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]

[Footnote 91: Mr. Gunn has recently detected among the newest red sandstones of Arran a small patch of volcanic rocks which may be of this age. Mr. A. Macconochie has also found what may be traces of a similar volcanic band below the Permian sandstones of Loch Ryan, in Wigtonshire.]

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.

[Footnote 92: See _Memoirs of Geol. Surv. Scotland_, Sheet 15 (1871), p. 35; Sheet 9 (1877), p. 31.]

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.

[Footnote 93: Explanation of Sheet 23, Geol. Surv. Scotland, p. 39.]

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.

[Footnote 94: Explanation of Sheet 14, Geol. Surv. Scotland, p. 22.]

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.

[Footnote 95: _Geology of Fife and the Lothians_, p. 34. In a reprint of this work, published in 1866, the venerable author briefly remarked in a footnote that he no longer believed in the second period of volcanic activity. This view was adopted in 1875 by Professor Judd, _Quart. Journ. Geol. Soc._ xxxi. p. 131. For the reasons stated in the text I believe Maclaren's original explanation of the structure of the hill to be correct.]

[Footnote 96: Forbes never published his views regarding Arthur Seat, but expounded them to his class, and explained them in diagrams, some of which are preserved in the Edinburgh Museum of Science and Art, in association with the specimens which he collected from the hill.]

[Footnote 97: Sheet 32, Geol. Survey of Scotland and descriptive Memoir. See also _Rep. Brit. Assoc._ 1867, address Geol. Sect., and Murchison's _Siluria_, 4th edit. p. 331.]

The older volcanic series of this hill has been broken through by the agglomerate which occupies a true neck, and is abruptly marked off from all the rocks older than itself. There is no trace of any of the older lavas or tuffs thickening towards this vent. On the contrary they are completely truncated by it, and their outcrops on the north side reappear from under the agglomerate on the south side. Their escarpments are wrapped round by the agglomerate which likewise fills the head of the hollow that had been previously worn by denudation out of the stratified deposits between the oldest lavas. There is thus a violent unconformability between the later and the older volcanic rocks of Arthur Seat.

The length of time indicated by this stratigraphical break must be great. There is no known discordance in the Carboniferous system of the Lothians, yet the Coal-measures, Millstone Grit, Carboniferous Limestone series and much of the Calciferous Sandstones were stripped from this hill before the eruption of the agglomerate. It will be shown in the sequel that a nearly similar amount of denudation preceded some of the probably Permian eruptions of Fife.

The agglomerate contains abundant fragments of the older volcanic series. Its matrix is a dull red gravelly detritus, crowded with blocks of all sizes up to a yard or more in diameter. It is pierced by a column or plug of basalt, which sends veins into it, and rises to the apex of the hill. A beautiful olivine-basalt forms the lateral mass of the Lion's Haunch, which rests on the agglomerate.

In general characters the agglomerate of Arthur Seat resembles that of some of the younger vents of Fife which pierce the Coal-measures and are connected with tuffs that lie unconformably on the Carboniferous Limestone. On these various grounds I think that it may be reasonably assigned to the same geological period.

That a new vent should be opened, after the lapse of one or more geological periods, on or near the site of more ancient volcanic orifices is an incident of which, as we have seen, the geological history of the British Isles furnishes a number of examples. It will be remembered that little more than a mile to the south of Arthur Seat lies the great vent of the Braid Hills, which in the time of the Lower Old Red Sandstone gave forth such a huge pile of lavas and tuffs. Volcanic energy thereafter entirely died away, and in this district was succeeded by a prolonged period of quiescence, during which the Lower Old Red Sandstone was upraised and extensively denuded, while the Upper Old Red Sandstone was deposited. At length, in the immediate neighbourhood, from one or more vents, the exact site of which is not certainly known, the older lavas and tuffs of Arthur Seat, Calton Hill and Craiglockhart Hill were erupted. Again, after another vast interval, a new volcano appeared, and the agglomerate and younger basalts of Arthur Seat were ejected from it. This is one of the most striking examples in this country of the remarkable persistence of volcanic energy in the same locality.

There is no evidence at Arthur Seat itself to fix the geological date of the last volcanic activity of the hill. If the group of younger rocks stood alone, with no other trace of post-Carboniferous eruptions in the surrounding district, a plausible conjecture as to its age would not be easily offered. But in reality it is not a solitary example of such rocks; for within sight, on the opposite side of the Firth of Forth, its counterparts may be seen. To the description of these numerous and clearer illustrations I now proceed.

The East of Fife is remarkable for a large assemblage of volcanic vents, which, unlike those in Ayrshire and Nithsdale, stand alone, their superficial ejections having been removed by denudation, and no connection being traceable between them and any Permian sandstones. The vents filled up with agglomerate and pierced with plugs and veins of basalt, rise through the Carboniferous rocks, but have left no record for precisely defining their geological age. On the one hand, it is quite certain that in this district volcanic eruptions took place during the earlier half of the Carboniferous period. To the north of Largo, and still more distinctly to the north-east of Leven, sections occur to show the contemporaneous outpouring of volcanic rocks during the time of the Carboniferous Limestone. The Leven section, seen in a ravine a little to the north-east of the town, is specially important. It presents a succession of red and green fine sandy tuffs, interstratified with fire-clays and sandstones, and containing a zone of basalt in the centre. These rocks lie not far from the top of the Carboniferous Limestone series.

On the other hand, there is equally clear proof of far later eruptions. From St. Andrews to Elie a chain of necks may be traced, having the same general characters, and piercing alike the Calciferous Sandstones, and the older part of the Carboniferous Limestone series. That these vents must in many cases be long posterior to the rocks among which they rise, is indicated by some curious and interesting kinds of evidence. They are often replete with angular fragments of shale, sandstone and limestone, of precisely the same mineral characters as the surrounding strata, and containing the same organic remains in an identical state of fossilization. It is clear that these strata must have had very much their present lithological aspect before the vents were opened through them. Again, the necks may often be observed to rise among much contorted strata, as, for example, along the crest of a sharp anticlinal arch, or across a synclinal basin. The Carboniferous rocks must thus have been considerably plicated before the time of the volcanic eruptions. In the next place, the vents often occur on lines of dislocation without being affected thereby. They must be posterior, however, not only to these dislocations, but also to much subsequent denudation, inasmuch as their materials overspread the rocks on each side of a fault without displacement. Hence we conclude with confidence, that a great deal of volcanic activity in the East of Fife must have been posterior to most, if not all, of the Carboniferous period.

In the neighbourhood of Largo, further important evidence is presented, confirming and extending this conclusion. The highest member of the Upper Coal-measures, consisting of various red sandstones, with red and purple clays, shales, thin coals and ironstones, is prolonged from the Fife coal-field in a tongue which extends eastward beyond the village of Lower Largo. It is well displayed on the shore, where every bed may be followed in succession along the beach for a space of nearly two miles. Two volcanic necks, presenting the same features as those which pierce the older portions of the Carboniferous system to the east, rise through these red rocks. We are thus carried not only beyond the time of the Carboniferous Limestone, but beyond the close of the very latest stage of the Carboniferous period in Central Scotland. Connected with these and other vents farther north, there is a large area of tuff which has been thrown out upon the faulted and greatly denuded Carboniferous rocks. It may be traced passing from the red Upper Coal-measures across the large fault which here separates that formation from the Carboniferous Limestone, and extending inland athwart different horizons of the latter series. Outlying fragmentary cakes of it may be seen resting on the upturned edges of the sandstones, shales and coal-seams, even at a distance of some miles towards the north-west, proving that the fragmentary materials discharged from the vents spread over a considerable area. The accompanying section (Fig. 208) may serve as an illustration of the relation between this sheet of bedded tuff and the underlying rocks.

Though interstratified volcanic rocks occur in the Carboniferous system of the East of Fife, no connection has been traced between them and any of the vents now referred to. While none of these vents can be proved to be of Carboniferous age, it is of course possible that such may be the true date of some of them. Others, nevertheless, and probably much the largest number, judged from the data just given, may be regarded as probably post-Carboniferous. Those which happen to rise through the uppermost Coal-measures do not appear to be distinguishable by any essential characters from those which pierce indifferently the Carboniferous Limestone series and Calciferous Sandstones of the East of Fife. They seem to be all one connected aggregate, resembling each other alike in their external characters, internal structure and component materials, and the limit of their age must be determined by the geological horizon of the youngest formation which they traverse. By this process of reasoning I reach the conclusion that this remarkable series of old volcanoes in the East of Scotland not improbably dates from the same time as that of Ayrshire and Nithsdale, already described.

Some idea of the importance and interest of the volcanic area of Eastern Fife may be gathered from the fact that in a space of about 70 square miles no fewer than 60 necks may be counted, and others are probably concealed below the drift-deposits which cover so much of the interior of the country. The area of this remarkable display extends from St. Andrews Bay and the Vale of the Eden southwards to the coast of the Firth of Forth between Lundin Links and St. Monans. All over the inland tract the necks form more or less marked eminences, of which the largest are conspicuous landmarks from the southern side of the Firth. But the distinguishing characteristic of the area is the display of the necks along the coast, where, in a series of natural dissections, their form, composition, internal structure and relations to the surrounding rocks have been laid open in such clearness and variety as have been met with in the volcanic records of no other geological period within the compass of these islands. As this district thus possesses a singular interest and value for the study of volcanic vents, I shall enter in some detail into the description of the sections so admirably laid bare.

As in Ayrshire, the necks in the East of Fife generally rise as isolated conical or dome-shaped hills, with smooth grassy slopes, but where a dyke or boss of basalt occurs in them, it usually stands out as a crag or knoll. Largo Law (Fig. 209) may be taken as a singularly perfect example of the cone-shaped neck. This hill, however, comprises more than one vent. The mass of tuff of which it consists probably includes at least three distinct funnels of discharge, and surrounding it there still remains a good deal of the fragmental material that gathered around these vents and is now seen to lie unconformably upon the Carboniferous formations (Fig. 208). There must be a total area of not much less than four square miles over which tuff occupies the surface of the ground.

While the Fife necks possess the great advantage of having been laid bare by the sea, their frequent small size on the coast allows their whole area to be examined. As illustrations of these little vents, two plates are here given from the coast-line to the east of St. Andrews, where a number of small necks of agglomerate have been planted among the plicated Calciferous Sandstones. In Fig. 210 the abrupt truncation of the sandstones by the volcanic rock is well shown. The strata on the right have been broken through, and the sea has indented a small gully along the wall of the old volcanic funnel. The sandstones in front, however, still adhere firmly to the agglomerate, which rises above them as a rugged mass of rock.

In Fig. 212 the edge of the vent can be traced partly in section and partly in plan for about half of its circumference. On the right hand, the actual wall of the funnel is visible where the false-bedded sandstones are sharply cut off by the agglomerate. In front the strata appear in plan on the beach, and their ledges can be seen to the left striking at the margin of the neck.

The shape of the Fife vents is, as usual, generally circular or oval; but is subject to considerable irregularity. The coast-section between Largo and St. Monans exposes many ground-plans of them, and permits their irregularities to be closely examined. The accompanying figure (Fig. 211) exhibits some characteristic forms. Eccentricities of outline no doubt arose from the irregular way in which the rocks yielded to the forces of explosion during the piercing of a volcanic orifice. This is often well shown by the veins and nests of tuff or agglomerate which have been forced into the rents or sinuosities of the orifices. In other cases, however, it is probable that, as among the Ayrshire necks, and those of Carboniferous age already cited, what appears now as one volcanic neck was the result of a shifting of the actual funnel of discharge, so that the neck really represents several closely adjacent vents. The case of Largo Law has been already noticed. The necks at Kellie Law (Fig. 213) show clearly the same structure, the Law itself (1) probably consisting of two contiguous vents, while a third (2) forms a smaller cone immediately to the east. Such a slight lateral displacement of the vent has been noticed at many Tertiary and recent volcanic orifices. In the island or peninsula of Volcanello, for example, three craters indicate successive shiftings of the vent, the most perfect of them marking the latest and diminishing phase of volcanic activity (Fig. 214, compare Fig. 29, vol. i., p. 70).

The Fife necks vary from only a few yards up to perhaps 4000 feet in diameter. One of the smallest and most completely exposed occurs on the shore at Newark Castle, near St. Monans. It measures only 60 yards in length by about 37 yards in breadth. A ground-plan of it is given in Fig 224. Still smaller is the neck at Buddo Ness, on the coast east of St. Andrews, which measures only 20 yards across.

From the way in which the vents have been dissected by the sea along the Fife coast, the geologist is enabled to study in minute detail the effects of the volcanic operations upon the strata through which the funnels have been drilled. Considerable variation may be observed in the nature and amount of change. Sometimes the orifice has been made without any noticeable alteration of the sandstones, shales and limestones, which retain their dip and strike up to the very wall of the chimney. Usually there is more or less jumbling and crushing of the stratification, and often a considerable amount of induration. As a typical example of these effects I give a section from the margin of the neck of tuff on the east side of Elie Harbour (Fig. 215). Here the sandstones and shales (_a_) have been doubled over and dragged down against the tuff (_b_). They have likewise been hardened into a kind of quartzite, and this alteration extends for about 20 to 30 feet from the edge of the neck.

The material which has filled up the vents is almost entirely fragmental, varying from a coarse agglomerate to a fine volcanic tuff. Some minor necks have been completely or in great part filled with angular debris of the ordinary rocks of the neighbourhood. In the western neck on the Largo shore, for example, which rises through the red rocks of the Upper Coal-measures, the material consists largely of fragments of red sandstone, clay and shale. Between Elie and St. Monans, some of the necks are filled almost wholly with debris of black shale and encrinal limestone.

There does not appear to be any relation between the diameter of a funnel and the size of the blocks that now fill it. Some of the larger necks, for example, consist of comparatively fine tuff. The Buddo Ness, on the other hand, though so small a vent, is packed with blocks of shale six feet long, while the sandstone through which the orifice has been drilled passes, as usual, into quartzite for several yards away from the edge. As an example of the general aspect presented by one of the coarse agglomerates in the necks of the Fife coast, a view is given in Fig. 216 of a portion of the neck at Ardross, about two miles east from Elie. This thoroughly volcanic accumulation is here shown to consist of blocks of all sizes heaped together without any definite arrangement.

Since the first stage in the history of the vents has been the perforation of the solid crust by explosion, and the consequent production of debris from the disrupted rocks, we may hope to detect underneath the pile of thoroughly volcanic ejections, traces of the first explosions. I have been much struck with the fact that in the East of Fife such traces may frequently be found here and there within the outer border of the vents. At Largo, and again between Elie and St. Monans, it may be noticed that the mass of material adhering to the wall of a neck, exposed in ground-plan upon the beach, often consists largely, or even wholly, of debris of sandstone, shale and limestone, while the central and chief mass is made up of green tuff or agglomerate, with occasional pieces of the surrounding stratified rocks scattered through it. It seems probable, therefore, that the sections of these Fife necks, laid bare on the present shore, do not lie far below the original crater-bottoms.

Some light might be expected to be thrown upon the phenomena in an active volcanic chimney by the condition of the fragments of recognizable sedimentary rocks imbedded in the ejected debris which has filled up the orifice. But the assistance from this source is neither so full nor so reliable as could be wished. In some of the Fife vents, indeed, the fragments of shale, sandstone and other sedimentary strata are so unchanged that they cannot on a fresh fracture be distinguished from the adjacent parent strata. The _Spirifers_, _Lingulæ_, crinoids, cyprid-cases, ganoid scales and other fossils are often as fresh and perfect in the fragments of rock imbedded in tuff as they are in the rock _in situ_. In some cases, however, distinct, and occasionally even extreme, metamorphism may be detected, varying in intensity from mere induration to the production of a crystalline texture. The amount of alteration has depended not merely upon the heat of the volcanic vent, but also in great measure upon the susceptibility of the fragments to undergo change and the duration of their exposure to it.

Dr. Heddle has computed the temperature to which fragments of shale, etc., in tuff-necks of the Fife coast have been subjected. He found that the bituminous shales have lost all their illuminants, and of organic matter have retained only some black carbonaceous particles; that the encrinal limestones have become granular and crystalline; that the sandstones present themselves as quartzite, and that black carbonaceous clays show every stage of a passage into Lydian-stone. He inferred from the slight depth to which the alteration has penetrated the larger calcareous fragments, that the heat to which they were exposed must have been but of short continuance. As the result of his experiments, he concluded that the temperature at which the fragments were finally ejected from the volcanic vents probably lay between 660° and 900° Fahr.[98]

[Footnote 98: _Trans. Roy. Soc. Edin._ vol. xxviii. p. 487.]

It may be perhaps legitimate to infer that, while the fragments that fell back into the volcanic funnel, or which were detached from the sides of the vent, after having been exposed for some time to intense heat under considerable pressure, would suffer more or less metamorphism, those, on the other hand, which were discharged by the æriform explosions from the cool upper crust, on the first outburst of a vent, would not exhibit any trace of such a change. Where, therefore, we meet with a neck full of fragments of unaltered stratified rocks, we may suppose it to have been that of a short-lived volcano; where, on the other hand, the fragments are few and much altered, they may mark the site of a vent which continued longer active. The metamorphism of the fragmentary contents of a volcanic funnel by the action of ascending vapours has already been described in the case of one of the vents of the Carboniferous plateaux (vol. i. p. 404).

One of the most curious and puzzling features in the contents of the tuff necks of the Fife coast is the occurrence there of crystals and fragments of minerals, often of considerable size, which do not bear evidence of having-been formed _in situ_, but have undoubtedly been ejected with the other detritus. Dr. Heddle has noticed the fact, and has described some of the minerals which occur in this way. The following list comprises the species which he and I have found:--

Hornblende, in rounded fragments of a glassy black cleavable variety. Augite, sometimes in small crystals, elsewhere in rounded fragments of an augitic glass. Orthoclase (Sanidine), abundant in worn twin crystals in the tuffs of the east of Fife. Plagioclase. Biotite. Pyrope, in the tuffs (and more rarely in the basalts) of Elie. Nigrine, common in some of the dykes, more rarely in the tuffs of Elie. Saponite, Delessite and other decomposition products. Semi-opal, one specimen found in the later (Permian?) agglomerate of Arthur's Seat. Asphalt, abundant at Kincraig, near Elie. Fragments of wood, with structure well preserved, may be included here.

Dr. Heddle has described from the neck of tuff at Kinkell, near St. Andrews, large twin crystals of a glassy orthoclase, which are invariably much worn, and preserve only rudely the form of crystals. He justly remarks that they have no connection with drusy cavity, exfiltration vein, or with any other mineral, and look as if a portion of their substance has been dissolved away. Internally, however, they are quite fresh and brilliant in lustre, though sometimes much fissured.[99]

[Footnote 99: _Trans. Roy. Soc. Edin._ vol. xxviii. p. 223.]

The tuffs at Elie are full of similar crystals. I obtained from one of the necks east of that village a specimen which measures 4 inches in length, 3-1/2 in breadth, and 2-1/4 in thickness, and weighs about 2 lbs. It is, however, a well-striated felspar. From the same tuff I procured an orthoclase twin in the Carlsbad form. All the felspar pieces, though fresh and brilliant internally, have the same rounded and abraded external appearance.

The fragments of hornblende form a characteristic feature in several of the Elie dykes (to be afterwards described), and in the neighbourhood of these intrusive rocks occur more sparingly in the tuff. It is a glossy-black cleavable mineral, in rounded pieces of all sizes, up to that of a small egg. Dr. Heddle obtained a cleavage angle of 124° 19', and found on analysis that the mineral was hornblende.[100]

[Footnote 100: _Op. cit._ xxviii. p. 522.]

Augite occurs sparingly in two forms among the rocks. I have obtained small crystals from the red agglomerate on the south side of Arthur Seat, recalling in their general appearance those of Somma. Lumps of an augitic glass have been found by Dr. Heddle, sometimes as large as a pigeon's egg, in two of the dykes at Elie, and in the tuff at the Kinkell neck, near St. Andrews. He observed the same substance at the Giant's Causeway, both in the basalt and scattered through one of the interstratified beds of red bole. Much larger rounded masses of a similar augitic glass, but with a distinct trace of cleavage, have already been referred to as occurring in a volcanic vent of Upper Old Red Sandstone age at John o' Groat's House.[101]

[Footnote 101: _Op. cit._ xxviii. pp. 481 _et seq._, and _ante_, vol. i. p. 352.]

Biotite is not a rare mineral in some tuffs. It may be obtained in Lower Carboniferous tuffs of Dunbar, in plates nearly an inch broad; but the largest specimen I have obtained is one from the same Elie vent which yielded the large felspar fragment. It measures 2-1/2 × 2 × 1/2 inches. These mica tables, like the other minerals, are abraded specimens.

That these various minerals were ejected as fragments, and have not been formed _in situ_, is the conclusion forced upon the observer who examines carefully their mode of occurrence. Some of them were carried up to the surface by liquid volcanic mud, and appear in dykes of that material like plums in a cake. But even there they present the same evidence of attrition. They assuredly have not been formed in the dykes any more than in the surrounding tuff. In both cases they are extraneous objects which have been accidentally involved in the volcanic rocks. Dr. Heddle remarks that the occurrence of the worn pieces of orthoclase in the tuff is an enigma to him. I have been as unable to frame any satisfactory explanation of it.[102]

[Footnote 102: Occasionally the crystals can be matched in some lava-form rock of the same volcanic area; but many of them have no such counterparts. See vol. i. p. 62 and _note_.]

It might have been thought that within the throat of a volcano, if in any circumstances, loose materials should have taken an indefinite amorphous aggregation. And, as has been shown in the foregoing chapters, this is usually the case where the materials are coarse and the vent small. Oblong blocks are found stuck on end, while small and large are all mixed confusedly together. But in numerous cases where the tuff is more gravelly in texture, and sometimes even where it is coarse, traces of stratification may be observed. Layers of coarse and fine material succeed each other, as they are seen to do among the ordinary interstratified tuffs. The stratification is usually at high angles of inclination, often vertical. So distinctly do the lines of deposit appear amid the confused and jumbled masses, that an observer may be tempted to explain the problem by supposing the tuff to belong not to a neck, but to an interbedded deposit which has somehow been broken up by dislocations. That the stratification, however, belongs to the original volcanic vents themselves is made exceedingly clear by some of the coast-sections in the East of Fife. On both sides of Elie, examples occur in which a distinct circular disposition of the bedding can be traced corresponding to the general form of the neck. The accompanying ground-plan (Fig. 217) represents this structure as seen in the neck which forms the headland at Elie harbour. Alternations of coarse and fine tuff with bands of coarse agglomerate, dipping at angles of 60° and upwards, may be traced round about half of the circle. The incomplete part may have been destroyed by the formation of another contiguous neck immediately to the east. To the west of Earlsferry another large, but also imperfect, circle may be traced in one of the shore necks. A quarter of a mile farther west rises the great cliff-line of Kincraig, where a large neck has been cut open into a range of precipices 200 feet high, as well as into a tide-washed platform more than half a mile long. The inward dip and high angles of the tuff are admirably laid bare along that portion of the coast-line. The section in which almost every bed can be seen, and where, therefore, there is no need for hypothetical restoration, is as shown in Fig. 218.

I have already referred to the frequently abundant pieces of stratified tuff, found as ejected blocks in vents filled with tuff, and to the derivation of these blocks from tuff originally deposited within the crater. There can, I think, be little hesitation in regarding the stratification of these Fife vents as a record of successive deposits of volcanic detritus inside the vents. The general dip inwards from the outer rim of the vent strikingly recalls that of some modern volcanoes. By way of illustration, I give here a section of part of the outer rim of the crater of the Island of Volcano, sketched by myself in the year 1870 while ascending the mountain from the north side (Fig. 220). The crater wall at this point consists of two distinct parts--an older tuff (_a_), which may have been in great measure cleared out of the crater before the ejection of the newer tuff (_b_). The latter lies on the outer slope of the cone at the usual angle of 30°. It folds over the crest of the rim, and dips down to the flat tuff-covered crater bottom, at an angle of 37°. These are its natural angles of repose.

Applying modern analogies of this kind, I have been led to conclude that the stratification so conspicuous in the tuff of the vents in the east of Fife and in the Carboniferous series of the Lothians belongs to the interior of the crater and the upper part of the volcanic funnel.[103] These stratified tuffs, on this view of their origin, must be regarded as remains of the beds of dust and stones which gathered within the crater and volcanic orifice, and which, on the cessation of volcanic action, sometimes remained in their original position, or were dislocated and slipped down into the cavity beneath. That the tuffs consolidated on slopes, perhaps quite as steep as those of Volcano, is now and then indicated by an interesting structure. The larger stones imbedded in the layers of tuff may be observed to have on their fronts in one direction a small heap of coarse gravelly debris, while fine tuff is heaped up against their opposite side. This arrangement doubtless points to deposit on a slope of loose debris, from which the larger blocks protruded so as to arrest the smaller stones, and allow the fine dust to gather behind.

[Footnote 103: Further illustrations of this characteristic structure of some vents will be found in the account of the Tertiary vents of the Faroe Isles in Chapter xli. See also the remarks in the introductory chapters, vol. i. p. 63.]

If the inference be correct, that the stratification here described belongs to the old craters or the upper parts of the funnels, it furnishes additional evidence of the wide interval of time that elapsed between the deposition of the Carboniferous strata and the outbreak of these vents. During that interval prolonged denudation reduced the upturned Carboniferous Limestone series to nearly its present form of surface, and any materials discharged from the vents over the surrounding ground would obviously lie with a violent unconformability on the rocks below.

The frequent great disturbance in the bedding of the tuff within the vents may be connected with some kind of collapse, subsidence or shrinkage of the materials in the funnel below. That a movement of this nature did take place is shown by the remarkable bending down of the strata round the margins of the vents, which has been already described.

The minor vents for the most part contain only fragmentary materials; but those of larger size usually present masses of lava in some characteristic forms. In not a few cases, the lava has risen in the central pipe and has hardened there into a column of solid rock. Subsequent denudation, by removing most of the cone, has left the top of this thick column projecting as a round knoll upon the hill-top. Arthur Seat presents a good example of this structure. Where the denudation has not proceeded so far, we may still meet with a remnant of the cake of lava which sometimes overflowed the bottom of a crater. The summit of Largo Law affords indications of this arrangement, the cone of tuff being there capped with basalt, evidently the product of successive streams, which welling out irregularly covered the crater bottom with hummocks and hollows (Fig. 226). The knolls are beautifully columnar, and sometimes show a divergent arrangement of the prisms.

But the most frequent form assumed by the lava in the necks is that of veins or dykes running as wall-like bands through the tuff or agglomerate. Many admirable examples may be cited from the shore between Largo and St. Monans. Two illustrations of them are given in Figs. 219 and 221. In Fig. 219 the dyke is about four feet broad, and is seen to present the common transverse jointing as it pursues its way through the tuff. White veins of calcite along its margin serve to define its limits. Its position in reference to the general body of the neck is shown in the ground-plan Fig. 224. The second instance (Fig. 221) is that of a dyke of basalt only one foot wide, which runs like a wall up the agglomerate of the Kincraig neck near Elie. It is seen at the bottom of the cliff projecting from the agglomerate; but higher up it has decayed, leaving its fissure as a gaping chasm. Here the general character of the pyroclastic material is well brought out. One or two large blocks may be seen imbedded in it, the largest lying above where the dyke bends away to the left.

The intruded masses vary in breadth from mere threadlike veins up to dykes several yards in breadth, which sometimes expand into large irregular lumps. They generally consist of some form of basalt; now and then, as at Ruddon Point, near Elie, they are amygdaloidal; and it may be observed among them, as among dykes in general, that where the amygdaloidal texture is developed, it is apt to occur most markedly in the central part of the vein, the amygdales running there in one or more lines parallel with the general trend of the mass.

That the basalt of these veins and dykes was sometimes injected in an extremely liquid condition is shown by its frequently exceedingly close homogeneous texture. Within the neck on the shore to the west of Largo, the basalt assumes in places an almost flinty character, which here and there passes into a thin external varnish of basalt-glass. A farther indication of the liquidity of the original rock seems to be furnished by the great number of included extraneous fragments here and there to be observed in the basalt.

But besides basalt, other materials may more rarely be detected assuming the form of dykes or veins within the necks. Thus, at the Largo neck just referred to, strings of an exceedingly horny quartz-felsite accompany the basalt--a remarkable conjunction of acid and basic rock within the same volcanic chimney. To the east of Elie some dykes, which stand out prominently on the beach from a platform worn by the sea in a neck, consist of an extremely compact volcanic mudstone, stuck full of the worn twin crystals of orthoclase and pieces of hornblende and biotite already noticed. So like is this rock to one of the decomposing basalts that its true fragmental nature may easily escape notice, and it might be classed confidently as a somewhat decayed basalt. A considerable amount of a similar fine compact mudstone is to be seen round the edges of some of the Elie vents. This material must have been injected into open fissures, where it solidified. There is further evidence of the presence of "mud-lava" in some of the vents of East Fife, where these orifices contain a remarkable compact volcanic sandstone, composed of the usual detritus, but weathering into spheroidal crusts, so as externally to be readily mistaken for some form of basalt.

A columnar arrangement may often be observed among the basalt dykes. When the vein or dyke is vertical, the columns of course seem piled in horizontal layers one above the other. The exposed side of the dyke then reveals a wall of rock, seemingly built up of hexagonal or polygonal, neatly fitting blocks of masonry, as in the Lower Carboniferous vent of the Binn of Burntisland (Figs. 166, 168). An inclination of the dyke from the vertical throws up the columns to a proportional departure from the horizontal. Sometimes a beautiful fan-shaped grouping of the prisms has taken place. Of this structure the Rock and Spindle, near St. Andrews, presents a familiar example (Fig. 222). Much more striking, however, though less known, is the magnificent basalt mass of Kincraig, to the west of Elie, where the columns sweep from summit to base of the cliff, a height of fully 150 feet, like the Orgues d'Expailly, near Le Puy in Auvergne. The general position of this basalt in the vent is represented in the section (B, Fig. 218). The curvature of the basalt is shown in Fig. 223, which is taken from the Elie side looking westward, beyond the intrusions, to the picturesque cliffs of tuff. The details of the cliff are given in Fig. 225.

That many of the dykes served as lines of escape for the basalt to the outer slopes of the cones is highly probable, though denudation has usually destroyed the proofs of such an outflow. A distinct radiation of the dykes from the centre of a neck is still sometimes traceable. This structure is most marked on the south cone of Largo Law, where a number of hard ribs of basalt project from the slopes of the hill. Their general trend is such that if prolonged they would meet somewhere in the centre of the cone. On the south-east side of the hill a minor eminence, termed the Craig Rock, stands out prominently (Fig. 209). It is oblong in shape, and, like the dykes, points towards the centre of the cone. It consists of a compact columnar basalt, the columns converging from the sides towards the top of the ridge. It looks like the fragment of a lava-current which flowed down a gully on the outer slope of the cone (B' in Fig. 226).

Veins of basalt are not confined to the necks, but may be seen running across the surrounding rocks. The shore at St. Monans furnishes some instructive examples of this character. As the veins thin away from the main mass of basalt they become more close-grained and lighter in colour, and when they enter dark shales or other carbonaceous rocks they pass, as usual, into the white earthy clay-like "white-trap." The influence of carbonaceous strata in thus altering basic dykes and sills may be instructively studied along the shore of the East of Fife. A good instance occurs near St. Monans Church (Fig. 227), where a vein of "white-trap" traverses black shales which have been somewhat jumbled.

In a modern volcano no opportunity is afforded of examining the contact of the erupted material with the rocks through which the vent has been opened. But in the basin of the Firth of Forth, within the area now under description, a numerous series of coast-sections lays bare this relation in the most satisfactory manner. The superincumbent cones of tuff have been swept away, and we can examine, as it were, the very roots of the old volcanoes. The margin of a neck or volcanic vent is thus found to be almost always sharply defined. The rocks through which the funnel has been drilled have been cut across, as if a huge auger had been sunk through them. This is well displayed in the beautifully perfect neck already cited at Newark Castle, near St. Monans (Fig. 224). The strata through which this neck rises consist of shales, sandstones, thin coal and encrinal limestones, dipping in a westerly direction at angles ranging from 25° to 60°. At the south end of the neck they are sharply truncated, as if by a fault. Elsewhere they are much jumbled, slender vein-like portions of the tuff being insinuated among the projecting strata. A large vertical bed of sandstone, 24 yards long by 7 yards broad, stands up as a sinuous reef on the east side of the vent (_s_). It is a portion of some of the surrounding strata, but, so far as can be seen at the surface, is entirely surrounded with agglomerate. Here and there the shales have been excessively crumpled, and at the north end have been invaded by a vein of basalt which, where it runs through them, assumes the usual clay-like character. The strata have been blown out, and their place has been occupied by a corresponding mass of volcanic agglomerate. But their remaining truncated edges round the margin of the orifice have undergone comparatively little alteration. In some places they have been hardened, but their usual texture and structure remain unaffected.

In a few examples, the progress of denudation has not advanced so far that the cone cannot still be partially made out amidst its surrounding masses of tuff. One of the most interesting of these is Largo Law, of which an outline has been given in Fig. 209. The accompanying section (Fig. 226) represents what appears to me to be the structure of this hill. Each of the two now conjoined cones was probably in succession the vent of the volcano. The southern and rather lower eminence, as already mentioned, is traversed by rib-like dykes of basalt, which point towards its top, where there is a bed of the same rock underlying a capping of tuff. On its eastern declivity lies the basalt stream already described (p. 87). The higher cone is surmounted by a cake of basalt which, as I have above suggested, may have solidified at the bottom of the latest crater. Of course all trace of the crater has disappeared, but the general conical form of the volcanic mass remains. Doubtless, still more of the old volcano would have been removed by denudation but for the protection afforded to the tuff by the intrusion of the basalt. The upper dotted lines in the figure are inserted merely to indicate hypothetically how the cone may originally have stood. On the west side the sheets of tuff which were thrown out over the surrounding country have been almost entirely removed, but on the east and south they still cover an extensive area. (See Fig. 208).

(2) _Sills._--In the Clyde coal-field and in the basin of the Firth of Forth, among the vast number of sills which there traverse the Carboniferous formations, it is possible that some belong to the Permian volcanic period (see vol. i. p. 474). Where the sheets have been intruded along horizons that lie below the upper stratigraphical limit of the puy eruptions, they may not unnaturally be held to belong to these manifestations of volcanic energy, though it is obviously quite conceivable that some of them may be of much later date. But where they lie above the highest platforms of Carboniferous lavas and tuffs, they may be assigned to a younger volcanic period. We know as yet of only two such periods after the deposition of the Carboniferous Limestone series in Scotland--Permian and older Tertiary. Unless, therefore, these higher sills formed part of some other display of subterranean activity which is not known to have culminated in eruptions at the surface, they must be looked upon as probably either Permian or Tertiary.

In the great coal-field of Stirlingshire and Lanarkshire, among the large sills that break into the Millstone Grit and the Coal-measures, one lies entirely in the Coal-measures, and covers about six square miles of ground, stretching from near Caldercruix Station, a little east of Airdrie, to near Kirk of Shotts, a distance of about four miles. A group of smaller sheets, possibly connected with the larger mass, runs for four miles further west to beyond New Monkland. Another chain of sills, which may also be part of the same great intrusion, extends from the Cant Hills, near the Kirk of Shotts, for more than eight miles in a north-easterly direction. The largest mass in this chain stretches from Blackridge, west of Bathgate, for upwards of three miles, covering an area of about three square miles and terminating on the north at the line of dislocation which has been followed by one of the east and west dykes. Another large sill, which appears nearly two miles further east on the north side of that dyke, lies on a lower stratigraphical horizon, for it cuts the Carboniferous Limestone series, and does not reach the top of the Millstone Grit. This sill is cut through by two of the later dykes.

That these great intrusions took place later than the deposition of the Coal-measures is obvious. There is no satisfactory evidence to enable us to decide to which of the two post-Carboniferous volcanic periods they may with most probability be assigned. As one of them is distinctly cut by dykes that have been referred to the Tertiary series, it might be plausibly argued that it at least is of pre-Tertiary date, and therefore probably Permian. On the other hand, as will be shown in a later chapter, some portion of the sills appears to be connected with the younger or Tertiary dykes. This problem must for the present remain unsolved.

In Ayrshire where, as already described, basic sills traverse the Permian volcanic series, other large intrusive sheets are found around the Permian basin. On the north side an important group of them, passing through the Coal-measures into the Carboniferous Limestone series, runs from Troon eastward for more than eight miles to beyond Craigie. On the south side a much more extensive series may be traced from the River Ayr southwards into the Dalmellington coal-field, and thence north-eastwards in a wide semicircular sweep into the coal-field of New Cumnock and Airds Moss. That some of these sills proceed from the Permian necks has been definitely ascertained, and this fact has been already alluded to in connection with the vents. I have little doubt that the great majority, if not the whole, of these intrusive sheets are to be referred to the Permian period.

Some of the sills must be later than a part of the Permian volcanic eruptions, for they are found in at least three places intercalated in the zone of lavas and tuffs. But no instance has been observed of their traversing the basin of Permian sandstone which overlies that zone, though a few dykes, possibly of Tertiary age, do cut this sandstone.