3. DYKES AND SILLS

Fig. 127.—Section across the vents Dumgoyn and Dumfoyn, and the edge of the Clyde plateau above Strathblane, Stirlingshire.1. Upper Old Red Sandstone; 2. Shales, cement-stones and sandstones ("Ballagan beds"); 3. White sandstone; 4. Andesite lavas; 5. Agglomerate (shown by the dotted portions), traversed by intrusive diabase.f, Fault.D.Late dolerite dyke.

Necks formed entirely of agglomerate are abundant among the vents connected with the plateaux. As examples of them I may refer to the series already mentioned as fronting the escarpment of the Clyde plateau from Fintry to Largs. Another interesting group rises through the Silurian and Old Red Sandstone rocks to the west of the escarpment of the Berwickshire plateau, that near Melrose forming one of the largest in Scotland.

Fig. 128.—Section through the large vent of the Campsie Hills.1. Andesite lavas; 2. Agglomerate and tuff; 3. Trachytic and andesitic intrusive rocks.

Fig. 129.—Diagrammatic section across the central vent of the Clyde plateau in Renfrewshire.1. Andesite lavas; 2. Agglomerates and fine tuffs often much altered; 3. Dykes of trachytic and andesitic rocks; 4. Later dykes of dolerite and basalt.

Illustrations of the varying structure of these vents are given in the accompanying figures. InFig. 127, a section is drawn through the two necks Dumgoyn and Dumfoyn, which have already been shown in outline and in ground-plan. The relation of these two vents to the neighbouring plateau to the right can here be seen.Fig. 128gives a section taken through thegreat vent of the Campsie Hills, with the minor adjacent necks of Dungoil, Bin Bairn, and the Meikle Bin.

The diagram inFig. 129is meant to convey in a general way what appears to be the structure of the central vent of the Renfrewshire plateaux, to be afterwards referred to. But, as already mentioned, the limits of the various rocks are too much obscured to allow an accurate delineation to be given of their areas and relations to each other. The Berwickshire plateau supplies abundant interesting examples of tuff necks which rise through the Old Red Sandstone many miles distant from the edge of the lavas. This structure is shown inFig. 130.

Fig. 130.—Section across Southern Berwickshire to show the relation of the volcanic plateau to the vents lying south from it.1. Upper Silurian strata; 2. Upper Old Red Sandstone; 3. The volcanic plateau; 4. Agglomerate and tuff of the vents; 5. Basalt and dolerite; 6. Lower Carboniferous strata.

Indications may occasionally be observed of an agglomerate vent having been first occupied by one kind of material and then, after being in great measure cleared out by explosions, having been subsequently filled up with another. As an example of this structure I may cite again the double neck of the Knock Hill a little to the north of Largs, of which the outline is shown inFig. 23, and the ground-plan inFig. 125, B. This hill rises from the red sandstone slopes that front the great Ayrshire plateau and forms a conspicuous cone the top of which is rather more than 700 feet above the sea. Its summit commands a remarkably extensive and interesting panorama of the scenery of the Clyde, but to the geologist perhaps the most striking feature in the landscape is the range of terraced hills behind, mounting up into the great vents of the Renfrewshire uplands. On these declivities the successive lava-streams that have built up the plateau can be seen piled over each other for a thickness of more than 1000 feet, and presenting their escarpments as parallel lines of brown crag with green slopes between.

The Knock has had its upper part artificially dressed, for lines of trench have been cut out of its rocks by some early race that converted the summit of the hill into a strongly intrenched camp. From the apex of the cone the ground falls rapidly westward into a hollow, beyond which rises a lower rounded ridge of similar materials. It is possible that this western ridge may really form part of the main hill, but the grass-covered ground does not afford sufficient exposures of the rocks to settle this point. From the contours of the surface, it may be inferred that there are two closely adjacent vents, and that the western and lower eminence is the older of the two. Thishill or ridge consists partly of a coarse agglomerate, and partly of veins and irregular protrusions of a dark, compact, slightly cellular lava. The stones in the fragmental rock are different olivine-basalts, or other basic lavas, and sandstones. The paste is rough, loose and granular. The sandstone fragments are much indurated and sometimes bleached.

The Knock itself is formed mainly of a remarkably coarse and strikingly volcanic agglomerate. Round the outside, and particularly on the south-east, the rock is finer in texture, compact, and gravelly, or like a mudstone, with few or no imbedded blocks, dull-green to red in colour, and breaking with a clean fracture which shows angular lapilli of various basalts or diabases. At the southern end of the neck, where the surrounding red sandstone can be seen within a few feet of the tuff, the latter is bright red in colour, and contains much debris of red sandstone and marl. Possibly this finer tuff, which is traceable as an irregular band round the outside of the neck, may mark an older infilling of the vent than the agglomerate of the centre; but there is no sharp line to be drawn between the two, though a hollow can sometimes be traced on the surface where they join.

The agglomerate of this locality is one of the most characteristic among the plateau-necks of the Clyde region. Its blocks sometimes measure from two to three feet in diameter. They consist almost wholly of a dark crystalline porphyritic olivine-basalt. These blocks are subangular in form, often with clean-fractured surfaces. Though occasionally slightly cellular, they are never slaggy so far as I could see, nor are any true scoriæ to be noticed among them. The blocks suggest that they were derived from the disruption of an already solidified mass of lava. The agglomerate is entirely without any trace of stratification.

Through this tumultuous accumulation of volcanic debris some irregular veins of olivine-basalt, sometimes glassy in structure, have been injected, and reach nearly to the summit of the hill. This intrusive material resembles generally some of the dark intrusive masses in the Dumbartonshire necks. Like these, it exhibits a tendency to assume a more or less distinctly columnar structure, its columns having the same characteristic wavy sides and irregular curvature. The intrusive rocks in the two eminences of the Knock may be paralleled among the stones in the agglomerate. The neck on its north-eastern side rises steeply from the red sandstones which it pierces, but which, although they are much jointed and broken, are not sensibly indurated. Unfortunately the actual junction of the igneous and sedimentary rocks is concealed under herbage.

Fig. 131.—Section of south end of Dumbuck Hill. East of Dumbarton.

As a rule, the fragmental materials of the plateau-necks are quite unstratified. Their included blocks, distributed irregularly through the mass, have evidently undergone little or no assortment after they fell back into the vents. Occasionally, however, a more or less distinct bedding of the agglomerate or tuff may be observed, the layers having a tendency to dip inward into the centre. One of the most conspicuous examples of this structure is to be found in the hill of Dumbuck, to the east of Dumbarton. This neck, which forms so prominent a featurein the landscape, presents a precipitous face towards the south, and allows the disposition of its component materials to be there seen. The agglomerate consists of a succession of rudely stratified beds of coarser and finer detritus, which on both sides are inclined towards the centre, where a plug of fine-grained olivine-basalt has risen and spread out into a columnar sheet above (Fig. 131). In general form this basalt resembles such intrusions as that of Largo Law, to be afterwards described (Fig. 226), where what may have been the hollow or bottom of the crater is filled with basalt.

Fig. 132.—Section across the East Lothian plateau to show the relative position of one of the necks.1. Lower Carboniferous sandstones and shales; 2. Red and green tuffs with a seam of limestone (l); 3. Band of basic sheets at the base of the lavas; 4. Trachytes; 5. Phonolite neck.

(b)Necks of Andesite, Trachyte, Dolerite, Diabase, or other massive Rock.—When the vents have been filled by the uprise of some molten rock, it is generally, as we have seen, of a more acid character than the ordinary lavas of the plateaux. Frequently it consists of some variety of trachyte or andesite, commonly of a dull yellow or grey tint and waxy lustre. Good examples may be seen among the remarkable group of necks on either side of the valley north of the village of Strathblane and in those above Bowling. The three great necks in East Lothian, already alluded to,—Traprain Law (Figs.132,133), North Berwick Law (Fig. 109), and the Bass Rock (Fig. 110)—are masses of phonolite and trachyte, obviously related to the trachytes of the adjacent plateau. A smaller but very perfect instance of a vent similarly filled is to be seen in the same neighbourhood on the shore to the east of North Berwick Law.[438]

[438]See "Geology of East Lothian,"Geological Survey Memoir, p. 40.

Examples occur where the funnels of eruption have been finally sealed up by the rise of more basic material, and this has happened even in a district where most of the lava-form necks consist of trachyte or some other intermediate lava. Thus, in the Campsie Fells, several such bosses appear, of which the most conspicuous forms the hill of Dungoil (1396 feet,Fig. 128). Further west, among the Kilpatrick Hills, bosses of this kind are still more numerous. The group of bosses near Ancrum and Jedburgh is mainly made up of olivine-dolerites and olivine-basalts (Fig. 130). This more basic composition of itself suggests that these bosses may be connected rather with the puy- than with the plateau-eruptions.

(c)Necks of Composite Character.—In not a few examples, the vents have been filled with agglomerate which has been pierced by a plug or veins of lava-form material. Many illustrations of this composite structure may be observed along the west front of the great escarpments from Fintry to Ardrossan (see Figs.124,125,127and128). In that region the intruded rock is often a dull yellowish or grey trachytic or andesitic material. Olivine-basalt is the chief rock intruded in the vents in the Dumbarton district. Among the Roxburghshire vents, where the injected material is commonly olivine-basalt or dolerite, it occasionally happens, as in Rubers Law, that the uprise of the lava has almost entirely cleared out or concealed the agglomerate, and in some of the bosses, where no agglomerate is now to be seen, the basalt may have taken its place (Fig. 130).

The largest and most interesting vents connected with this type of Carboniferous volcano, are those which occur within the limits of the plateaux, where they are still surrounded with lavas and tuffs that probably came out of them. Of these by far the most extensive and remarkable lies among the high moorlands of Renfrewshire between Largs and Lochwinnoch, where the ground rises to more than 1700 feet above the sea (seeFig. 129). This area, as already remarked, is unfortunately much obscured with drift and peat, so that the limits of its rocks cannot be so satisfactorily traced as might be desired. I think it probable that several successive vents have here been opened close to each other, but their erupted ashes probably cannot be distinguished. Over a space measuring about four miles in length by two and a half in breadth, the rocks exposed at the surface are fine tuffs, breccias and coarse agglomerates, largely made up of trachytic, andesitic or felsitic material, and pierced by innumerable protrusions of various andesitic, trachytic or felsitic rocks in bosses and veins, as well as also by dykes of a more basic kind, such as dolerites and basalts. Some of the tuffs present a curiously indurated condition; and they are frequently much decayed at the surface.[439]Another large mass of tuff and agglomerate lies a little to the south-west of the main area.

[439]This tract of ground was mapped for the Geological Survey by Mr. R. L. Jack, now in charge of the Geological Survey of Queensland. See Sheet 31,Geological Survey of Scotland.

After the explosions ceased, by which the vents were opened and the cones of debris were heaped up, heated vapours would in many cases, as in modern volcanoes, continue for a long while to ascend in the funnels. The experiments of Daubrée on the effects of water and vapour upon silicates under great pressure and at a low red heat, have shown how great may be the lithological changes thereby superinduced. It is improbable that where a mass of tuff and lava, lying deep within a volcanic vent, was thoroughly permeated with constantly ascending heated vapours, it should escape some kind of change. I am inclined to attribute to this cause the frequent conversion of the sandstones round the walls of the vents into quartzite. The most remarkable example of metamorphism within a vent which I have observed among the plateaux, occurs in the heart of the Campsie Fells, where, instead of forming a prominence, the neck is marked by a great hollow, measuring about a mile in length and half a mile in breadth (Fig. 128).[440]It is occupied mainly by a coarse tumultuous agglomerate, like that of other necks in the same district, but with a matrix rather more indurated, and assuming in certain parts a crystalline texture, so as to be at first sight hardly distinguishable from some of the surrounding andesites. Even in this altered condition, however, its included fragments may be recognized, particularly blocks of sandstone which have been hardened into quartzite. Numerous small veins of pink and yellow trachyte traverse the agglomerate, and are found also cutting the bedded andesites that encircle it.

[440]See Explanation to Sheet 31,Geological Survey of Scotland, par. 21 (1878).

Fig. 133.—View of Traprain Law from the south, a phonolite neck of the Garleton Plateau.

Intrusive masses both in the form of dykes and of sills are of frequent occurrence in connection with the Carboniferous volcanic plateaux. From the variety of their component materials it may be inferred that these rocks belong to different ages of intrusion.

Dykes.—The great majority of the Dykes consist of trachyte or of andesite, resembling in lithological characters the material of the necks and doubtless connected with its uprise. There occur also dykes of diabase, basalt or dolerite. Some of the latter, especially those which run for many miles, cutting every rock in the districts in which they occur, and crossing large faults without deviation, are certainly long posterior to the plateau volcanic period. Whether the small inconstant dykes of more basic composition, found in the same districts with the trachytes, are to be looked upon as part of the volcanic phenomena of the plateaux, is a question to which at present no definite answer can be given. I shall have occasion to show that in the next volcanic period the lavas that flowed from the puys are more basic than most of those of the plateaux, and that they are associated with more basic dykes and sills. In Roxburghshire, where it is so difficult to distinguish between the denuded vents of the two periods, the dark heavy olivine-basalts and dolerites of the bosses may possibly belong rather to the later than to the earlier volcanic episode. And if that be their true age, the dykes of similar material may be connected with them. At the same time it must be remembered that the earliest eruptions of the plateaux were markedly basic, that many vents in the plateaux are pierced by basic intrusions, and that basic dykes may have been associated with the uprise of the same magma.

The dykes occur in considerable numbers and in two distinct positions, though these may be closely related to each other: 1st, among the rocks outside and beneath the plateau-lavas, or cutting these lavas; and 2nd, in and around the vents.

1. Among the rocks which emerge from under the Carboniferous volcanic plateaux, dykes are sometimes to be observed in considerable numbers. They may be compared to the far more extensive series connectedwith the Tertiary basalt-plateaux, like which they may have had a close relation to the actual building up of the successive sheets of andesite, trachyte and basalt that were erupted at the surface. They are particularly well developed in the Clyde plateau, where by extensive denudation they have been admirably exposed. I would especially refer to those that traverse the tract of red sandstones which underlie the volcanic series along the flanks of the great escarpments from Fintry to Strathblane and Dumbarton, and between Gourock and Ardrossan. These dykes have been dissected by the sea along both sides of the estuary of the Clyde and in the islands of Cumbrae. In these islands and in Bute they have recently been mapped in great detail for the Geological Survey by my colleague, Mr. W. Gunn, who has supplied me with notes of his observations on the subject, from which the following summary is compiled.

"There are at least four distinct groups of intrusive rocks in the Greater Cumbrae. The oldest of these is trachytic in character, and occurs both as dykes and sheets, which run generally in the same E.N.E. direction. The rock is usually pinkish in colour, sometimes grey or purplish. A specimen from the dyke of the Hawk's Nest, north of Farland Point, analyzed by Mr. Teall, was found to contain 11 per cent of alkalies, principally potash, while the percentages of lime and iron were very low. Sometimes these rocks are fine in grain with only a few porphyritic orthoclase crystals, though numerous small crystals of this mineral are found with the aid of the microscope. These red trachyte dykes are almost confined to the Upper Old Red Sandstone, rarely entering the overlying white Calciferous Sandstones, and never invading the plateau-lavas. They are therefore probably of early Carboniferous age.

"The next group follows the same general direction, but clearly traverses the trachytes, and must therefore be of later date. The dykes of this group are the most numerous of the whole, the greater part of the island being intersected by them. In the north-east corner about 40 of them may be counted in half a mile of coast-line, some being of large size. All of them which can be clearly made out are porphyritic olivine-basalts of the type of the Lion's Haunch at Arthur's Seat. They are generally grey in colour and finer at the edges than in the centre, which is often coarsely porphyritic and amygdaloidal. Olivine seems always characteristic, but has often been replaced by hæmatite or calcite. In Bute a good many dykes have been mapped to the north of Kilchattan Bay resembling this basalt series of Cumbrae, and running in the same direction. But they appear to be all porphyritic andesites. The second group of dykes, though it cuts the first and is thus proved to be later in date, is nevertheless confined within the same stratigraphical limits. It may thus belong nearly to the same period of intrusion.

"The dykes of the third group are dolerites without olivine, and follow on the whole an east and west direction. They cut both of the two foregoing sets of dykes, and likewise the lavas of the plateau. They must thus belong to a far later period of intrusion. They may be connected withother dykes and sills on the mainland, which traverse the Coal-measures, and would thus be not older than late Carboniferous or Permian time.

"The fourth group of dykes intersects all the others, and is probably of Tertiary age. The prevalent direction of these dykes in the Cumbraes is N.N.W." The Tertiary dykes are more fully described in Chaptersxxxiv.andxxxv.

The great group of tuffs which underlies the lavas of the East Lothian plateau is traversed by numerous dykes and sills, of which many good examples may be seen in the coast-cliffs of North Berwick. Among these rocks are beautiful olivine-basalts with singularly fresh olivine, as on the shore at North Berwick. Some of them are still more basic, as in the case of a limburgite intrusion at the Gin Head, Tantallon Castle.

Fig. 134.—Veins and dykes traversing the agglomerate and tuff of the great Renfrewshire vent.

2. In the necks, dykes are sometimes abundant, and they may be observed occasionally to traverse the surrounding lavas. They consist of similar materials to those found outside the plateaux. Some of the larger necks are intersected by a network of dykes and veins. The great vent or group of vents among the uplands of Renfrewshire, already described (Fig. 129), furnishes some admirable examples of this characteristic volcanic feature. An illustration from that locality forms the subject ofFig. 134. The agglomerate which fills the large hollow among the Campsie Hills may be quoted as another illustration (Fig. 128). Further instances will be found in some of the sections given in preceding pages (see Figs.124,125,127). The general aspect of a dyke in the volcanic series is shown inFig. 135.

Fig. 135.—"The Yellow Man," a dyke in volcanic tuff and conglomerate on the shore a little east of North Berwick.

TheSillsassociated with the plateau-type of Carboniferous volcanic action form a less prominent feature than they do among the earlier Palæozoic formations or in the puy-type which succeeded them. They consist in general of short lenticular sheets of andesite or trachyte, like the necks and dykes in proximity to which they commonly appear. The best area for the study of them is the ground which stretches out from the base of the great escarpments of the Campsie, Kilpatrick and Ayrshire Hills (Fig. 136), where, among the agglomerate-vents and abundant dykes, intrusive sheets have likewise been injected between the bedding-planes of the red sandstones. But these sheets are of comparatively trifling dimensions. Very few of them reach a mile in length, the great majority falling far short of that size. In the Cumbraes and in Bute, Mr. Gunn has observed that the trachytic, olivine-basalt and dolerite dykes are apt to pass into intrusive sheets. That the sills, as well as the dykes and bosses of the same material, are not of older date than the lavas of the plateaux is proved by the manner in which they pierce these lavas, especially towards the bottom of the series. The general absence of basic sills, when we consider how thick a mass of these rocks has sometimes been poured out in the plateaux, is not a little remarkable. Only in the basin of the Firth of Forth do we encounter thick basic sills near the plateaux, such, for instance, as Salisbury Crags at Edinburgh. But it is doubtful whether they ought not rather to be classed with the sills of the puys, to be afterwards described.

Fig. 136.—Trachytic sills, Knockvadie, Kilpatrick Hills.1. Upper Old Red Sandstone; 2. "Ballagan Beds"; 3. Tuffs; 4. Lavas of the Plateau; 5. Agglomerate of necks; 6. Trachyte sills; 7. Dolerite dyke (? Tertiary).

The relative geological date when the eruptions of each plateau ceased can fortunately be determined with much more precision than the time of their beginning. The Hurlet Limestone, so well known as the lowest thick calcareous seam in the Carboniferous Limestone series, of which it is generally taken as the base, can be identified over the whole of Central Scotland, and thus forms an excellent stratigraphical horizon, from which the upward termination of the volcanic sheets underneath it can be measured.

When the volcanic episode of the plateau-eruptions came to an end, such banks or cones as rose above the level of the shallow sea which then overspread Central Scotland were brought beneath the water, as I have already remarked, either by prolonged denudation or more probably in largepart by the continued subsidence of the region. The downward movement may possibly for a time have been accelerated, especially in some districts. Thus the Hurlet Limestone, though usually not more than five or six feet thick, increases locally to a much greater thickness. At Petersfield, near Bathgate, for example, it is between 70 and 80 feet in depth, while at Beith, in North Ayrshire, it increases to 100 feet (Fig. 137), which is the thickest mass of Carboniferous Limestone known to exist in Scotland. At both of these localities the limestone lies upon a series of volcanic rocks, and we may perhaps infer that the subsidence advanced there somewhat more rapidly or to a greater extent, so as to form hollows in which the limestone could gather to an abnormal depth. The water would appear to have become for a time tolerably free from mechanical sediment. The limestone is hence comparatively pure, and is extensively quarried all over the country for industrial purposes. It is a crinoidal rock, abounding in many species of corals, brachiopods, lamellibranchs, and gasteropods, with trilobites, cephalopods, and fishes.

Fig. 137.—Section across the edge of the Clyde plateau, south-east of Beith.1. Plateau-lavas; 2. Tuffs and volcanic conglomerates; 3. Hurlet Limestone; 4. Coal-bearing strata above the limestone; 5. Dolerite dyke.

A variable thickness of strata intervenes between the top of the volcanic series and the Main Limestone. Sometimes these deposits consist in large measure of a mixture of ordinary sandy and muddy material with the washed-down tuff of the cones, and probably with volcanic dust and lapilli thrown out by the latest eruptions. Thus along the flank of the hills from Barrhead to Strathavon, yellow and green ashy sandstones, grits and conglomerates are succeeded by ordinary sandstones, black shales and ironstones, while here and there true volcanic tuff and conglomerate make their appearance.[441]Further west, in the Kilbirnie district, the limestone lies directly on the tuffs that rest upon the andesites (Figs.137,138).

[441]Explanation of Sheet 22,Geol. Surv. Scotland, p. 12.

Fig. 138.—Section across the upper part of the Clyde plateau at Kilbirnie, Ayrshire.1 1. Plateau-lavas; 2 2. Tuffs; 3 3. Hurlet Limestone; 4. Black-band Ironstone.ff. Faults.

But perhaps the most striking contrast between adjacent localities in regard to the distance between the limestone and the top of the volcanic series is to be observed along the southern front of the Campsie Fells. In spite of the abundant faults which have there so broken up the regular sequence of the rocks, we can see that at Banton and Burnhead the limestone lies almost immediately on the volcanic series (Fig. 139). But a little to the westward, sandstones, conglomerates, shales and thin limestones begin to intervene between the volcanic series and the Hurlet Limestone and swell out so rapidly that on Craigmaddie Muir and South Hill of Campsie, only some five miles off, they must form a total thickness of not less than from 600 to 800 feet of ordinary non-volcanic deposits, chiefly thick pebbly sandstones (Fig. 140). Such local variations not improbably serve to indicate hollows on the flanks of the plateaux that were filled up with detritus before the depression and clearing of the water that led to the deposition of the Hurlet Limestone.

Fig. 139.—Section across the upper surface of the Clyde volcanic plateau, Burnhead, north-west of Kilsyth.1. Lavas of the plateau; 2. Tuffs; 3. Hurlet Limestone; 4. Hosie's Limestone;f, Fault.

Fig. 140.—Section across the upper surface of the Clyde volcanic plateau at Campsie.1. Shales, sandstones, cement-stones, etc. ("Ballagan Beds"); 2. Lavas of the plateau; 3. Thick white sandstone and conglomerate; 4. Hurlet Limestone; 5. Hosie's Limestone;f. Fault.

Fig. 141.—Section across western edge of the Garleton plateau.1. Trachyte lavas of the plateau; 2. Calciferous Sandstones; 3. Hurlet Limestone.

I have already remarked that the eruptions of the plateau period lasted longer in the western than in the eastern parts of the region. In the Garleton district, where the peculiar viscous trachytic lavas probably gave rise to a more uneven surface or more prominent cones than was usual among the andesitic plateaux, the eruptions ceased some time before thedeposition of the Hurlet Limestone. As the area sank, the successive zones of the Calciferous Sandstones crept over the flanks of the trachytes, until at last they had completely buried these rocks before the limestone spread over the area (Fig. 141). In consequence, probably, of the uneven surface of this plateau, there is here a strong overlap of the higher part of the Calciferous Sandstones. On the west side of the volcanic area there can hardly be more than some 200 feet of strata between the top of the trachytic series and the limestone, while on the south side there must be greatly more than that thickness. This structure probably indicates that the Garleton volcanoes became extinct after having piled up a mass of tuffs and lavas to such a height that its summits were not submerged until the area had subsided 800 or 1000 feet in the waters, over the floor of which the Calciferous Sandstones were laid down. Hence, in spite of the proximity of the lavas to the limestone, there may have been a vast interval of time between their respective epochs, as has been already suggested with regard to other plateaux. This subject will be again referred to in discussing the relative chronology of the plateaux and puys.

In the Berwickshire and Solway districts, the extinction of the plateau-vents appears to have taken place at a still earlier part of the Carboniferous period, for there the andesites, while they rest on the Upper Old Red Sandstone, are covered with at least the higher group of the Calciferous Sandstones (Fig. 142). The equivalent of the Hurlet Limestone of Central Scotland must lie many hundred feet above them.

The submergence of the plateaux, and their entombment under the thick Carboniferous Limestone series, did not mark the close of volcanic activity in Central Scotland during Carboniferous time. The plateau-type of eruption ceased and was not repeated, but a new type arose, to which I would now call the reader's attention.

Fig. 142.—Section across the Solway plateau from Birrenswark to Kirtlebridge.1. Upper Silurian strata; 2. Upper Old Red Sandstone; 3. Plateau-lavas; 4. Calciferous Sandstones and Carboniferous Limestone series; 5. Trias.

Back to Index Next