The Ancient Volcanoes of Great Britain, Volume 1 (of 2)

We have now to consider the manner in which the various volcanic products have been grouped around and within the orifices of discharge. The first feature to arrest the eye of a trained geologist who approaches them as they are displayed in one of the ranges of hills in Central Scotland is the bedded aspect of the rocks. If, for example, he looks eastward from the head of the Firth of Tay, he marks on the right hand, running for many miles through the county of Fife, a succession of parallel escarpments, of which the steep fronts face northwards, while their long dip-slopes descend towards the south. On his left hand a similar but higher series of escarpments, stretching far eastwards into Forfarshire, through the chain of the Sidlaw Hills, repeats the same features, but in opposite directions. If he stands on the alluvial plain of the Forth, near Stirling, and looks towards the north, he can trace bar after bar of brown rock and grassy slope rising from base to summit of the western end of the Ochil Hills. If, again, from any height on the southern outskirts of the city of Edinburgh, he lets his eye range along the north-western front of the chain of the Pentland Hills, especially towards evening, he can follow the same parallel banding as a conspicuous feature on each successive hill that mounts above the plain. Or if, as he traverses the west of Argyllshire, he comes in sight of the uplands of Lorne, he at once recognizes the terraced contours of the hills between Loch Awe and the western sea, presenting so strange a contrast to the rugged and irregular outlines of the more ancient schist and granite mountains all around (see Fig. 99).

i. BEDDED LAVAS AND TUFFS

On a nearer inspection, the dominant topographical features are found to correspond with a well-marked stratification of the whole volcanic series. Where two sheets of andesite are separated by layers of tuff, sandstone or conglomerate, a well-marked hollow will often be found to indicate the junction-line; but even where the lavas follow each other without such interstratifications, their differences of texture and consequent variations in mode and amount of weathering usually suffice to mark them off from each other, and to indicate their trend along the surface in successive terraces. Even where the angles of inclination are high, the bedded arrangement can generally be detected.

It is in the picturesque and instructive coast-sections, however, that the details of this bedded structure are most clearly displayed. On both sides of the country, along the shores of Ayrshire on the west, and those of Kincardineshire and Forfarshire on the east, the volcanic group has been admirably dissected by the waves. The lava-beds have been cut in vertical section, so that their structure and their mode of superposition, one over another, can be conveniently studied, while at the same time, the upper surfaces of many of the flows have been once more laid bare as they existed before they were buried under the sedimentary accumulations of the waters in which they were erupted.

Though distinctly bedded, the Lavas show little of the regularity and persistence so characteristic of those of Carboniferous and of Tertiary time. Some of them are not more than from four to ten feet thick, and generally, on the coast-cliffs, they appear to be less than fifty feet. A continuous group of sheets can sometimes be traced for ten miles or more from the probable vent of discharge.

That many of these lavas were erupted in a markedly pasty condition may be inferred from certain of their more prominent characteristics. Sometimes, indeed, they appear as tolerably dense homogeneous masses, breaking with a kind of prismatic jointing; but more frequently they are strongly amygdaloidal, and sometimes so much so that, as already stated, the amygdales form the larger proportion of their bulk. Where the secondary infiltration-products have weathered out, the rough scoriform rock looks as if it might only recently have been erupted. In a few instances I have observed an undulating rope-like surface, which reminded me of well-known Vesuvian lavas. Usually the top and bottom of each sheet assume a strikingly slaggy aspect, which here and there is exaggerated to such an extent that between the more solid and homogeneous parts of two consecutive flows an intermediate band occurs, ten or twelve feet thick, made up of clinker-like lumps of slag, the interspaces being filled in with hardened sand. In some cases these agglomeratic layers may actually consist in part of ejected blocks; but the way in which many of the lavas have cooled in rugged scoriaceous surfaces is as conspicuous as on any modern coulée. The loosened slags, or the broken-up cakes and blocks of lava, have sometimes been caught up in the still moving, pasty current, which has congealed with its vesicles drawn out round the enclosed fragments, giving rise to a mass that might be taken for a breccia or agglomerate. Now and then we may observe that the upper slaggy portion of a sheet has assumed a bright red colour from the oxidation of its ferruginous minerals; and from the contrast it thus presents to the rest of the rock we may perhaps legitimately infer that the disintegration took place before the outflow of the next succeeding lava. If this inference be well founded, and it is confirmed by other evidence which will be subsequently adduced, it points to the probable lapse of considerable intervals of time between some of the outflows of lava.

But perhaps the most singular structure displayed by these lavas is to be seen in the manner in which they are traversed by and enclose portions of sandstone. Since I originally observed this feature on the Ayrshire coast, near Turnberry Point, many years ago,[342] I have repeatedly met with it in the various volcanic districts of the Lower Old Red Sandstone across the whole of the Midland Valley of Scotland. The first and natural inference which a cursory examination of it suggests is that the molten rock has caught up and carried along pieces of already consolidated sandstone. But a little further observation will show that the lines of stratification in the sandstone, even in what appear to be detached fragments, are marked by a general parallelism, and lie in the same general plane with the surface of the bed of lava in which the sandy material is enclosed. In a vertical section the sandstone is seen to occur sometimes in narrow dykes with even, parallel walls, but more usually in irregular twisting and branching veins, or even in lumps which, though probably once connected with some of these veins, now appear as if entirely detached from them (Fig. 65). Frequently, indeed, the nodular slaggy andesite and the sandstone are so mixed up that the observer may hesitate whether to describe the mass as a sandstone enclosing balls and blocks of lava, or as a scoriaceous lava permeated with hardened sand. A close connection may be traced between these sandstone-inclosures and the beds of sandstone interstratified between the successive lavas. We can follow the sandy material downwards from these intercalated beds into the andesites below them. On exposed upper surfaces of the lava, an intricate reticulation of sandstone veins may be noticed, in each of which the stratification of the material runs across the veins, showing sometimes distinct current-bedding, but maintaining a general parallelism with the bedding of the volcanic sheets and their fragmentary accompaniments (Fig. 66). If we could remove the sandstone-veinings and aggregates, we should find the upper surfaces of these igneous masses to present a singularly fissured and slaggy appearance, reminding us of the rugged, rent and clinker-loaded slopes of a modern viscous lava, like some of those in the Atrio del Cavallo on Vesuvius. There cannot, therefore, be any doubt that the sandstone, so irregularly dispersed through these lavas, was introduced originally as loose sand washed in from above so as to fill the numerous rents and cavernous interspaces of the volcanic rock. A more striking proof of the subaqueous character of the eruptions could hardly be conceived. This interesting feature in lavas erupted under water is not confined to the volcanic series of the Old Red Sandstone. We shall find that it is hardly less distinct among the basic lavas of the Permian series both in Scotland and in Devonshire.

A remarkable exception to the general type of dark basic and intermediate lavas is furnished by the pale, decomposing felsites of the Pentland and Dolphinton Hills. Those which issue from the great eruptive centre of the Braid Hills, alternate with the andesites and the diabases, gradually diminishing like these in a southward direction and dying out in some six or seven miles. Beyond the limits of these lavas, another similar thick group was erupted from a separate vent at the northern end of the Biggar district near Dolphinton. The same occurrence has been ascertained also in the area of the Ochil chain. Fuller reference will be made to these interesting rocks in the descriptions to be afterwards given of the structure and history of the volcanic areas of the Pentland Hills, the Biggar centre and the Ochil Hills.

It is certainly a notable feature in the volcanism of Old Red Sandstone time that from the same, or from closely adjoining vents, lavas should be alternately poured forth, differing so much from each other, alike in chemical composition and petrographical characters, as andesites and diabases on the one hand, and felsites on the other. Additional examples, from widely different geological systems, will be cited in subsequent pages. It will be shown that even in the very latest volcanic period in Britain, that of older Tertiary time, highly basic and markedly acid materials were ejected from the same centres of eruption.

The part taken by the Tuffs in the structure of the ground agrees with what might have been expected in the accompaniments of extremely slaggy and viscid lavas. These pyroclastic intercalations are, in most of the volcanic districts, comparatively insignificant in amount, by far the largest proportion of solid material ejected from the various vents having consisted of streams of lava. Round or within some of the vents the fragmentary materials attain a remarkable coarseness, as may be seen in the great agglomerates of Dumyat, near Stirling, the largest of which is more than 700 feet thick. These massive accumulations doubtless represent a long series of explosive discharges from the summit of the lava column in one or more adjacent vents. Traced away from the orifices of emission, the tuffs rapidly grow finer in grain, less in thickness, and more mixed with ordinary detritus, until they pass into ordinary non-volcanic sediment or die out between the lava-sheets.

Good sections, showing the nature and arrangement of the thin intercalations of andesite-tuff between the successive outpourings of lava, may be examined on the coast. Thus, near Turnberry Point, in Ayrshire, upwards of a dozen successive flows of lava, with their sandy and ashy intervening layers, are exposed in plan upon the beach, and partly also in section along the cliffs on which the ruins of the historic castle of Turnberry stand. (Figs. 95, 96, 97). Again, along the coast of Forfarshire, from the Red Head to Montrose, the numerous sheets of andesite are separated by layers of dull purplish tuff passing into conglomerate, with blocks of porphyrite a yard or more in diameter.

The most remarkable interstratified tuffs in the Lower Old Red Sandstone are the felsitic varieties. Those which proceed from the great vent of the Braid Hills, extend south-westwards for eight or nine miles, and their peculiar materials, mixed with ordinary sediment, may be traced several miles further. They occur in successive sheets, which, from a maximum thickness and number at the north end, gradually thin away southwards, like the felsitic lavas which they accompany, and from the explosion of which they no doubt were derived. They consist to a large extent of extremely fine volcanic dust, and since they are generally much decomposed, it is often, as already remarked, hardly possible to distinguish between them and the equally decayed felsites. In some parts of the hills they present a distinct fissile bedding; but still more satisfactory is the occasional fine brecciated structure which they assume, when they are seen to consist of angular lapilli of different felsites.

The amount of volcanic material ejected from the more important vents was much greater than the height of the present hills would lead us to suppose. The rocks have generally been tilted into positions much more inclined than those which they originally occupied, so that to measure their actual thickness we must take a line approximately perpendicular to the dip. In this way we ascertain that the accumulated mass of lavas and tuffs immediately outside the vent at the north end of the Pentland Hills must be at least 7000 feet thick, for the base of the series is concealed under the unconformable overlap of the Lower Carboniferous Sandstones, while the top is cut off by a fault which brings down the Carboniferous formations against the eastern flank of the hills. Probably not less voluminous is the pile of ejected material in the Ochil Hills, where, though the base of the whole is concealed by the fault which throws down the coal-field, some 6500 feet of lavas, tuffs and conglomerates can be seen. There were thus, during the time of the Lower Old Red Sandstone, more than one volcano in Central Scotland which might be compared in bulk of ejected material to Vesuvius.

That the eruptions were mainly subaqueous is indicated, as I have shown, by the intercalated bands of sandstone and conglomerate between the successive lavas, as these are traced away from the centres of discharge, and likewise, even more impressively, by the hardened sand which has been washed into former fissures and crevices in the lava. But that, in some cases, the volcanic cones were built up above the surface of the lake may be legitimately inferred from the remarkable volcanic conglomerates which occur, more particularly in the great chain of the Ochil and Sidlaw Hills. These thick accumulations of well-rounded and water-worn blocks are interspersed between sheets of andesite, and are mainly made up of andesite fragments. Impressive sections of them may be seen along the Kincardineshire coast. The conglomerates are sometimes so remarkably coarse, many of their blocks exceeding two feet in diameter, and so rudely bedded, that it is only by noting the position of oblong boulders that one can make out the general direction of the stratification. In their smooth rounded forms, these blocks resemble the materials of storm-beaches on an exposed coast. The trituration of the andesite fragments has given rise to a certain amount of green paste, which firmly wraps round the stones, and retains casts of them after they have dropped out. It is further deserving of remark that while in some districts, as in the central Ochils, the materials were entirely derived from the destruction of volcanic rocks, in others a large proportion of non-volcanic materials is mingled with the debris of the lavas. South of Stonehaven, for example, large boulders of quartzite form a conspicuous feature in the conglomerates, of which in places they make up quite half of the total constituents. There can be little doubt, I think, that the materials of these coarse detrital accumulations were gathered together as shingle-beaches, and were derived in part from volcanic cones which had risen above the level of the lake. They seem to suggest considerable degradation of these cones by breaker-action, whereby blocks of rock a yard or more in diameter could be rounded and smoothed.

Another inference deducible from such conglomerates, and to which I have already alluded, is that considerable intervals of time took place between some of the eruptions. Round the vents, indeed, where the successive sheets of volcanic material follow each other continuously, it is perhaps impossible to form any definite opinion as to the relative chronological value of the lines of separation between different ejections. But where some hundreds of feet of coarse conglomerate, chiefly composed of well-rounded andesite blocks, intervene between two streams of lava, we may conclude that the interval between the outpouring of these streams must have been of considerable duration. Other evidence of a similar tendency may be recognized in the intercalation of groups of varied sedimentary accumulations, such as those which were deposited over the site of Eastern Forfarshire and Kincardineshire during the time that elapsed between two successive floods of lava. In the Den of Canterland, for example, in the midst of the volcanic sheets we find interesting evidence of one of these intervals of quiescence, during which layers of fine olive shales were laid quietly down, while macerated vegetation, drifting over the lake-bottom, was buried with remains of fishes, and abundant gally-worms (Kampecaris, Archidesmus), washed from the neighbouring land.[343] So undisturbed were the conditions of deposition that calcareous sediment gathered round some of the organisms and encased them in limestone nodules.

In some of the districts the discharges of volcanic material were so abundant or so continuous that no recognizable deposition of ordinary sediment has taken place between them. Thus, at the north end of the Pentland Hills the rocks are entirely of volcanic origin, and though, as we trace them southwards away from the centre of eruption, they diminish in thickness, they include hardly any interstratified sandstones and conglomerates until they finally begin to die out.

The distances to which the lavas and tuffs have been erupted from the chief vents of a district vary up to 15 or 20 miles. Those of the Pentland Hills extend from the Braid Hill vent for 10 miles to the south-west. Those of the Biggar centre stretch for about 16 miles to the north-east. Those of the Ochil Hills, which probably came from a number of distinct vents, can be traced for nearly 50 miles.

ii. VENTS

On the whole the actual vents of the volcanoes of Lower Old Red Sandstone time are less clearly distinguishable than those of subsequent volcanic periods. This deficiency doubtless arises from the geological structure of the districts in which the formation is chiefly developed. Thus, in the great Midland Valley of Scotland, where the Old Red Sandstone covers a large part of the surface, the vents seem to have been placed along the central parts of the long trough rather than among the older rocks on either margin. Hence they are in large measure buried either under the volcanic and sedimentary accumulations of their own period or under Carboniferous strata.

Certain bosses of massive rocks lying well within the volcanic area may with some confidence be regarded as the sites of eruptive centres. They occur either singly or in groups, and may be specially noticed along the chain of the Ochil and Sidlaw Hills. Yet it seems to me probable that these visible bosses, even if we are correct in regarding them as marking the positions of true vents, do not indicate the chief orifices of discharge. If we consider their size and their distribution with reference to the areas of lava and tuff discharged at the surface, we are rather led to look upon them as subsidiary vents, the more important orifices, from which the main bulk of the eruptions took place, being still concealed under the Carboniferous rocks of the Midland Valley. The bosses which rise through different portions of the volcanic series are obviously not the oldest or original vents. The great felsitic mass of Tinto in Lanarkshire (Fig. 93), indeed, pierces strata which lie near the base of the Lower Old Red Sandstone, but the smaller cone of Quothquan in its neighbourhood appears in the midst of the lavas (Fig. 92). In the south-western part of the Ochil chain the bosses or necks are chiefly small in size, seldom exceeding half a mile in diameter. They have been filled sometimes with crystalline, sometimes with fragmental materials. Two of them, containing the remarkable granophyric quartz-diorite already referred to, emerge from among the tuffs in a low part of the series, immediately above the village of Tillicoultry in Clackmannan (Fig. 68). Two or three more, which are occupied by orthophyres and quartz-felsites, pierce the volcanic group a few miles to the west of Loch Leven. The whole of the visible bosses of the Ochil Hills may be regarded as one connected group, subsidiary to the main orifices which lay rather further to the south and west. More particular reference to this district will be made in the following chapter (p. 303).

Vents which have been filled up with agglomerate, and which thus furnish the most obvious proofs of their connection with the eruptions of the volcanic series, though not frequent, may be observed in a number of the volcanic districts. Their fragmentary materials generally consist mainly of the detritus of andesites or diabases like those which form the bedded lavas. But where more acid lavas have risen to the surface, fragments of felsite may occur more or less abundantly. In the great vent of the Braid Hills the tuffs and breccias are almost wholly acid. Non-volcanic materials may often be found in the agglomerates, and occasionally even to the exclusion of volcanic detritus. Thus, in the far north of Scotland several examples occur among the Shetland Isles of necks filled entirely with blocks of the surrounding flagstones and sandstones. Such cases, as has been already pointed out, probably represent incompleted volcanoes, when the explosive vapours were powerful enough to drill orifices in the crust of the earth and eject the shattered debris from them, but were not sufficiently vigorous or lasting to bring up any solid or liquid volcanic material to the surface. These Shetland examples are further noticed on p. 345.

Necks of agglomerate in the Lower Old Red Sandstone vary in size from a great orifice measuring two miles across to little plugs only a few yards in diameter. They may be found in limited numbers in most of the volcanic districts. No examples have been observed rising through older rocks than the Old Red Sandstone, all the known instances being eruptive through some part of the volcanic series or of the sandstones, and therefore not belonging to the earliest eruptions.

The largest, and in some respects the most interesting, vent in the Lower Old Red Sandstone, that of the Braid Hills near Edinburgh, described in Chapter xx., covers an area of more than two square miles, and is filled with felsitic breccias and tuffs, through which bosses and veins of acid and basic rocks have been injected. It completely truncates the bedded lavas and tuffs of the Pentland Hills, and not improbably marks the chief centre from which these rocks were erupted. Several smaller necks rise a little beyond its southern margin, marking, perhaps, lateral cones on the main volcano.

In the small area of Lower Old Red Sandstone lying between Campbeltown and the Mull of Cantyre, several necks of agglomerate occur, which have been partly dissected by the waves along the shore, thus revealing their internal structure and their relation to the surrounding conglomerates. An account of them will be found at p. 311. One of the series, which lies back from the coast-line, forms a prominent rounded hill measuring about 400 yards in its longest diameter. Its general contour is represented in Fig. 82.

Of the eruptive bosses of massive rock outside the limits of the Old Red Sandstone which may be plausibly referred to the volcanic phenomena of the period, though they cannot be proved to be actually part of them, the most notable are the bosses of granite and other acid material which rise through the Silurian strata of the Southern Uplands of Scotland.[344] The largest are the well-known masses of Galloway (Fig. 69), with which must be grouped the bosses near New Cumnock, that of the Spango Water (Fig. 94), and those of Cockburn Law and Priestlaw in Lammermuir, together with a number of masses of felsitic material scattered over the same region, such as the Dirrington Laws of Berwickshire (Fig. 70). These bosses present some points of structure in common with true vents. They come like great vertical columns through highly-folded and puckered strata, and, as they truncate the Llandovery and Wenlock formations, they are certainly younger than the greater part of the Upper Silurian series. They must be later, too, than the chief plication and cleavage of these strata; but they are older than the Upper Old Red Sandstone or basement Carboniferous rocks which contain pebbles of them. Their date of eruption is thus narrowed to the interval between the later part of the Upper Silurian period and the beginning of the Upper Old Red Sandstone. I have myself little doubt that they are to be associated with the volcanic epoch we are now considering, as it was the only known great episode of igneous activity in this region during the interval within which the protrusion of these granites must have taken place. In the Cheviot Hills, indeed, we have evidence of the eruption of a large mass of augite-granitite through the porphyrite-lavas of the Lower Old Red Sandstone, with abundant veins projecting from it into them, as will be narrated in later pages.[345]

Not improbably many other granite protrusions throughout the British Isles are to be referred to the volcanic operations of the Lower Old Red Sandstone. Such are those of the Lake District, notably that of Shap,[346] the granites of Newry and Leinster in the east of Ireland, which are later than the Silurian rocks and older than the Carboniferous Limestone, and the younger Grampian granites, which pierce the presumably Arenig belt along the Highland border. Whether or not these granitic protrusions were connected with superficial volcanic discharges of which no remains have survived, they seem to indicate the wide extent and remarkable vigour of the subterranean igneous action of this geological period.

Viewed as a whole, the materials which now occupy the vents of the volcanic chains in the Lower Old Red Sandstone of the British Isles are more acid than the lavas erupted at the surface. In the Pentland district, indeed, and in some other areas this acid material was ejected at intervals in abundant discharges of dust and lapilli and in outflows of felsitic lavas, while between these successive discharges copious streams of diabasic and andesitic lavas, either from the same or from some closely-adjoining vent, were poured out. Throughout the whole region, however, as a closing phase of the volcanic history, the acid magma rose after the outpouring of the more basic lavas and filled such chimneys of the volcanoes as were not already blocked with agglomerate. It was probably after these pipes were plugged that the final efforts of volcanic energy were expended in the protrusion of the acid material as sills between the bedding-planes of the surrounding rocks, and as dykes and veins in and around the vents.

iii. SILLS AND DYKES

Nowhere throughout the volcanic tracts of the Lower Old Red Sandstone is there any such development of sills as may be seen beneath the Silurian volcanic sheets of North Wales. Those which occur are most abundant in the Lanarkshire district, to the north-west and south-west of Tinto, and in the south of Ayrshire. From the village of Muirkirk to the gorge of the Clyde, below the Falls, the Upper Silurian and Lower Old Red Sandstone strata are traversed by numerous intrusive sheets of pink and yellow felsite, quartz-porphyry, minette, lamprophyre and allied rocks, which are no doubt to be regarded as part of the volcanic phenomena with which we are here concerned. In the south of Ayrshire, between the villages of Dalmellington and Barr, there is a copious development of similar sills, especially along one or more horizons near the base of the Old Red Sandstone. Garleffin Fell, Glenalla Fell, Turgeny and other heights are conspicuous prominences formed of these rocks; above the sills lie thick conglomerates and sandstones on which the great andesite-sheets rest.

In the Pentland Hills, as will be described in Chapter xx., a massive felsitic sill forms a conspicuous feature along the north side of the chain, and there are probably others which have not yet been separated from the felsitic tuffs and orthophyres which they so much resemble.

Perhaps the most remarkable acid sills in the Old Red Sandstone of Britain are those which occur at the extreme northern end of the region among the volcanic phenomena of the Shetland Isles (Figs. 71, 72). The largest of them, consisting mainly of granite and felsite, is believed to reach a length of 20 and a breadth of from three to four miles.[347]

A group of sills composed of a bright red quartz-porphyry has been traced along the southern flanks of the Highlands for upwards of 18 miles.[348] This rock, already referred to as the "Lintrathen porphyry," lies chiefly among the conglomerates and sandstones, but also intersects the lavas, and may be later than the Old Red Sandstone (p. 277). An extension of it is found even on the north side of the boundary fault, cutting the andesites which there lie unconformably on the schists.

Examples, however, occur of sills much less acid in composition. In the Dundee district, for instance, the intrusive sheets are andesites and diabases. They send veins into and bake the sandstones among which they have been intruded, and are sometimes full of fragments of such indurated sandstone, as may be well seen on the northern shore of the Firth of Tay, west of Dundee.

A conspicuous characteristic of most of the volcanic tracts of the Lower Old Red Sandstone is the comparative scarcity of contemporaneous dykes. In the band of acid sills between Muirkirk and the Clyde, a considerable number of dykes have been mapped, which must be regarded as due to the same series of movements and protrusions of the magma that produced the adjacent sills. Throughout the length of the Southern Uplands dykes of felsite, minette, lamprophyre, vogesite and other varieties, which may also be connected with the volcanic phenomena of the Lower Old Red Sandstone, not infrequently occur among the Silurian rocks. On the Kincardineshire coast, south of Bervie, a number of dykes of pink quartz-porphyry traverse the conglomerates and sandstones. The coast south of Montrose displays some singularly picturesque sections, where a porphyry dyke running through andesitic lavas and agglomerates stands up in wall-like and tower-like projections. On the shore at Gourdon, as well as inland, intrusive dykes of serpentine occur. A line of these, possibly along the same fissure, has been traced for more than a dozen of miles from above Cortachy Castle to near Bamff. But there is no evidence to connect them with the volcanic phenomena of the Old Red Sandstone. Not improbably they belong to a later geological period.

One would expect to meet with a network of dykes in and around the volcanic vents; but even there they are usually not conspicuous either for number or size. In the great vent of the Braid Hills only a few have been noticed. In the Ochil Hills groups of dykes of felsite and andesite may be observed, especially near the necks. They are fairly numerous in the neighbourhood of Dollar (see Fig. 68). One of the most abundant series yet observed traverses the tract around the granite boss of the Cheviot Hills, from which many dykes of granite, felsite, quartz-porphyry and andesite radiate. This district will be more fully referred to in Chapter xxi. Another remarkable development of dykes occurs in Shetland (Fig. 72), where they consist of granite, felsite and rhyolite, and are associated with the acid sills above referred to.