The close of the Carboniferous portion of the geological record in Britain is marked by another of those great gaps which so seriously affect the continuity of geological history. No transitional formation, such as in other countries marks the gradation from the Carboniferous into the succeeding period, has been definitely recognized in this country. The highest Carboniferous strata are here separated from all younger deposits by an unconformability, indicating the lapse of vast periods of time whereof, within the British area, no chronicle has been preserved.

When we pass from the Carboniferous system to that which comes next to it in order of time, we soon become sensible that great changes in geography, betokening an immense interval, took place between them. The prolonged subsidence during which the Coal-measures were accumulated, not only carried down below sea-level all the tracts over which the Carboniferous system was deposited, but possibly submerged the last of the islets, which, like those of Charnwood Forest, had survived so many geological changes. Eventually, however, and after what may have been a vast period of quiescence, underground movements began anew, and the tracts of Coal-measures were unequally ridged up into land. The topography thus produced appears to have resulted in the formation of a series of inland seas somewhat like those of the Old Red Sandstone, but probably less in area and in depth. In these basins the water seems to have been on the whole unfavourable to life, for the red sand and mud deposited in them are generally unfossiliferous, though, when the conditions became more suitable, calcareous or dolomitic sediment accumulated on the bottom, to form what is now known as the "Magnesian Limestone," and muddy sediment was deposited which is now the "Marl Slate." In these less ferruginous strata, betokening a less noxious condition of water, various marine organisms are met with.[87]

The vegetation of the land surrounding these basins was still essentially Palæozoic in character. It presented a general resemblance to that of Carboniferous time, but with some notable differences. The jungles of Sigillaria seem to have disappeared, while on the other hand, conifers increased in number and variety. The sediments of the water-basins have handed down only a scanty remnant of the animal life of the time. Along the sandy shores walked various amphibians which have left their footprints on the sand. A few genera of ganoid fishes have been found in some of the shales, and a comparatively poor assemblage of crinoids and molluscs has been obtained from the Magnesian Limestone. To the geological period distinguished by these geographical and biological characters the name of Permian is assigned.

In his survey of the progress of volcanic history in the area of Britain, the geologist finds that the long period of quiescence indicated by the deposition of the Coal-measures, and probably also by the unconformability between the Coal-measures and the Permian formations, was at length terminated by a renewed volcanic outbreak, but on a singularly diminished scale and for a comparatively brief period of time. Whether, had the Permo-Carboniferous strata which connect the Coal-measures with the Permian formations on the Continent been found in this country, they would have filled up the gap in the geological record, and would have supplied any trace of contemporaneous volcanic action, cannot even be surmised. All that we know is that, after a vast interval, and during the deposition of the breccias and red sandstones which unconformably overlie the Coal-measures, a few scattered groups of little volcanoes appeared in the area of the British Isles.

It is unfortunate that in those districts where these volcanic relics have been preserved, the stratigraphical record is singularly imperfect, and that on the eastern side of England, where this record is tolerably complete, there are no intercalated volcanic rocks. The latter occur in tracts where the strata are almost wholly destitute of fossils, and where therefore no palæontological evidence is available definitely to fix the geological age of the eruptions. Nevertheless there is usually ample proof that the strata in question are much later than the Coal-measures, while their geological position and lithological characters link them with the undoubted Permian series of the north-east of England. They may, however, belong to a comparatively late part of the Permian period, if indeed some of them may not be referable to the succeeding or Triassic period.

The comparatively feeble and short-lived volcanoes now to be described are found in two regions wide apart from each other. The more important of these lies in the south-west and centre of Scotland. A second group rose in Devonshire. It is possible that a third group appeared between these two regions, somewhere in the midlands. The evidence for the history of each area will be given in a separate section in the following pages.

i. GENERAL CHARACTERISTICS—NATURE OF MATERIALS ERUPTED

The chief district for the display of volcanic eruptions that may be assigned to the Permian period lies in the centre of Ayrshire and the valleys of the Nith and Annan. But, for reasons stated below, I shall include within the same volcanic province a large part of the eastern half of the basin of the Firth of Forth (see Map V.).

Unfortunately the interesting volcanic rocks now to be considered have suffered severely from the effects of denudation. They have been entirely removed from wide tracts over which they almost certainly once extended. But this enormous waste has not been wholly without compensations. The lavas and tuffs ejected at the surface, and once widely spread over it, during the deposition of the red sandstones, have been reduced to merely a few detached fragments. But, on the other hand, their removal as a superficial covering has revealed the vents of discharge to an extent unequalled in any older geological system, even among the puys of the Carboniferous period. The Permian rocks, escaping the effects of those great earth-movements which dislocated, plicated and buried the older Palæozoic systems of deposits, still remain for the most part approximately horizontal or only gently inclined. They have thus been more liable to complete removal from wide tracts of country than older formations which have been protected by having large portions of their mass carried down by extensive faults and synclinal folds, and by being buried under later sedimentary accumulations. We ought not, therefore, to judge of the extent of the volcanic discharges during Permian time merely from the small patches of lava and tuff which have survived in one or two districts, but rather from the number, size and distribution of the vents which the work of denudation has laid bare.

The evidence for the geological age of the volcanic series now to be described is less direct and obvious than most of that with which I have been hitherto dealing. It consists of two kinds. (a) In the first of these comes the series of lavas and tuffs just referred to as regularly interstratified with the red sandstones, which, on the grounds given in the next paragraph, it is agreed to regard as Permian. (b) Connected with these rocks are necks which obviously served as vents for the discharge of the volcanic materials. They pierce not only the Coal-measures, but even parts of the overlying bedded lavas. So far there is not much room for difference of opinion; but as we recede northward from Ayrshire and Nithsdale, where the intercalation of the volcanic series in the red sandstones is well displayed, we enter extensive tracts where these interstratified rocks have disappeared and only the necks remain. All that can be positively asserted regarding the age of these necks is that they must be later than the rocks which they pierce. But we may inferentially connect them with the interstratified lavas and tuffs by showing that they can be followed continuously outward from the latter as one prolonged group, having the same distribution, structure and composition, and that here and there they rise through the very highest part of the Coal-measures. It is by reasoning of this kind that I include, as not improbably relics of Permian volcanoes, a large number of vents scattered over the centre of Scotland, in the East of Fife.

The red sandstones among which the volcanic series is intercalated cover several detached areas in Ayrshire and Dumfriesshire. Lithologically they present a close resemblance to the Penrith sandstone and breccias of Cumberland, the Permian age of which is generally admitted. They lie unconformably sometimes on Lower and Upper Silurian rocks, sometimes on the lower parts of the Carboniferous system, and sometimes on the red sandstones which form the highest subdivision of that system. They are thus not only younger than the latest Carboniferous strata, but are separated from them by the interval represented by the unconformability. On these grounds they are naturally looked upon as not older than the Permian period. The only palæontological evidence yet obtained from them in Scotland is that furnished by the well-known footprints of Annandale, which indicate the existence of early forms of amphibians or reptiles during the time of the deposition of the red sand. The precise zoological grade of these animals, however, has never yet been determined, so that they furnish little help towards fixing the stratigraphical position of the red rocks in which the footprints occur.

The stratigraphical relations of the red sandstones of Ayrshire and Nithsdale were discussed by Murchison, Binney and Harkness.[88] These observers noticed certain igneous rocks near the base of the sandstones, to which, however, as being supposed intrusive masses, they did not attach importance. They regarded the volcanic tuffs of the same district as ordinary breccias, which they classed with those of Dumfries and Cumberland, though Binney noticed the resemblance of their cementing paste to that of volcanic tuff, and in the end was doubtful whether to regard the igneous rocks as intrusive or interstratified.

In the year 1862, on visiting the sections in the River Ayr, I recognized the breccia as a true volcanic tuff. During the following years, while mapping the district for the Geological Survey, I established the existence of a series of contemporaneous lavas and tuffs at the base of the Permian basin of Ayrshire, and of numerous necks marking the vents from which these materials had been erupted. An account of these observations was published in the year 1866.[89] Since that time the progress of the Survey has extended the detailed mapping into Nithsdale and Annandale, but without adding any new facts of importance to the evidence furnished by the Ayrshire tract.[90]

The materials erupted by the Scottish Permian volcanoes display a very limited petrographical range, contrasting strongly in this respect with the ejections of all the previous geological periods. They consist of lavas generally more or less basic, and often much decayed at the surface; and of agglomerates and tuffs derived from the explosion of the same lavas.

The lavas are dull reddish or purplish-grey to brown or almost black rocks; sometimes compact and porphyritic, but more usually strongly amygdaloidal, the vesicles have been filled up with calcite, zeolites or other infiltration. The porphyritic minerals are in large measure dull red earthy pseudomorphs of hæmatite, in many cases after olivine. These rocks have not yet been fully studied in regard to their composition and microscopic structure. A few slides, prepared from specimens collected in Ayrshire and Nithsdale, examined by Dr. Hatch, were found to present remarkably basic characters. One from Mauchline Hill is a picrite, composed chiefly of olivine and augite, with a little striped felspar. Others from the Thornhill basin in Dumfriesshire show an absence of olivine, and sometimes even of augite. The rock of Morton Castle consists of large crystals of augite and numerous grains of magnetite in a felspathic groundmass full of magnetite. Around Thornhill are magnetite-felspar rocks, composed sometimes of granular magnetite with interstitial felspar. Throughout all the rocks there has been a prevalent oxidation of the magnetite, with a consequent reddening of the masses.

The pyroclastic materials consist of unstratified agglomerates and tuffs, generally found in necks, and of stratified tuffs, which more or less mingled with non-volcanic material, especially red sandstone, are intercalated among the bedded lavas or overlie them, and pass upward into the ordinary Permian red sandstones.

The agglomerates, though sometimes coarse, never contain such large blocks as are to be seen among the older Palæozoic volcanic groups. Their composition bears reference to that of the bedded lavas associated with them, pieces of the various basalts, andesites, etc., which constitute these lavas being recognizable, together with others, especially a green, finely-vesicular, palagonitic substance, which has not been detected among the sheets of lava. In general the agglomerates contain more matrix than blocks, and pass readily into gravelly tuffs. A series of specimens collected by me from necks which pierce the Dalmellington coal-field has been sliced and examined under the microscope by Mr. Watts, who finds it to consist of basic tuffs, in which the lapilli include various types of olivine-basalt, sometimes glassy, sometimes palagonitic, and occasionally holocrystalline, also pieces of grit, shale and limestone. In one case a crinoid joint detached from its matrix was noticed. A specimen from Patna Hill consists of "a clear irregularly cracked aggregate of carbonates and quartz with hornblende, and its structure reminds one of that of olivine. The hornblende is in small irregular patches surrounded by the clear mineral, and is probably a replacement of a pyroxene, perhaps diallage." If this stone was once an olivine nodule, the agglomerate might in this respect be compared with some of the tuffs of the Eifel so well known for their lumps of olivine.

The stratified tuffs are generally more or less gravelly deposits, composed of lapilli varying in size from mere grains up to pea-like fragments, but with numerous larger stones and occasional blocks of still greater dimensions. They often pass into a tough dull compact mudstone. In colour they are greenish or reddish. They have been largely derived from the explosion of lavas generally similar to those of which fragments occur in the agglomerates. They often contain non-volcanic detritus, derived from the blowing up of the rocks through which the vents were opened. Occasionally they include also various minerals such as pyrope, black mica, sanidine, augite, and others which appear to have been ejected as loose and often broken crystals. This character is more fully described in regard to its occurrence among the necks of the east part of Fife.

The intrusive rocks, probably referable to the same volcanic period, consist chiefly of dolerites and basalts which occur as dykes, sills and bosses, and are more particularly developed in the south-west of Ayrshire.

ii. GEOLOGICAL STRUCTURE OF THE VOLCANIC DISTRICTS

1. Ayrshire, Nithsdale and Annandale

(1) Interstratified Lavas and Tuffs.—It will be convenient to consider first the volcanic chronicle as it has been preserved in the south-west and south of Scotland, where the existence of Permian volcanoes in Britain was first recognized. The volcanic rocks in the middle of the Ayrshire coal-field rise from under a central basin of red sandstone, which they completely enclose. Their outcrop at the surface varies up to about a mile or rather more in breadth, and forms a pear-shaped ring, measuring about nine miles across at its greatest width (Map V.).[91]

This volcanic ring runs as a tract of higher ground encircling the hollow in which the Permian red sandstones lie, and forming a marked chain of heights above the Carboniferous country around. It is built up of a succession of sheets of different lavas, with occasional partings of tuff or volcanic breccia, which present their escarpments towards the coal-field outside, and dip gently into the basin under the inner trough of brick-red sandstones. Good sections of the rocks are exposed in the ravines of the River Ayr, particularly at Ballochmyle, in the Dippol Burn near Auchinleck House, and in the railway cutting near Mossgiel.

That these are true lava-flows, and not intrusive sills, is sufficiently obvious from their general outward lithological aspect, some of them being essentially sheets of slag and scoriæ. Their upper surfaces may be found with a fine indurated red sand wrapping round the scoriform lumps and protuberances, and filling in the rents and interspaces, as in the case of the Old Red Sandstone lavas already referred to. As an example of these characteristics, I may cite the section represented in Fig. 200. At the bottom lies a red highly ferruginous and coarsely amygdaloidal basalt (a). Over it comes a volcanic conglomerate three feet thick, made up of balls of vesicular lava like that below, wrapped in a brick-red sandy matrix (b). Lenticular bands of sandstone without blocks occur in the conglomerate, and others lie in hollows of its upper surface (c). This intercalation of detrital material is followed by another basic lava (d), about six feet thick, highly amygdaloidal in its lower and upper parts, more compact in the centre. The amygdales and joints are largely filled with calcite. The slaggy bottom has caught up and now encloses some of the red sand of the deposit below. Another lava from three to six feet thick next appears (e), which is remarkable for its slaggy structure, and is so decomposed that it crumbles away. Like the others it is dull-red and ferruginous and full of calcite. It must have been at the time of its outflow a sheet of rough slag that cracked into open fissures. That it was poured out under water is again shown in the same interesting way just referred to, by the red sand which has been washed into the interspaces between the clinkers and has filled up the fissures, in which it is stratified horizontally between the walls. Above this band, and perhaps passing into it as its slaggy base, lies another more compact lava (f) like the lower sheets.

Throughout the series of lavas, as indicated in the foregoing section, traces of the pauses that elapsed between the separate outflows may be seen in the form either of layers of red sandstone or of tuff and volcanic breccia. Here and there, under the platform of bedded lavas, the brick-red sandstone is full of fragments of slag and fine volcanic dust. But the most abundant accumulation of such detritus is to be seen at the top of the volcanic series, where it contains the records of the closing phases of eruption. Thick beds of tuff and volcanic breccia occur there, interleaved with seams of red sandstone, like the chief mass of that rock, into which they gradually pass upward. Yet, even among the sandstones above the main body of tuff, occasional nests of volcanic lapilli, and even large bomb-like lumps of slag, point to intermittent explosions before the volcanoes became finally extinct and were buried under the thick mass of red Permian sandstone.

There is good reason to believe that both the volcanic sheets and the red sandstones overlying them, instead of being restricted to an area of only about 30 square miles, once stretched over the lowlands of Ayrshire; and not only so, but that they ran down Nithsdale, and extended into several of its tributary valleys, if indeed, they were not continuous across into the valley of the Annan.[92] Traces of the lavas and tuffs are to be found at intervals over the area here indicated. The most important display of them, next to their development in Ayrshire, occurs in the vale of the Nith at Thornhill, whence they extend continuously up the floor of the Carron Valley for six miles. They form here, as in Ayrshire, a band at the base of the brick-red sandstones, and consist mainly of bedded lavas with the basic characters above referred to. These lavas, however, are followed here by a much thicker development of fragmental volcanic materials. Abundant volcanic detritus is diffused through the overlying sandstones, sometimes as a gravelly intermixture, sometimes in large slaggy blocks or bombs, and sometimes in intercalated layers of tuff, while an occasional sheet of one of the dull red lavas may also be detected. The final dying-out of the volcanic energy in a series of intermittent explosions, while the ordinary red sandy sediment was accumulating, is here also admirably chronicled. As an illustration of these features the accompanying section is given (Fig. 201). The last of the lavas (a) presents an uneven surface against which the various kinds of detritus have been laid down. First comes a coarse volcanic breccia (b) made up of angular and subangular blocks of different lavas imbedded in a matrix of red ashy sand. This deposit is succeeded by a band of dull red tufaceous sandstone, evidently formed of ordinary red sandy sediment, into which a quantity of volcanic dust and lapilli fell at the time of its accumulation. Some of the ejected blocks which lie inclosed in the finer sediment are upwards of a foot in length. A more vigorous discharge of fragmental material is shown by the next bed (d), which consists of a coarse nodular tuff, mingled with a little red sandstone and crowded with blocks of the usual lavas. Beyond the locality of this section these tuffs are found to pass up insensibly into the ordinary Permian sandstone.

But we can detect the edges of yet more distant streams of lava emerging from under the red sandstones and breccias to the east of the Nith. On the farther side of the Silurian ridge that forms the eastern boundary of the Nith valley, above which it rises some 700 or 800 feet, there is preserved at the bottom of the valley of the Capel Water, which flows into Annandale, another small outlier of a similar volcanic band. Three miles to the south-east of it two little fragments of the volcanic group lie on the sides of a small tributary of the Water of Ae. Since these may serve as a good illustration of the extent to which denudation has reduced the area of the Permian volcanic series, a section of the locality is here given (Fig. 202). The general foundation rocks of the country are the Silurian greywackes and shales in highly inclined and contorted positions (a). Each outlier has, as its basement material, a volcanic breccia (bb) in which, together with the usual lava-fragments, are mingled pieces of the surrounding Silurian strata. In the smaller outlier lying to the north-east, this detrital layer is only about one foot thick. It is overlain by a slaggy amygdaloid of the usual character (cc), which in the lower outlier is covered with boulder clay (d). There can be little doubt that these detached fragments were once united in a continuous sheet of lava which filled the valley of the Water of Ae and that of its tributary. That the lava stretched down the Ae valley for some distance is proved by the occurrence of another outlier of it two miles below.

But there is still additional evidence for the wide extension of these volcanic sheets. It appears to be certain that they stretch far to the eastward, under the Permian sandstones of the Lochmaben basin of Annandale, for breccias largely made up of pieces of the bedded lavas are found close to the northern edge of the basin on the west side of the River Annan. To this remarkable adherence of the lavas and tuffs to the bottom of the Permian valleys I shall afterwards more specially refer.

The thickness of the whole volcanic group cannot be very accurately determined. It reaches a maximum in the Ayrshire basin, where, at its greatest, it probably does not exceed 500 feet, but is generally much less; while in the Nithsdale and Annandale ground the detached and much denuded areas show a still thinner development.

(2) Vents.—One of the most interesting features in this south-western district of Scotland is the admirable way in which the volcanic vents of Permian time have been preserved. Their connection with the lavas and tuffs can there be so clearly traced that they serve as a guide in the interpretation of other groups of vents in districts where no such connection now remains. In Ayrshire, the lower part of the Permian volcanic band is pierced by several small necks of agglomerate. There cannot, I think, be any doubt that these necks mark the positions of some of the vents from which the later eruptions took place. Immediately beyond them necks of precisely similar character rise through the upper division of the Coal-measures. There can be as little hesitation in placing these also among the Permian vents. And thus step by step we are led away from the central lavas, through groups of necks preserving still the same features, external and internal, and rising indifferently through rocks of any geological age from the Coal-measures backward. Thus, although if we began the investigation at the outer limits of the chain of necks, we might well hesitate as to their age, yet, when we can fix their geological position in one central area, we are, I think, justified in classing, as parts of one geologically synchronous series, all the connected groups that retain the same general characteristics. It is to denudation that we owe their having been laid bare to view; but at the same time, denudation has removed the sheet of ejected materials which may have originally connected most of these vents together.

In this regard, it is most instructive to follow the vents south-eastwards from the Ayrshire basin into Nithsdale for a distance of some eighteen miles. If we traced them down that valley to Sanquhar, without meeting with any vestige of superficial outflows to mark their stratigraphical position, we might possibly hesitate whether the age of those which are so far removed from the evidence that would fix it should not be left in doubt. But if we continued our traverse only a few hundred yards farther, we should find some fragmentary outliers of the Permian lavas capping the Upper Coal-measures; and if we merely crossed from the Nith into the tributary valley of the Carron Water, we should see preserved in that deep hollow a great series of Permian lavas, tuffs and agglomerates. It is only by a happy accident that here and there these superficial volcanic accumulations have not been swept away. There was probably never any great thickness of them, but they no doubt covered most, if not all, of the district within which the vents are found.

The Permian necks are, on the whole, smaller than those of the Carboniferous period. The largest of them in the Ayrshire and Nithsdale region do not exceed 4000 feet in longest diameter; the great majority are much less in size, while the smallest measure 20 yards, or even less. Those of Fife, to be afterwards described, exhibit a wider range of dimensions, and have the special advantage of being exposed in plan along the shore.

These necks, from their number and shapes, form a marked feature in the scenery. They generally rise as prominent, rounded, dome-shaped, or conical hills, which, as the rock comes close to the surface, remain permanently covered with grass (Figs. 203 and 204). Such smooth green puys are conspicuous in the heart of Ayrshire, and likewise further south in the Dalmellington coal-field, where some of them are locally known as "Green Hill," from their verdant slopes in contrast to the browner vegetation of the poorer soil around them (Fig. 203).

As in those of older geological periods, the necks of this series are, for the most part, irregularly circular or oval in ground-plan, but sometimes, like those of the Carboniferous system, they take curious oblong shapes, and occasionally look as if two vents had coalesced (Fig. 205). Here and there also the material of the vents has consolidated between the walls of a fissure or the planes of the strata, so as to appear rather as a dyke than as a neck. Descending, as usual, vertically through the rocks which they pierce, the necks have the form of vertical columns of volcanic material, ending at the surface in grassy rounded hillocks or hills.

In almost all cases, the necks of the Ayrshire region consist of a gravelly tuff or agglomerate, reddish or greenish in colour, made up of blocks of such lavas as form the bedded sheets, together with fragments of the stratified rocks through which the chimneys have been blown out. Thus, in some of the necks, pieces of black shale are abundant, as at Patna. In other cases, there are proofs of the derivation of the stones from much greater depths, as in the Green Hill of Waterside, where fragments of fine greywacke are not infrequent, probably derived from the Silurian formations which lie deep beneath the Carboniferous and Old Red Sandstone series.

The fragmentary material of the necks is generally unstratified, but a rude stratification may sometimes be noticed, the dip being irregularly inward at high angles towards the middle of the vent. This structure, best seen in the vents of the Fife coast, as will be shown in the sequel, may be detected in some of the necks of the Dalmellington district.

Occasionally some form of molten rock has risen in the funnel, and has partially or wholly removed or concealed the agglomerate. This feature is especially noticeable among the necks that pierce the Dalmellington coal-field. Portions of basic lavas traverse the agglomerate or intervene between it and the surrounding strata. These have probably in most cases been forced up the wall of the funnel, while here and there sills run outward from the necks into the surrounding Coal-measures. Sometimes a thin sheet of lava, adhering to the wall of a funnel, may be the remnant of a mass of rock that once filled up the orifice. In one of the necks of the Muirkirk Coal-field, which was pierced by a mine driven through it from side to side, fingers and sheets of "white trap," or highly altered basalt, were found to run out from the neck into the surrounding strata.[93] Dark heavy basalt, or some still more basic rock, has here and there filled up a vent. As so many of the necks rise through the coal-fields, opportunities are afforded of studying the effects of volcanic action upon the coal-seams, which for some distance from them have been destroyed.

Another feature, which can be recognized from the information obtained in mining operations, is that, in the great majority of instances, no connection is traceable between the positions of the vents and such lines of dislocation as can be detected at the surface or in the underground workings. Some vents, indeed, have evidently had their positions determined by lines of fault, as, for instance, that of the Green Hill below Dalmellington. Yet in the same neighbourhood a number of other examples may be found where the volcanic funnels seem to have avoided faults, though these exist close to them.

In this south-western district of Scotland upwards of sixty distinct vents have been mapped in the course of the Geological Survey. They run from the north of Ayrshire to the foot of the Southern Uplands, and descend for some distance the vale of the Nith. The area over which they are distributed measures roughly about forty miles from north-west to south-east, and at its greatest breadth twenty miles from south-west to north-east. Within this tract the vents are scattered somewhat sporadically in groups, sometimes numbering twenty necks in a space of sixteen square miles, as in the remarkable district of Dalmellington.

In considering their distribution we cannot but be impressed by the striking manner in which these necks keep to the valleys and low grounds. I have already alluded to this characteristic, as shown by the volcanoes of the Old Red Sandstone and Carboniferous periods. But it is displayed by the Permian volcanoes in a still more astonishing way. Beginning at the northern end of the long chain of necks in the West of Scotland, we find a row of them on the plains fronting the volcanic plateau of the Ardrossan, Dunlop and Stewarton Hills. Thence we may follow them, as single individuals or in small groups, across the broad lowland of Ayrshire, southward to the very base of the great chain of the Southern Uplands. There, a cluster of some two dozen of them may be seen rising out of the Carboniferous rocks on the low grounds, but they abruptly cease close to the base of the hills; not one has been detected on the adjacent Silurian heights. Moreover, if we turn into the valleys that lead away from the great Ayrshire plain to the interior, we find necks of the same character in these depressions. They ascend the valley of Muirkirk, and may be met with even at its very head, near the base of the Hagshaw Hills. Again, on the floor of the remarkable transverse valley trenched by the Nith across the Southern Uplands, Permian necks pierce the Coal-measures, while the outlying fragments of bedded lava show that these vents flooded the bottom of that valley with molten rock. Turning out of Nithsdale into the long narrow glen of the Carron Water, we observe its floor and sides to be covered with the sheets of lava and tuff already noticed. And so travelling onward from the vale of the Nith into that of the Capel Water, thence into the Water of Ae and across into the great strath of Annandale, we may detect, if not actual vents, at least the beds of lava and layers of volcanic detritus that were ejected from them.

All along these valleys, which were already valleys in Carboniferous time, traces of the volcanic activity of this epoch may be detected. But, so far as I am aware, in not a single case has any vent been observed to have been opened on the high surrounding ridges. There has obviously been a determining cause why the volcanic orifices should have kept to the plains and the main valleys with their tributaries, and should have avoided the hills which rise now to heights of 500 to 1000 feet or more above the bottoms of the valleys that traverse them. It might be said that the valleys follow lines of fracture, and that the vents have been opened along these lines. But my colleagues in the Geological Survey, as well as myself, have failed, in most cases, to find any evidence of such dislocations among the rocks that form the surface of the country, while it is sometimes possible to prove that they really do not exist there.

Though only a few scattered patches of the Permian bedded lavas and tuffs have been preserved, enough is left to indicate that the vents were active only in the early part of the period represented by the Scottish Permian red sandstones, for it is entirely in the lower part of these strata that volcanic rocks occur. The eruptions gradually ceased, and the sheets of ejected material, probably also the volcanic cones, were buried under at least several hundred feet of red sandstone. Whether or not any portion of the erupted material was for a time built up above the level of the water, there seems to be no question that the vents were, on the whole, subaqueous.

3. Sills.—The phenomena of sills and dykes are less clearly developed among the Permian volcanic rocks of the Ayrshire basin than among those of older formations. In the section exposed in the course of the River Ayr at Howford Bridge, a coarsely crystalline dolerite which extends for nearly 300 yards up the stream, cuts the Permian lavas, of which it encloses patches as well as pieces of sandstone. At the contact, the rock becomes fine-grained (Fig. 206). Through the coarsely crystalline material run long parallel "segregation veins" of a paler, more acid substance, as among the Carboniferous sills. Similar rocks are well seen in the Dippol Burn near Auchinleck House.

Passing outward into the Coal-measures, we encounter a much larger display of similar intrusive sheets. The best district for the study of these sills lies around Dalmellington. The Coal-measures are there traversed by many intrusions, which have produced great destruction among the coal-seams. Some of the rocks are extremely basic, including a beautiful picrite like that of Inchcolm (Letham Hill, near Waterside). The age of these sills must be later than the Coal-measures into which they have been injected. Some of them are obviously connected with the agglomerate-necks, and the whole or the greater number should thus probably be assigned to the Permian period.[94] The phenomena of intrusion presented by these rocks reproduce the appearances already described in connection with the basic intrusive sheets of Carboniferous age.

2. Basin of the Firth of Forth

The other district of Southern Scotland, where traces of volcanic action later in age than the Coal-measures may be observed, lies in the basin of the Firth of Forth (Map V.). They include no bedded lavas, and only at one locality do any relics of a covering of stratified tuffs overspread the Carboniferous formations. The evidence for the old volcanoes consists almost entirely of necks of tuff, which mark the position of vents of eruption.

(1) Vents.—On the south side of the estuary of the Forth there is only one neck which may be plausibly placed in this series. It forms the upper part of Arthur Seat, at Edinburgh. This hill has already been cited as consisting of two distinct portions. The lower, built up of bedded tuffs, basalts and andesites, forms part of the Midlothian volcanic plateau of Carboniferous time. The vent from which these materials were ejected must lie at some little distance, and its site has not been certainly ascertained. The upper part of the hill is formed of a distinct group of rocks which has now to be described.

The geological structure of Arthur Seat has long been well known. It served as a theme for discussion in the Neptunist and Plutonist controversy, and was often referred to in the various mineralogical or geognostical writings of the time. The first thorough examination of it as a relic of ancient volcanic action was that of Charles Maclaren, published in 1839.[95] This author clearly recognized the later age and unconformable position of the coarse mass of agglomerate pierced by the basalt of the apex, and pointed out the evidence of the upheaval and denudation of the older volcanic series during a long interval of repose before the latest eruptions took place. Subsequently Edward Forbes suggested that the upper part of the hill might be of Tertiary age.[96] Thereafter I mapped the ground in detail for the Geological Survey, entirely confirming the observations of Maclaren.[97] In the end it seemed to me that the interval between the two epochs of volcanic activity might not be so great as Forbes had supposed; and after tracing the Permian vents of Ayrshire, I came to the conclusion that the younger unconformable agglomerate of Arthur Seat was not improbably Permian.