CHAPTER XVII

DISTRIBUTION OF THE VOLCANIC CENTRES IN THE LOWER OLD RED SANDSTONE—CHARACTERS OF THE MATERIALS ERUPTED BY THE VOLCANOES

i.DISTRIBUTION OF VOLCANIC CENTRES

The area within which volcanic rocks belonging to the Lower Old Red Sandstone appear is one of the most extensive regions over which the volcanic eruptions of any geological period can be traced in the British Isles (Map I.). Its northern limit reaches as far as the islet of Uya in Shetland, and its southern appears in England in the Cheviot Hills—a distance of about 250 miles. But volcanic rocks of probably corresponding age occur even as far to the south as the hills near Killarney. The most easterly margin of this area is defined by the North Sea on the coast of Berwickshire, and its extreme western boundary extends to near Lough Erne in the north of Ireland—a distance of some 230 miles. If we include the post-Silurian bosses and dykes, like those of Shap, and likewise the Devonian volcanic rocks of Devon and Cornwall, as contemporaneous with those of the Old Red Sandstone, the area of eruption will be greatly enlarged. But leaving these out of account for the present, and confining our attention to the Lower Old Red Sandstone series, we find that, within the wide limits over which the volcanic rocks are distributed, a number of distinct and often widely separated centres of eruption may be traced. Taking these as they lie from north to south, we may specially enumerate the following:—

1. The Shetland and Orkney Islands, together with the basin of the Moray Firth. This region includes several distinct volcanic groups, of which the most northerly extends through the centre to the north-western headlands of the mainland of Shetland, another lies in the island of Shapinshay, one of the Orkneys, while at least two can be recognized on the south side of the Moray Firth. To this wide region of Old Red Sandstone I have given the general designation of "Lake Orcadie."[322]

[322]Trans. Roy. Soc. Edin.vol. xxviii. (1878), p. 354.

2. The basin of Lorne, on the west of the mainland of Argyllshire, extending from Loch Creran to Loch Melfort and the hills on the west side of Loch Awe.

3. The great central basin of Scotland, which, for the sake of distinctness, I have called "Lake Caledonia,"[323]stretching between the Highlands and the Southern Uplands, from the east coast south-westwards across Arran and the south end of Cantire into Ireland as far as Lough Erne. Numerous distinct volcanic groups occur in this great basin, and their volcanic history will be discussed in detail in later chapters (seeMap III.).

[323]Op. cit.

4. The basin of the Cheviot Hills and Berwickshire, with these hills as the chief area, but including also other tracts, probably independent, which are cut off by the sea along the eastern coast of Berwickshire between St. Abb's Head and Eyemouth.

5. The Killarney tract, including the hills lying around Lough Guitane in the east of County Kerry.

At the outset we may take note of a feature in the volcanic history of Britain, first prominently noticeable in the records of the Old Red Sandstone, and becoming increasingly distinct during the rest of the long sequence of Palæozoic eruptions, namely, the persistence with which the vents have been opened in the valleys and have avoided the high grounds. I formerly dwelt on this relation, with reference to the Carboniferous volcanic phenomena,[324]but the observation may be greatly extended. With regard to the Old Red Sandstone of Central Scotland, though the lavas and tuffs that were discharged over the floor of the sheet of water which occupied that region gradually rose along the flanks of the northern and southern hills, yet it was on the lake-bottom and not among the hills that the orifices of eruption broke forth.

[324]Trans. Roy. Soc. Edin.vol. xxix. (1879), p. 454.

So far as I am aware, no undoubted vents of the age of the Lower Old Red Sandstone have been detected among the high grounds of the Highlands on the one hand, or among the Silurian uplands on the other, although a fringe of the lavas may be traced here and there along the base of the hills.[325]In some cases, doubtless, the position of the valleys may have been determined by lines of fault that might well serve as lines of relief along which volcanic vents would be opened. But in many instances it can be proved that, though the vents have risen in valleys and low grounds, they have not selected lines of fault visible at the surface, even when these existed in their neighbourhood. Any fissures up which the volcanic ejections made their way must have lain at great depths beneath the formations that now form the surface rocks.

[325]Certain remarkable necks of breccia have been detected by Mr. J. R. Dakyns rising through the schists at the upper end of Loch Lomond; but there is not sufficient evidence to connect them with the volcanic series of the Lower Old Red Sandstone. Some of the younger granite bosses are not improbably to be referred to this volcanic series. The latest granites of the eastern Grampians, as already stated, have lately been found by Mr. Barrow cutting the band of probably Lower Silurian strata along the southern border of the Highlands. Those of Galloway are younger than the Upper Silurian formations, which they invade, and older than the conglomerates of the Upper Old Red Sandstone, which contain pebbles of them. These eruptive bosses will be further discussed in the sequel.

ii.CHARACTERS OF THE MATERIALS ERUPTED BY THE VOLCANOES

A general summary of the petrographical characters of the igneous rocks of the Lower Old Red Sandstone may here find a place. Further details will be given in the account of "Lake Caledonia," which is the typical area for them; but, on the whole, the prevailing types in one region are found to be repeated in the others.

1.Bedded Lavas.—Beginning with the lavas which were poured out at the surface, we have to notice a considerable range of chemical composition among them, although, as a rule, they are characterized by general similarity of external appearance. At the one end, come diabases and other ancient forms of basalt or dolerite, wherein the silica percentage is below or little above 50. By far the largest proportion of the lavas, however, are porphyrites or altered andesites, having about 60 per cent of silica. With these are associated lavas containing more or less unstriped felspar and a somewhat higher proportion of silica, which may be grouped as trachytes, though no very sharp line can be drawn between them and the andesites. In the Pentland Hills, and some other areas, orthophyres flowed out alternately with the more basic lavas, and were associated with felsitic tuffs and breccias.

It is noteworthy that the lava-sheets of the Lower Old Red Sandstone, if we consider the character of the prevalent type, hold an intermediate grade between the average chemical composition of those of Silurian and of those of later Carboniferous time. On the one hand, they rarely assume the character of thoroughly acid rocks, like the nodular rhyolites of the Bala and Upper Silurian series;[326]on the other hand, they seldom include such basic lavas as the basalts, so common among the puy-eruptions of the Carboniferous system, and never, so far as I know, contain varieties comparable to the "ultra-basic" compounds which I shall have occasion to allude to as characteristic of a particular volcanic zone in that system.

[326]The only examples known to me are those of Benaun More and other hills in County Kerry.

(a) TheDiabase-lavasare typically developed in the chain of the Pentland Hills, where they form long bands intercalated between felsitic tuffs—a remarkable association, to which I shall make more detailed reference in a later chapter. They range in texture from a compact dark greenish base to a dull earthy amygdaloid. One of their most remarkable varieties is a fine-grained green porphyry, with large flat tabular crystals of plagioclase arranged parallel to the direction of flow (Carnethy Hill). Most of them, however, are more or less amygdaloidal, and some of them (Warklaw Hill) strongly so. The following analyses, made in the laboratory of the Royal School of Mines under the direction of Prof. E. Frankland, show the chemical composition of some of the diabases of the Pentland Hills:[327]—

[327]For analyses of some Shetland diabases of Old Red Sandstone age, see Mr. R. R. Tatlock,Trans. Roy. Soc. Edin.vol. xxxii. (1887), p. 387.

[328]There was a trace of manganous oxide in this specimen.

(b) TheAndesites, or, as they were formerly called,Porphyrites, which constitute by far the largest proportion of the lavas, have a characteristic but limited range of lithological varieties. The prevailing type presents a close-grained, rather dull texture, and a colour varying from pinkish grey, through many shades of green and brown, to purplish red, which last is, on the whole, the predominant hue. Minute lath-shaped felspars may frequently be detected with the naked eye on fresh surfaces, while scattered crystals, which are generally hæmatitic pseudomorphs after some pyroxene, occasionally after hornblende or mica, may often be observed. The usual porphyritic constituents are plagioclase felspars, occasionally in abundant tabular crystals measuring half an inch or more across, also one or more pyroxenes (augite, enstatite), and sometimes brown or black mica. Where large felspar-crystals occur in a compact green matrix, the rock assumes a resemblance to theverde antiqueof the ancients.[329]One of the Cheviot andesites lying at the bottom of the series is distinguished by its large and abundant plates of black mica.[330]

[329]An instance of this rock occurs in Kincardineshire, from which the large flat twins of labradorite have been analyzed by Dr. Heddle (Trans. Roy. Soc. Edin.vol. xxviii. (1879), p. 257).[330]C. T. Clough, "The Cheviot Hills,"Mem. Geol. Survey(1888), p. 12.

[329]An instance of this rock occurs in Kincardineshire, from which the large flat twins of labradorite have been analyzed by Dr. Heddle (Trans. Roy. Soc. Edin.vol. xxviii. (1879), p. 257).

[330]C. T. Clough, "The Cheviot Hills,"Mem. Geol. Survey(1888), p. 12.

The texture of the andesites occasionally becomes faintly resinous, where a considerable proportion of glass still remains undevitrified, as in the well-known varieties from the Cheviot Hills, and in another pitchstone-like rock from above Airthrey Castle in the Ochil Hills, near Bridge of Allan. It sometimes presents a nodular or coarsely perlitic character, weathering out in nut-like balls, like the rock of Buckham's Wall Burn in the Cheviot Hills.[331]Much more frequent is a well-developed amygdaloidal structure, which indeed may be said to be the most obvious characteristic of these rocks as a whole. The steam-vesicles, now filled with agate, quartz, calcite or zeolite, vary in size from mere granules up to large irregular cavities a foot or more in diameter. Where the kernels are coated with pale-green earth and lie in a dark brown matrix, they give rise to some of the most beautiful varieties of rock in any volcanic series in this country, as may be seen onthe Ayrshire coast at Culzean and Turnberry. Some rocks contain the vesicles only as rare individuals, others have them so crowded together as to form the greater part of the cubic contents of the mass. When the infiltration-products have weathered out, some of the amygdaloids present a striking resemblance to recent slaggy brown lavas; lumps of them must have been originally light enough to float in water.

[331]Ibid.p. 11.

My colleague in the Geological Survey, Mr. J. S. Grant Wilson, some years ago made for me a large series of determinations of the specific gravity of the volcanic rocks of the Lower Old Red Sandstone of Scotland. He found that the andesites collected from various districts to illustrate the more typical varieties of rock averaged about 2·66. He also made a series of chemical analyses of a number of the same rocks from the Cheviot Hills, where they are well preserved. The results are shown in the following table:—

[332]This is CO2.

The microscopic structure of the andesites of the Lower Old Red Sandstone has been partially investigated, especially those of the Cheviot Hills, by Mr. Teall[333]and by Dr. Petersen,[334]who both give chemical analyses of the rocks. Much, however, still remains to be done before our knowledge of this branch of British petrography can be regarded as adequate. The groundmass in some of the rocks consists mainly of a brown glass with a streaky structure (as in the well-known variety of Kirk Yetholm, and in the rock, still more like pitchstone, from near Airthrey Castle in the Ochil chain); more usually it has been devitrified more or less completely by the appearance of felspathic microlites, until it presents a confused felspar aggregate. The porphyritic felspars are often large, generally striped, but sometimes including crystals that show no striping. They are frequently found to be full of inclusions of the base, and these sometimes consist of glass. The ferro-magnesian constituents are usually rather decomposed, being now represented by chloritic pseudomorphs; but augite, and perhaps still more frequently enstatite, may be recognized, or its presence may beinferred among them. The beautiful resinous or pitchstone-like rock from near Airthrey Castle has been found by Mr. Watts to be a glassy hypersthene-augite-andesite, since among its phenocrysts of plagioclase, augite and hypersthene both occur. Magnetite is commonly traceable, and apatite may be occasionally detected. As the result of decomposition, calcite, chlorite and limonite are very generally diffused through the rocks.[335]

[333]Geol. Mag. for 1883, pp. 100, 145, 252.[334]Mikroskopische und chemische Untersuchungen am Enstatit-porphyrit aus den Cheviot Hills, Inaug. Dissert. Kiel, 1884. Descriptions have also been published of detached rocks from other districts, such as those by Prof. Judd and Mr. Durham of specimens from the Eastern Ochils,Quart. Journ. Geol. Soc.vol. xlii. (1886), p. 418.[335]Dr. F. H. Hatch supplied notes on microscopic structure which are incorporated in the text, together with particulars afterwards furnished by Mr. Watts.

[333]Geol. Mag. for 1883, pp. 100, 145, 252.

[334]Mikroskopische und chemische Untersuchungen am Enstatit-porphyrit aus den Cheviot Hills, Inaug. Dissert. Kiel, 1884. Descriptions have also been published of detached rocks from other districts, such as those by Prof. Judd and Mr. Durham of specimens from the Eastern Ochils,Quart. Journ. Geol. Soc.vol. xlii. (1886), p. 418.

[335]Dr. F. H. Hatch supplied notes on microscopic structure which are incorporated in the text, together with particulars afterwards furnished by Mr. Watts.

(c) The lavas which may be separated asTrachytesoffer no distinctive features externally by which they may be distinguished from the andesites. Indeed, both groups of rocks appear to be connected by intermediate varieties. In the Cheviot Hills some of the lavas are found, on microscopic examination, to contain a large admixture of unstriped porphyritic felspars, which can occasionally be recognized as sanidine in Carlsbad twins. The groundmass is sometimes a brown glass, but is usually more or less completely devitrified, portions of it being inclosed in the large felspars. Chlorite, pseudomorphic after augite or enstatite, may be detected, and sometimes a brown mica. A specimen of one of these rocks, from a locality to the north-west of Whitton, near Jedburgh, was found by Mr. J. S. Grant Wilson to have the following composition:—

(d) Acid rocks such asFelsitesandRhyolitesare rare among the lavas poured out at the surface during the time of the Lower Old Red Sandstone. They occur in the Pentland Hills, also near Dolphinton in the Biggar district, and in the Ochil Hills near Auchterarder, associated with extensive accumulations of felsitic tuffs and breccias. They are usually so much decomposed that it is hardly possible to procure fresh specimens of them. Some of them display beautiful flow-structure. They appear to be generally orthoclase-felsites or orthophyres. Dull, fine-grained to flinty in texture, they hardly ever display free quartz, so that they can seldom be placed among the typical rhyolites, though in their banded flow-structure they often strongly resemble some lithoid varieties of these rocks, especially such varieties as that represented inFig. 9.

Mr. Watts, to whom I submitted, for microscopic examination, a number of specimens from the Pentland and Ochil Hills, has found them to "consist of a brown felsitic groundmass in which are embedded a generation of small stumpy prisms of orthoclase and a set of larger phenocrysts, generally consisting of orthoclase and plagioclase in equal proportions. Brown mica is usually present and zircons are not uncommon." The rocks,when they undergo weathering, pass into the varieties formerly comprised under the name claystone.

The only nodular felsite of this age which I have met with is that of Lough Guitane among the "Dingle Beds," near Killarney, to which reference will be made in later pages.

2.Intrusive Bosses, Sills and Dykes.—While the interbedded lava-sheets are mainly andesites, the intrusive rocks are generally more acid, and most of them may be grouped under the convenient head of felsites. Some intrusive andesites, and even more basic rocks, do indeed occur in dykes and sills, as well as also filling vents. But the rule remains of general application over the whole country that the materials which have consolidated in the volcanic orifices of the Old Red Sandstone, or have been thrust among the rocks in dykes, bosses or sills, are decidedly acid. In this series of rocks a greater range of types may be traced than among the extrusive lavas. At the one end we find true granites or granitites, as in the intrusive bosses of Spango Water and of Galloway, which, for reasons which I will afterwards adduce, may with some probability be assigned to the volcanic history of the Lower Old Red Sandstone period. Among the bosses, many of which probably mark the positions of eruptive vents, orthophyres are especially prominent. These rocks frequently contain no mica, but, on the other hand, they sometimes show abundant quartz in their groundmass. The augite-granitite of the Cheviot Hills, so well described by Mr. Teall, has invaded the bedded andesites of that region.[336]A similar rock has been noticed by my brother, Prof. James Geikie, associated with the Lower Old Red Sandstone volcanic rocks of the east of Ayrshire. A remarkable petrographical variety has been mapped by Mr. B. N. Peach, rising as a small boss through the lower part of the great lava-sheets of the Ochil Hills, above Tillicoultry. It is a granophyric quartz-diorite, which, under the microscope, is seen to be composed of short, thick-set prisms of plagioclase, with abundant granophyric quartz, a pleochroic hypersthene, and needles of apatite. Sometimes the pyroxene is replaced by green chloritic pseudomorphs.[337]

[336]Geol. Mag.for 1883, pp. 100, 145, 252; andBritish Petrography, pp. 272, 278.[337]Notes by Dr. Hatch.

[336]Geol. Mag.for 1883, pp. 100, 145, 252; andBritish Petrography, pp. 272, 278.

[337]Notes by Dr. Hatch.

At the other end of the series come the felsites, quartz-porphyries, mica-porphyrites, minettes, vogesites, "hornstones" and "claystones" (or decayed felsites), which have a close-grained texture, often with porphyritic felspars, quartz or black mica, generally a whitish, pale buff, orange, pink or purplish-grey colour, and a specific gravity of about 2·55.[338]

[338]The intrusive "porphyry" of Lintrathen in Forfarshire (which may be younger than the Old Red Sandstone) is a bright red rock with porphyritic felspar, quartz, white mica and a very singular black mica (Mr. Teall'sBritish Petrography, p. 286).

Though I class these rocks as intrusive, I am not prepared to assert that in none of the instances where they occur as sheets may they possibly have been erupted at the surface as lavas. In one or two cases the evidence either way is doubtful, but as the great majority of the acid rocks can beshown to be intrusive in their behaviour, I have preferred to keep them all in the same category. I am prepared to find, however, that, as so vast an amount of felsitic debris was ejected to form the tuffs, more of this material may have flowed out in streams of lava than is at present recognized.

The following table shows the chemical composition of some acid sills and dykes from the Lower Old Red Sandstone, as determined in the laboratory of Prof. E. Frankland:[339]—

[339]Two analyses of rhyolites from Shetland by Mr. Tatlock will be found inTrans. Roy. Soc. Edin.vol. xxxii. (1887), p. 387. Their silica percentage is 72·32 and 73·70. An analysis of a quartz-felsite from the Cheviot Hills by Mr. T. Waller is given in the Geological Survey Memoir on the Cheviot Hills, p. 25. The proportion of silica in this rock is 67·9.

[340]This specimen also yielded 0·13 of ferrous oxide, and 2·42 of carbon dioxide.

The rock of Tinto, which may be considered typical of the prevailing acid intrusive rocks of the series, presents several slightly different varieties. Dr. Hatch, as the result of his examination of a number of microscopic slides prepared from specimens taken by me from various parts of the hill, found some to be minettes, showing small isolated crystals of orthoclase and rare flakes of biotite, sometimes granules of quartz, imbedded in a brown, finely microlitic groundmass of felspar powdered over with calcite; while other specimens had a granular instead of a microlitic groundmass, and contained a considerable amount of quartz in addition to the constituents just mentioned. A conspicuous knob on the south side of Tinto, called the Pap Craig, is a mass of augite-diorite, which has risen through the other rocks[341](seeFig. 93). The sills in the same region show still further differences. Some are true "felspar-porphyries," and "quartz-porphyries" varying in the relative abundance and size of their porphyritic orthoclase and quartz, while others, by the introduction of hornblende or pseudomorphs after that mineral, pass into vogesites.

[341]This rock differs considerably from the other intrusive masses in its neighbourhood. Dr. Hatch found it to be composed chiefly of lath-shaped striped felspar, with some granular augite, magnetite and interstitial quartz.

Basic sills and bosses are chiefly developed among the Ochil and Sidlaw Hills. They may generally be classed as diabases. But sometimes their pyroxenic constituent is partly hypersthene, as in a coarsely crystalline boss about a mile south of Dunning, which has been determined by Mr. Wattsto "consist of augite and hypersthene imbedded in and occurring amongst large plagioclase prisms. Some iron-ore is also present; the rock is a hyperite."

3.Tuffs and Agglomerates.—The fragmental materials, ejected from or filling up the vents, vary from the finest compacted dust up to some of the coarsest agglomerates in this country. In general they consist mainly of detritus of andesite, and have been derived from the blowing up of already consolidated masses of that rock. The fragments are usually angular, and range from minute grains up to blocks as large as a cottage. The tuffs are often more or less mixed with ordinary non-volcanic sediment, and as they are traced away from the centres of eruption they pass insensibly into sandstones and conglomerates.

But while, as might be expected, the tuffs are most commonly made up of debris of the same kind of lavas as those that usually form the sheets which were poured out at the surface, they include also bands of material derived from the destruction of much more acid rocks. Throughout the chain of the Ochil Hills, for example, in the midst of the bedded andesite-lavas, many of the thin courses of fine tuff consist largely of felsitic fragments, with scattered felspar crystals. The most remarkable examples of this nature, however, are to be met with at the great vent of the Braid Hills, in the chain of the Pentland Hills which runs south-westward from it, and in the Biggar volcanic district still further south. These acid tuffs are generally pale flesh-coloured or lilac in tint, and compact in texture, but, like the felsitic lavas from which they were derived, they are apt to weather into yellow or buff "claystones." The finer varieties are so compact as to present to the naked eye no distinguishable grains; they might be mistaken for felsites, and indeed, except where they contain recognizable fragments of rock or broken crystals of felspar, can hardly be discriminated from them. They consist of an exceedingly fine compacted felsitic dust. Here and there, however, the scattered crystals of felspar and small angular fragments of felsite, which may be detected in them, increase in number until they form the whole of the rock, which is then a brecciated tuff or fine volcanic breccia, made up of different felsites, among which, even with the naked eye, delicate flow-structures may be detected. In these pale acid tuffs, fragments of different andesites may often be observed, which increase in number as the rocks are traced away from the main vents of eruption.

At my request my colleague, Mr. George Barrow, determined the silica percentages in a few specimens which I selected as showing some of the more characteristic varieties of these tuffs from the Braid and Pentland Hills. His results are exhibited in the following table:—

From these analyses it may be inferred that the average amount of silica in the more typical varieties is between 70 and 75 per cent. The last specimen in the table, with its abnormally high percentage of acid, must be regarded as an exceptional variety, where there has either been an excessive removal of some of the bases, or where silica has been added by infiltration.

The microscopic examination of these rocks has not added much to the information derivable from a study of them in the field. In their most close-grained varieties, as above remarked, they are hardly to be distinguished from felsites. But they generally show traces of the minute detrital particles of felsite of which they are essentially composed. The brecciated varieties exhibit finely-streaked flow-structure in some of the fragments. Pieces of andesite, grains of quartz, and other extraneous ingredients appear in these rocks towards the southern limits of the volcanic area of the Pentland Hills, where the acid tuffs are associated with and pass laterally and vertically into ordinary non-volcanic sedimentary strata. Further details as to the part which these tuffs play in the volcanic history of the regions wherein they occur will be given in later pages.

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