In addition, there is a considerable amount of interstitial matter, which is partly colourless glass, and partly shows a slight reaction between crossed nicols." Another example of the same type of rock occurs as a plug or boss in the tuff-vent of the Hill of Beath, and a further display of the limburgite type is to be seen in Dunearn Hill near Burntisland.
Although olivine-basalts of the Dalmeny type are most frequently met with as interstratified lavas, they also occur as bosses and sills. The typical example from Dalmeny is itself intrusive. Other illustrations are to be found in the Castle Rock of Edinburgh and in the sheets near Crossgates and Blairadam in Fife. The presence or absence of olivine, however, may sometimes be a mere accident of cooling or otherwise. I have shown that in the same mass of rock at Blackburn a gradation can be traced from a rock largely composed of altered olivine into one consisting mainly of felspar with but little olivine, and another example occurs in the picrite-sill between Edinburgh and Cramond. Dr. Stecher has ascertained that the marginal portions of the sills in the basin of the Firth of Forth, which cooled first and rapidly, and may be taken, therefore, to indicate the mineral composition of the rock at the time of extrusion, are often rich in olivine, while that mineral may be hardly or not at all discernible in the main body of the rock.[447]
[Footnote 447: Dr. Stecher, Tschermak's Mineralog. Mittheil. vol. ix. (1887) p. 193. Proc. Roy. Soc. Edin. vol. xv. (1888) p. 162.]
Of the ordinary and characteristic dolerites without olivine which constitute most of the intrusive masses, the various types enumerated in the tabular arrangement are abundantly developed in Central Scotland. Thus the normal ophitic type is displayed by the uppermost sill of the Burntisland series, and by the rock which forms the plug of the Binns Hill neck in Linlithgowshire. The Ratho type is well seen in the large sill at Ratho, likewise in the extensive intrusive sheets in the west of Linlithgowshire as at Muckraw and Carribber. The Burntisland sill type is shown by the lower sills of Burntisland and by some others in the same region, especially by that of Colinswell, and by another on the shore east from the Poorhouse, near Kinghorn. The great boss among the Bathgate Hills likewise displays it. The Bowden Hill type occurs in well-marked development at Bowden Hill, three miles south-west of Linlithgow, and in the massive sill at St. Margaret's, west from North Queensferry.
The non-olivine-bearing basalts are found in various bosses and sheets in the basin of the Firth of Forth. Thus the Binny Craig type occurs in the prominent and picturesque sill from which it is named, likewise among the intrusive sheets near Kirkcaldy, in Fife. Sometimes the same mass of rock displays more than one type of structure, as in the case of the great Galabraes neck among the Bathgate Hills wherein both the Tholeiite and Burntisland sill types may be recognized.
Some of the sills in West Lothian, as I pointed out many years ago, contain bitumen and give off a bituminous odour when freshly broken. They have been injected into bituminous shales or coal-seams.[448]
[Footnote 448: Geol. Survey Memoir on Geology of Edinburgh (Sheet 32, Scotland), p. 46.]
2. Andesites.--Rocks referable to this series appear to have been of rare occurrence among the puy-eruptions. Examples of them containing as much as 60 per cent of silica occur among the lavas of the Limerick basin. Some of the necks and what may be sills in the same district likewise consist of them.
3. Trachytes and Quartz-bearing Rocks.--Acid rocks, as I have already said, are extremely rare among the puy-eruptions. The only important examples known to me are those around the Limerick basin, where they rise apparently in old vents and form conspicuous rounded or conical hills. These rocks have been examined microscopically by Mr. W. W. Watts. One of the most interesting varieties, which occurs at the Standing Stone near Oola, was found by him to show quartz enclosing ophitically the felspars which, with well-terminated prisms, project into it. Further west, near Knockaunavoher, another boss occurs with conspicuous quartz. These rocks have much in common with trachytes but have a wholly crystalline structure. They will be described in the account of the Limerick basin.
B. Tuffs
The fragmental rocks connected with the puy-eruptions form a well-marked group, easily distinguishable, for the most part, from the tuffs of the plateaux. They vary from exceedingly fine compacted dust or volcanic mud, through various stages of increasing coarseness of texture, to basalt-conglomerates and tumultuous agglomerates.
The fragmentary material found in the necks of the puys is generally an agglomerate of a dull dirty-green colour. The matrix ranges from a fine compact volcanic mud to a thoroughly granular detritus, and sometimes shows a spheroidal concentric structure in weathering. In this matrix the lapilli are distributed with great irregularity and in constantly varying proportions. They consist in large measure of a pale yellowish-green, sometimes pale grey, very basic, finely vesicular, devitrified glass, which is generally much decomposed and cuts easily with the knife. This highly basic substance is a kind of palagonite. So minute are its vesicles that under the microscope a thin slice may present a delicate lace-like network of connected walls, the palagonite occupying much less space than the vesicles. The material has been a finely frothed-up pumice.
Besides this generally distributed basic pumice, the stones in the agglomerate of the necks likewise include fragments of older volcanic grits or tuffs, blocks of basalt or diabase, as well as pieces of the Carboniferous strata of the district, especially shale, sandstone and limestone. Not infrequently also, they comprise angular blocks of fossil wood.
The materials which fill the necks are generally much coarser than those that form intercalated beds. But while in numerous cases huge blocks of basalt and large masses of sandstone, shale, limestone, ironstone or other strata may be seen wrapped up in a matrix of coarse basalt-tuff, in not a few instances the material in the necks may be observed to consist of a tuff quite as fine as that of the interstratified bands. Such necks appear to mark the sites of tuff-cones where only fine ashes and lapilli were ejected, and where, after sometimes a brief and feeble period of activity, the orifice became extinct.
The bedded tuffs interstratified with the ordinary Carboniferous strata do not essentially differ in composition from the material of the necks. They are basalt- (diabase-) tuffs and basalt- (diabase-) conglomerates, usually dull green in colour and granular in texture, the lapilli consisting in great measure of various more or less decayed basalts, but containing the same highly vesicular basic glass or pumice above referred to. They are mainly to be distinguished by their conspicuous stratification, and especially by their rapid alternations of coarser and finer material, by the intercalation of shales, limestones, sandstones or ironstones in them, and by the insensible gradations by which they pass both vertically and laterally into ordinary sediments. Occasional large blocks or bombs, indicating some paroxysm of explosion, may be observed even among the finer tuffs, shales and other strata, which round the sides of these masses have had their layers bent down by the fall of heavy blocks.[449] Many of the bedded tuffs contain fossils, such as crinoids, corals, brachiopods, fish-teeth or macerated fragments of land-plants. Coal-seams also are occasionally interstratified among them.
[Footnote 449: Ante, p. 36, and Figs. 15 and 151. See also _Geol. Mag._ i. (1864), p. 22; Trans. Roy. Soc. Edin. vol. xxix. (1879) p. 515.]
Of the finer kinds, the best example is furnished by a remarkable group of "green and red marls" which lie above a seam of coal (Houston Coal) in the Calciferous Sandstones of West Lothian.[450] These strata, which differ much from any of the rocks with which they are associated, are exceedingly fine in grain, dull sage-green and brownish or chocolate-red in colour, not well laminated like the shales, but breaking under the influence of weathering into angular fragments, sometimes with a conchoidal fracture. They look like indurated mud. Mr. H. M. Cadell, who has recently re-examined them in connection with a revision of the Geological Survey Map (Sheet 32) has found them passing into ordinary granular tuff.
[Footnote 450: Memoir on Sheet 32 Geol. Surv. Scotland (1861), p. 42. The stratigraphical position of these "Houston Marls," as they are locally called, is indicated in Fig. 155.]
Palagonitic-tuff is of frequent occurrence. It is met with in the Firth of Forth district,[451] and Mr. Watts has detected fragments of palagonite among the tuffs of the Limerick basin.
[Footnote 451: Trans. Roy. Soc. Edin. vol. xxix. (1819) p. 515.]
XXVII. GEOLOGICAL STRUCTURE OF THE CARBONIFEROUS PUYS OF SCOTLAND
1. Vents: Relation of the Necks to the Rocks through which they rise--Evidence of the probable Subærial Character of some of the Cones or Puys of Tuff--Entombment of the Volcanic Cones and their Relation to the Superficial Ejections. 2. Bedded Tuffs and Lavas--Effects of Subsequent Dislocations. 3. Sills, Bosses and Dykes.
The puy-type of volcanic hill differs widely in one respect from those which we have hitherto been considering. In the earlier epochs of volcanism within the British area, it is the masses of material discharged from the vent, rather than the vents themselves which arrest attention. Indeed, so copiously have these masses been erupted that the vents are often buried, or their positions have been rendered doubtful, by the uprise in and around them of sills and bosses of molten rock. But among the Carboniferous puys the vent is often the only record that remains of the volcanic activity. In some cases we know that it never ejected any igneous material to the surface. In others, though it may be filled with volcanic agglomerate or tuff, there is no record of any shower of such detritus having been discharged from it. In yet a third class of examples, we see that lava rose in the vent, but no evidence remains as to whether or not it ever flowed out above ground. Other cases occur where beds of lava or of tuff, or of both together, have been intercalated in a group of strata, but with no trace now visible of the vent from which they came. The most complete chronicle, preserving at once a record of the outflow of lava, of the showering forth of ashes and bombs, and of the necks that mark the vents of eruption, is only to be found in some of the districts.
I shall therefore, in the present instance, reverse the order of arrangement followed in the previous chapters, and treat first of the vents, then of the materials emitted from them, and lastly of the sills and dykes.
i. VENTS
A large number of vents rise through the Carboniferous rocks of Scotland. Some of these are not associated with any interbedded volcanic material, so that their geological age cannot be more precisely defined than by saying that they must be later than the particular formations which they pierce. Some of them, as I shall endeavour to show, are in all probability of Permian age. But many, from their position with reference to the nearest intercalated lavas and tuffs, are to be regarded as almost certainly belonging to the Carboniferous period. Those which are immediately surrounded by sheets of lava and tuff, similar in character to the materials in the vents themselves, may without hesitation be connected with these sheets as marking the orifices of discharge.
The vents of the puys are in general much less than those of the plateaux. Their smallest examples measure only a few yards in diameter, their largest seldom much exceed half a mile.[452]
[Footnote 452: The following measurements of necks belonging to the puy-eruptions in different parts of Scotland are taken from the 6-inch field-maps of the Geological Survey:--Saline Hill, Fife, 6000 × 4000 feet; Binn of Burntisland, 3500 × 1500; Hill of Beath, Fife, 2900 × 1550; Binns Hill, Linlithgowshire, 4800 × 2200; Tor Hill, Ecclesmachan, Linlithgowshire, 1900 × 1000 (Fig. 155); Great Moor, near Maiden Pap, Roxburghshire, 2600 × 2400; Tinnis Hill, Liddesdale, 1500 × 1000; Roan Fell, Liddesdale, 300 × 200; Hadsgarth Burn, Liddesdale, 250 × 200; Dalbate Burn, 250 × 120. In some cases, especially in those of the larger necks, it is probable that the tuff belongs to more than one funnel. Thus the Binn of Burntisland almost certainly includes two necks, a smaller one to the west and a much larger one to the east. Saline Hill may also conceal more than one vent. But in the continuous mass of tuff at the surface it is at present impossible to determine precisely the number and boundaries of the several orifices.]
The dislocations of the Carboniferous system are probably on the whole later than its volcanic phenomena. It is at least certain that the lavas and tuffs of the puys have been extensively faulted, like the surrounding sedimentary strata, and the vents seldom show any apparent relation to faults. It may sometimes be observed, however, that the vents are arranged in lines suggestive of fissures underneath. A remarkable instance of the linear distribution is furnished by the chain of necks which extends from the Vale of the Tweed at Melrose south-westwards across the watershed and down Liddesdale. The most notable part of this line lies among the uplands to the east of the Old Mosspaul Inn at the head of the Ewes Water. A string of masses of agglomerate has there solidified in a fissure among the Silurian greywackes and shales, running in a north-easterly direction for several miles. The largest connected mass of agglomerate is 4700 feet long, and from 350 to 600 feet broad (see No. 1 in Fig. 22). That this curious vent, or connected line of vents along a great fissure, belongs to the puy-eruptions of Liddesdale is shown by the abundant fragments of yellow sandstone and cement-stone which occur in the agglomerate.[453]
[Footnote 453: These facts were ascertained by Mr. Peach in mapping the ground for the Geological Survey. See Sheet 17, Scotland.]
Most frequently the vents are distributed irregularly in groups. As examples of this arrangement I may cite those of the west of Fife, of West Lothian and of the north of Ayrshire.
A convenient classification of the vents may be made by dividing them into four groups according to the nature of the material that now fills them: 1st, Necks of non-volcanic debris; 2nd, Necks of tuff and agglomerate; 3rd, Necks of similar materials, but with a central plug of basalt; 4th, Bosses of basalt or other lava, without agglomerate or tuff.
1. Necks of Non-volcanic Debris.--In a few instances the orifices of eruption have been filled up entirely with non-volcanic debris. They have served as funnels for the discharge of explosive vapours only, without the expulsion of any solid volcanic materials. At least no trace of fragmentary lavas is met with in them, nor are any beds of tuff or lava intercalated among the surrounding strata. Some interesting examples of this kind were laid bare in the open ironstone-workings near Carluke in Lanarkshire. They were circular in ground-plan, descended vertically into the strata, and were somewhat wider at the top of the quarry than at the bottom. They were filled with angular pieces of Carboniferous sandstone, shale, limestone, ironstone and other rocks, these materials being rudely arranged with a dip towards the centre of the neck, where the blocks were largest in size. Though no fragments of igneous rocks were observed among the debris, a few string-like veins of "white trap," or altered basalt, were seen to traverse the agglomerate here and there. The necks and the strata surrounding them were highly impregnated with pyrites and sulphate of lime.[454]
[Footnote 454: Prof. Jas. Geikie, Mem. Geol. Surv. Scotland, Explanation of Sheet 23, p. 39.]
A vent of the same nature, but on a much larger scale, has been mapped by Mr. Peach in the south of Perthshire, near East Grange, where it rises through the higher coal-bearing part of the Carboniferous Limestone series (Fig. 143). It has been encountered in the mining of coal and ironstone, and its cross-section has been ascertained in the underground workings which have been carried up to its margin. It measures 1500 feet in diameter from east to west and 2000 feet from north to south. It does not appear ever to have emitted any ashes or lava. Mr. Peach found it filled with dark sandy crumbling clays, full of fragments of sandstone, shale and coal. These sediments are arranged in layers that dip in the same general direction as the strata surrounding the vent. They contain abundant calcareous nodules of all sizes from that of a hazel-nut up to concretions 18 feet in diameter. The clays likewise include many of the common shells and crinoids of the Carboniferous Limestone sea, and the same fossils are enclosed in the nodules. A remarkable feature in this vent is the occurrence of abundant vertical rents, which have been filled partly with the same material that forms the nodules, and partly with sandstone.
The formation of the neck took place after the deposition of the Index Limestone, and probably about the time of the accumulation of the next limestone, which lies immediately to the west somewhat higher in the series. It would appear that the eruption which produced this funnel gave forth only gaseous explosions, and occurred on the sea-floor; that the low crater-walls were washed down to such an extent that the sea entered and carried some of its characteristic organisms into the lagoon or maar within; further, that as the silt gathered inside, successive subsidences occurred, whereby the sediment was rent by cracks into which sand and calcareous mud were washed from above.[455]
[Footnote 455: The vent is shown in Sheet 39, Geol. Surv. Scotland.]
Many necks occur wherein non-volcanic materials, though not forming the whole of the agglomerate, make up by far the larger part, with only a slight admixture of volcanic tuff between them. Among the Burntisland necks of Fife, for instance, abundant fragments of the well-marked cyprid limestone and shale may be observed, while at Niddry in Linlithgowshire blocks several yards in length, and consisting of different layers of shale and cement-stone still adhering to each other, may be seen imbedded at all angles in the tuff.
Where only the debris of non-volcanic rocks occupies a vent, we may infer that the volcanic action was limited to the explosion of steam, whereby the rocks were dislocated, and an orifice communicating with the surface was drilled through them, and that while no true volcanic rock in such a case appeared, the pipe was filled up to perhaps not far from the surface by the falling back of the shattered detritus. A little greater intensity or farther prolongation of the volcanic action would bring the column of lava up the funnel, and allow its upper part to be blown out as dust and lapilli; while still more vigorous activity would be marked by the rise of the lava into rents of the cone or its actual outflow at the surface. Every gradation in this scale of progress may be detected among the Carboniferous volcanoes of the basin of the Firth of Forth.
2. Necks of Tuff and Agglomerate.--The majority of the necks connected with the puys consist of tuff or agglomerate. Externally they generally appear as smooth rounded grassy hills that rise disconnected from other eminences. In some districts their materials consist of a greenish granular often stratified tuff, enclosing rounded balls of various basic lavas and pieces of sandstone, shale, limestone or other strata through which they have been drilled. This is their usual character in the Forth region. But in some cases, the tuff becomes a coarse agglomerate, made up partly of large blocks of basalt and other volcanic rocks and partly of the sedimentary strata around them, of which large masses, many cubic yards in bulk, may be seen. Among the enclosed fragments it is not unusual to find pieces of older stratified tuff. These resemble in general petrographical character parts of the tuff among which they are imbedded. Sometimes they have been
[Illustration: Fig. 144.--View of the Binn of Burntisland--a volcanic neck of agglomerate. (This illustration and Figs. 145, 152, 153, 164, 166 and 168 are from photographs taken by Mr. Robert Lunn for the Geological Survey.)] derived from previous tuffs which, interstratified among the sedimentary strata, had been broken up by the opening of a new vent. But probably in most cases they should be regarded as portions of the volcanic debris which, having solidified inside the crater, was blown out in fragments by subsequent explosions. In a modern volcano a considerable amount of stratified tuff may be formed inside the crater. The ashes and stones thrown out during a period of activity fall not only on the outer slopes of the cone, but on the steep inner declivities of the crater, where they arrange themselves in beds that dip at high angles towards the crater bottom. This feature is well seen in some of the extinct cones in the Neapolitan district. In some of the Scottish puys the tuff is stratified and has tumbled down into a highly inclined or vertical position (Fig. 145).
As a good illustration of the variety and relative proportions of the ejected blocks in the green tuff of the puy-vents, I may cite the following table of percentages which I took many years ago in the tuff which rises through the Cement-stone group on the beach at the Heads of Ayr.
Diabase and basalt 57 per cent. Older tuff 3 " Andesite (probably from Old Red Sandstone volcanic series of the neighbourhood) 14 " Limestone (cement-stone, etc.) 20 " Shale 3 " Sandstone 2 " Fossil wood 1 " ---- 100
While many examples might be cited where no molten rock of any kind has risen in the vents, or where at least all the visible materials are of a fragmentary character, yet small veins and dykes of basalt have not infrequently been injected into the tuff or agglomerate. These seldom run far, and usually present a more or less tortuous course. Thus, on the south front of the Binn of Burntisland (Figs. 166, 168) a number of basalt-dykes, which vary in breadth from five or six feet to scarcely so many inches, bifurcate and rapidly disappear in the tuff, one of them ascending tortuously to near the top of the cliff. They at once recall the appearance of the well-known dykes in the great crater wall of Somma.
Though not by any means the largest or most perfect of the vents in the basin of the Firth of Forth, the Binn of Burntisland, of which a view is given in Fig. 144, may be cited in illustration of their general characters. It presents in detail some of the most strikingly volcanic aspects of scenery anywhere to be seen in that region. Consisting of a dull green granular volcanic tuff, it rises abruptly out of the Lower Carboniferous formations to a height of 631 feet above the sea. The southern edge of this neck has been so extensively denuded, that it presents steep craggy slopes and rugged precipices, which descend from the very summit of the cone to the plain below--a vertical distance of nearly 500 feet. Here and there the action of atmospheric waste has hollowed out huge crater-like chasms in the crumbling tuff. Standing in one of these, the geologist can realize what must have been the aspect of the interior of these ancient Carboniferous volcanic cones. The scene at once reminds him of the crater-walls of a modern or not long extinct volcano. The dull-green rudely stratified tuff rises around in verdureless crumbling sheets of naked rock, roughened by the innumerable blocks of lava, which form so conspicuous an element in the composition of the mass. The ribs or veins of columnar basalt run up the declivities as black shattered walls. The frosts and rains of many centuries have restored to the tuff its original loose gravelly character. It disintegrates rapidly, and rolls down the slopes in long grey lines of volcanic sand, precisely as it no doubt did at the time of its ejection, when it fell on the outer and inner declivities of the original cone. Some of these features may be partly realized from Fig. 145, which represents a portion of the south front of the hill. Sections of this neck are given in Figs. 149 and 159.
(3) Necks of Tuff or Agglomerate with a Central Plug of Basalt or other Lava.--It has often happened that, after the explosions in a vent have begun to decrease in vigour, or have at last ceased, lava has risen in the chimney and finally sealed it up. In such cases the main mass of the rock may consist of tuff or agglomerate, which the enfeebled volcanic activity has been unable to expel from the orifice, while a plug of basalt, dolerite, or even more basic material, of much smaller dimensions, may have risen up the pipe in the centre or towards one side. Binns Hill, West Lothian, the Beath and Saline Hills of Fife, and Tinnis Hill in Liddesdale are good examples of this structure. (See Figs. 26, 148, 149 and 174).
(4) Necks of Basalt, Dolerite, etc.--In other cases no fragmental material is present in the vent, or possibly traces of it may be seen here and there adhering to the walls of the funnel, the prevailing rock being some form of lava. Necks of this kind are much less frequent in the puy- than in the plateau-type. But examples may be found in several districts. The most striking with which I am acquainted are those which form so picturesque a group of isolated cones around the volcanic basin of Limerick, to be afterwards described (Figs. 195, 196). The vents there have been filled by the uprise of much more acid rocks than the lavas of the basin, for, as I have already stated, they include even quartziferous trachytes. In the basin of the Firth of Forth some prominent bosses of basalt probably mark the sites of former vents, such as Dunearn Hill in Fife, the Castle Rock of Edinburgh, and Galabraes Hill near Bathgate. Some striking vents which occur in the Jedburgh district, in the debateable land between the plateau series on the east and the puy-series on the west, show the nearly complete usurpation of the funnel by basalt, but with portions of the tuff still remaining visible.
Relation of the Necks to the Rocks through which they rise.--A remarkable feature among the Carboniferous and Permian vents of central Scotland is presented by the effect which has been produced on the strata immediately surrounding them. In the interior of the country this effect is often
[Illustration: Fig. 145.--View of part of the cliffs of vertical agglomerate, Binn of Burntisland.] concealed by herbage, but where the rocks have been laid bare by the sea it may be most instructively studied. In such shore-sections, a singular change of dip is often observable among the strata round the edge of a vent. No matter what may be the normal inclination at the locality, the beds are bent sharply down towards the wall of the neck, and are frequently placed on end. This structure (shown in Figs. 24, 143, 147, 148 and 149) is precisely the reverse of what might have been anticipated, and can hardly be due to upward volcanic explosions. It is frequently associated with considerable metamorphism in the disturbed strata. Shales are converted into porcellanite or various jaspery rocks, according to their composition. Sandstones pass into quartzite, with its characteristic lustrous fracture. It is common to find vents surrounded with a ring of this contact-metamorphism, which, from the hardness and frequently vertical or highly inclined bedding of its strata, stands up prominently on the beach (as in Figs. 126 and 210), and serves to mark the position of the necks from a distance.
I have not been able to find an altogether satisfactory explanation of this inward dip of the strata around vents. Taking it in connection with the metamorphism, I am inclined to believe that it arose after the close of the long-continued volcanic action which had hardened the rocks around the volcanic pipe, and as the result of some kind of subsidence within the vent. The outpouring of so much tuff and lava as escaped from many of the volcanoes would doubtless often be apt to produce cavities underneath them, and on the decay of volcanic energy there might be a tendency in the solid or cavernous column filling up the funnel, to settle down by mere gravitation. So firmly, however, did much of it cohere to the sides of the pipe, that if it sank at all, it could hardly fail to drag down a portion of these sides. So general is this evidence of downward movement in all the volcanic districts of Scotland where the necks have been adequately exposed, that the structure may be regarded as normal to these volcanic vents. It has been observed among the shore-sections of the volcanoes of the Auckland district, New Zealand. Mr. C. Heaphy, in an interesting paper upon that district, gives a drawing of a crater and lava-stream abutting on the edge of a cliff where the strata bend down towards the point of eruption, as in the numerous cases in Scotland.[456]
[Footnote 456: Quart. Journ. Geol. Soc. 1860, vol. xvi. p. 245.]
Evidence for the probable subærial Character of some of the Cones or Puys of Tuff.--From the stratigraphical data furnished by the basin of the Firth of Forth, it is certain that this region, during a great part of the Carboniferous period, existed as a wide shallow lagoon, sometimes overspread with sea-water deep enough to allow of the growth of corals, crinoids, and brachiopods; at other times, shoaled to such an extent with sand and mud as to be covered with wide jungles of a lepidodendroid and calamitoid vegetation. As volcanic action went on interruptedly during a vast section of that period, the vents, though generally submarine, may occasionally have been subærial. Indeed, we may suppose that the same vent might begin as a subaqueous orifice and continue to eject volcanic materials, until, as these rose above the level of the water, the vent became subærial. An instance of a submarine vent has been cited from the Perthshire coal-field (p. 426).
Among the evidence which may be collected to show that some Carboniferous volcanoes probably rose as insular cones of tuff above the surrounding waters, the structure of the tuff in many necks may be cited, for it suggests subærial rather than subaqueous stratification. The way in which the stones, large and small, are grouped together in lenticular seams may be paralleled on the slopes of many a modern volcano. Another indication of this mode of origin is supplied by the traces of wood to be met with in some of the tuff-necks. The vents of Fife and Linlithgowshire contain these traces sometimes in great abundance. The specimens are always angular fragments, and are frequently encrusted with calcite.[457] Sometimes they present the glossy fracture and clear ligneous structure shown by sticks of well-made wood charcoal. In a neck at St. Magdalen's, near Linlithgow, the wood fragments occur as numerous black chips. So far as can be ascertained from the slices already prepared for the microscope, the wood is always coniferous. These woody fragments seldom occur in the interstratified tuffs or in the associated strata where Stigmaria, Lepidodendron, etc., are common. They are specially characteristic of the necks and adjacent tuffs. The parent trees may have grown on the volcanic cones, which as dry insular spots would support a different vegetation from the club-mosses and reeds of the surrounding swamps. As the fragments occur in the tuffs which, on the grounds already stated, may be held to have been deposited within the crater, they seem to point to intervals of volcanic quiescence, when the dormant or extinct craters were filled with a terrestrial flora, as Vesuvius was between the years 1500 and 1631, when no eruptions took place. Some of the cones, such as Saline Hill and the Binn of Burntisland, may have risen several hundred feet above the water. Clothed with dark pine woods, they would form a notable feature in the otherwise monotonous scenery of central Scotland during the Carboniferous period.
[Footnote 457: The largest I have observed is a portion of a stem about two feet long and six inches broad, in the (Permian?) neck below St. Monan's church.]
Entombment of the Volcanic Cones and their relation to the bedded Lavas and Tuffs.--From the facts above detailed, it is evident that in most cases the necks represent, as it were, the mere denuded stumps of the volcanoes. As the puys took their rise in areas which, on the whole, were undergoing a movement of subsidence, they were eventually submerged and buried under sedimentary accumulations. Their loose ashes would be apt to be washed down and strewn over the sea-bottom, so that only the lower and inner part of a cone might remain. We can hardly hope to discover any of the actual craters among these volcanic relics. The cones having been submerged and buried under many hundred feet of sediment, their present position at the surface is due to subsequent elevation and prolonged denudation. It is obvious that there must still be many buried cones which the progress of denudation has not yet reached. Some of these have been revealed in the course of mining operations. Valuable seams of coal, ironstone and oil-shale in the Scottish Carboniferous Limestone and Calciferous Sandstone series are extensively worked, and in the underground operations many illustrations of former volcanic action have been met with. The most remarkable instances of the discovery of buried volcanoes have occurred in the Dalry coal-field in the north of Ayrshire. In one pit-shaft about a mile and a half to the south-west of the village of Dalry, a thickness of 115 fathoms of tuff was passed through, and in another pit 90 fathoms of similar tuff were sunk into before the position of the black-band ironstone of that mineral field was reached by driving levels through the tuff into the sedimentary strata outside of it. Only a short distance from these thick piles of tuff, their place is entirely taken up by the ordinary sedimentary strata of the district. The working-plans of the mines show the tuff to occur in irregular patches and strips, between which the ironstone is workable. From these data we perceive that the shafts have in some cases been sunk directly upon the tops of puys of tuff, which were, in one case, nearly 700 feet, and in another instance, 540 high[458] (Fig. 146).
[Footnote 458: Explanation of Sheet 22, Geol. Surv. of Scotland, p. 16.]
It is obvious that from the condition of a completely buried and concealed cone every stage may be expected to occur up to the deeply worn-down neck representing merely the stump of the volcanic column. The subjoined diagram (Fig. 147) may serve to illustrate this process of gradual re-emergence.
When, in the progress of denudation, a volcanic cone began to show itself from under the cover of removed strata, it would still for a time maintain its connection with the sheets of tuff or of lava which, when active, it had erupted. A number of examples of this structure may be observed in the basin of the Firth of Forth, where the degradation of the surface has not yet proceeded so far as to isolate the column of agglomerate or tuff from the sheets of tuff that were strewn around the old volcano. In such cases, the actual limits of the vent are still more or less concealed, or at least no sharp line can be drawn between the vent and its ejections. As an illustration of this connection of a volcanic pipe with the materials ejected from it over the surrounding country I would cite Saline Hill in the west of Fife. That eminence rises to a height of 1178 feet above the sea, out of a band of tuff which can be traced across the country for fully three miles. Numerous sections in the water-courses show that this tuff is regularly interbedded in the Carboniferous Limestone series, so that the relative geological date of its eruption can be precisely fixed. On the south of Saline Hill, coal and ironstone, worked under the tuff, prove that this portion of the mass belongs to the general sheet of loose ashes and dust, extending outwards from the original cone over the floor of the sheet of water in which the Carboniferous Limestone series of strata was being deposited. But the central portion of the hill is occupied by one or more volcanic pipes. A section across the eminence from north-west to south-east would probably show the structure represented in Fig. 148. Immediately to the east of the Saline Hill lies another eminence, known as the Knock Hill, which marks the site of another eruptive vent. A coal-seam (the Little Parrot or Gas Coal) is worked along its southern base, and is found to plunge down steeply towards the volcanic rocks. This seam, however, is not the same as that worked under the Saline Hill, but lies some 600 feet below it. Probably the whole of the Knock Hill occupies the place of a former vent.
A further stage of decay and denudation brings before us the entire severance of the volcanic column from the materials that were ejected from it. An excellent example of this isolation of the neck in the midst of surrounding masses of tuff and lava which proceeded from it is presented by the Binn of Burntisland, to which I have already alluded. A section across that eminence gives the geological structure represented in Fig. 149. The dip of the rocks away from the volcanic pipe at this locality has been produced long after the volcanic phenomena had ceased. The arch here shown is really the prolongation and final disappearance of the great anticlinal fold of which the Pentland Hills form the axis on the opposite side of the Firth. But if we restore the rocks to a horizontal, or approximately horizontal position, we find the Binn of Burntisland rising among them in one or more necks, which doubtless mark centres of volcanic activity in that district. A series of smaller neck-like eminences runs for two miles westward.
Striking as the forms of many of the necks are, and much as their present conical forms resemble those of active and extinct volcanoes, the evidence of extensive denudation proves that these contours are not the original outlines of the Carboniferous vents, but are in every case the result of prolonged waste. What we now see is a section of the volcanic chimney, and the conical form is due to the way in which the materials filling the chimney have yielded to the forces of denudation.
ii. BEDDED TUFFS AND LAVAS
During at least the earlier part of the period of the puys, in some districts or from certain vents, such as those of East Fife, Western Midlothian, Eastern Linlithgowshire, Northern Ayrshire, Heads of Ayr and Lower Eskdale, only fine tuff seems to have been thrown out, which we now find intercalated among the surrounding strata. These eruptions, neither so vigorous nor so long-continued as those of the plateaux, never gave forth such thick and widespread sheets of fragmentary materials as those associated with the plateaux in East Lothian and the north-east of Ayrshire. A single discharge of ashes seems in many cases to have been the sole achievement of one of those little volcanoes; at least only one thin band of tuff may be discoverable to mark its activity.
The tuff of these solitary bands is seldom coarse in texture. It usually consists of the ordinary dull green paste, with dust and lapilli of basic pumice. The local variations in the tuffs of the puys generally arise mainly from differences in the composition, size and numbers of the included ejected blocks. Generally the most abundant stones are pieces of different diabases, or basalts; then come fragments from the surrounding Carboniferous strata, from older tuffs and rarely from rocks of much deeper-seated origin.
Now and then the eruptions of tuff have consisted of extremely fine volcanic dust, which, mingling with water, took the form of a compact mudstone, as in the case of the Houston Marls (p. 423), which remind one of a volcanic mud. But in most localities the discharge of tuff, though for a time it may have completely obscured the ordinary contemporaneous sedimentation, was intermittent, so that in the intervals between successive showers of detritus, the deposition of non-volcanic sediment went on as usual. Hence it is that bands of tuff, whether they lie among lavas or among sedimentary formations, are apt to contain interstratifications of sandstone, shale, limestone or other detrital deposit, and to pass insensibly into these. The extremely gentle gradation from volcanic into non-volcanic sediment, and the occasional reappearance of thin partings of tuff bring vividly before the mind the slow dying out of volcanic energy among the Carboniferous lagoons.
The comparatively quiet character of the volcanic explosions, and the contemporaneous undisturbed deposition of sediment during the earlier part of the puy period, are exemplified in many sections throughout the areas above enumerated, as will be more fully illustrated in subsequent pages. Two typical examples may suffice for this general statement of the characters of the discharges of tuff in the puy-eruptions. In the Linlithgowshire quarry represented in Fig. 150, where about ten feet of strata have been exposed, a black shale (1) of the usual carbonaceous character, so common in the Oil-shale series of this region, may be seen at the bottom of the section. It is covered by a bed of nodular bluish-grey tuff (2) containing black shale fragments. A second black shale (3) is succeeded by a second thin band of fine pale yellowish tuff (4). Black shale (5) again supervenes, containing rounded fragments of tuff, perhaps ejected lapilli, and passing up into a layer of tuff (6). It is evident that we have here a continuous deposit of black shale which was three times interrupted by showers of volcanic dust and stones. At the close of the third interruption, the deposition of the shale was renewed and continued, with sufficient slowness to permit of the segregation of thin seams and nodules of clay ironstone round the decomposing organic remains of the muddy bottom (7). A fourth volcanic interlude now took place, and the floor of the water was once more covered with tuff (8). But the old conditions of deposit were immediately afterwards resumed (9); the muddy bottom was abundantly peopled with ostracod crustaceans, while many fishes, whose coprolites have been left in the mud, haunted the locality. At last, however, a much more serious volcanic explosion took place. A coarse agglomeratic tuff (10), with blocks sometimes nearly three feet in diameter, was then thrown out, and overspread the lagoon.[459]
[Footnote 459: See Geol. Surv. Memoir of Edinburgh, p. 45. These tuffs are further described on pp. 465 et seq.]
The second illustration may be taken from the admirable coast-section between Burntisland and Kinghorn, where the number of intercalations of tuff is very great. Besides thicker well-marked bands, successive innumerable thin layers occur there among the associated zones of sedimentary strata which separate the sheets of basalt. The character of these tuff-seams may be inferred from the following details of less than two feet of rock at Pettycur Point:--
Tuff 1·5 inch. Limestone 0·2 " Tuff 0·5 " Shale 0·2 " Tuff 0·1 " Shale and tuff 1·0 " Shale 0·2 " Limestone 0·5 " Shale full of volcanic dust 3·5 " Shaly limestone 1·5 " Laminated tufaceous limestone 2·0 " Limestone in thin bands, with thin laminæ of tuff 0·8 " Granular tuff 0·6 " Argillaceous limestone, with diffused tuff 0·9 " Fine granular tuff 0·7 " Argillaceous limestone, with diffused tuff 1·5 " Laminated limestone 0·1 " Limestone, with parting of granular tuff in middle 0·9 " Tufaceous shale 2·0 " Limestone 0·4 " Shaly tuff 1·25 " Laminated limestone 0·1 " Tuff 1·2 " ----- 21·65 inches.
Such a section as this brings vividly before the mind a long-continued intermittent feeble volcanic action during pauses between successive outbursts of lava. In such intervals of quiescence, the ordinary sediment of the lagoons accumulated, and was mixed up with the debris supplied by occasional showers of volcanic dust. In this Fife volcanic series, thin layers of sandstone, streaked with remains of the Carboniferous vegetation; beds of shale full of cyprid-cases, ganoid scales, and fragmentary ferns; thin beds of limestone, and bands of fire-clay supporting seams of coal, are interleaved with strata of tuff and sheets of basalt. Now and then a sharp discharge of larger stones is seen to have taken place, as in the case of the block many years ago described by me as having fallen and crushed down a still soft bed of coal (Fig. 151).[460]
[Footnote 460: Geol. Mag. vol. i. p. 22. This Fife coast-section is given in full at p. 470.]
The Fife coast-section from which these details are taken supplies almost endless instances of the varying characters of the pyroclastic materials of the puy-eruptions. The very same cliff, bank or reef will show at one point an accumulation of excessively coarse volcanic debris and at another thin laminæ of the finest dust and lapilli. These rapid gradations are illustrated in Fig. 152, which is taken from the east end of the Kingswood Craig. The lower part of the declivity is a coarse agglomerate which passes upward into finer tuff.
Besides the thin partings and thicker layers of tuff which, intercalated among the sedimentary strata of the Carboniferous system, mark a comparatively feeble and intermittent volcanic activity, we meet in some localities with examples where the puys have piled up much thicker accumulations of fragmentary material without any intercalated streams of lava, or interstratified sandstone, shale or limestone. Thus the widespread Houston marls above described reach a thickness of some 200 feet. The vents of the Saline Hills in Fife covered the sea-floor with volcanic ashes to a depth of several hundred feet. In the north of Ayrshire the first eruptions of the puys have formed a continuous band of fine tuff traceable for some 15 miles, and in places at least 200 feet thick.
Where volcanic energy reached its highest intensity during the time of the puys, not only tuffs but sheets of lava were emitted, which, gathering round the vents, formed cones or long, connected banks and ridges. Of these there are four conspicuous examples in Scotland--the hills of the Burntisland district, the Bathgate Hills, the ground between Dalry and Galston in north Ayrshire, and a broken tract in Liddesdale. Nowhere in the volcanic history of this country have even the minutest details of that history been more admirably preserved than among the materials erupted from puys in these respective districts.
Lava-cones, answering to solitary tuff-cones among the fragmental eruptions, do not appear to have existed, or, like some of those in the great lava-fields of Northern Iceland and Western America, must have been mere small heaps of slag and cinders at the top of the lava-column, which were washed down and effaced during the subsidence and entombment of the volcanic materials. The lavas never occur without traces of fragmentary discharges. Two successive streams of basalt may indeed be found at a given locality without any visible intercalation of tuff, but proofs of the eruption of fragmental material will generally be observed to occur somewhere in the neighbourhood, associated with one or both of them, or with other lavas above or below them.
Where the phenomena of the puys have been most typically developed, lavas and tuffs succeed each other in rapid succession, with numerous or occasional interstratifications of ordinary sediment. Perhaps the most complete and interesting example of this association is to be found on the coast between Burntisland and Kirkcaldy, where, out of a total thickness of rock which may be computed to be between 1500 and 2000 feet, it will probably be a fair estimate to say that the igneous materials constitute four-fifths, or from
[Illustration: Fig. 153.--Alternations of basalt and tuff with shale, etc., Kingswood Craig, Burntisland.] 1200 to 1600 feet. The lavas are varieties of basalt ranging in character from a black compact columnar to a dirty green earthy cellular or slaggy rock. Each separate flow may be on the average about 20 or 30 feet in thickness. Columnar and amorphous sheets succeed each other without any interposition of fragmentary material (Fig. 171). But along the junctions of the separate flows layers of red clay, like the bole between the basalts of the Giant's Causeway, may frequently be noticed. The characteristic slaggy aspect of the upper parts of these ancient coulées is sometimes remarkably striking. The full details of this most interesting section will be given in later pages (p. 470). But some of its more characteristic external features may be understood from the views which are presented in Figs. 152, 153, 170, 171.
The general bedded character of the volcanic series is well shown in Fig. 153, which represents the alternations of lavas and tuffs in the Kingswood Craig two miles to the east of Burntisland. The harder basalts will be seen to project as bold crags while the tuffs and other stratified deposits between them give rise to grassy slopes and hollows. A nearer view of the alternation of lavas and tuffs with non-volcanic sedimentary deposits is supplied in Fig. 170, which is taken from a part of the Fife coast a little further to the east than the last illustration. Here one of the limestones of the Carboniferous Limestone series is overlain with shale and tuff, which, being easily disintegrated, have been cut away by the waves, leaving the lava above to overhang and fall off in blocks. The columnar structure of some of the basalts of this coast is well brought out in Fig. 171, which shows further how the columns sometimes merge into an amorphous part of the same sheet.
These Fife basalts illustrate admirably the peculiarities of the sheets of lava which are intercalated among the Carboniferous strata. They show how easy it generally is to discriminate between such sheets and intrusive sills. The true lavas are never so largely crystalline, nor spread out in such thick sheets as the sills; they are frequently slaggy and amygdaloidal, especially towards the top and bottom, the central portion being generally more fine-grained and sometimes porphyritic. Where most highly cellular they often decompose into a dull, earthy, dirty-green rock. Where they form a thick mass they are usually composed of different beds of varying texture. Except the differences between the more compact centre and the slaggy layer above and below, the bedded lavas do not present any marked variation in composition or structure within the same sheet. A striking exception to this rule, however, is furnished by the Bathgate "leckstone" already described.[461] This mass forms a continuation of the great basaltic ridge of the Bathgate Hills, and though its exact relations to the surrounding strata are concealed, it appears to be an interbedded and not an intrusive sheet. The remarkable separation of its constituent minerals into an upper, lighter felspathic layer, and a lower, heavier layer, rich in olivine, augite and iron-ores, is a structure which might be more naturally expected to occur in a sill. An instance of its development in an undoubted sill will be described further on. Nevertheless, if we follow the trend of the volcanic band of the Bathgate Hills southward for only two miles beyond the picrite quarry, we find in the Skolie Burn a rock in many respects similar, and quarried for the same purpose of building oven-soles. This "leckstone" is there seen to be surmounted by a group of calcareous shales and thin limestones. The section laid bare in the stream is represented in Fig. 154. Immediately above the diabase, which is highly cellular, lies a green felspathic sandstone or shale containing detached fragments of the amygdaloid together with Lingulæ and other shells. There seems no reason to doubt that this is a true interstratified lava.[462]
[Footnote 461: Trans. Roy. Soc. Edin. xxix. (1879) p. 504.]
[Footnote 462: Trans. Roy. Soc. Edin. xxix. (1879), pp. 505-507.]
Where the puys attained their greatest development in Scotland, they rose in the shallow lagoons, and here and there from deeper parts of the sea-bottom, until by their successive discharges of lavas and tuffs they gradually built up piles of material, which, in the Linlithgow and Bathgate district, may have been nearly 2000 feet in thickness. It must be remembered, however, that the eruptions took place in a subsiding area, and that even the thickest volcanic ejections, if the downward movement kept pace with the volcanic activity, need not have grown into a lofty volcanic hill. Indeed, largely as the lavas and tuffs bulk in the geology of some parts of Central Scotland, their eruption does not seem to have seriously interfered with the broader physical changes that were in progress over the whole region. Thus the subsidence which led to the spread of a marine and limestone-making fauna over much of Central Scotland included also the volcanic districts. The limestones, formed of crinoids, corals and other marine organisms, extended over the submerged lavas and tuffs, and were even interstratified with them.
While the volcanic materials are found to replace locally the ordinary Carboniferous sedimentary strata, it is interesting in this regard to note that, during pauses in the volcanic activity, while the subsidence doubtless was still going on, some groups of sandstones, shales or limestones extended themselves across the volcanic ridges so as to interpose, on more than one platform, a mass of ordinary sediment between the lavas or tuffs already erupted and those of succeeding discharges, and thus to furnish valuable geological chronometers by which to define the stratigraphical horizons of the successive phases of volcanic energy.
The volcanic banks or ridges not improbably emerged as islets out of the water, and were sometimes ten miles or more in length. Their materials were supplied from many separate vents along their surface, but probably never attained to anything approaching the elevation which they would have reached had they been poured out upon a stable platform. This feature in the history of the volcanic ridges is admirably shown by the fact just referred to, that recognizable stratigraphical horizons can sometimes be traced right through the heart of the thickest volcanic accumulations. One of the largest areas of basalts and tuffs connected with the puys is that of the Linlithgow and Bathgate Hills, where, as already remarked, a depth of some 2000 feet of igneous rocks has been piled up. Yet several well-known seams of stone can be traced through it, such as the Hurlet Limestone and the Index Limestone (Fig. 155). Only at the north end, where the volcanic mass is thickest and the surface-exposures of rock are not continuous, has it been impossible to subdivide the mass by mapping intercalations of sedimentary strata across it. It would thus seem that, even where the amplest accumulations gathered round the puys, they formed low flat domes, rather than prominent hills, which, as subsidence went on and the tuff-cones were washed down, gradually sank under water, and were buried under the accumulating silt of the sea-floor.
As a detailed illustration of the manner in which the growth of organically-formed limestones and the deposit of ordinary sediment took place concurrently with the occasional outflow of lava-streams over the sea-bottom, I may cite the section presented in another Linlithgowshire quarry (Fig. 156). At the bottom of the group of strata there exposed, a pale amygdaloidal, somewhat altered basalt (A) marks the upper surface of one of the submarine lavas of the period. Directly over it comes a bed of limestone (B) 15 feet thick, the lower layers of which are made up of a dense growth of the thin-stemmed coral Lithostrotion irregulare. The next stratum is a band of dark shale (C) about two feet thick, followed by about the same thickness of an impure limestone with shale seams (D). The conditions for coral and crinoid growth were evidently not favourable, for this argillaceous limestone was eventually arrested first by the deposit of a dark mud, now to be seen in the form of three or four inches of a black pyritous shale (E), and next by the inroad of a large quantity of dark sandy mud and drift vegetation, which has been preserved as a sandy shale (F), containing Calamites, Producti, ganoid scales and other traces of the life of the time. Finally, a great sheet of lava, represented by the uppermost amygdaloid (G), overspread the area, and sealed up these records of Palæozoic history.[463]
[Footnote 463: Geol. Surv. Mem. "Geology of Edinburgh," p. 58.]
Among the phenomena associated with the Carboniferous volcanoes mention may, in conclusion, be made of the evidence for the former existence of thermal springs and saline sublimations or incrustations. Among the plateau-tuffs of North Berwick, as has been already pointed out (p. 390), a fœtid limestone has been quarried, which bears indications of having been deposited by springs, probably in connection with the volcanic action of the district. The lower limestones of Bathgate furnish abundant laminæ of silica interleaved with calcareous matter, the whole probably due to the action of siliceous and calcareous springs connected with the active puys of that district. Some portions of the limestone are full of cellular spaces, lined with chalcedony.[464] A saline water has been met with among the volcanic rocks to the west of Linlithgow, in a bore which was sunk to a depth of 348 feet in these rocks without reaching their bottom. The water that rose from the bore-hole was found to contain as much as 135 grains of chloride of sodium in the gallon. It is not improbable that this salt was originally produced by incrustations on the Carboniferous lavas immediately after their eruption, as has happened so often in recent times at Vesuvius, and that it was then buried under succeeding showers of tuff and streams of lava.[465]
[Footnote 464: Ibid. p. 49, et seq.]
[Footnote 465: Proc. Roy. Soc. Edin. vol. ix. p. 367. Besides chloride of sodium the water contained also chlorides of calcium, magnesium and potassium, carbonates of lime and magnesia, sulphate of lime, and other ingredients in minute proportions.]
Subsequent Dislocation of Bedded Lavas and Tuffs.--As the interstratified volcanic materials were laid down in sheets at the surface, they necessarily behave like the ordinary sedimentary strata, and have undergone with them the various curvatures and fractures which have occurred since Carboniferous times. Notwithstanding their volcanic nature, they can be traced and mapped precisely as if they had been limestones or sandstones. This perfect conformability with the associated stratified rocks is strikingly seen in the case of the sheets of lava which lie imbedded in the heart of the great volcanic ridge of Linlithgowshire. The overlying strata having been removed from their surface for some distance, and the ground having been broken by faults, these volcanic rocks might at first be taken for irregular intrusive bosses, but their true character is that shown in Fig. 157, where by a succession of faults, with a throw in the same direction, the upper basalts of Bonnytoun Hill are gradually brought down to the level of the Firth of Forth.
iii. SILLS, BOSSES AND DYKES
One of the characteristic features of Central Scotland is the great number, and often the large size and extraordinary persistence, of the masses of eruptive, more or less basic material, which have been injected among the Carboniferous strata. The precise geological age of these intrusions cannot, of course, be more exactly defined than by stating that they are younger than the rocks which they traverse, though in many cases their association with the necks, lavas and tuffs is such as to show that they must be regarded as part of the Carboniferous volcanic phenomena.
Sills.--With regard to the sills I have been led, for the following reasons, to connect the great majority of them with the puys, though some are certainly of far later date, while others should possibly be assigned to the plateaux.
In the first place, the sills obviously connected with the plateaux are in great measure intermediate, or even somewhat acid rocks, while those of the puy series are much more basic. It is hardly possible, however, in all cases to decide to which series a particular sill should be assigned. This difficulty is particularly manifest in the western part of Midlothian, where the plateau of that district exhibits such frequent interruption, and where it often consists only of a single basaltic sheet. To the west of it lie the abundant puys with their lavas and tuffs, and between the two volcanic areas numerous sills of dolerite and diabase make their appearance. In the difficulty of deciding to which series these sills should be referred, it will be convenient to consider them with those of the puys.
A remarkable illustration of the contrast in petrographical character between the typical sills of the plateaux and those of the puys is furnished by the chain of the Campsie Fells, where, on the north side, among the Calciferous Sandstones which emerge from under the andesitic lavas of the Clyde plateau, many intrusive sheets and bosses of trachytic material may be seen, while on the southern side come the great basic sills which, from Milngavie by Kilsyth to Stirling, run in the Carboniferous Limestone series (Fig. 158). A similar contrast may be observed in Renfrewshire between the trachytic sills below the plateau-lavas south of Greenock and the basic sills above these lavas in the Carboniferous Limestone series around Johnstone and Paisley.
In the second place, the more basic sills, as a rule, appear on platforms higher in stratigraphical position than the plateaux, and wherever this is their position there cannot be any hesitation in deciding against their association with the older phase of volcanic activity.
In the third place, the basic sills often occur in obvious connection with the vents or bedded lavas and tuffs of the puy series. A conspicuous example of this dependence is supplied by the intrusive sheets of Burntisland, underlying the basalts and tuffs of that district in the immediate neighbourhood of some of the vents from which these bedded rocks were erupted (Fig. 159).
In the fourth place, even where no visible vents appear now at the surface near the sills, the latter generally occupy horizons within the stratigraphical range indicated by the interbedded volcanic rocks. It must be remembered that all the Carboniferous vents were deeply buried under sedimentary deposits, and that large as is the number of them which has been exposed by denudation, it is probably much smaller than the number still concealed from our view. The sills are to be regarded as deep-seated parts of the volcanic protrusions, and they more especially appear at the surface where the strata between which they were injected crop out from under some of the higher members of the Carboniferous system. Thus the remarkable group of sills between Kilsyth and Stirling (Fig. 158) may quite possibly be connected with a group of vents lying not far to the eastward, but now buried under the higher parts of the Carboniferous Limestone, Millstone Grit and Coal-measures. Again, the great series of sills that gives rise to such a conspicuous range of hills in the north and middle of Fife may have depended for its origin upon the efforts of a line of vents running east and west through the centre of the county, but now buried under the Coal-measures. Some vents, indeed, have been laid bare in that district, such as the conspicuous groups of the Saline Hills and the Hill of Beath, but many more may be concealed under higher Carboniferous strata further east.
In the fifth place, the materials of which the sills consist link them in petrographical character with those that proceeded from the puys. The rocks of the intrusive sheets in West Lothian, Midlothian and Fife are very much what an examination of the bedded lavas of the puys in the same region would lead us to expect. There is, of course, the marked textural difference between masses of molten rock which have cooled very slowly within the crust of the earth and those which have solidified with rapidity at the surface, the sills being for the most part much more coarsely crystalline than the lavas, and more uniform in texture throughout, though generally finer at the margins than at the centre. There is likewise the further contrast arising from differences in the composition of the volcanic magma at widely-separated periods of its extravasation. At the time when the streams of basalt flowed out from the puys its constitution was comparatively basic, in some localities even extremely basic. Any sills dating from that time may be expected to show an equal proportion of bases. But those which were injected at a long subsequent stage in the volcanic period may well have been considerably more acid.
In actual fact the petrographical range of the sills reasonably referable to the puy-eruptions varies from picrite or limburgite to dolerite without olivine. The great majority of these sheets in the basin of the Firth of Forth, where they are chiefly displayed, are dolerites (diabases), sometimes with, but more frequently without, olivine. They include all the more coarsely crystalline rocks of the region, though occasionally they are ordinary close-grained basalts. Their texture may be observed to bear some relation to their mass, so far at least as that, where they occur in beds only two or three feet or yards in thickness, they are almost invariably closer-grained. A cellular or amygdaloidal texture is seldom to be observed among them, and never where they are largely crystalline. This texture is most often to be found in thin sills which have been injected among carbonaceous shales or coals. These intrusive sheets are generally finely cellular, and more or less decayed ("white trap").
Differences of texture may often be observed within short distances in the same sill, and likewise considerable varieties in colour and composition. The most finely crystalline portions are, as usual, those along the junction with the stratified rocks, the most crystalline occurring in the central parts of the mass. A diminution in the size of the crystalline constituents may be traced not only at the base, but also at the top of a sheet, or at any intermediate portion which has come in contact with a large mass of the surrounding rock. A good illustration is supplied by the intrusive sheet at Hound Point (Fig. 160), to the east of South Queensferry, where some layers of shale have been involved in the igneous rock, which becomes remarkably close-grained along the junction.[466] This change in texture and absence of cellular structure form a well-marked distinction between these sheets and those which have flowed out at the surface as true lava-streams.
[Footnote 466: See Hay Cunningham's "Essay," p. 66, and plate ix.; and Geol. Survey Memoir on "Geology of Edinburgh," p. 114.]
Some of the larger doleritic sills display a somewhat coarsely crystalline texture in their central portions, and occasionally present a notable micropegmatitic aggregate, which plays the part of interstitial substance enclosing the other minerals. Mr. Teall has referred to the frequent occurrence of this structure in the coarser parts of the Whin Sill of the north of England.[467] It occurs also in a marked degree in the Ratho sill and in some portions of the great doleritic sill of which the crags of Stirling form a part.[468]
[Footnote 467: British Petrography, p. 208.]
[Footnote 468: Mr. H. W. Monckton. Quart. Journal Geol. Soc. vol. li. (1895), p. 482.]
But beside the differences in texture, mainly due to varying rates of cooling, the sills sometimes exhibit striking varieties of composition in the same mass of rock. These variations are more especially noticeable among the larger sills, and particularly where the material is most markedly basic. The special type of differentiation, so noticeable in the Bathgate diabase and picrite mass already alluded to, is likewise well exhibited in an intrusive sheet or group of sheets, recently exposed at Barnton, in the cutting of a railway from Edinburgh to Cramond[469] (Fig. 161). The intrusive nature of the several bands of igneous rock which occur here is made quite evident by the alteration they have produced upon the shales with which they have come in contact. It is the uppermost and most extensive of these sills which specially deserves notice, for the differentiation of its constituents. It stretches along the cutting for several hundred yards at an angle of dip of about 15°. At the western or upper part of the mass its actual contact with the superincumbent sedimentary strata is not visible, but as the igneous rock is there a good deal finer in grain than elsewhere, its upper surface cannot be many feet distant. The upper visible portion is a light well-crystallized dolerite with a rudely bedded structure, the planes dipping westwards at 15°. About 20 or 30 feet below the upper visible termination of the mass, the dark ferro-magnesian minerals begin rapidly to increase in relative proportion to the pale felspar, and the rock consequently becomes dark-greenish brown. The change is particularly noticeable in certain bands which run parallel with the general dip. There is no definite line between the pale and dark body of the rock, the two graduating into each other and the darker part becoming deeper in colour, heavier and more decomposing, until it becomes a true typical picrite. Even in this ultra-basic portion the same rude bedding or banding may be observed.
[Footnote 469: This rock has been described by Mr. J. Henderson and Mr. Goodchild, Trans. Geol. Soc. Edin. vi. (1893) pp. 297, 301, and by Mr. H. W. Monckton, Quart. Journ. Geol. Soc. l. (1894) p. 39. Mr. Goodchild recognized the occurrence of picrite, and Mr. Monckton has described the succession of rocks, and given a diagram of them.]
Veins in which felspar predominates over the darker minerals traverse the rock, sometimes parallel with the bedding, sometimes across it. They vary from less than an inch to a foot in width, sometimes dividing and enclosing parts of the surrounding mass. But that they are on the whole contemporaneous with the sill itself, and not long subsequent injections, is shown by the way in which the dark ferro-magnesian minerals project from the picrite into the veins and lock the two together.
But besides these injections, which doubtless represent the last and more acid portions of the magma injected into the basic parts before the final consolidation of the whole, there are to be observed irregular concretionary patches, of similar character to the veins, distributed through the picrite. On the other hand, towards its base the sill becomes a coarse dolerite round which the picrite is wrapped, and which encloses a detached portion of that rock.
It is deserving of note that while the ultra-basic portion descends almost to the very bottom of the sill, the lowest five feet show the same change as occurs at the top of the mass. There the felspar rapidly begins to predominate over the darker minerals, and the dolerite into which the rock passes shows a fine-grained margin adhering firmly to the shales on which it rests. This lower doleritic band, showing as it does the effect of chilling upon its under surface, may be due to more rapid cooling and crystallization, while in the overlying parts the mass remained sufficiently mobile to allow of a separation of the heavier minerals from the felspars, which appear in predominant quantity towards the top. It must be frankly admitted, however, that we are still very ignorant of the causes which led to this separation of ingredients in a few sills, while they were entirely absent or non-efficient in most of them.
The intrusive character of the Carboniferous sills of Central Scotland and their contact-metamorphism have been fully described, and some of them have become, as it were, "household words" in geology.[470] Exposed in so many fine natural sections in the vicinity of Edinburgh, they early attracted the notice of geologists, and furnished a battle-ground on which many a conflict took place between the Plutonist and Neptunist champions at the beginning of the present century.
[Footnote 470: See, for instance, Maclaren's Geology of Fife and the Lothians, 1839; Hay Cunningham's Essay, previously cited; Geological Survey Memoir on the Geology of Edinburgh (Sheet 32), 1861; Mr. Allport, Quart. Journ. Geol. Soc. vol. xxx. (1874) p. 553; Teall, British Petrography, p. 187; E. Stecher, Contacterscheinungen an schottischen Olivindiabasen, Tschermak's Mineralog. Mittheil. vol. ix. (1887) p. 145; Proc. Roy. Soc. Edin. vol. xv. (1888) p. 160.]
As the sills frequently lie in even sheets perfectly parallel with the bedding of the strata between which they have been injected, care is required in some cases to establish that they are of intrusive origin. One of the most obvious tests for this purpose is furnished by the alteration they produce among the strata through which they have made their way, whether these lie above or below them. The strata are sometimes crumpled up in such a manner as to indicate considerable pressure. They are occasionally broken into fragments, though this may have been due rather to the effects of gaseous explosions than to the actual protrusion of melted rock. But the most frequent change superinduced upon them is an induration which varies greatly in amount even along the edge of the same intrusive sheet. Sandstones are hardened into quartzite, breaking with a smooth clear glistening fracture. Coals are converted into a soft sooty substance, sometimes into anthracite. Limestones acquire a crystalline saccharoid structure. Shales pass generally into a kind of porcellanite, but occasionally exhibit other types of contact-metamorphism. Thus below the thick picrite sill at Barnton, near Edinburgh, the shales have assumed a finely concretionary structure by the appearance in them of spherical pea-like aggregates.
Another proof of intrusion is to be found in the manner in which sills catch up and completely enclose portions of the overlying strata. The well-known examples on Salisbury Crags (Fig. 162) are paralleled by scores of other instances in different parts of the same region.
Moreover, sills do not always remain on the same horizon; that is, between the same strata. They may be observed to steal across or break through the beds, so as to lie successively between different layers. No more instructive example of this relation on a small scale could be cited than that of the intrusive sheet which has been laid open in the Dodhead Limestone Quarry, near Burntisland. As shown in the accompanying figure (Fig. 163), this rock breaks through the limestone and then spreads out among the overlying shales, across which it passes obliquely.
Among the larger sills this transgressive character is seen to be sometimes manifested on a great scale. Thus, along the important belt of intrusive rocks that runs from Kilsyth to Stirling, the Hurlet Limestone lies in one place below, in another above, the invading mass, but in the intervening ground has been engulphed in it. Similar evidence of the widely separate horizons occupied by different parts of the same sill is supplied at Kilsyth, where the intrusive sheet lies about 70 or 80 fathoms below the Index Limestone, while at Croy, in the same neighbourhood, it actually passes above that seam.[471]
[Footnote 471: Explanation of Sheet 31, _Geological Survey of Scotland_, §§ 43 and 83.]
Other interesting evidence of the intrusive nature of the Carboniferous dolerite sills of Central Scotland is supplied by the internal modifications which the eruptive rock has undergone by contact with the strata between which it has been thrust. These alterations, though partly visible to the naked eye, are best studied in thin slices with the aid of the microscope. Tracing the variations of an intrusive dolerite outwards in the direction of the rocks which it has invaded, we perceive change first in the augite. The large crystals and kernels of that mineral grow smaller until they pass into a granulated form like that characteristic of basalts. The large plates and amorphous patches of titaniferous iron or magnetite give place to minute particles, which tend to group themselves into long club-shaped bodies. The labradorite continues but little affected, except that its prisms, though as defined, may not be quite so large. The interstitial glassy groundmass remains in much the same condition and relative amount as in the centre of the rock.
Along the line of contact, while the dolerite becomes exceedingly close-grained, its felspar crystals are still quite distinct even up to the very edge. But they become fewer in relative number, and still smaller in size, though an occasional prism two or three millimetres in length may occur. They retain also their sharpness of outline, and their comparative freedom from enclosures of any kind. They tend to range themselves parallel with the surface of the contact-rock. The augite exists as a finely granular pale green substance, which might at first be taken for a glass, but it gives the characteristic action of augite with polarized light. It is intimately mixed through the clear glass of the groundmass, which it far exceeds in quantity. The iron oxides now appear as a fine granular dust, which is frequently aggregated into elongated club-shaped objects, as if round some inner pellucid or translucent microlite. In patches throughout the field, however, the oxides take the form of a geometrically perfect network of interlacing rods. This beautiful structure, described and figured by Zirkel and others,[472] is never to be seen in any of the dolerites, except close to the line of contact with the surrounding rocks. It occurs also in some of the dykes. I have not succeeded in detecting any microlites in the sandstones at the edge of a dolerite sheet, though I have had many slices prepared for the purpose.
[Footnote 472: _Mikroskopische Beschaffenheit der Mineralien und Gesteine_, p. 273; Vogelsang's Krystalliten.]
Where one of the dolerite sills has invaded sandstone, there is usually a tolerably sharp line of demarcation between the two rocks, though it is seldom easy to procure a hand-specimen showing the actual contact, for the stone is apt to break along the junction-line. Where, however, the rock traversed by the igneous mass is argillaceous shale, we may find a thorough welding of the two substances into each other. In such cases the dolerite at the actual contact becomes a dark opaque rock, which in thin slices under the microscope is found to be formed of a mottled or curdled segregation of exceedingly minute black grains and hairs in a clear glassy matrix, in which the augite and felspar are not individualized. But even in this tachylyte-like rock perfectly formed and very sharply defined crystals of triclinic felspar may be observed ranging themselves as usual parallel to the bounding surfaces of the rock. These characters are well seen in the contact of the intrusive sheet of dolerite with shale and sandstone at Hound Point (Fig. 160).
Another instructive example is furnished by the small threads which proceed from the dolerite of Salisbury Crags, and traverse enclosed fragments of shale (Fig. 162). Some of these miniature dykes are not more than one-eighth of an inch in diameter, and may therefore easily be included, together with part of the surrounding rock, in the field of the microscope. The dolerite in these ramifications assumes an exceedingly fine texture. The felspar is the only mineral distinctly formed into definite crystals. It occurs in prisms of an early consolidation, sometimes one-fifth of an inch long, and therefore readily recognizable by the naked eye. These prisms are perfectly shaped, contain abundant twin lamellæ, and show enclosures of the iron of the base. They had been already completely formed at the time of injection; for occasionally they may be observed projecting beyond the wall of the vein into the adjacent shale or sandstone, and they have ranged themselves parallel to the sides of the vein.[473] The black ground, from which these large well-defined crystals stand out prominently, consists of a devitrified glass, rendered dark by the multitude of its enclosed black opaque microlites. These are very minute grains and rudely feathered rods, with a tendency to group themselves here and there into forms like portions of the rhombohedral skeletons of titaniferous iron. So thoroughly fused and liquid has the dolerite been at the time of its injection, that little threads of it, less than 1/100 of an inch in diameter, consisting of the same dark base, with well-defined felspars, may be seen isolated within the surrounding sedimentary rock. Minute grains and rounded portions of the latter may also be noticed in the marginal parts of the dolerite.
[Footnote 473: The infusibility of the felspar was well shown in some experiments on the rocks of the neighbourhood of Edinburgh, made at my request by Dr. R. S. Marsden, who subjected some of these rocks to fusion at the laboratory of the University of Edinburgh. Microscopic sections were prepared of the products obtained. The basalt of Lion's Haunch is peculiarly instructive. Its large labradorite crystals have resisted the intense white heat which, continued for four hours, has reduced the rest of the minerals to a perfect glass. We can thus well understand how large definite crystals of felspar should have survived or appeared in dykes and veins while the rock was still thoroughly liquid. The glass obtained from the Lion's Haunch rock is of a honey-yellow, and contains translucent tufted microlites. The iron forms beautiful dendritic films in the cracks. Altogether, the glass presents a strong resemblance to the palagonitic substance so abundant among the lapilli in the tuffs of the vents.]
It is thus evident that specimens taken from the edge of an intrusive sheet, where the rock has rapidly chilled and solidified, represent to us an earlier stage in the history of the whole mass than specimens taken from its central portions. In fact, a series of samples collected at short intervals from the outer contact to the inner mass shows, as it were, the successive stages in the consolidation of the molten rock.
From the observations just described, it appears that the triclinic felspars began to assume the shape of large definite crystals before any of the other minerals. These felspars already existed when the molten mass forced its way among the shales, for they can be seen lying with their long axes parallel to the surface of shale, precisely as, in the well-known flow-structures, they behave round a large crystal embedded in the heart of a rock. A few feet from where the consolidation was not so rapid, the iron oxides have grouped themselves into incipient crystalline forms and skeleton crystals; the felspar crystals abundantly occur, and the augite has been left in the finely granular condition. Still further towards the interior of the mass, the normal character of the dolerite is gradually assumed.[474]
[Footnote 474: For a further and more detailed investigation of the contact phenomena of the Carboniferous doleritic sills of the basin of the Firth of Forth, see the papers by Dr. Stecher, quoted on p. 451.]
Where a sill has been injected among carbonaceous shales and coals it has undergone great alteration along the contact, and if the sheet is only a few inches or feet thick, the change extends throughout its whole mass. Black basalts and dolerites, in such circumstances, pass into a substance like a white or pale yellow clay, which at first might be mistaken for some band of fire-clay intercalated among the other sediments. But evidence of actual intrusion may usually be found, as where the igneous rock has caught up or broken through the adjacent strata, besides altering them. Such "white traps," as they have been called, have long been familiar in the coal-fields of Scotland and Central England.
As a good illustration of the behaviour of such thin sills among carbonaceous shales I give here a section (Fig. 165) exposed in the old limestone quarry of Hillhouse, south of Linlithgow. At the bottom lies the Hurlet Limestone which has once been extensively mined at this locality. Above it comes a group of black shales which in turn are surmounted by a sheet of beautifully columnar basalt. The shales seem at first sight to include two layers of pale fire-clay, each only a few inches in thickness. Closer inspection, however, will show that these two thin intercalations are really sills which, though on the whole parallel with the bedding of the shale, may be seen to cut across it, and even at one place to send a finger into it. The upper example may also be observed to diminish rapidly in thickness in one direction.
The dimensions of the sills vary within tolerably wide limits. Although here and there the injected material dwindles down to an inch or less in thickness, running away even into threads, it more usually forms sheets of considerable depth. The rock of Salisbury Crags, for example, is fully 150 feet thick at its maximum. That of Corstorphine Hill is probably about 350 feet. The great sheet which runs among the lower limestones from Kilsyth by Denny to Stirling has been bored through to a depth of 276 feet, but as the bore started on the rock, and not in overlying strata, some addition may need to be made to that thickness.
The spheroidal weathering so characteristic of basic eruptive rocks is nowhere more characteristically displayed than among the great doleritic sills of the basin of the Firth of Forth. As an illustration of this structure an example is taken here from the large sheet at North Queensferry (Fig. 164).
While one is struck with the great size and extent of some of the sills connected with the puys, as compared with the small and local sheets underneath the plateaux, there is a further fact regarding them that
[Illustration: Fig. 166.--Dyke cutting the agglomerate of a neck. Binn of Burntisland.] deserves remark--their capricious distribution. Their occurrence seems to have little or no relation to the measure of volcanic energy as manifested in superficial eruptions. Thus in the north of Ayrshire, where a long band of lavas and tuffs, pointing to vigorous activity, lies at the top of the Carboniferous Limestone series, and where the strata underneath it are abundantly exposed at the surface, the sills occur as thin and inconstant bands in the central and eastern parts of the district only. The bedded lavas and tuffs at the head of the Slitrig Water have no visible accompaniment of sills. On the other hand, in the Edinburgh and Burntisland districts, the sills bear a large proportion to the amount of bedded lavas and tuffs, while in the Bathgate and Linlithgow district, where the superficial eruptions were especially vigorous and prolonged, the sills are of trifling extent.
It would seem from these facts that the extent to which the crust of the earth round a volcanic orifice is injected with molten rock, in the form of intrusive sheets between the strata, does not depend upon the energy of the volcano as gauged by its superficial outpourings, but on other considerations not quite apparent. Possibly, the more effectively volcanic energy succeeded in expelling materials from the vent, the less opportunity was afforded for subterranean injections. And if the protrusion of the sills took place after the vents were solidly sealed up with agglomerate or lava, it would doubtless often be easier for the impelled magma to open a way for itself laterally between the bedding-planes of the strata than vertically through the thick solid crust. The size and extent of the sills may thus be a record of the intensity of this latest phase of the volcanic eruptions.
* * * * *
Bosses.--The rounded, oval or irregularly shaped masses of igneous rock included under this head are found in some cases to be only denuded domes of sills, as, for example, in the apparently isolated patches in the oil-shale district of Linlithgowshire, which have been found to unite under ground. (Compare Fig. 157). In other instances, bosses possibly, or almost certainly, mark the position of volcanic funnels, as at the Castle Rock of Edinburgh, Dunearn Hill, Burntisland, and Galabraes, near Bathgate. But many examples occur which can only be regarded as the exposed ends of irregular bodies of molten material which has been protruded upwards into the
[Illustration: Fig. 168.--Side of columnar basalt-dyke in the same agglomerate as in Fig. 166.] Carboniferous formations. The area between Edinburgh and Linlithgow and the hills of the north of Fife furnish many examples.
The connection between bosses and intrusive sheets is instructively exhibited in a railway cutting to the west of Edinburgh, where the section shown in Fig. 167 may be seen. In the space of a few yards no fewer than four distinct bands of diabase traverse the shale, thickening rapidly in one direction and uniting with a large boss of more coarsely crystalline material. Such connections must exist in all sills, for the material injected as a sheet between stratified formations cannot but be united to some column, dyke or irregular protrusion which descends to the parent magma in the interior. But it is very rarely that the geologist is permitted to see them.
* * * * *
Dykes take a comparatively unimportant place in the eruptive phenomena of the puys. They occur in some numbers, but on a small scale, among the tuff vents, and there they can without much hesitation be set down as part of the phenomena of eruption through these pipes. The Binn of Burntisland, which has been so often referred to in this Chapter, may again be cited as a typical vent for the display of this series of dykes (Figs. 149 and 159). Two additional illustrations from this locality are here given. In Fig. 166 a dyke of compact black basalt is seen on the right hand running up the steep slopes of the agglomerate. Some of these dykes are distinctly columnar, the columns diverging from the walls on each side. Where the encasing agglomerate has been removed by the weather, the side of the dyke presents a reticulated network of prism-ends. A narrow basalt-dyke of this character near the top of the Binn vent is represented in Fig. 168.
But dykes also occur apart from vents and without any apparent relation to these. They are sometimes associated with sills and bosses in such a manner as to suggest that the whole of these forms of injected material belong to one connected series of intrusions. Among the Bathgate Hills, for example, from which I have already cited instances of sills and a boss, the section represented in Fig. 169 occurs. Yet in this same district there is a group of large east and west dykes which cut all the other rocks including the bedded lavas and tuffs, and must be of later date than the highest part of the Coal-measures (Fig. 155).
It is difficult to ascertain the age of the dykes which rise through the Carboniferous formations at a distance from any interbedded sheets of lava and tuff, or from any recognizable vent. The south-east and north-west dykes, increasing in number as they go westward, and attaining a prodigious development in the great volcanic area of Antrim and the Inner Hebrides, are probably of Tertiary date.[475] Others may possibly be Permian, while a certain number may reasonably be looked upon as Carboniferous. In petrographical characters the latter resemble the dolerites and basalts (diabases) of the finer-grained sills.
[Footnote 475: These are fully described in Chapters xxxiv. and xxxv.]
XXVIII. ILLUSTRATIVE EXAMPLES OF THE CARBONIFEROUS PUYS OF SCOTLAND
The Basin of the Firth of Forth--North Ayrshire--Liddesdale.
Though many of the geological details of each of the Scottish districts of Puys have been given in the foregoing pages, it will be of advantage to describe in connected sequence the structure and geological history of a few typical areas. By far the fullest and most varied record of this phase of volcanic activity has been preserved in the basin of the Firth of Forth; but the north of Ayrshire and the district of Liddesdale furnish also many interesting characteristics.
1. BASIN OF THE FIRTH OF FORTH
Reference has already been made to the remarkable peculiarity in the development of the lower part of the Carboniferous system in this district.[476] Elsewhere throughout Scotland the Cement-stone group and the plateau lavas are immediately overlain by the Carboniferous Limestone series. But in the basin of the Firth of Forth a varied succession of strata, more than 3000 feet in thickness, intervenes between the Cement-stones and the Hurlet Limestone. The lower portion of this thick mass of sediment may represent a part of the Cement-stone group of other districts, but even if some deduction is made on this account there remain many hundred feet of stratified deposits, for which there does not appear to be any stratigraphical equivalent elsewhere in Scotland. The distinguishing features of this series of strata are the thick zones of white sandstone, with occasional bands of fine conglomerate, the abundant seams of dark shale, often highly carbonaceous (oil-shales), the cyprid limestones and the seams of coal. Such an association of deposits may indicate a more humid climate and more varied conditions of denudation and deposition than are presented by the typical Cement-stones. The muddy floor of the shallow water must, in many places, have supported a luxuriant growth of vegetation, which is preserved in occasional seams and streaks of coal. Numerous epiphytic ferns grew on the subærial stems and branches of the lycopodiaceous trees. Large coniferae clothed the higher grounds, from which the streams brought down copious supplies of sediment, and whence a flood now and then transported huge prostrate trunks of pine. In the lagoons animal life abounded. Cyprids swarmed to such a degree as to form by their accumulated remains bands of limestone, which in the well-known Burdiehouse seam sometimes attain a thickness of 70 feet. Fishes of many genera haunted the waters, for their scales, bones and coprolites are found in profusion among the shales and limestones.
[Footnote 476: See Maclaren's "Geology of Fife and the Lothians," the Memoirs of the Geological Survey of Scotland, on Sheets 31 and 32, and my Memoir, already cited, Trans. Roy. Soc. Edin. vol. xxix. (1879) p. 437.]
When the puys began their activity, this district was gradually dotted over with little volcanic cones. At the same time it was affected by the general movement of slow subsidence which embraced all Central Scotland. Cone after cone, more or less effaced by the waters which closed over it, was carried down and buried under the growing accumulation of sediment. New vents, however, continued to be opened elsewhere, throwing out for a time their showers of dust and stones, and then lapsing into quiescence as they sank into the lagoon. Two groups of volcanoes emitted streams of lava and built up two long volcanic ridges--those of Fife and West Lothian.
The occasional presence of the sea over the area is well shown by the occurrence of thin bands of limestone or shale, containing such fossils as species of Orthoceras, Bellerophon and Discina, which suffice to prove the strata to be stratigraphical equivalents of the Lower Limestone shale, and part of the Carboniferous Limestone of England (Fig. 170). Yet the general estuarine or freshwater character of the accumulations seems satisfactorily established, not only by the absence of undoubtedly marine forms from most of the strata, but by the abundance of cyprids and small ganoids, the profusion of vegetable remains, and the occasional seams of coal.
The portion of the Forth basin within which the puys are displayed extends from near Leven in Fife, on the north, to Crosswood Burn near the borders of Lanarkshire, on the south, a distance of about 36 miles, and from near Culross in Fife and the line of the Almond River between Stirlingshire and Linlithgowshire, on the west, to the island of Inchkeith on the east, a distance of about 16 miles (Map IV.). But these limits do not precisely mark the original boundaries of the eruptions. To the north and south, indeed, we can trace the gradual dying out of the volcanic intercalations, until we reach ground over which no trace of either lavas or tuffs can be detected. To the east, the waters of the Firth conceal the geology of a considerable area, the island of Inchkeith with its bedded lavas and tuffs showing that these rocks extend some way farther eastwards than the position of that island. But in Midlothian there is no evidence that any of the puy-eruptions took place to the east of the line of the Pentland Hills. On the west side, the volcanic rocks dip under the Millstone Grit and Coal-measures, so that we do not know how far they extend in that direction. But as the Carboniferous Limestone series, when it rises again to the surface on the west side of the Stirlingshire coal-field, is destitute of included lavas and tuffs, the westward limit of the eruptions cannot lie much beyond the line of the River
[Illustration: Fig. 170.--Junction of amygdaloidal basalt with shales and limestone, Shore, half a mile east from Kinghorn, Fife. (From a photograph by Mr. R. Lunn.)] Almond. We shall probably be within the mark if we set down the original area over which puys broke out and spread abroad their lavas and tuffs as covering about 300 square miles of the lagoons and jungles of Central Scotland.
I have already shown that the range in geological time of the puy-eruptions in this district extends from a low horizon among the Calciferous Sandstones through the Carboniferous Limestone series, up to nearly the level of the Calmy Limestone, which lies not far from the top of that series. The vertical thickness of strata between these two stratigraphical limits, when there are no intercalated volcanic rocks, is probably more than 4000 feet.
The vents from which the volcanic materials were ejected, so far as they are now to be observed at the surface, may be divided into two groups, one lying to the north, the other to the south of the Firth of Forth. The northern or Fife group may be followed over an area 15 miles long, and about three miles broad. Some fifteen separate vents may be recognized in it, distributed chiefly at the two ends of the belt, a cluster of about six rising around Burntisland, while another of nearly as many appears at Saline. The characters of some of these necks have been already given in the foregoing pages.
The southern or West Lothian group includes about a dozen vents which are scattered over an area of some 60 square miles, extending from the coast-line between Borrowstounness and Queensferry southwards to Bathgate and Uphall. In this group Binns Hill, a mile long by almost half a mile broad, and rising to a height of nearly 300 feet above the sea, forms the most prominent individual. But the vents are generally smaller in the southern than in the northern group.
The manner in which the vents have been left filled with volcanic material has been described in previous pages. Most of them are occupied by tuff or agglomerate. In many cases the neck consists entirely of pyroclastic detritus, as in most of the vents of eastern Linlithgowshire and many of those in Fife. Not infrequently a column of basalt has risen in the funnel and solidified there, as exemplified by Binns Hill and Saline Hill, or the basalt has filled rents in the tuff and now appears in dykes like those on the Binn of Burntisland (Figs. 148, 149, 159, 166, 168).
But it is possible that in some cases vents may be represented by bosses of basalt or dolerite, unaccompanied by any agglomerate or tuff. Perhaps the best example of this suggested origin is supplied by Galabraes Hill, which rises through the Hurlet limestone and the volcanic series of the Bathgate Hills, about a mile north-east from the town of Bathgate. This eminence rises to a height of 940 feet above the sea, and consists of a rudely elliptical boss of basalt, measuring 3500 feet in its greater and 3000 feet in its minor axis. It disrupts the sedimentary and volcanic series, which can be traced up to it on all sides. Some of the smaller circular or elliptical bosses in eastern Linlithgowshire and western Fife may perhaps belong to the same category. But undoubtedly most of the intrusive basalts and dolerites of this volcanic region are sills.
Over the greater part of the district, only fine tuffs were ejected. These occur as interstratifications among the ordinary sediments, and vary from mere thin partings, marking the feeblest and briefest explosions, up to continuous accumulations several hundred feet thick. As an example of the least vigorous emission of tuff I may refer to the sections already given on pp. 437, 438. The thicker bands are well illustrated by that which lies some way above the Houston Coal, between Drumcross and West Broadlaw in Linlithgowshire, and by the great mass of tuff which occurs immediately below the Calmy Limestone on the River Avon near Linlithgow Bridge, and which may be 300 feet thick.
It is a striking characteristic of the tuffs that they may be met with in their solitary beds intercalated in the midst of ordinary sediments at a distance from any other trace of volcanic activity, their parent vents not being visible. I may cite in illustration an interesting case in the Swear Burn, near the southern end of the volcanic district. A band of tuff about ten feet thick lies there intercalated in a group of dark shales and thin coals. Below it there is a seam of coal four inches thick, and among the blue shales overlying it is another coal ten inches thick. The tuff is pale green, almost white in colour, fine in texture, like a volcanic mud, while some of its component beds, one foot in thickness, are made up of fine laminæ and are even false-bedded. We might infer that the volcanic vent lay at some distance, so that only the finest dust fell over the swamps in which the coal-vegetation was accumulating, but for the presence of occasional blocks of basalt one foot in diameter scattered through the tuff. When the eruptions ceased, the deposition of ordinary mud and the accumulation of plant-remains went on as before, and animal life crowded in again over the spot, for between the partings of the coal above the tuff, abundant fragments of eurypterids and scorpions may be found.
One of the most extensive volcanic discharges of fragmentary material was that which produced the "Houston marls" already referred to. These strata appear to mark a peculiar phase in the volcanic history of the Lower Carboniferous rocks of the Firth of Forth, when exceedingly fine ash, or perhaps even volcanic mud, was erupted in considerable quantity. The "marls" attain in some places a thickness of nearly 200 feet, and can be traced through most of the eastern part of Linlithgowshire, over an area of perhaps more than 50 square miles. This volcanic platform, which has been followed in mining for oil-shale, is one of the most extensive among the puy-eruptions. The material, which probably came from one or more vents among the Bathgate Hills, is not always of equal fineness, but passes into and even alternates with ordinary granular tuff. Thus in the Niddry Burn, above Ecclesmachan, the dull sage-green and brownish red Houston marls contain a few inconstant layers of green tuff, in which may be noticed pieces of black shale and lapilli of the usual basic pumice. Not far to the west, beyond Wester Ochiltree, and thus probably nearer to the active vents that ejected the dust and ashes, the Houston marls are replaced by or include a bedded granular tuff or basalt-agglomerate, which lies above the 2-feet coal of the district. The matrix of this rock is in part a dull green granular mudstone, wrapping round the lapilli and ejected stones, which, when they fall out under the action of the weather, leave casts of their forms behind them. The enclosed fragments vary in size up to blocks three feet in diameter, and consist in great measure of a compact volcanic grit, composed of a fine mud mixed with minute fragments of black shale, grains of sand and flakes of mica. There are likewise blocks of cement-stone and shale. Thin courses of black shale interlaminated with the tuff show its bedding.
The thickest and most continuous accumulations of tuff occur round some of the larger tuff cones, particularly round the Saline Hills, and in the Burntisland district. In the first-named area the copious eruptions of fragmentary material brought the volcanic history there to an end; but around Burntisland they were only the prelude to a prolonged and varied series of discharges.
I have already remarked that in the area of the puys of the Forth-basin, while the majority of the vents were tuff-cones, and emitted only fragmentary discharges, there were two well-marked tracts where lavas were poured out extensively and during a long geological interval. One of those lies in the southern, the other in the northern area.
The southern or Linlithgowshire lava-ridge forms now what are known as the Bathgate and Linlithgow Hills. The lavas extend for about 14 miles from north to south, dying out in both directions, while their present visible breadth is about three miles at its widest part. The highest summit reaches a height of about 1000 feet above the sea. The structure of this long ridge reveals an interesting record of volcanic eruptions. It consists mainly of sheets of basalt, sometimes separated by layers of tuff (Fig. 155). But on one or two horizons the volcanic rocks cease, and ordinary sedimentary deposits take their place. As has been already stated, the Main or Hurlet Limestone can be traced through the heart of the volcanic masses. This seam attains there an exceptional thickness of as much as 70 to 80 feet, and is nowhere more abundantly fossiliferous. During its deposition there seems to have been a subsidence of the area, together with a cessation of volcanic activity for a time. The crinoids, corals, brachiopods, bryozoa, lamellibranchs, gasteropods, cephalopods and fishes, which swarmed in the clear water, built up a thick calcareous layer above the lavas and tuffs of the sea-bottom.
Among the sandstones and shales that cover the limestone, bands of tuff make their appearance, indicating the renewal of volcanic activity. These are immediately surmounted by another thick pile of basalt-sheets. Subsequently, during pauses in the eruptions, while the general subsidence continued, renewed deposits of sediment spread over the submerged volcanic bank. One of the longest periods of quiescence was that during which the coals and even the Index Limestone of Bathgate crept northwards over the sunken lavas and tuffs. But the whole of the ridge does not seem to have disappeared at that time under water, at least these intercalated strata have not been traced across the thick pile of volcanic material near Linlithgow. During the final period of eruption, the outpouring of lava and discharge of ashes, neither in united thickness nor in horizontal extent, equalled those which had preceded them. When the volcanoes ceased their activity, the area continued to sink, and over the submerged lavas marine organisms crowded the sea-floor, so as to build up the Calmy Limestone. After that time volcanic action seems to have become extinct in this region, for no trace of any intercalated lava or tuff has been detected either in the overlying Millstone Grit or in the Coal-measures. The total thickness of rock in the Linlithgowshire volcanic ridge is about 2200 feet. It will probably not be an exaggeration to place the proportion of lava and tuff in that depth of material at nearly 2000 feet.
The northern or Fifeshire district over which lavas were abundantly erupted stretches along the coast from Aberdour to Kirkcaldy and inland to near Lochgelly, as well as seawards to Inchkeith. It may comprise an area of about 30 square miles. In many respects this is the most important locality in Britain for the study of Carboniferous volcanic history. The sea has cut an admirable coast-section in which the structures of the rocks are laid bare. The bottom and top of the whole volcanic series can be seen. The vents and their relation to the lavas and tuffs that were emitted from them may easily be made out; while the interstratification of well-known seams of rock in the Scottish Carboniferous system permits the sequence and chronology of the whole volcanic series to be traced with great clearness.
Most of these features have already been described in foregoing pages, for the district is a typical one for the study of Carboniferous volcanic phenomena. Thus the group of vents about Burntisland has been illustrated by the Binn of Burntisland rising among the bedded lavas and tuffs. The characters of the Carboniferous basalt-sheets have been enumerated, together with their intercalated layers of tuff and bole, and their fine partings of ashy material that was thrown out over the lagoons during the intervals between two outbursts of lava. But it may be of service if I insert here a detailed section of the whole volcanic series as it is displayed along the coast-section between Burntisland and Kinghorn. The lowest intercalated lavas of that section lie a little above the horizon of the Burdiehouse Limestone, and are thus probably rather earlier than those of Linlithgowshire. The highest reach up to the base of the Hurlet Limestone. The volcanic energy manifestly died out here long before it ceased on the south side of the Firth. Yet so vigorous was its activity while it continued, that it piled up one of the thickest masses of volcanic material anywhere to be seen among the puy-eruptions of the British Isles. The following tabular statement of the alternations of material in this great mass in descending order, was drawn up by me on the ground many years ago, before the construction of fortifications and other changes partly concealed the rocks.
Section of the Volcanic Series below the Hurlet or Main Limestone on the Coast of Fife, west of Kinghorn, in descending order[477]
[Footnote 477: The succession of rocks in this interesting coast-section was briefly given by Dr. P. Neill in his translation of Daubuisson's Basalts of Saxony, Edinburgh, 1814, note f, p. 215. He was secretary of the Wernerian Society, and in his enumeration the Wernerian terminology is used without a hint that any single band in the whole series is of volcanic origin.]
75. Reddish and white sandstones.
74. Shale with hard ribs of limestone and ironstone nodules. Fossils abundant.
73. Limestone, crinoidal, 8 or 9 feet.
72. Blue shale, becoming calcareous towards the top, where shells are plentiful.
71. Reddish false-bedded sandstones, with bands of reddish and blue shale.
70. Basalt in two sills separated by 2 or 3 feet of sandstone and shale.
69. Dark fissile sandy shale, passing up into white shaly sandstone, and including a thin parting of impure coal.
68. Limestone (Hurlet or Main Seam) in a number of bands having a united thickness of 25 feet. Abundant fossils.
67. Black shale becoming calcareous at top, and then enclosing abundant Productus, etc., 8 or 10 feet.
66. Red and green tufaceous marl and tuff. About 30 feet.
65. Basalt, the lower part strongly amygdaloidal.
64. Tufaceous red marl and tuff; comparatively coarse below, becoming finer above, 3 or 4 feet.
63. Basalt, earthy and amygdaloidal, with an irregular bottom involving masses of the shales below.
62. Dark calcareous shale and dull green tufaceous marly shale, 2 or 3 feet.
61. Crinoidal limestone in several bands with a united thickness of 10 feet.
60. Shale, 1 foot.
59. Fine green sandy tuffs in a number of bands of varying coarseness, about 6 feet.
58. Dark shale with abundance of Aviculopecten immediately under the tuffs above, 1½ feet.
57. Soft, light, marly shale with fragmentary plants, 1½ feet.
56. Dark fissile shale, full of fish-scales, plants, etc., 3 feet.
55. Basalt, rudely columnar, dark fine-grained in centre, becoming highly amygdaloidal and scoriaceous at bottom and top.
54. Basalt, like the sheet above, vesicular at top and bottom, with a parting of red clay on top.
53. Fissile rippled sandy shale, with plants, having a red and green marly parting at the top, 12 or 14 feet.
52. Basalts; a group of beds, probably in part sills, involving three bands of sandstone or quartzite.
51. Quartzite--a hard white altered sandstone, 2 to 3 feet.
50. Basalt, light green, earthy, amygdaloidal.
49. Sandstones and shales with plants, 25 feet.
48. Basalt, with a highly amygdaloidal central band. There may be several sheets here.
47. Green tufaceous shale and marl, 1 foot.
46. Basalt, dark, firm and amygdaloidal.
45. Sandstones and shales with plants.
44. Basalt forming west side of Kinghorn Bay, and including more than one sheet. The rock is very black, compact, irregularly columnar, with the usual amygdaloidal earthy band at the base, and forms the crag called the Carlinehead Rocks. An irregular and inconstant band of dull green tufaceous shale, sometimes 2 feet thick, serves to separate two of the basalt-sheets. Below it lies a remarkable scoriaceous almost brecciated basalt, which has been broken up on cooling in such a manner that at first it might be mistaken for a volcanic conglomerate.
43. Basalt, a compact black solid rock, with a vesicular and amygdaloidal bottom, about 40 feet. This sheet runs out into the promontory of Kinghorn Ness.
42. Basalt, firm, compact and highly amygdaloidal throughout, 15 feet.
41. Basalt, earthy, amygdaloidal and scoriaceous in the upper part, compact below.
40. Red tufaceous marl, clay or bole, a few inches thick.
39. Basalt: one of the most compact sheets of the whole series, about 40 feet. The top is formed of a thick zone of scoriaceous and brecciated material, the bottom is singularly uneven owing to the very irregular surface of the underlying bed.
38. Basalt more or less scoriaceous throughout, especially at the bottom, the vesicles being drawn out round the slag-like blocks.
37. Green tufaceous shales with bands of fine green tuff, 7 to 8 feet. The lower bands consist of a gravelly tuff passing up into a fine volcanic mudstone, with scattered lapilli of basalt an inch or more in diameter.
36. Basalt, with an upper, earthy and highly amygdaloidal portion, 30 feet.
35. Tufaceous sandstone and shale, 6 to 8 feet.
34. Basalt, in a thick bed, having an earthy, slaggy top and a scoriaceous bottom.
33. Basalt, very slaggy below with a compact centre.
32. Basalt, like that below it.
31. Basalt, firm, compact, black rock, with a rough, green earthy band, from 6 inches to a foot, at the bottom, and becoming again very slaggy at the top.
30. Green shale like that below the underlying limestone, a few inches in thickness.
29. Coarse, green, sandy tufaceous limestone, averaging 1 foot in thickness.
28. Black shale with plants, 12 or 14 feet, becoming green and tufaceous at the top.
27. Basalt--the most striking of the whole section--a fine compact black olivine-bearing rock, beautifully columnar, 30 feet. The columns reach to within a foot of the bottom of the bed and cease about 10 feet from the top, the upper portion of the bed being massive, with vesicles which are drawn out parallel to the bedding of the series. The lowest part of the bed is a broken brecciated band, 3 or 4 inches thick. (See Fig. 171.)
26. Black shale with fragmentary plants, 3 feet.
25. Basalt, with plentiful olivine, 12 to 16 feet. The base is not highly scoriaceous, but finely vesicular. Towards the top it becomes green, earthy and roughly brecciated. It partly cuts out the tuff underneath.
24. Tuff, green, fine-grained and well-stratified, consisting chiefly of fine volcanic dust, but becoming coarser towards the top, where it contains lapilli and occasional bombs of highly vesicular lavas.
23. Black carbonaceous shale, 3 feet; approaching to the character of an impure coal in the lower part, and becoming more argillaceous above with some thin nodular calcareous bands.
22. Green tuff, 12 feet, well stratified and fine-grained, with minute lapilli of highly vesicular basic lavas; becomes shaly at the bottom.
21. Basalt, compact, amygdaloidal, with highly vesicular upper surface, 20 feet.
20. Basalt, hard, black and full of olivine; an irregular bed 3 to 6 feet thick.
19. Basalt, dull brownish-green to black, full of kernels and strings of calcite, and showing harder and softer bands parallel with upper and under surfaces, which give it a stratified appearance.
18. Basalt, some parts irregularly compact, others earthy and scoriaceous. The distinguishing feature of this bed is the abundance of its enclosed fragments of shale, ironstone and limestone, which here and there form half of its bulk. The roughly scoriaceous upper portion is especially full of these fragments. In the ironstone balls coprolites may be detected, and occasional pieces of plant-stems are embedded in the basalt. This lava has evidently broken up and involved some of the underlying strata over which it flowed. This rock overhangs Pettycur Harbour.
17. Shales and limestone bands more or less tufaceous, 8 to 10 feet, with plants, cyprids, etc. The intercalation of fine partings of tuff in this band has been already cited on p. 438, as an illustration of the feeble intermittent character of many of the volcanic explosions between successive outflowings of lava.
Owing to a change in the direction of strike the rocks now wheel round and for a time run nearly parallel with the coast-line, while they are partly concealed by blown sand and herbage. The shales and limestones just mentioned are not constant, and are soon lost, but about a quarter of a mile westward a band of tuff begins on the same horizon or near it, and increases in thickness towards the west, where it is associated with other sediments. The shore ceases to furnish a continuous section, so that recourse must be had to the craggy slopes immediately to the north, where the rocks can be examined on a cliff face (Fig. 153). There the tuff just referred to, together with some overlying bands of sandstone, is seen to pass under the group of basalts last enumerated. It is a green, stratified rock, perhaps 60 feet thick at its maximum, but dying out rapidly to north-west and south-east. It encloses balls of basalt and subangular and rounded fragments of sandstone, limestone and shale. A mass of coarse volcanic agglomerate which is connected with it and cuts across the ends of some of the basalts below, probably marks the position of the vent from which the tuff was ejected (Fig. 152).
16. Black and grey shales forming a thin band at the summit of King Alexander's Crag.
15. Basalt, dark compact rock, with an upper and lower highly scoriaceous and amygdaloidal band, 15 feet.
14. Black shales, tufaceous green shales, sandstone, and 6 inches of coal, forming a group of strata about 12 feet thick between two basalts; plants and cyprids abundant. (The coal seam is shown in Fig. 151.)
13. Basalt, dull, earthy and highly amygdaloidal, with abundant calcite in kernels and veins; about 15 feet, but varying in thickness.
12. Basalt, forming a well-marked bed from 12 to 25 feet thick. It is a compact black olivine-bearing rock, sparingly amygdaloidal, but showing the usual dull green, earthy scoriform base. The upper surface is singularly irregular, having, in flowing, broken up into large clinker-like blocks, which are involved in the immediately overlying basalt. The bottom also is very uneven, for the basalt has in some places cut out the underlying shales, so as to rest directly upon the basalt below.
11. Black shale, varying up to 6 inches, but sometimes entirely removed by the overlying lava-stream.
10. Basalt, containing large irregularly spheroidal masses of hard black finely vesicular material enclosed in more earthy and coarsely vesicular rock. The vesicles are sometimes elongated parallel to the bedding, but have often been drawn out round a spheroid; some of them measure nearly a foot in length by 2 or 3 inches in breadth. The upper surface is uneven and coarsely amygdaloidal.
9. Basalt, hard black, with abundant olivine, and a columnar structure.
8. Green shale, 6 inches to 1 foot, much baked and involved in the overlying basalt.
7. Basalt, dull-green, earthy, amygdaloidal, varying from 10 to 40 feet in thickness.
6. Blue shale, disappearing where the basalt above it unites with that below.
5. Basalt with olivine, forming a thick irregular bed, which in some places is black and compact, in others green, earthy and amygdaloidal. The upper part is particularly cellular.
4. Sandstones forming a thick group of beds which have long been quarried for building-stone at the Grange and elsewhere.
3. Black shales.
2. Limestone (Burdiehouse).
1. Sandstones, shales and thin limestones forming the strata at Burntisland through which the sills of that district have been injected (Fig. 159).
The phenomena of sills are abundantly developed among the Carboniferous rocks of the basin of the Firth of Forth, and some of the more remarkable examples in this district have been already cited. Taking now a general survey of this part of the volcanic history, I may observe that if the sills are for a moment considered simply as they appear at the surface, and apart from the geological horizons on which they lie, they form a wide ring surrounding the Falkirk and Stirlingshire coal-field.
Beginning at the Abbey Craig, near Stirling, we may trace this ring as a continuous belt of high ground from Stirling to the River Carron. Thence it splits up into minor masses in different portions of the Carboniferous system, and doubtless belonging to different periods of volcanic disturbance, but yet sweeping as a whole across the north-eastern part of the Clyde coal-field, and then circling round into Stirlingshire and Linlithgowshire. There are no visible masses to fill up the portion of the ring back to Abbey Craig. But through Linlithgowshire and the west of Edinburghshire a number of intrusive sheets form an eastward prolongation of the ring. Large as some of these sheets are at the surface, for they sometimes exceed two or three square miles in area, a much larger portion of their mass is generally concealed below ground. Mining operations, for example, have proved that in the south-east of Linlithgowshire areas of intrusive rock which appear as detached bosses or bands at the surface are connected underneath as portions of one continuous sill, which must be several square miles in extent.
But it is in Fife that the sills reach their greatest development among the Carboniferous rocks of Scotland (Fig. 172). A nearly continuous belt of them runs from the Cult Hill near Saline on the west, to near St. Andrews on the east, a distance of about 35 miles. This remarkable band is connected with a less extensive one, which extends from Torryburn on the west, to near Kirkcaldy on the east. In two districts of the Fife region of sills, a connection seems to be traceable between the intrusive sheets and volcanic vents, at least groups of necks are found in the midst of the sills. One of these districts is that of the Saline Hills already described, the other is that of Burntisland. In the latter case the evidence is especially striking, for the vents are connected above with bedded lavas and tuffs, while below lie three well-marked sills (Fig. 159).
It is certainly worthy of remark that sills are generally absent from those areas where no traces of contemporaneous volcanic activity are to be found. No contrast in this respect can be stronger than that between the ground to the east and west of the old axis of the Pentland Hills. In the western district, where the puys are so well displayed, sills abound, but in the eastern tract both disappear.
Another question of importance in dealing with the history of these sills is their stratigraphical position. By far the larger proportion of them lies in the Carboniferous Limestone series. Thus the great sill between Stirling and Kilsyth keeps among the lower parts of that series. On the same general horizon are the vast sheets of dolerite which stretch through Fife in the chain of the Cult, Cleish, and Lomond Hills on the one side, and in the eminences from Torryburn to Kinghorn on the other, though the intrusive material sometimes descends almost to the Old Red Sandstone. In Linlithgowshire and Edinburghshire, as well as in the south of Fife, the sills traverse the Calciferous Sandstone groups.
If the horizons of the sills furnished any reliable clue to their age, it might be inferred that the rocks were all intruded during the Carboniferous period, and as most of them lie beneath the upper stratigraphical limit of the puy-eruptions, the further deduction might be drawn that they are connected with these eruptions. I have little doubt that in a general sense both conclusions are well-founded. But that there are exceptions to the generalization must be frankly conceded. On close examination it will be observed that the same intrusive mass sometimes extends from the lower into the upper parts of the Carboniferous groups. Thus, in the west of Linlithgowshire, a large protrusion which lies upon the Upper Limestones, crosses most of the Millstone Grit, and reaches up almost as high as the Coal-measures. Again, in Fife, to the east of Loch Leven, a spur of the great Lomond sill, crossing the Carboniferous limestone, advances southward into the coal-field of Kinglassie, In Stirlingshire and Lanarkshire numerous large dolerite sheets have invaded the Millstone Grit and Coal-measures, including even the upper red sandstones, which form the top of the Carboniferous system in this region. It is thus obvious that if the puy-eruptions in the basin of the Forth ceased towards the close of the deposition of the Carboniferous Limestone series, there must have been a subsequent injection of basic lava on a gigantic scale in central Scotland. I shall recur to this subject in Chapter xxxi.
2. NORTH OF AYRSHIRE
In this part of the country another group of puys and their associated tuffs and lavas may be traced from near Dairy on the west, to near Galston on the east (Map IV.). The length of the tract is about sixteen miles, while its breadth varies from about a furlong to nearly a mile and a half. I have had occasion to allude to this marked band of volcanic materials which here intervenes between the Carboniferous Limestone and the Coal-measures, and from its position appears to mark the latest Carboniferous volcanoes. Its component rocks reach a thickness of sometimes 600 feet, and as they dip southwards under the Coal-measures, they may extend for some distance in that direction. They have been met with in borings sunk through the northern part of the Irvine coal-field. Even what of them can be seen at the surface, in spite of the effects of faults and denudation, shows that they form one of the most persistent platforms of volcanic rock among the puy-eruptions of Scotland.
Where best developed this volcanic band has a zone of tuff at the bottom, a central and much thicker zone of bedded basalts, and an upper group of tuffs, on which the Coal-measures rest conformably. A few vents, probably connected with it, are to be seen at the surface between Fenwick and Ardrossan. But others have been buried under the Carboniferous sedimentary rocks, and, as already described, have been discovered in the underground workings for coal and ironstone (p. 434). These mining operations have, indeed, revealed the presence of far more volcanic material below ground than would be surmized from what can be seen at the surface. Here and there, thin layers of tuff appear in brook-sections, indicating what might be conjectured to have been trifling discharges of volcanic material. But the prosecution of the ironstone-mining has proved that, at the time when the seam of Black-band Ironstone of that district was accumulated, the floor of the shallow sea or lagoon where this deposition took place was dotted over with cones of tuff, in the hollows between which the ferruginous and other sediments gathered into layers. That seam is in one place thick and of good quality; yet only a short distance off it is found to be so mixed with fine tuff as to be worthless, and even to die out altogether.[478]
[Footnote 478: See Explanation of Sheet 22, Geol. Surv. of Scotland, pars. 29, 33, 45.]
3. LIDDESDALE
A remarkable development of puys lies in that little-visited tract of country which stretches from the valleys of the Teviot and Rule Water south-westwards across the high moorland watershed, and down Liddesdale. Through this district a zone of bedded olivine-basalts and associated tuffs runs in a broken band which, owing to numerous faults and extensive denudation, covers now only a few scattered patches of the site over which it once spread. The geological horizon of this zone lies in the Calciferous Sandstones, many hundred feet above the position of the top of the plateau-lavas (Map IV.).
So great an amount of material has been here removed by denudation that not only has the volcanic zone been bared away, but the vents which supplied its materials have been revealed in the most remarkable manner over an area some twenty miles long and eight miles broad. Upwards of forty necks of agglomerate may be seen in this district, rising through the Silurian, Old Red Sandstone, and lowest Carboniferous rocks. It fills the geologist with wonder to meet with those stumps of old volcanoes far to the west among the Silurian lowlands, sometimes fully ten miles away from the nearest relic of the bedded lavas connected with them.[479] That these vents, though they rose through ground which at the time of their activity was covered with the volcanic series of the plateaux, do not belong to that series, but are of younger date, has been proved in several cases by Mr. Peach. He has found that among the blocks composing their agglomerates, pieces of the sandstones, fossiliferous limestones and shales of the Cement-stone group, overlying the plateau-lavas, are to be recognized. These vents were therefore drilled through some part at least of the Calciferous Sandstones, which are thus shown to have extended across the tract dotted with vents. After the volcanic activity ceased, fragments of these strata tumbled down from the sides into the funnels. Denudation has since stripped off the Calciferous Sandstones, but the pieces detached from them, and sealed up at a lower level in the agglomerates, still remain. Mr. Peach's observations indicate to how considerable an extent sagging of the walls of these orifices took place, with the precipitation not merely of blocks, but of enormous masses of rock, into the volcanic chimneys. In one instance, between Tudhope Hill and Anton Heights, a long neck, or perhaps group of necks, which crosses the watershed, shows a mass of the red sandstone many acres in extent, and large enough to be inserted on the one-inch map, which has fallen into the vent (Fig. 175).
[Footnote 479: They have been recognized and mapped by Mr. B. N. Peach for the Geological Survey. See Sheets 11 and 17, _Geol. Surv. Scotland_.]
The materials ejected from the Liddesdale vents include both basaltic lavas and tuffs. The former are sometimes highly vesicular, especially along the upper parts of the flows. They are thickest towards the north, and in Windburgh Hill attain a depth of at least 300 or 400 feet. In that part of the district they form the lower and main part of the volcanic series, being there covered by a group of tuffs. But a few miles southwards, not far to the west of Kershopefoot, they die out. The tuffs then form the whole of the volcanic band which, intercalated in a well-marked group of limestones, can be followed across the moors for some six miles into the valley of the Esk, where an interesting section of them and of the associated limestone and shales is exposed (Fig. 174). At Kershopefoot, a higher band of basic lava overlies the Kershopefoot limestone, and can be traced in scattered patches both on the Scottish and English side of the Border.
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Transcriber Note
Minor typos corrected. Several tables were rotated so they will fit in the limits of the text format. Text rearranged to avoid split paragraphs at full page images.