iii. SCOTLAND

From the centre of England we must in imagination transport ourselves into the Southern Uplands of Scotland, where a widely distributed series of Silurian volcanic rocks has been preserved. It was, until recently, supposed that the Silurian system north of the Tweed contains no contemporaneously erupted volcanic rocks. Yet, as far back as the year 1860, I pointed to the abundant existence of volcanic detritus in these strata throughout the southern counties as a probable indication of volcanic activity at the time and in the area within which the strata were deposited.[170] Some years later, when the microscope had been introduced as an aid to field-geology, I sliced some of the Silurian sediments of that region and found them, particularly certain shales and grits of Moffatdale, to contain a large admixture of perfectly fresh unworn felspar crystals, which I felt tolerably certain had been supplied by volcanic explosions. As no trace, however, had then been detected of an intercalated volcanic group in any part of the Silurian series of the south of Scotland, I used at that time to speculate on the possibility of the volcanic detritus having been wind-borne from the volcanoes of the Lake District. I had at that time no suspicion that its source was rather to be sought under my feet. The presence of volcanic rocks underneath the uplands of the south of Scotland would have been a welcome explanation of the frequent felspathic composition of many of the Silurian greywackes and shales of that region, and particularly the abundance of andesitic and felsitic fragments in them.

[Footnote 170: _Trans. Roy. Soc. Edin._ xxii. (1860), p. 636.]

It had been long known that the Scottish Silurian formations, besides having undergone extensive plication, have also been injected by protrusions of igneous material of various kinds. The intrusive character of many of these is so obvious that a similar origin was attributed even to those bosses which could not be proved to be intrusive. Recent work of the Geological Survey, however, and more especially the numerous and careful traverses of my friend and colleague Mr. Peach, have revealed the unlooked-for and important fact that a large number of these supposed intrusions are really portions of a volcanic group brought up on the crests of anticlinal folds, and laid bare by denudation. This group can be traced for at least 100 miles from north-east to south-west over a belt of country sometimes 30 miles broad. Its original limits cannot be ascertained, but they obviously exceeded those within which the rocks can now be seen. Nevertheless the present boundaries embrace an area of nearly 2000 square miles. This Palæozoic volcanic region is thus one of the most extensive in the British Isles. Owing, however, to the constant plication of the strata, and the wide space which the overlying sedimentary deposits are thus made to cover, the volcanic group only comes occasionally into view, and thus occupies but a mere fraction of the superficial extent of the region over which its scattered outcrops appear. These exposures, sometimes only a few square yards in extent, may always be looked for where the anticlinal folds bring up a sufficiently low portion of the Silurian system; they prove that a vast volcanic floor underlies the visible Lower Silurian grits and shales over the length and breadth of the Southern Uplands of Scotland.

Without anticipating details which will properly appear in the official _Memoirs_ of the Geological Survey, I may briefly indicate the visible boundaries of the volcanic group, and refer to some of the localities where it may best be seen. The most easterly points where it has been recognized by Mr. Peach stand on the crests of some sharp anticlinal folds near St. Mary's Loch and near Leadburn and Winkstone in Peeblesshire. Farther westwards it appears at many places along the northern border of the Silurian territory, as at Romanno Bridge, Wrae, Kilbucho, Culter Water and Abington, the length and breadth of each exposure depending partly on the breadth of the anticline and partly on the depth to which it has been cut down by denudation. Near Sanquhar the volcanic series opens out for a breadth of more than a mile, and is seen at intervals across the wild moorlands of Carrick, until from the Stinchar valley it widens out seaward and occupies much of the coast-line of Ayrshire between Girvan and the mouth of Loch Ryan. It probably rises again along a fold near Portpatrick, and it is seen at various points along the southern borders of the Silurian uplands, as near Castle-Douglas, at Glenkiln, Bell Craig near Moffat, and the head of Ettrickdale.

The best sections are those exposed along the coast to the north and south of Ballantrae. When that ground was first examined by the Geological Survey, the hypothetical views in regard to metamorphism already referred to were in full ascendant, and the rocks were mapped on the same general principles as those which had been followed in Wales. Professor Bonney, however, a few years later recognized the true igneous nature of many of the rocks. He found among them porphyrite lavas and agglomerates which he regarded as of Old Red Sandstone age, likewise intrusive serpentines and gabbros.[171]

[Footnote 171: _Quart. Journ. Geol. Soc._ vol. xxxiv. (1878), p. 769.]

The volcanic rocks of this wide district include both lavas and their pyroclastic accompaniments, as well as intrusive sills and bosses of various materials. They have recently been studied by Mr. J. J. H. Teall, and full descriptions of them by him will appear in a forthcoming volume of the _Memoirs_ of the Geological Survey. He has ascertained that though generally more or less decomposed, the lavas would be classed by German petrographers as diabases and diabase-porphyrites. The former are compact dark-green non-porphyritic rocks, often containing numerous small spherical amygdales; while the latter are markedly porphyritic, enclosing large phenocrysts of more or less altered plagioclase, often measuring half an inch across. These two groups of rock are connected by transitional varieties. They were probably, in the first instance, composed of plagioclase, augite, iron-ores, and a variable quantity of imperfectly crystallized interstitial matter.

Some of these rocks closely resemble in outward appearance the andesites ("porphyrites") of the Old Red Sandstone of the district not many miles to the north, that is, fine purplish-red rocks with a compact base through which porphyritic felspars are abundantly scattered. Occasionally they are markedly slaggy, and show even a ropy surface, while the breccias associated with them contain blocks of similar slag.

But the most characteristic external feature of these lavas is their tendency to assume irregularly-elliptical, sack-like or pillow-shaped forms. On a weathered face they sometimes look like a pile of partially-filled sacks heaped on each other, the prominences of one projecting into corresponding hollows in the next. The general aspect of this structure is shown in Fig. 12, which represents a face of rock about eight feet high and six feet broad. The rocks exhibiting this peculiarity are usually finely amygdaloidal, and it may be observed that the vesicles are grouped in lines parallel to the outer surface of the pillow-like block in which they occur. The diagram in Fig. 51 represents in ground-plan a surface about twelve feet square on the shore immediately to the south of the mouth of the River Stinchar. In the heart of the spheroids enclosed fragments of other lavas are sometimes observable.

This singular structure has already (p. 184) been referred to as strikingly displayed in a rock at the top of Cader Idris. It is found in dark basic lavas probably of Arenig age, which will be afterwards referred to as occurring along the southern flanks of the Scottish Highlands and also in the north of Ireland. It has been observed by Mr. Teall among the rocks of the Lizard, and has been described as occurring in Saxony and California.[172] In these different localities it is associated with jaspers and cherts, some of which contain abundant Radiolaria. The same structure has been found among the variolitic diabases of Mont Genèvre,[173] and likewise in some modern lavas, as in that of Acicastello already referred to (_ante_, p. 26).

[Footnote 172: Mr. J. J. H. Teall, _Roy. Geol. Soc. Cornwall_, 1894, p. 3. Mr. L. Ransome, _Bull. Depart. Geol. University of California_, vol. i. p. 106.]

[Footnote 173: Messrs. Cole and Gregory, _Quart. Journ. Geol. Soc._ vol. xlvi. (1890), p. 311.]

The volcanic agglomerates and breccias, in the south-west of Ayrshire, attain a great development in several centres probably at or near the original volcanic vents. They present several distinct petrographical types. The remarkable neck-like hill of Knockdolian in the Stinchar Valley is made of a coarse breccia composed mainly of angular pieces of dull greyish-green fine-grained diabase. The breccias and agglomerates of Bennane Head in some parts consist largely of broken-up shales, flinty mudstone, black radiolarian flint or chert, and abundant fragments of andesites and felsites. In other parts the volcanic material predominates, including angular and subangular fragments of various somewhat basic lavas, lumps of vesicular slag and pieces of pumice. Here and there much calcite is diffused through the matrix in strings, veins and patches, which enclose the lapilli. The agglomerate north of Lendalfoot possesses a greenish, somewhat serpentinous matrix, through which immense numbers of tabular felspar crystals are scattered. Similar crystals also occur abundantly in embedded blocks of one of the purplish diabase-porphyrites, which occurs in mass on the shore and inland, and closely resembles the rock of Carnethy in the Old Red Sandstone volcanic series of the Pentland Hills.

Yet another and very distinct type of agglomerate is to be seen on the Mains Hill south-east of Ballantrae. It is a coarse rock, enclosing blocks up to a yard or more in diameter, of a fine compact purplish porphyrite, with large crystals of plagioclase and smaller ones of augite. In some places immense numbers of the small lapilli in the matrix consist of an extremely fine vesicular pumice. Small perfect and larger broken crystals of augite are likewise abundant in some of the greenish, more basic parts of the mass. These greenish serpentinous parts and the numerous augite crystals point to the explosion of some tolerably basic pyroxenic lava. A similar dark green, almost black, rock, with augite crystals, which sometimes measure a quarter of an inch in diameter, occurs near Sanquhar in Nithsdale. It presents a close resemblance to the agglomerate of Rhobell Fawr, already alluded to. So far as these Scottish agglomerates have yet been microscopically examined, they have been found to be composed of crystals, crystal-fragments, and lapilli derived partly from lavas similar to those above described, and partly from felsitic and other rocks which have not yet been observed here in the form of lavas.

The finer tuffs show likewise a considerable range of composition. According to Mr. Peach's observations along the south-eastern parts of the volcanic area, the ejected materials have consisted largely of fine dust (probably in great measure felsitic), which towards the north-east is gradually interleaved with ordinary sediment till the ashy character disappears. As I have already remarked, there is reason to believe that the overlying greywackes and shales derived part of their material either directly from volcanic explosions or from the attrition of banks of lavas and tuffs exposed to denudation.

But besides the interstratified lavas and fragmental rocks there occur numerous intrusive masses which are so intimately associated with the volcanic series that they may with little hesitation be regarded as forming part of it. They consist of various gabbros and serpentines, which are especially developed where the volcanic series comes out in greatest force in the south-west of Ayrshire. They also include more acid intrusions which, as in the case of the rock of Byne Hill, near Girvan, even assume the characters of granite.

The dying out of the volcanic material towards the north-east probably indicates that the vents of the period lay rather in the central or south-western parts of the district. Unfortunately, the limited extent of the exposures of the rocks makes it a hopeless task to search for traces of these vents over by far the largest part of the area. There are two localities, however, where the search may be made with better prospect of success. One of these is a tract to the north of Sanquhar in Nithsdale, which still requires to be studied in detail with reference to the sequence and structure of its volcanic rocks. The other area is that south-western part of Ayrshire which has been already cited as displaying so large a development of the volcanic series. Here the coast-sections reveal the intercalation of fossiliferous bands which show the true stratigraphical horizons of the lavas and tuffs. Under Bennane Head, Professor Lapworth some years ago found, in certain hardened black shales, a group of graptolites which mark an undoubted Arenig platform.[174] Recently the ground has been carefully re-examined by Messrs Peach and Horne, who have detected a number of other fossiliferous zones which confirm and extend previous observations. They have also been able to unravel the complicated structure of the volcanic series, and to represent it on the 6-inch maps of the Geological Survey, of which a reduction on the scale of 1 inch to a mile is now in course of preparation. The following tabular summary, taken partly from notes made by myself during a series of traverses of the ground with Mr. Peach when the revision was begun, and partly from memoranda supplied by that geologist himself, may suffice as a general outline of the volcanic history of this exceedingly interesting and important region.

[Footnote 174: _Geol. Mag._ 1889, p. 22.]

Llandovery. }Pentamerus grit. }Conglomerate (Mulloch Hill).

Caradoc. {Shales, sandstones, grits, etc. (Ardmillan, Balcletchie). {Thick conglomerate (Byne Hill, Bennane, etc.). {Thick fossiliferous limestone (Stinchar, Girvan). (On this horizon { come the perlitic felsites and soda-felsites of Winkstone and Wrae.) {Sandstone (_Orthis confinis_) passing down into thick conglomerate.

[Unconformability.]

Upper Llandeilo. }Green mudstones, grits and greywackes. }Thin band of dark mudstone with Upper Llandeilo graptolites.

Arenig and Lower and Middle Llandeilo. {Group of Radiolarian cherts (about 70 feet) with alternating tuffs. {Tuff or volcanic conglomerate, with occasional lava-flows. {Black shale (10 feet) with Arenig graptolites. {Volcanic breccias around local centres (Knockdolian, etc.). {Thick group of porphyrite and diabase lavas. {Red flinty mudstones with Arenig graptolites. {Porphyrites, etc. {Fine tuffs, etc., with Lower Arenig fossils. {Diabase lavas, etc. (base not seen).

It will be noticed from this table that the bottom of the volcanic series is not reached, so that no estimate can be formed of its full thickness, nor on what geological platform it begins. Possibly its visible portions represent merely the closing scenes of a long volcanic history, which, over the area of the south of Scotland, extended into Cambrian time, like the contemporary series of Cader Idris.

Among the lowest lavas there are interstratified courses of fine tuffs, flinty shales and thin limestones, which sometimes fill in the hollows between the pillow-like blocks above referred to. Among the characteristic Lower Arenig graptolites of these intercalated layers are _Tetragraptus bryonoides_, _T. fruticosus_, _T. quadribrachiatus_, and _T. Headi_ together with _Caryocaris Wrightii_. Considerable variation is to be seen in the development of the upper part of the volcanic series. In some places the lavas ascend almost to the top; in others, thick masses of breccia or agglomerate take their place. These fragmentary materials are locally developed round particular centres, which probably lie near the sites of active vents whence large quantities of pyroclastic material were discharged. One of the volcanic centres must have been situated close to the position of Knockdolian Hill already referred to. The exceedingly coarse breccia of that eminence is rudely stratified in alternations of coarser and finer material, which was probably to some extent assorted under water around the cinder-cone that discharged it. The date of the explosions of this hill has been ascertained by Mr. Peach from the intercalation of black shales containing Arenig graptolites among the breccias. Another vent lay somewhere in the immediate neighbourhood of the Mains Hill agglomerate, if not actually on part of the site of that rock. Though probably not more than a mile from the Knockdolian volcano, and belonging to the same epoch of eruption, this vent, to judge from the peculiarities of its ejected material, must have been quite distinct in its source. A third vent lay somewhere in the immediate vicinity of Bennane Head, and threw out the extraordinary masses of agglomerate and the sheets of lava seen on the coast at that locality. A fourth may be traced by its separate group of fine tuffs on the coast three miles south of Ballantrae.

A feature of singular interest in the material erupted from these various centres of activity consists in the evidence that the explosions occurred at intervals during the deposition of the Lower Silurian formations, and that these formations were successively disrupted by submarine explosions. Mr. Peach has found, for example, abundant pieces of the peculiar and easily recognized radiolarian cherts imbedded in the volcanic series. That these cherts were deposited contemporaneously with the volcanic eruptions is proved by their intercalation among the breccias. Yet among these very breccias lie abundant fragments of chert which must have already solidified before disruption. It is thus evident that this siliceous ooze not only accumulated but set into solid stone on the sea-floor, between periods of volcanic outburst, and that such an occurrence took place several times in succession over the same area.

These facts derive further interest from the organic origin of the chert. It is now some years since Mr. Peach and his colleagues observed that between the Glenkiln Shale with its Upper Llandeilo graptolites and the top of the volcanic group in the central part of the Silurian uplands, alternations of green, grey or red shaly mudstones and flinty greywackes are interleaved with fine tuffs, and are specially marked by the occurrence in them of nodules and bands of black, grey and reddish chert. This latter substance, on being submitted to Dr. Hinde, was found by him to yield twenty-three new species of Radiolaria belonging to twelve genera, of which half are new. It thus appears that during the volcanic activity there must have been intervals of such quiescence, and such slow, tranquil sedimentation in clear, perhaps moderately deep water, that a true radiolarian ooze gathered over the sea-bottom.[175]

[Footnote 175: _Ann. Mag. Nat. Hist._ (1890), 6th ser. vi. p. 40.]

That the deposition of this ooze probably occupied a prolonged lapse of time seems clearly indicated by the evidence of the fossils that occur below and above the cherts. The graptolites underneath indicate a horizon in the Middle Arenig group, those overlying the cherts are unmistakably Upper Llandeilo. Thus the great depth of strata which elsewhere constitute the Upper Arenig and Lower and Middle Llandeilo subdivisions is here represented by only some 60 or 70 feet of radiolarian cherts. These fine siliceous, organic sediments probably accumulated with extreme slowness in a sea of some depth and over a part of the sea-floor which lay outside the area of the transport and deposit of the land-derived sediment of the time.[176]

[Footnote 176: _Annual Report of the Geol. Surv. for 1895_, p. 27 of reprint.]

As an illustration of some of the characteristic features in the succession of deposits in the volcanic series of the south-west of Ayrshire, the accompanying section (Fig. 54) is inserted. In descending order we come first upon a group of greywackes and grey shattery mudstones (_a_), followed by grey-green and dark banded cherts, containing Radiolaria and much plicated. Next comes a group of dark-grey, black and red cherts, with numerous partings and thin bands of tuff and volcanic conglomerate (_c_). The siliceous bands were certainly deposited during the volcanic eruptions, and they are moulded round the rugose, slaggy upper surface of the band of lavas (_d_) on which they directly lie. These lavas have the sack-like or pillow structure already described, and they enclose lumps of chert containing Radiolaria. A few yards to the west of the line of section bands of nodular tuff are interposed between the top of the lavas and the overlying cherts, with which also they are interstratified. These tuffs contain blocks of lava six inches or more in diameter. Below the belt of lavas come black cherts and shales (_e_) succeeded below by volcanic breccias and tuffs (_f_) alternating with shales in thin inconstant courses. These coarse detrital rocks are thoroughly volcanic in origin, and they contain fragments of the black cherts which lie still lower in the series. The whole depth of strata represented in this section does not amount to much more than 100 feet.

While in some parts of the Ayrshire district the coarse breccias that accumulated around their parent vents form most of the upper part of the volcanic series, in others the lavas are succeeded by fine tuffs which are intercalated among the ordinary sediments, and show a gradual decline and cessation of volcanic energy. South of Ballantrae, for example, the lavas occupy more than two miles of coast, in which space they display hardly any intercalations of sedimentary material, though they show more or less distinctly that they consist of many separate flows. Where they at last end, bands of nodular and fine tuff make their appearance, together with bands of ashy shale and the characteristic zone of the red radiolarian cherts or flints. Above these, in conformable sequence, come bands of black shale, containing abundant Upper Llandeilo graptolites, overlain by greenish or olive-coloured shaly mudstones, which pass upward into a thick overlying group of greywackes.

In this section the alternation of fine pyroclastic with ordinary sediment shows that the volcanic eruptions in the southern part of the Ballantrae district came to an end by a slowly-lessening series of explosions. The ashy material gradually dies out, and does not reappear all through the thick group of sandy and muddy sediments which here overlies the volcanic series.

We thus learn from the evidence of the Ayrshire sections that volcanic action was in full vigour in the south-west of Scotland during the Arenig period, but gradually died out before the end of the Llandeilo period. The rocks in which this volcanic history is chronicled have been very greatly disturbed and plicated, so that though from their frequently vertical position they might be thought to attain a vast depth, they very possibly do not exceed 500 feet in thickness.

As the volcanic series is followed north-eastwards it exhibits a gradual diminution in extent and variety, but this may be at least partly due to the much less depth of it exposed on the crests of the narrow anticlines that bring it to the surface. There is evidence in that region that the eruptions did not everywhere terminate in the Llandeilo period, but were in some districts prolonged into the age of the Bala rocks. Thus in the neighbourhood of Sanquhar volcanic breccias, tuffs and lavas have been found by Messrs. Peach and Horne intercalated in strata apparently belonging to the Bala group. Again, in the district of Hartfell, a moderately coarse volcanic agglomerate occurs in the heart of the so-called "barren mudstones" of the Hartfell black-shale group, which, from its graptolites, is placed on the horizon of the Bala rocks. At Winkstone, Hamilton Hill, and Wrae in Peeblesshire, perlitic felsites and soda-felsites have been detected by Messrs. Peach and Horne and determined by Mr. Teall. They are associated with the Bala limestone, which in some of its conglomeratic bands contains pebbles of felsite.

The intrusive rocks which accompany the Lower Silurian volcanic series of the south of Scotland are best displayed in the south-west of Ayrshire, between Girvan and Ballantrae, where they appear to be on the whole later than at least the great mass of the interstratified lavas and tuffs. The most abundant rocks and the earliest to be injected are complex basic masses which include serpentine, olivine-enstatite rock, troctolite, gabbro and other compounds, all which may be different modifications of the same original basic magma. They do not show a finer texture where they respectively meet, nor any other symptom of having been subsequently intruded into each other, though they do exhibit such structures along their lines of contact with the surrounding rocks, into which they are intrusive. These more basic masses have subsequently been invaded by irregular bosses and dyke-like protrusions, which, when small, are fine-grained dolerites, but when in larger bodies take the form of gabbro, sometimes exhibiting a mineral banding and foliated structure. These banded varieties much resemble the banded Tertiary gabbros of Skye and some parts of the Lewisian gneiss.

At the Byne Hill, near Girvan, a large intrusive boss or ridge displays on its outer margin a fine-grained texture, where it comes in contact with the serpentine. Further inwards it becomes a fine dolerite, passing into gabbro and increasing in coarseness of grain as well as in acidity of composition, through stages of what in the field would be called diorite and quartz-diorite, into a central granitic rock, whereof milky or blue quartz forms the prominent constituent. The intrusive rocks of this district have generally been injected parallel to the stratification-planes, and take on the whole the form of sills.

Some time after the close of the volcanic episode in the Silurian period of the south of Scotland, the rocks were locally subjected to considerable disturbance and elevation, whereby parts of the volcanic series were exposed to extensive denudation. Hence the overlying unconformable Caradoc conglomerates are in some places largely made up of the detritus of the volcanic rocks. It is interesting to find this evidence of waste during the very next stage of the Silurian period, for it affords good evidence that the extensive sheets of intrusive material could not have had any large amount of overlying strata resting upon them at the time of their injection. Pieces of these intrusive rocks, such as the serpentine, occur abundantly in the Caradoc conglomerates, some of which indeed are almost wholly composed of their detritus. Probably the total thickness of the overlying cover of rock under which the sills were injected did not amount to as much as 200 or 300 feet. Yet we see that among the sills were coarse gabbros and granitoid rocks. We may therefore infer that for the injection of such intrusive masses, great depth and enormous superincumbent pressure are possibly not always necessary.

During the progress of the Geological Survey along the southern borders of the Highlands, a remarkable group of rocks has been observed, intervening as a narrow interrupted strip between the schistose masses to the north and the great boundary-fault which brings the Old Red Sandstone in vertical strata against them. Between Cortachy in Forfarshire and Stonehaven on the east coast, these rocks have been mapped by Mr. G. Barrow, who has carefully worked out their relations. They appear again between Callander and Loch Lomond, where their extent and structure have been mapped by Mr. C. T. Clough. For the purpose of our present inquiry two chief features of interest are presented by these rocks. They include a group of sedimentary strata among which occur bands of jasper or chert containing radiolaria, and one of their most conspicuous members is a series of volcanic rocks consisting chiefly of dolerites and basalts, some of which have been much crushed and cleaved, but in which vesicular structures can still occasionally be recognized.

The striking resemblance of both the aqueous and igneous members of this marginal strip of rocks along the Highland border to the Arenig cherts and their accompanying lavas in the south of Scotland, the remarkable association of the same kinds of material in the same order of sequence, the occurrence of radiolaria in the siliceous bands in both regions, furnish strong presumptive evidence that a strip of Arenig rocks has been wedged in against the Highland schists.

In many respects, these dull green diabasic lavas of the Highland border resemble those of the Ayrshire coast. In particular, the same peculiar sack-like or pillow-shaped masses are conspicuous in the Forfarshire ravines. As in Ayrshire, igneous materials underlie the cherts which are doubled over and repeated by many successive folds. Unfortunately, it is only a narrow strip of these probably Arenig lavas that has been preserved, and no trace has been detected of tuffs, agglomerates or necks. If, however, we may regard the rocks as truly of Arenig age, they furnish interesting additional proof of the wide extent of the earliest Silurian volcanoes. The distance between the last Arenig volcanic outcrop in the Southern uplands and the band of similar lavas along the margin of the Highlands is about 50 miles. If the volcanic ejections were continuous across the intervening tract, the total area over which the lavas and tuffs of the Arenig volcanoes were distributed must be increased by at least 6000 square miles in Scotland.

But it is in the north of Ireland that this northern extension of what may probably be regarded as an Arenig series of volcanic rocks attains its greatest development. Of this Irish prolongation a brief account is given in Chapter xiv., where the whole of the Silurian volcanic rocks of the island are discussed.