XIV. THE SILURIAN VOLCANOES OF IRELAND
Abundant as are the volcanic records of the Silurian period in England, Wales and Scotland, the description of them would be incomplete without an account of those of Ireland. The eruptions of Arenig, Llandeilo and Bala time, which we have followed from the south of Caermarthenshire to the borders of the Scottish Highlands, had their counterparts all down the east of Ireland. The Irish register of them, however, supplies some details which are less clearly preserved in the sister island. But the most distinctive feature of the Silurian volcanic history in Ireland is the preservation of memorials of eruptions during the Upper Silurian period. In no part of Great Britain has any unquestionable trace been found of volcanic activity during that part of the geological record, the last eruptions of which the age is known being those of the Bala rocks. But in the south-west of Ireland there is evidence that for a time active vents appeared over the sea-floor on which the earlier deposits of Upper Silurian time were laid down.
I. The Lower Silurian Series
i. Eruptions probably of Arenig Age
It is in that part of Ireland which lies east of a line drawn from Strabane to Dungarvan Harbour that the records of Lower Silurian volcanic activity are to be found. In the north the development of volcanic rocks resembles that in Scotland, in the south it corresponds rather with the volcanic districts of Wales.
The Irish Silurian volcanic rocks have been traced with more or less detail on the maps of the Geological Survey. Since these maps were published, however, great advances have been made in the study of the petrography of volcanic rocks, as well as in the art of tracing their structure upon maps. Much, therefore, now remains to be done to bring our knowledge of the older volcanic history of Ireland abreast of that of the rest of the British Isles. In the following summary I have had to rely mainly on my own traverses of the ground, guided by the maps and memoirs of the Survey, and with the personal assistance of some of my colleagues.
The remarkable zone of crushed cherts, igneous rocks and sandstones, probably of Lower Silurian age, which I have referred to (p. 201) as wedged in between the schists and the Old Red Sandstone along the southern margin of the Highlands of Scotland, reappears in Ireland. It occupies an area in the County Tyrone, about 24 miles long and about 9 miles broad at the broadest part, but disappearing towards the north-east and south-west.[268] Lying between the Palæozoic formations on the south and the schists on the north, it occupies a similar position to the Scottish belt, but presents a much broader area, and thus affords greater facilities for examining the rocks. It presents the same indefinite or faulted boundaries as in Scotland, so that its relations to the rocks along its flanks have not been satisfactorily determined. That the rocks of this area are older than the Silurian strata to the south of them seems to be established by the occurrence of fragments of them in these strata, and that they are younger than the schists may be inferred from their non-foliated character. But they have undoubtedly undergone considerable crushing by powerful terrestrial movements which have placed them in their present position.
[Footnote 268: This area was mapped by Mr. J. Nolan for the Geological Survey, and was described by him in the Geol. Mag. for 1879. I visited it in company with my colleagues, Mr. B. N. Peach and Mr. A. M'Henry, in 1890 and again in 1894. My first conclusion was that the volcanic rocks should be regarded as part of the schistose series lying to the north of them (Pres. Address Geol. Soc. 1891, p. 77). But on the second visit, after having studied the rocks of the border of the Scottish Highlands, I formed the opinion stated in the text.]
The special feature of interest in this Irish area is the remarkable development of volcanic materials which is there to be seen, spreading over a far wider area than in Scotland. The rocks include lavas associated with tuffs and agglomerates, likewise a varied series of intrusive masses.
The lavas are chiefly dull greenish, fine-grained rocks, having the general character of diabases and "porphyrites." They are sometimes quite slaggy, and where the amygdaloidal kernels remain, these are usually of calcite. Under the microscope, the diabases show in some parts that their lath-shaped felspars, and the augite which these penetrate, are tolerably fresh, while in other parts fibrous chlorite, granular epidote and veins of calcite bear witness to the metamorphism which they have undergone.
One of the most conspicuous features in some of these lavas is the occurrence of the same sack-like or pillow-shaped structure which has been already referred to as so marked among the Arenig lavas of Scotland. Though the vesicles of these rocks are often quite uncrushed, showing that there has been no general subsequent deformation of the whole mass, there occur local tracts where evidence of considerable movement may be noticed. Thus close to a mass of gneiss, and elsewhere along their margin, the lavas are apt to be much jointed and broken with numerous lines of shear, along which the crushed material assumes more or less of a schistose structure. Yet in the solid cores between these bands of crushing the original forms of the vesicles are retained.
These greenish lavas are occasionally interleaved with grey flinty mudstones, cherts and red jaspers, which are more particularly developed immediately above. In lithological character, and in their relation to the diabases, these siliceous bands bear the closest resemblance to those of Arenig age in Scotland. But no recognizable Radiolaria have yet been detected in them.
Besides the more basic lavas, there occur also, but less abundantly, platy felsitic rocks which have suffered much from shearing, and consequently have acquired a fissile slaty structure.
The agglomerates are made up of angular, subangular and rounded fragments imbedded in a matrix of similar composition. This matrix has in places become quite schistose, and then closely resembles some parts of the "green schists" of the Scottish Highlands. Of the inclosed stones the great majority consist of various felsites, which, weathering with a thick white opaque crust, are internally close-grained, dull-grey or even black, sometimes showing flow-structure, and of all sizes up to eight inches in diameter or more. There are also fragments of the basic lavas, and likewise pieces of chert and jasper. On many of the rocky hummocks no distinct bedding can be made out in the agglomerate, but in others the rock is tolerably well stratified.
The tuffs are fine silky schistose rocks, and seem to have been largely derived from basic lavas. They have suffered more than any of the other rocks from mechanical deformation, for they pass into green chloritic schists. Some portions of them are not unlike the slaty tuffs of Llyn Padarn in Caernarvonshire.
Accompanying the fragmental volcanic rocks, some ordinary sedimentary intercalations are to be found--red shales and pebbly quartzites, that seem to have escaped much crushing. The true order of succession in the volcanic series has not yet been determined. But apparently above this series come some dark shales, such as might yield graptolites, pale grits and occasional limestones.
Later than the lavas and the pyroclastic material are various intrusive masses, which in bands and bosses form numerous craggy hills throughout the area. So far as I have been able to observe, these rocks include two groups. Of these the older consists of basic injections, such as gabbros and allied rocks, some of which remind me of the so-called "hypersthene-rock" of Lendalfoot, in Ayrshire. The coarser varieties, as at Carrickmore or Termon rock, are sometimes traversed by fine-grained veins from an inch to several feet in breadth. Portions of the slaggy diabases may be observed inclosed in these intrusive masses. The younger group is of more acid composition (granite, quartz-porphyry, etc.), and sends veins into the older.
ii. Eruptions of Llandeilo and Bala Age
Into the east of Ireland the Lower Silurian rocks are prolonged from Scotland, from the Lake District and from Wales. Though greatly concealed under younger formations across the breadth of the island, and occasionally interrupted by what are regarded as older strata of Cambrian age, they nevertheless occupy by much the larger part of the maritime counties from Belfast Lough to the southern coast-line of Waterford, even as far as Dungarvan Harbour. With the same lithological types of sedimentary deposits as in other parts of the United Kingdom, they carry with them here also their characteristic records of contemporaneous volcanic action. Though nowhere piled into such magnificent mountain-masses as in Westmoreland and North Wales, these records become increasingly abundant and interesting as they are traced southwards, until they are abruptly terminated by the coast-line along the south of the counties of Wexford and Waterford.
While much remains to be done, both in the field and in the laboratory and microscope-room, before our acquaintance with the Irish Silurian volcanic rocks is as complete as our knowledge of their equivalents in other portions of the United Kingdom, a serious preliminary difficulty must be recognized in the fact that the several geological horizons of these rocks have only been approximately fixed. Great difficulty was experienced by the Geological Survey in drawing any satisfactory line between the Llandeilo and Bala formations. This arose not so much from deficiency of fossil evidence as from the way in which the fossils of each group seemed to occur in alternating bands in what were regarded as a continuous series of strata. Indeed, in some localities it almost appeared as if the occurrence of one or other facies of fossils depended mainly on lithological characters indicative of original conditions of deposit, for the Llandeilo forms recurred where black shales set in, while Bala forms made their reappearance where calcareous and gritty strata predominated.[269] More recent work among the Silurian formations in England and Scotland, however, indicates that the parallel repetition of the two types of fossils is due to rapid and constant plication of the rocks, whereby the two formations, neither of them, perhaps, of great thickness, have been folded with each other in such a way that without the evidence of an established sequence of fossils, or the aid of continuous sections, it becomes extremely difficult to make out the stratigraphical order in any district. When the ground is attacked anew in detail, with the assistance of such palæontological and lithological horizons as have permitted the complicated structure of the southern uplands of Scotland to be unravelled, we may be enabled to tabulate the successive phases of the volcanic history of the region in a way which is for the present impossible. We have as yet no palæontological evidence that in the Silurian region of the east of Ireland, which extends from Belfast Lough to the south coast of County Waterford, any of the anticlinal folds bring up to the surface a portion of the Lower Arenig formation, though possibly some of the lowest visible strata may be of Upper Arenig age. A considerable part of the region must be referred to the Llandovery and other Upper Silurian formations, but the precise limits of the two divisions of the Silurian system have not yet been determined, except for the region north of Dublin, which has recently been re-examined for the Geological Survey by Mr. F. W. Egan and Mr. A. M'Henry.
[Footnote 269: Jukes was disposed to regard the two faunas as essentially coeval, but inhabiting different kinds of sea-bottom. See his note, Explanation of Sheets 167, 168, 178, 179, p. 30.]
These observers have ascertained that, as in Southern Scotland, by far the larger part of the Silurian region of the north-east of Ireland is occupied by strata belonging to the upper division of the system. The Lower Silurian formations, including the Llandeilo and Bala groups, form a belt varying up to six miles in breadth, which stretches from the coast of Down, between the mouth of Belfast Lough and Copeland Island, in a south-westerly direction to near the valley of the Shannon in County Longford. South of this belt the Lower Silurian rocks rise to the surface only here and there on the crests of anticlinal folds, and it is in these scattered "inliers" that the volcanic and intrusive rocks are found. So far as the available evidence goes, the volcanic history of this part of Ireland is entirely to be assigned to Lower Silurian time, and more especially to the interval between the beginning of the Llandeilo and the close of the Bala period. I must for the present content myself with this general limit of geological chronology, and make no attempt to trace the relative antiquity of the igneous rocks in the several districts in which they are distributed.[270]
[Footnote 270: The task of revising the Irish maps and tracing out the respective areas of Upper and Lower Silurian rocks over the whole island is now in progress by the Geological Survey, Mr. Egan and Mr. M'Henry being entirely engaged on it.]
Viewing the volcanic region of Eastern Ireland as a whole, we are first struck by the feebleness of the manifestations of eruptivity in the north, and their increasing development as we advance southwards. At the northern end of the Silurian area in County Down, thin bands of "felstone" and "ash" have been mapped by the Geological Survey as interstratified with the highly inclined and plicated Silurian rocks.[271] As the latter are plainly a continuation of the strata which have been mapped out zone by zone in the south of Scotland, their igneous intercalations may be looked upon as probably equivalents of some of those in the Silurian districts of Wigtonshire and Kirkcudbrightshire. But in County Down no representative has yet been detected of the Arenig and Llandeilo volcanic series of the southern uplands of Scotland. Nor has more precise petrographical examination confirmed the reference of any of the igneous rocks in the Silurian area of that district to truly contemporaneously intercalated volcanic rocks. All the eruptive material appears to be of an intrusive character. It occurs in the form of dykes of lamprophyre or mica-trap belonging to the groups of minettes and kersantites. Nothing definite is known of the age of these intrusions: they are possibly referable to the time of the Lower Old Red Sandstone.[272]
[Footnote 271: See Sheet 49 Geol. Survey, Ireland, and Explanation thereto (1871), pp. 16, 37, 39. The so-called "ashes" of the Explanation are probably parts of dykes which have been more or less crushed.]
[Footnote 272: Guide to the Collection of Rocks and Fossils belonging to the Geological Survey of Ireland, by Messrs. M'Henry and Watts, Dublin, 1895, p. 74.]
Far in the interior several bands of "felspathic ash" and "massive agglomerate" are shown on the Survey map as running through the counties of Monaghan and Cavan.[273] In one locality south of the Drumcalpin Loughs a large exposure of this ash is visible: "brown crumbly beds, with small rounded pebbles, give place to a massive bed of agglomerate, the enclosed blocks of which are always of one species of felstone, sometimes measuring 10 × 12 × 18 inches, and not always rounded." South of Carrickatee Lough, and a few miles farther to the south-west, near Lackan Bridge, considerable exposures of these rocks occur. One crag in particular displays a thickness of more than 70 feet of "tough flaky breccias," "thick agglomerates with small and large blocks of felstone," and "thin beds of fine pale green compact grit without pebbles, and a few flags." "One of the flaky beds contains numerous white worn crystals of felspar"; "the imbedded blocks of felstone are of the usual kind--pale compact matrix showing dark oblong patches, vesicular and amygdaloidal, the cavities being filled with chlorite."
[Footnote 273: Sheet 69 Geol. Survey, Ireland, and Explanation of Sheets 68 and 69, pp. 9, 13, 15.]
Further south a more extensive area of igneous rocks has been mapped on the borders of Louth and Meath, where, according to the Geological Survey map, a group of lavas and tuffs extends for about twelve miles near Slane.[274] Other bands of "ash" and "felstone" have been mapped in the Silurian area south of Drogheda. Thus at Hilltown, west from the racecourse, a "bluish crystalline felstone, showing in places lines of viscous flow," is stated to be overlain by "indurated felspathic ash and tuff, felstone, and indurated shale" in alternating beds.[275] On a recent visit to this locality I found that the "porcellanite or indurated shale" is a greenish-grey chert, full of Radiolaria and finely-diffused volcanic dust. This association of radiolarian chert with contemporaneous volcanic activity is of much interest, as showing the extension of the same physical conditions of the Lower Silurian sea from Scotland into Ireland. The Lower Llandeilo age of the volcanic intercalations in County Meath is further indicated by the occurrence of Didymograptus Murchisoni in grey shales in the same neighbourhood with the radiolarian cherts. In the Lower Silurian district of Balbriggan numerous intrusive bosses and sills have been mapped by the Geological Survey. I have found, however, that among these rocks there occur bands of volcanic breccia, containing abundant angular fragments of a minutely-vesicular pumice, and also that some of the diabase-masses display the pillow-structure and amygdaloidal texture. Hence, though most of the igneous rocks are no doubt intrusive, they appear to include lavas and tuffs of Bala age.
[Footnote 274: Ibid. Sheets 81 and 91. These rocks are chiefly augitic andesites, a few are basalts, and some seem related to felstones. Probably many of them are intrusive sills of uncertain age. The "ashes" contain fragments of felsite and porphyrite often of considerable size (Guide to Irish Rock-Collection, p. 36).]
[Footnote 275: Ibid. Sheets 91 and 92 and Explanation to these Sheets (1871), p. 10; Guide to Irish Rock-Collection, p. 36. Some of these lavas are andesites, others are felsites. Mr. M'Henry has contended that certain "ashes" and "agglomerates," particularly those exposed on the coast at Portraine, opposite Lambay Island, are "crush-conglomerates" due to terrestrial disturbances, which have affected both intrusive igneous rocks and the sedimentary series into which these have been injected.]
When the numerous Silurian cores of the mountain-groups in the interior of Ireland shall have been searched for traces of contemporaneous volcanic action, it is not improbable that these will be found. One of the smaller Silurian inliers which diversify the great Carboniferous plain, that of the Chair of Kildare, has long been known to have igneous rocks associated with its abundantly fossiliferous Bala limestone.[276] On recently visiting this locality I found that, besides the amygdaloidal and porphyritic andesites and basalts described by Jukes and Du Noyer, the fossiliferous conglomerates contain pebbles of rocks like those of the Chair, together with worn crystals of felspar, while intercalated with them are thin courses of volcanic tuff. There is thus evidence here of contemporaneous volcanic activity during the accumulation of the Bala group of strata. The limited area over which the rocks are exposed, however, affords merely a glimpse of this volcanic centre.
[Footnote 276: See Explanation to Quarter Sheet 35 N.E. (Sheet 119 of newer numeration) of Geol. Survey Ireland (1858), p. 16. (See note, p. 256.)]
Crossing over the broad belt of Carboniferous Limestone through which the Liffey flows into Dublin Bay, we come to the great continuous tract of older Palæozoic rocks which stretches southward to the cliffs of Waterford. Through this tract runs the huge ridge of the Wicklow and Carlow granite. On the west side of this intrusive mass, bands of "greenstone-ash," as well as "felspathic ashes," have been traced among the Silurian rocks by the Geological Survey. But it is on the south-east side of the granite that the volcanic intercalations are best displayed. Indeed, from Wicklow Head to Dungarvan Harbour there is an almost continuous development of igneous rocks, rising into rocky eminences, trenched into ravines by the numerous streams, and laid bare by the waves in fine coast-cliffs. It is in this south-eastern region, comprising the counties of Wicklow, Wexford and Waterford, that the Irish Lower Silurian igneous rocks can best be studied.
There are obviously various distinct centres of eruption in this long belt of country. The Rathdrum and Castletimon tract forms one of these. Another of less size culminates in Kilpatrick Hill, a few miles to the southward. Arklow Head marks the position of a third. The lavas and tuffs which set in a few miles to the south of that promontory, and may be said to extend without interruption to the south coast, were probably thrown out by a series of vents which, placed along a north-east and south-west line, united their ejections into one long submarine volcanic bank. There can be no doubt that the most active vents lay at the southern end of the belt, for there the volcanic materials are piled up in thickest mass, and succeed each other with comparatively trifling intercalations of ordinary sedimentary material. Some of these vents, as I shall relate in the sequel, have been cut open by the sea along a range of precipitous cliffs.
The comparatively feeble character of the volcanic energy during Lower Silurian time over the greater part of the south-east of Ireland is shown by the great contrast between the thickness of the volcanic intercalations there and in Wales and the Lake country, but still more strikingly by innumerable sections where thin interstratifications of fine tuff or volcanic breccia occur among the ordinary sedimentary strata, and are sometimes crowded with Bala fossils. Some interesting illustrations of this feature are to be seen in the Enniscorthy district, where layers of fine felsitic tuff, sometimes less than an inch in thickness, lie among the shales. In some of the tuffs the lapilli are fragments of trachytic or andesitic rocks.
A striking example of rapid alternations of pyroclastic material with ordinary sediment lies far to the south in County Waterford, close to Dunhill Bridge, where a group of fine volcanic breccias and grits has been laid bare by quarrying.[277] These strata consist of coarser and finer detritus, enclosing angular fragments of felsites and grey and black shale. The felsite-lapilli vary in texture, some of them presenting beautiful flow-structure. The stones are stuck at random through each bed, the largest being often at the bottom. The beds of breccia vary from a few inches to a foot or more in thickness. There can, I think, be little doubt that each of these breccia-bands points to a single volcanic explosion, whereby felsitic fragments were thrown out, mingled with pieces of the Silurian strata through which the vents were drilled. In a vertical thickness of some fifty feet of rock there must thus be a record of ten or twelve such explosions.
[Footnote 277: See Explanation of Sheets 167, 168, 178 and 179, Geol. Surv. Ireland, p. 56.]
Nearer the active vents the fragmental deposits become, as usual, coarser and thicker. But I have not observed any thick masses of tuff like those of North Wales. So far as my examination has gone, the tuffs are mainly felsitic. The so-called "greenstone-ash" of the Survey maps is certainly in many cases not a true tuff. This term was proposed by Jukes for certain apple-green to olive-brown flaky fissile rocks only found "in association with masses of greenstone."[278] Some years ago I had occasion to make a series of traverses in Wicklow and Wexford, and then convinced myself that in that part of the country the "greenstone-ashes" were probably crushed bands of basic sills. Dr. Hatch has proved this to be their origin from a series of microscopic slides prepared from specimens collected by himself on the ground.[279] In other cases the "greenstone-ashes" seem to be excessively-cleaved or sheared felsites, which have acquired a soapy feel and a dull green colour; but they also do include true tuffs. Thus, in one instance, at Ballyvoyle cross-roads, in the south of County Waterford, a "greenstone-ash" is a dull green tuff full of fragments of felspar (chiefly plagioclase) and pieces of dark andesitic lavas. Another example may be found to the west of the Metal Man, near Tramore, where the tuff is full of fragments of felspar and shale cemented in a greenish-yellow material which may be palagonite.
[Footnote 278: Explanation of Sheets 129, 130, p. 13 (1869).]
[Footnote 279: Explanation of Sheets 138, 139.]
The felsites of the south-east of Ireland form by much the largest proportion of the whole volcanic series. They occur as lenticular sheets from a few feet to several hundred feet in thickness, and occasionally traceable for some miles. On the whole, they are compact dull grey rocks, weathering with a white crust. A geologist familiar with the contemporary lavas of North Wales cannot fail to be struck with the absence of the coarse flow-structure so often characteristic of the felsites in that region. This structure, indeed, is not entirely absent from the Irish rocks, but it occurs, so far, at least, as I have seen, rather as a fine streakiness than in the bold lenticular bands so common in Caernarvonshire. In like manner the nodular structure, though not entirely absent, is rare.[280]
[Footnote 280: In Waterford nodular felsites occur with concretions varying from the size of a pea to several inches in diameter. Explanation to Sheets 167, 168, 178 and 179, p. 11.]
Until these felsites have been subjected to more detailed investigation, little can be said as to their petrography, and as to the points of resemblance or difference between them and those of other Lower Silurian districts in the United Kingdom. An important step, however, in this direction was taken by Dr. Hatch, who studied them on the ground, in the laboratory, and with the microscope. He found that some of them were soda-felsites or keratophyres (with albite as their felspar), that others were potash-felsites (with orthoclase as their felspar), while a third group contained both soda and potash, the last-named greatly preponderating.[281] The existence of soda-felsites had not been previously detected among British volcanic rocks, and it remains to be seen how far they may occur in the large and somewhat varied group of rocks combined under the general term "felsites." Dr. Hatch believed that these rocks probably graduate into the normal or orthoclase felsites; but it has not yet been possible to test this view on the ground, nor to ascertain whether there is any essential difference between the mode of occurrence of the two types.
[Footnote 281: Explanation of Sheets 138, 139, p. 49; and Geol. Mag. 1889, p. 545.]
Besides the more abundant felsites, occasional bands of andesite have been detected. Various other eruptive rocks occur, probably in most or all cases intrusive. Such are quartz-mica-diorites, quartz-diorites, augite-diorites or proterobases, dolerites, gabbros, diabases and epidiorites.[282]
[Footnote 282: Guide to Irish Rock-Collections, pp. 34, 35.]
I have said that the chief theatre of eruption lay towards the south-west end of the volcanic belt of the south-east of Ireland. The coast-line of County Waterford, from Tramore westward to Ballyvoyle Head--a distance of nearly fifteen miles--presents, perhaps, the most wonderful series of sections of volcanic vents within the British Islands. No one coming from the inland is prepared for either the striking character of the cliff scenery or the extraordinary geological structure there presented, for the country is, on the whole, rather featureless, and much of it is smoothed over and obscured by a covering of drift, through which occasional knobs of the harder felsites protrude. The cliffs for mile after mile range from 100 to 150 or 200 feet in height, and present naked vertical walls of rock, trenched by occasional gullies, through which a descent may be made to the beach. Throughout the whole distance agglomerates and felsites succeed each other in bewildering confusion, varied here and there by the intercalation of Lower Silurian shales and limestones involved and pierced by the igneous rocks. Hardly any bedded volcanic material is to be recognized from one end to the other. The sea has laid bare a succession of volcanic vents placed so close to each other that it will be difficult or impossible to separate them out. A careful study and detailed mapping of this marvellous coast-section, however, is a task well worthy of the labour of any one desirous of making himself acquainted with some of the conditions of volcanism during older Palæozoic time.
At the east end of the section, black shales containing Llandeilo graptolites, and calcareous bands full of Bala fossils, dip westward below a group of soda-felsites and felsitic tuffs, which seem to lie quite conformably on these strata. Here, then, we start with proof that the volcanic eruptions of this locality began during some part of the Bala period. But immediately to the west, these bedded igneous rocks are broken through by a neck of coarse agglomerate stuck full of chips and blocks of shale, some of them a foot long, with abundant fragments of scoriform and flinty felsites. Some columnar dykes of dolerite cut through the neck, and a larger intrusion seems to have risen up the same funnel. The bedded tuffs appear again for a short distance, but they are soon replaced by a tumultuous mass of agglomerate. And from this part of the coast onwards for some distance all is disorder.
The agglomerates are crowded with blocks of various felsites and micro-granites sometimes 18 inches in diameter, many of them presenting the most exquisite streaky flow-structure. The angularity of these stones and the abrupt truncation of their lines of flow prove that they were derived from the shattering of already consolidated rocks. In other places the ejected materials consist almost wholly of black shale fragments, but with an intermixture of felsite-lapilli.
It is difficult to convey an adequate idea of the way in which the agglomerates are traversed by dykes, veins and bosses of various felsites, and of how these break in endless confusion through each other. Some of the intrusive rocks are compact and amorphous, others are vesicular, others close-grained and columnar. Again and again they present the most perfect flow-structure, and it is noticeable that the lines of flow follow the inequalities of the walls of the fissure up which the rock has ascended, and not only so, but even of the surfaces of detached blocks of shale or felsite which have been caught up and enclosed in the still moving mass.
A few of these intrusive rocks were examined in thin slices by Dr. Hatch. Most of them appear to be soda-felsites, but they include also rather decomposed rocks, some of which are probably diorites and quartz-diorites. Occasionally, thoroughly basic dykes (dolerite) may be observed.
In the midst of this tumultuous assemblage of volcanic masses, representing the roots of a group of ancient vents, there occur occasional interspaces occupied by ordinary stratified rocks. In the eastern part of the section these consist mainly of black shale, sometimes with calcareous bands, from which a series of Bala fossils has been obtained.[283] A very cursory examination suffices to show that these intercalations do not mark pauses in the volcanic eruptions. They are, in fact, portions of the marine accumulations under the sea-floor through which the vents were blown; they have been tossed about, crushed and invaded by dykes and veins of felsite.
[Footnote 283: But see the Geol. Survey Memoir on Sheets 167, 168, 178 and 179, Ireland (1865), p. 28, for a description of the association of Bala and Llandeilo fossils on that coast-line.]
But certain other intercalated strips of stratified rocks present a special interest, for they bring before us examples of volcanic ashes that gathered on the sea-floor, but which were disrupted by later explosions. Thus, at the Knockmahon headland, well-bedded felspathic grits and ashy shales occur, thrown in among the general mass of eruptive material. As I have already remarked, it is difficult or impossible to fix the horizons of the stratified patches that are involved among the igneous ejections of this coast-section, save where they contain recognizable fossils, but the intercalation of true bedded tuffs among them is a proof that volcanic action had been in operation there long before the outbreak of the vents which are now laid bare along the cliffs.
In the south-east of Ireland there is the usual association of acid and basic sills with the evidence of a superficial outpouring of lavas and ashes. But these intrusive masses play a much less imposing part than in Wales. They may be regarded, indeed, as bearing somewhat the same proportion to the comparatively feeble display of extrusive rocks in this region that the abundant and massive sheets of Merionethshire and Caernarvonshire do to the enormous piles of lavas and tuffs which overlie them.
Among the acid intrusive sheets the most conspicuous are those mapped by the Survey as "elvans." These rocks, as they occur in Wicklow and Wexford, have been examined by Dr. Hatch, who finds them to be micro-granitic in structure, occasionally exhibiting micropegmatitic or granophyric modifications.[284] The true stratigraphical relations of these rocks have not yet been adequately investigated. Those of them which occur on the flanks of the great granite ridge are not improbably connected with that mass, and if so are much younger than the Lower Silurian volcanoes.[285]
[Footnote 284: Explanation of Sheets 138 and 139, p. 53.]
[Footnote 285: The Leinster granite is certainly later than the Lower Silurian rocks and older than the Carboniferous rocks of the south-east of Ireland. It may belong to the great epoch of granite protrusion during the Old Red Sandstone period.]
The basic sills, or "greenstones," consist largely of diabase, frequently altered into epidiorite; they include also varieties of diorite.[286] That they were intruded before the plication and cleavage of the rocks among which they lie is well shown by their crushed and sheared margins where they are in thick mass, and by their cleaved and almost schistose condition where they are thinner. The intense compression and crushing to which they have been subjected are well shown by the state of their component minerals, and notably by the paramorphism of the original augite into hornblende.
[Footnote 286: Dr. Hatch, op. cit. p. 49.]
The scarcity of dykes associated with Silurian volcanic action is as noticeable in the south-east of Ireland as it is in Wales. I have observed a considerable number, indeed, but they are confined to the line of old vents on the Waterford coast, and, but for the clear cliff-sections cut by the sea, they would certainly have escaped observation, for they make no feature on the ground in the interior. They are sometimes distinctly columnar, and vary from less than a foot to many yards in width. They traverse both the agglomerates and the intrusive felsites. Most of them are of felsite, sometimes cellular; but in some cases they are dolerites. There is obviously no clue to the dates of these dykes.
That some at least of the vents along the south coast of County Waterford may be vastly younger than the Lower Silurian rocks through which they have forced their way is suggested, if not proved, by a section which is in some respects the most extraordinary of the whole of this remarkable series. The occurrence of a group of red strata was carefully noted by the late Mr. Du Noyer at Ballydouane Bay, when he was engaged in carrying on the Geological Survey of that part of the country. At first he regarded them as belonging to the Old Red Sandstone, which comes on in great force only a few miles to the west; but he subsequently arrived at the belief that they are really an integral part of the Lower Silurian rocks of the district. Professor Jukes had previously expressed himself in favour of this latter idea, which was thought to receive support from the occurrence of some reddish strata in the Lower Silurian rocks of Tagoat, County Wexford.[287]
[Footnote 287: Explanation of Sheets 167, 168, 178 and 179 of the Geological Survey of Ireland (1865), pp. 10, 59.]
The occurrence of red rocks among Silurian strata, which are not usually red, might quite reasonably be looked for in the neighbourhood of Old Red Sandstone, Permian or Triassic deposits. If these deposits once spread over the Silurian formations, a more or less decided "raddling" of the latter may have taken place. But in the present instance, though the Old Red Sandstone begins not many miles to the west, no such explanation of the colour of the strata is possible. The cliffs of Ballydouane Bay consist of red sandstone, red sandy shale and conglomerate. The red tint is of that dull chocolate tone so characteristic of the Lower Old Red Sandstone. The conglomerates are immense accumulations of ancient shingle, consisting largely of pieces of white vein-quartz and quartzite, sometimes a foot long and often well water-worn. Some of the sandy beds are full of large scales of white mica, as if derived from some granitic or schistose region at no great distance. Taken as a whole, the strata are much less indurated and broken than the Silurian grits and shales of the district; some of them, indeed, weather into mere incoherent sand that crumbles under the fingers. There does not appear to be any positive proof that the red rocks are truly bedded with the ordinary Silurian strata, the junctions being faulted or obscured by intrusive igneous masses.
Nowhere in the British Islands, so far as I am aware, is there a similar group of strata among the Lower Silurian rocks. If they belong to so ancient a series, they show that in the south of Ireland, during Lower Silurian time, there arose a set of peculiar physical conditions precisely like those that determined the accumulation of the Old Red Sandstone in the same region at a later geological period. And in that case it is hardly possible to conceive that these conditions could have been confined to the extreme south of Ireland. We should certainly expect to meet with evidence of them elsewhere, at least in the same Silurian region.[288]
[Footnote 288: The nearest approach of any Silurian group of strata to the character of these conglomerates is furnished by the remarkably coarse conglomerates, boulder-beds and pebbly grits of the Bala and Llandovery series in the region between Killary Harbour and Lough Mask, to which further reference is made in a later part of this chapter.]
While I hesitate to express a decided opinion in opposition to the conclusions of such experienced observers as Jukes and Du Noyer, I incline to believe that the rocks in question really belong to the Old Red Sandstone. If such shall finally be determined to be their geological position, they will supply evidence that some at least of the volcanic vents of the coast-line cannot be older than the Old Red Sandstone. They are pierced by masses of soda-felsite and by a coarse red agglomerate containing abundant pieces of felsite. These volcanic rocks belong to the same type as those which break through the undoubted Silurian rocks on either side. They may thus come to prove a recrudescence of volcanic energy in this same district at a much later geological period; and a new problem will arise to task the skill of the most accomplished field-geologist and petrographer--to unravel the structure and history of this chain of volcanic vents, and, in so doing, to detect and separate the eruptions of Lower Silurian time from those of the Lower Old Red Sandstone.
In the far west of Ireland, another group of Lower Silurian volcanoes has left its remains in the mountainous tract of country between the western shores of Lough Mask and Killary Harbour.[289] There appear to have been at least three separate centres of eruption along a line stretching in a north-easterly direction for about 16 miles from the western end of Lough Nafooey to the hamlet of Derrindaffdery beyond Tourmakeady, where the older rocks are unconformably overlain by the lower Carboniferous strata. As shown by the mapping of the Geological Survey, the most northerly area, which may be called the Tourmakeady centre, has a breadth of about a mile, and dies out southward after a course of nearly six miles. About a mile to the south-west of the last visible prolongation of its rocks, we encounter a second volcanic centre which occupies an area of about a square mile in the valley of Glensaul. The third centre stretches from the western shores of Lough Mask across Lough Nafooey, where it forms a mass of high rugged ground, and reaches a length of some six or seven miles before it finally dies out.[290]
[Footnote 289: This group was placed in the Upper Silurian series by the officers of the Geological Survey who mapped the region (see Sheets 84, 85, 94 and 95 of the Geological Map of Ireland and accompanying Explanation), and on their testimony I formerly referred to the volcanic rocks as of Upper Silurian age. Mr. Baily, however, had pointed out that the limestone associated with the lavas and agglomerates contains Bala fossils. Yet, in spite of this palæontological testimony, the fossils were considered to be "derivative," and the rocks were removed from the series of formations to which they would naturally be assigned. A recent examination of the ground, in company with Mr. J. R. Kilroe of the Geological Survey, has satisfied me that the volcanic rocks are interstratified with sedimentary deposits of Bala age, and must consequently be grouped with the rest of the Lower Silurian series of Ireland. The results of this examination are given in the text.]
[Footnote 290: These areas were carefully mapped for the Survey by Mr. Nolan, and the lines of division marked by him fairly represent the general distribution of the rocks.]
The rocks in each of these three areas are similar. One of their distinguishing features is the intercalation among them of a fossiliferous limestone and calcareous fossiliferous tuffs, which contain well-preserved species of organisms characteristic of the Bala division of the Lower Silurian rocks.[291] There cannot be any question that these organisms were living at the time the strata in which their remains occur are found. The most delicate parts of the sculpture on Illænus Bowmanni and Orthis elegantula are well preserved. Nor have the limestones been pushed into their present places by volcanic agency, or by faults in the terrestrial crust. They are not only regularly intercalated among the volcanic rocks, but the limestone in some places abounds in volcanic dust, while above it come calcareous tuffs, also containing the same fossils. It is thus clearly established that the volcanic series now to be described has its geological age definitely fixed as that of the Bala period.
[Footnote 291: See the list of fossils as determined by Mr. Baily in Explanatory Memoir to accompany Sheets 73, 74, 83 and 84 of the Geological Survey of Ireland, p. 68 (1876).]
The lavas of the Lough Mask region consist of felsites and andesites with rocks of probably more basic composition. The felsites are generally quartziferous porphyries, which occupy a considerable space in each of the three districts. To what extent they are intrusive rather than interstratified remains for investigation. Some of them have undoubtedly invaded other members of the volcanic series. But, on the other hand, fragments of similar quartz-porphyries and felsites abound in the intercalated bands of volcanic breccia.
The andesites and more basic lavas are finely-crystalline or compact, dull-green to chocolate-purple rocks, often resembling the "porphyrites" of the Old Red Sandstone. Some of them are strongly vesicular, the cavities being filled with calcite on fresh fracture, though empty on weathered surfaces. The sack-like or pillow structure, already referred to as characteristic of many Lower Silurian lavas, appears conspicuously among some of these rocks. At Bohaun, nine miles south from Westport, where a prolongation of the volcanic series rises to the surface from under the overlying coarse conglomerates, I observed that, owing to the compression which the rocks have there undergone, the pillow-shaped blocks have been squeezed together into rudely polygonal forms, while their vesicles have been greatly drawn out in the direction of tension. Where the rocks have been still more sheared, the distinct pillow-shaped blocks with their vesicular structure disappear, while the more fine-grained crusts that surround them have been broken up and appear as fragments involved in a matrix of green schist.
Intercalated with the lavas are numerous bands of volcanic breccia and fine tuff. The stones in these breccias consist chiefly of various felsites with andesites and more basic lavas. But pieces of jasper, chert, shale and grit are not infrequent. In some places abundant blocks of black shale are to be noticed, probably derived from the Llandeilo group which exists below, and which has here and there been ridged up to the surface in the midst of the volcanic rocks.[292] Near Shangort I noticed in one of these breccias one block measuring 12 feet, another 20 feet in length and 3 or 4 feet thick, composed of alternating bands of grit and slate. It is interesting to note that these strata had already undergone cleavage before disruption, the bands of slate being strongly cleaved obliquely to the bedding. None of the Llandeilo or other rocks in the neighbourhood display this structure. The blocks seem to have been derived from some deeper group of strata. They are laid down parallel with the rude bedding of the breccia in which they lie.
[Footnote 292: In re-examining this region, Mr. Kilroe has found in the stream west of the monastery, Tourmakeady, an uprise of graptolitic black shale containing forms belonging to the very lowest Llandeilo or Upper Arenig strata, and a similar band above Leenane, Killary Harbour.]
The fine tuffs and thin ashy limestones associated with the thicker band of limestone show the renewal of volcanic explosions after the interval marked by the calcareous deposit which is sometimes 20 or 40 feet thick. In many places this limestone is brecciated and much mingled with volcanic dust and lapilli. At Shangort, for example, the thick tolerably pure limestone is truncated on the west and north sides by a coarse agglomerate probably filling a volcanic vent. A few hundred yards further north, beyond the interrupting agglomerate, the limestone reappears on the same line of strike, but is then found to be nodular and brecciated and much mingled with volcanic detritus. It lies among ashy grits and tuffs.
The general structure of the ground occupied by the Lough Mask volcanic rocks is diagrammatically represented in Fig. 64. The thickness of the volcanic series must amount to many hundred feet, but it has not been precisely determined. The uppermost parts of the series pass under a great thickness of coarse conglomerates and pebbly grits which form the ridge of Formnamore, and stretch thence westwards along Killary Harbour and through the Mweelrea mountains. These strata are classed as the Upper Silurian on the Geological Survey map. Since, however, they conformably overlie rocks containing Bala fossils, and in the Killary district include green shales which have yielded fossils of the same age, they doubtless belong in large part to the Lower Silurian division. The remarkable coarseness of these conglomerates towards the south, and their rapid passage into much finer grits and shales towards the north, probably indicate that they were formed close to the shores of a land composed of schistose rocks, quartzite and granite, of which the mountainous tracts of Connemara are the last relics.
A base to the volcanic series is found in the occasional uprise of a short axis of Llandeilo, or perhaps even upper Arenig strata, containing bands of dark chert and black graptolitic shales. Unfortunately the relations of these underlying rocks to the volcanic masses are not very clear, being obscured by superficial accumulations and also by faulting. It is thus hardly possible to be certain whether they pass up conformably into the base of the volcanic series, or are covered by it unconformably.
The position of this isolated volcanic district in the far west of Ireland, the abundance, variety and thickness of the erupted materials, and the definite intercalation of these materials in the Bala or highest division of the Lower Silurian series, acquire a special interest from the history of the nearest Silurian volcanic area which has now to be described--that of the western shores of the Dingle promontory.
II. The Upper Silurian Series
The latest volcanic eruptions of Silurian time yet definitely known took place during the accumulation of the Wenlock and Ludlow rocks in the far west of Ireland. No satisfactory record of any contemporaneous phenomena of a like kind has yet been met with in any other Upper Silurian district in the British Isles, unless at Tortworth in Gloucestershire, as above described. So far as at present known, only one centre of activity has been preserved. It lies among the headlands of Kerry, where the land projects furthest west into the stormy Atlantic. The occurrence of volcanic rocks in this remote area and their geological horizon have been clearly indicated on the maps of the Geological Survey. More than thirty years, however, have elapsed since some of the mapping was done, and we must therefore be prepared to find it, more especially in its petrography, capable of modification and improvement now.
In the country known as the Dingle promontory, these traces of contemporaneous volcanic rocks are to be observed at various localities and on several horizons. To the east, near Anascaul, on the northern shore of Dingle Bay, some tuffs occur in what are believed to be Llandovery strata. But it is on the western coast, among the headlands and coves that lie to the north and south of Clogher Head, that the best sections are to be seen. The succession of the rocks in this locality was well worked out by Du Noyer, and the Memoir prepared by him, with the general introduction by Jukes, is an invaluable guide to the geologist who would explore this somewhat inaccessible region.[293] The most important correction that will require to be made in the work arises from a mistake as to the true nature of certain rocks which were described as pisolitic tuffs, but which are nodular felsites.
[Footnote 293: Sheets 160 and 171 of the one-inch map, and Memoir on Sheets 160, 161, 171 and 172.]
By far the most striking geological feature of this singularly interesting and impressive coast-line is to be found in the interstratification of lavas with bands of tuff among abundantly fossiliferous strata which, from their organic contents, are unmistakably of the age of the Wenlock group. These lavas occur in a number of sheets, separated from each other by tuffs and other fragmental deposits. They thus point to a series of eruptions over a sea-bottom that teemed with Upper Silurian life. They consist for the most part of remarkably fine typical nodular felsites. The nodules vary in dimensions from less than a pea to the size of a hen's egg. They are sometimes hollow and lined with quartz-crystals. They vary greatly in number, some parts being almost free from them and others entirely made up of them. The matrix, where a fresh fracture can be obtained, is horny in texture, and often exhibits an exceedingly beautiful and fine flow-structure. On weathered faces there may be seen thick parallel strips and lenticles of flow-structure like those of the Snowdon lavas. The upper portions of some of the sheets enclose fragments of foreign rocks. The microscopic examination of a few slices cut from these lavas shows them to be true felsites (rhyolites) composed of a microcrystalline aggregate of quartz and felspar, with layers and patches of cryptocrystalline matter, and only occasional porphyritic crystals of orthoclase and plagioclase.
The pyroclastic rocks associated with these lavas vary from exceedingly fine tuff to coarse agglomerate. Some of the finer tuffs contain pumiceous fragments and pieces of grey and red shale; they pass into fine ashy sandstones and shales, crowded with fossils, and into gravelly breccias made up of fragments of different volcanic rocks.
But the most extraordinary of these intercalated fragmental strata is a breccia or agglomerate, about 15 feet thick, which lies in a thick group of fossiliferous dull-yellow, ashy and ochreous sandstones. The stones of this bed consist chiefly of blocks of different felsites, varying up to three feet in length. Some of them show most perfect flow-structure; others are spongy and cellular, like lumps of pumice. The calcareous sandstone on the top of the breccia is crowded with fossils chiefly in the form of empty casts, and the same material, still full of brachiopods, crinoids, corals, etc., fills up the interstices among the blocks down to the bottom of the breccia, where similar fossiliferous strata underlie it.
Nowhere has the volcanic history of a portion of Palæozoic time been more clearly and eloquently recorded than in this remote line of cliffs swept by the gales of the Atlantic. We see that the ordinary sedimentation of Upper Silurian time was quietly proceeding, fine mud and sand being deposited, and enclosing the remains of the marine organisms that swarmed over the sea-bottom when volcanic eruptions began. First came discharges of fine dust and small stones, which sometimes fell so lightly as not seriously to disturb the fauna on the sea-floor, but at other times followed so rapidly and continuously as to mask the usual sediment and form sheets of tuff and volcanic gravel. Occasionally there would come more paroxysmal explosions, whereby large blocks of lava were hurled forth until they gathered in a thick layer over the bottom. But the life that teemed in the sea, though temporarily destroyed or driven out, soon returned. Corals, crinoids and shells found their way back again, and fine sediment carried their remains with it and filled up the crevices. The ejected volcanic blocks are thus enclosed in a highly fossiliferous matrix.
A succession of lava-streams, of which the strongly-nodular sheet of Clogher Head is the thickest and most conspicuous, mark the culmination of the volcanic energy, and show how at this late part of the Silurian period felsites that reproduce some of the most striking peculiarities of earlier time were once more poured out at the surface. A few more discharges of tuff and the outflow of a greenish flinty felsite brought this series of eruptions to an end, and closed in Britain the long and varied record of older Palæozoic volcanic activity.[294]
[Footnote 294: As this sheet is passing through the press, the interesting paper by Messrs. S. H. Reynolds and C. J. Gardiner, "On the Kildare Inlier" has appeared (Quart. Journ. Geol. Soc. vol. lii. p. 587). These authors give petrographical details regarding the lavas, which they show to be andesites and basalts of Bala age, associated with highly fossiliferous tuffs.]
TO ACCOMPANY SIR ARCHIBALD GEIKIE'S "ANCIENT VOLCANOES OF BRITAIN" Map II
MAP OF THE SILURIAN VOLCANIC DISTRICTS OF NORTH WALES
Reduced from the Maps of the Geological Survey.
The Edinburgh Geographical Institute -- Copyright -- J. G. Bartholomew.
XV. THE DEVONIAN VOLCANOES
Throughout the whole region of the British Isles, wherever the uppermost strata of the Silurian system can be seen to graduate into any later series of sedimentary deposits, they are found to pass up conformably into an enormous accumulation of red sandstones, marls, cornstones, and conglomerates, which have long been grouped together under the name of "Old Red Sandstone." In England and Wales, in Scotland and in Ireland, this upward succession is so well shown that at first British geologists were naturally disposed to believe it to represent the normal order of the geological record. When, however, Sedgwick and Murchison demonstrated that in the counties of Devon and Cornwall a very different group of strata contained an abundant assemblage of organic remains, including types which Lonsdale showed to be intermediate between those of the Silurian and the Carboniferous systems; when, moreover, this palæontological facies of the south-west of England, termed by its discoverers "Devonian," was found to be abundantly developed on the Continent, and to be there indeed the prevalent stratigraphical type of the formations intervening between Silurian and Carboniferous, geologists began to perceive that the Old Red Sandstone must be regarded as the record of peculiar local conditions of sedimentation, while the Devonian type was evidently the more usual development of the same geological period.
From the remote Shetland Isles, across the whole of Scotland and England, down to the northern shores of the Bristol Channel, the Old Red Sandstone maintains its general characters. Nowhere, indeed, are these characters more typically developed than in South Wales, where many thousands of feet of red sediments, almost entirely devoid of organic remains, emerge from under the escarpments of Carboniferous Limestone, and stretch into broad uplands until they are lost at the top of the Silurian system.
But when the geologist crosses the Bristol Channel to the opposite shores of North Devon, he encounters a remarkably different assemblage of rocks. It is true that he has not yet been able to detect there any equivalents of the uppermost Silurian strata of Glamorganshire, nor does he find any conspicuous band of Carboniferous Limestone, such as that which encircles the Welsh Coal-field. He is thus unable to start from a known definite horizon in the attempt to work out the order of succession, either in an upward or downward direction. Lithological characters likewise afford him no means of establishing any satisfactory parallelism. As he follows the Devonian strata, however, he finds them to disappear conformably under the Culm-measures, which, though strangely unlike the Carboniferous strata on the opposite coast, are yet proved by their fossils to belong to the Carboniferous system. Hence the Devonian type, like the Old Red Sandstone, is proved to be immediately anterior to, and to graduate into, the Carboniferous rocks.
There is no stratigraphical change in Britain so rapid and complete as that from the Old Red Sandstone on the one side of the Bristol Channel to the Devonian series on the other. No satisfactory explanation has yet been found for this sudden transformation, which still remains one of the unsolved problems in British geology.
As the observer follows the Devonian assemblage across the land to the southern coast-line, he is conscious that its general characters, both lithological and palæontological, depart more and more from the type of the Old Red Sandstone, and approach more closely to the common Devonian facies of the Continent. He is forced to admit that the Old Red Sandstone, notwithstanding its extensive development in Britain, must be regarded as an exceptional type of sedimentation, while the Devonian facies represents that which is most widely prevalent, not only in Europe, but generally over the world.
The broad estuary of the Bristol Channel unfortunately conceals from view the tract which lies between the typical Old Red Sandstone of Glamorganshire and the typical Devonian formations of Devonshire. Whether this intervening space of some fifteen miles was occupied by a physical barrier, which separated the respective areas of deposit of these two types, or the circumstances of sedimentation in the one region merged insensibly into those of the other, must remain matter for speculation.
The geographical conditions betokened by the Old Red Sandstone will be considered in the next chapter. There can be no doubt that those indicated by the Devonian system were marine. The organic remains so plentifully distributed through the argillaceous and arenaceous sediments of that system, and so crowded together in its limestones, were obviously denizens of the open sea. Yet the only tract of Britain over which this sea can be shown to have spread was the south of England. To the north of that belt, the site of Britain during Devonian time appears to have been partly land and partly wide water-basins in which the Old Red Sandstone was deposited.
In that half terrestrial half lacustrine territory that stretched northwards to beyond the Shetland Isles, many volcanoes were active, of which the chronicles will be described in later pages. The most southerly of these centres of eruption yet known was the district of the Cheviot Hills. Throughout the rest of England and Wales no trace of any contemporary volcanic action has been detected in the Old Red Sandstone. It is true that over most of that region rocks of this age have been concealed under younger formations. Yet throughout Wales, where the Old Red Sandstone attains so vast a thickness, and covers so wide an area, it has not yet yielded a vestige of any contemporaneous volcanic eruptions.
But over the sea-floor that covered the south of England, and stretched thence into the heart of Europe, abundant volcanoes have left behind them proofs of their activity. The first geologist who recognized these proofs and traced their extent on the ground appears to have been De la Beche, who, by his detailed maps and careful description of the igneous rocks of Devonshire, did so much to advance the study of ancient volcanic action. This great pioneer not only determined the former existence of Devonian volcanoes, but he was likewise the first to detect and map the volcanic rocks associated with the Carboniferous and "New Red Sandstone" formations of the same region. The broad outlines traced by him among the volcanic products of these three geological periods in the south-west of England still remain but little changed. Nor are they likely to be much improved until the ground is resurveyed on a larger and more accurate map, and with better petrographical equipment than were available in his day.
Not long after the observations of De la Beche came those of A. C. Godwin-Austen, who devoted much time to a sedulous exploration of the rocks of South Devon, and satisfied himself that contemporaneous volcanic sheets were intercalated among the limestones of that district. "The coral limestones," he says, "are in many places superincumbent on great sheets of volcanic materials, with which, in some instances, as at North Whilborough, they alternate." He pointed out that the interstratified volcanic rocks are of two periods, one Devonian and the other Carboniferous.[295]
[Footnote 295: Trans. Geol. Soc. 2nd ser. vol. vi. (1842), pp. 465, 470, 473.]
In his Geological Maps of Devon and Cornwall, which are to the present time those issued by the Geological Survey, De la Beche made no attempt to discriminate between the varieties of igneous rocks, save that the basic "greenstones" were distinguished from the acid bosses of granite and the elvans. But in his classic "Report" much more detail was inserted, showing that he clearly recognized the existence both of volcanic ashes and of lavas, as well as of intrusive sheets. At the outset of his account of the "Grauwacke," he remarks that the sedimentary deposits are accompanied with igneous products, "a portion of which may also be termed sedimentary, inasmuch as it would seem to have been deposited in beds among contemporaneous rocks of the former description by the agency of water, after having been ejected from fissures or craters in the shape of ashes and cinders, precisely as we may now expect would happen with the ashes and cinders ejected from volcanoes, particularly insular and littoral volcanoes, into the sea."[296] Again he speaks of "two kinds of trappean rocks having probably been erupted, one in the state of igneous fusion, and the other in that of ash, during the time that the mud, now forming slates, was deposited, the mixtures of volcanic and sedimentary materials being irregular from the irregular action of the respective causes which produced them; so that though the one may have been derived from igneous action, and the other from the ordinary abrasion of pre-existing solid rocks, they were geologically contemporaneous."[297] He recognized the origin of the amygdaloidal varieties of rock, and by dissolving out the calcite from their cells showed how close was their resemblance to modern pumice.[298]
[Footnote 296: "Report on the Geology of Cornwall, Devon and West Somerset," Mem. Geol. Survey, 1839, p. 37.]
[Footnote 297: Op. cit. p. 57.]
[Footnote 298: Op. cit. pp. 57, 61.]
Since these early researches many geologists have studied the igneous rocks of Devonshire. I would especially refer to the labours of Mr. Allport,[299] the late J. A. Phillips,[300] Mr. Rutley,[301] the late Mr. Champernowne,[302] Mr. W. A. E. Ussher,[303] Mr. Hobson,[304] and General M'Mahon.[305] Mr. Champernowne in particular has shown the abundance of volcanic material among the rocks of Devonshire, and the resemblance which in this respect they offer to the Devonian system of North Germany.
[Footnote 299: Quart. Journ. Geol. Soc. xxxii. (1876), p. 418.]
[Footnote 300: Op. cit. xxxi. (1875) p. 325, xxxii. (1876) p. 155, xxxiv. (1878) p. 471.]
[Footnote 301: "Brent Tor," Mem. Geol. Surv. p. 18; _Quart. Journ. Geol. Soc._ lii. (1896), p. 66.]
[Footnote 302: See in particular his last paper "On the Ashprington Volcanic Series of South Devon," Quart. Journ. Geol. Soc. vol. xlv. (1889), p. 369.]
[Footnote 303: This geologist has spent many laborious years in the investigation of the geology of Devonshire, and has published numerous papers on the subject, in the Transactions of the Devonshire Association and of the Royal Cornwall Geological Society, in the Proceedings of the Somersetshire Archæological and Natural History Society, and of the Geologists' Association, in the Geological Magazine, and the Quarterly Journal of the Geological Society. Reference may especially be made to his Memoir in the last named journal, vol. xlvi. (1890), p. 487.]
[Footnote 304: Quart. Journ. Geol. Soc. xlviii. (1892), p. 496.]
[Footnote 305: Op. cit. xlix. (1893), p. 385.]
Unfortunately the geological structure of the Palæozoic rocks of the South-west of England has been complicated to an amazing extent by plication and fracture, with concomitant cleavage and metamorphism. Hence it is a task of extreme difficulty to trace out with any certainty definite stratigraphical horizons, and to determine the range of contemporaneous volcanic action. Mr. Ussher has shown with what success this task may be accomplished when it is pursued on a basis of minute mapping, combined with a sedulous collection and determination of fossils.[306] But years must necessarily elapse before such detailed work is carried over the whole Devonian region, and probably not till then will the story of the volcanic history of the rocks be adequately made out.
[Footnote 306: See Memoir cited in a previous note.]
In the meantime, it has been established that while there is a singular absence of igneous rocks in North Devon, a strip of country extending from Newton Abbot and Torquay westwards by Plymouth across Cornwall to Penzance contains abundant records of volcanic action. It has not yet been possible to map out, among what were formerly all grouped together as "greenstones," the respective limits of the bedded lavas and the tuffs, to distinguish the true sills, and to fix on the position of the chief vents of eruption. So intense have been the compression and shearing of the rocks that solid sheets of diabase have been crushed into fissile schists, which can hardly be distinguished from tuffs. Moreover, owing perhaps in large measure to the mantle of red Permian (or Triassic) strata, which has been stripped off by denudation from large tracts of this region once overspread by it, the Devonian rocks have been deeply "raddled," or stained red. But probably one of the main sources of difficulty in studying the petrography of the area is to be found in the results of atmospheric weathering. Devonshire lies in that southern non-glaciated strip of England, where the rocks have been undergoing continuous decay since long before the Ice Age. No ploughshare of ice has there swept off the deep weathered crust, so as to leave hard surfaces of rock, fresh and bare, under a protecting sheet of boulder-clay. It is seldom that a really fresh piece of any igneous rock can be procured among the lanes and shallow pits of Devon, where alone, for the most part, the materials are exposed.
Much, therefore, remains to be done, both in the stratigraphy and petrography of the Devonian volcanic rocks of this country. To the late J. A. Phillips geology is indebted for the first detailed chemical and microscopical investigation of these rocks. He clearly showed the truly volcanic origin of many of the so-called "greenstones." He believed that certain "slaty blue elvans," which he found to have a composition identical with that of altered dolerites, might be highly metamorphosed tuffs, and that others might have been originally sheets of volcanic mud. After studying the chemical composition and minute structure of a large collection of "greenstones," he demonstrated that in all essential particulars they were dolerites, though somewhat altered from their original character.[307] Subsequently they were studied by Dr. Hatch, who found the fresher specimens generally to possess an ophitic structure, while some are granular, others porphyritic.[308]
[Footnote 307: See especially Quart. Journ. Geol. Soc. vols. xxxii. and xxxiv.]
[Footnote 308: A few of the eruptive rocks of Devonshire have recently been studied by K. Busz. He finds most of his specimens (chiefly from the Torquay district) to be varieties of diabase, but describes a palæopicrite from Highweek near Newton Bushel, and a kersantite from South Brent on the south-east edge of Dartmoor (Neues Jahrb. 1896, p. 57).]
Although the rocks have undergone so much crushing, solid cores of them, showing the original structure, may be obtained, also examples of the amygdaloidal, vesicular or slaggy character. They occur in sheets either singly or in groups, and appear generally to be regularly interstratified in the slates and grits. While some of these intercalations, especially the amygdaloidal sheets, may be true superficial lavas, it can hardly be doubted that others are sills, especially those which assume the crystalline structure and composition of gabbros, and show an entire absence of the vesicular structure. But no one has yet attempted to separate the two types from each other.
With these rocks are associated abundant diabase-tuffs (schalstein), frequently mingled with ordinary non-volcanic detrital matter, and shading off into the surrounding grits and slates. There is thus clear evidence of the outpouring of basic lavas and showers of ashes during the Devonian period in the south-west of England, under conditions analogous to those which characterized the deposition of the Devonian system in Nassau and the Harz.
The exact range of these eruptions in geological time has still to be ascertained. So far as at present determined, volcanic activity was not awakened during the accumulation of the Lower Devonian formations. It was not until the sporadic coral-reefs and shell-banks had grown up, which form the basement limestones of the Middle Devonian group, that the first eruptions took place. As Godwin-Austen, Champernowne and Mr. Ussher have shown, some of these reefs were overwhelmed with streams of lava or buried under showers of ashes. The volcanic discharges, however, were peculiarly local, probably from many scattered vents, and never on any great scale. Some districts remained little or not at all affected by them, so that the growth of limestone went on without interruption, or at least with no serious break. In other areas again the place of the limestone is taken by volcanic materials.
The chief epoch of this volcanic action, marked by the "Ashprington Volcanic Series," appears to have occurred about midway in the Middle Devonian period. But in certain districts it extended into Upper Devonian time. Intrusive sills of diabase may mark the later phases of the volcanic history. But the occurrence of such sills even in the Upper Devonian rocks, and the alteration of the strata in contact with them (spilosite), point to the continuance or renewal of subterranean disturbance even in the later Devonian ages, if not in subsequent geological time. That volcanic activity accompanied the deposition of the Carboniferous rocks of Devonshire has long been well known (see Chapter xxix.).
XVI. THE VOLCANOES OF THE OLD RED SANDSTONE
Geological Revolutions at the close of the Silurian Period--Physical Geography of the Old Red Sandstone--Old Lake-basins, their Flora and Fauna--Abundance of Volcanoes--History of Investigation in the Subject.
We now enter upon the consideration of the records of a notable era in the geological evolution of north-western Europe. Up to the close of the Silurian period the long history embodied in the rocks presents a constant succession of slowly sinking sea-floors. Wide tracts of ocean stretched over most of Europe, and across the shifting bottom, sand and mud, washed from lands that have long vanished, spread in an ever-accumulating pile. Now and then, some terrestrial movement of more than usual potency upraised this monotonous sea-bed, but the old conditions of ceaseless waste continued, and fresh sheets of detritus were thrown down upon the broken-up heaps of older sediment. All through the vast cycles of time denoted by these accumulations of strata, generations of sea-creatures came and went in long procession, leaving their relics amidst the ooze of the bottom. Genera and families, once abundant, gradually died out, and gave place to others, the onward march of life being slow but uninterrupted. Of the land of the time or of the plants and animals that lived on its surface, hardly anything is known. The chronicles that have come down to us are almost wholly records of the vicissitudes of the ocean-bed.
Over the centre and south of Europe, the marine conditions of Silurian time were prolonged, as we have seen, into the next period, when the Devonian formations were deposited. In that wide region, no marked break has been traced between either the sedimentation or the animal life of the Silurian and Devonian periods. But in the north-west of Europe a striking departure took place from the protracted monotony of marine conditions. By a series of terrestrial movements that affected the area lying to the north of the line of the Bristol Channel, and extended not only to the furthest limit of the British Isles, but probably as far as Norway, and perhaps even into northern Russia, the previous widespread conditions of marine sedimentation were entirely altered. Instead of the fine oceanic silts and sands with their abundant organic remains, and the thick limestones with their masses of coral and crowds of crinoids, there were now laid down, over these northern regions, vast piles of deep red sediment, from which traces of animal life are almost wholly absent. The shelving land against which these ferruginous sands and gravels gathered can still in part be recognized. As the observer follows its margin, notes the varying local peculiarities of its sediment, and detects, sometimes in great abundance, remains of the vegetation which clothed it, the conviction grows in his mind that the remarkable contrast between these deposits, known as the Old Red Sandstone, and those of the Silurian and Devonian systems is not to be accounted for by any mere rearrangement of the sea-bottom, or redistribution of the land that supplied that sea-bottom with sediment. It has long been the general belief among geologists that the subterranean movements which, over the greater part of Britain, brought the deposition of the Upper Silurian formations to a close, led to a total alteration of the geography of the region affected, that the sea-floor was elevated, and that, over the upraised tract, large lakes or inland seas were eventually formed, in which the peculiar sediments of the Old Red Sandstone were accumulated.
The records of this series of geographical changes are too fragmentary to enable us to follow, except in a very general way, the sequence of events in the transformation of the Silurian sea into the peculiar topographical conditions in which the Old Red Sandstone was laid down. While there was a widespread elevation of the sea-floor, and of such ridges of insular land as may have risen above sea-level, the upheaval appears to have been of a somewhat complicated kind, and to have been combined with many local subsidences. The area of disturbance was probably thrown into a series of parallel ridges and troughs, the former continuing to be pushed upward, while the latter tended to subside. The ridges thus became land surfaces, and their prolonged elevation may have more or less compensated for the denudation to which, on their emergence, they were necessarily exposed. The troughs, on the other hand, which sank down, may in many cases have subsided below the sea. But where the general upheaval of the crust was most pronounced, some of the depressions would be isolated above sea-level and become lake-basins in the terrestrial areas.
Of some of these water-basins the outlines can still in some measure be defined. The rocks that rose into hills around them, and from which their enormous accumulations of detritus were derived, still partially survive. We can explore these piles of sediment, and from them can form some idea of the condition of the water in the lakes, and the nature of the vegetation on the surrounding land. The frequent occurrence and exceeding coarseness of the conglomerates, which appear on many successive horizons throughout the deposits of these basins, probably indicate contemporaneous terrestrial disturbances. The same causes that led to the wrinkling of the crust into parallel ridges and troughs no doubt still continued in operation. From time to time the ridges, much worn down by prolonged denudation, were pushed upward, either by gradual uprise or by more rapid jerks. The troughs may in like manner have been still affected by their old tendency to subsidence. Hence, in spite of the effects of degradation and deposition, it is possible that the ridges may not, on the whole, have varied much in height, nor the basins in depth, during the time when thousands of feet were stripped off the land and strewn in detritus over the bottoms of the lakes.
Let us try to realize a little more definitely the general aspect of the region in which the Old Red Sandstone water-basins lay. As the axes of the folds into which the crust of the earth was thrown ran in a north-east and south-west direction, they gave this trend to the lakes and to the tracts of land that separated them. These intervening ridges must in some instances have been hilly or even mountainous. Thus, the Scottish Highlands rose between two of the lakes, and poured into them an abundant tribute of gravel, sand and silt. The terrestrial vegetation of the time has been partially preserved. The hills seem to have been clothed with conifers, while the lower slopes and swamps were green with sigillariæ, lepidodendra and calamites. One of the most characteristic plants was Psilophyton, of which large matted sheets were drifted across the lakes and entombed in the silt of the bottom. A grass-like vegetation, with long linear leaves, seems to have grown thickly in some of the shallows of the lakes.
Of the land animals we have still less knowledge than of the vegetation. Doubtless various forms of insect life flitted through the woodlands, though no relics of their forms have yet been recovered. But the remains of myriapods have been found in Forfarshire.[309] These early relics of the animal life of the land inhabited the woodlands, like our modern gally-worms, and were swept down into the lakes, together with large quantities of vegetation.
[Footnote 309: Mr. B. N. Peach, Proceedings of Royal Physical Society of Edinburgh, vol. vii. (1882).]
Some of the lakes, especially in the earlier part of their history, abounded in eurypterid crustacea. These animals inhabited the seas in Upper Silurian time, and appear to have been isolated in the water-basins of the Old Red Sandstone. Certain species of Pterygotus, a Silurian genus found also in the Lower Old Red Sandstone, reached a length of six feet. But the most abundant forms of animal life were fishes. These furnish additional evidence in favour of the lacustrine nature of the waters in which they lived. Such characteristically marine forms as the sharks and rays of the Silurian seas were replaced by genera of Acanthodians, Ostracoderms, Dipnoids, Teleostomes, Placoderms, and Palæoniscids, which abounded in the more northerly waters. The distinctive outward characters of many of these early vertebrates were their bony scales and plates. Some of them had their heads encased in an armature of bone, of large size and massive thickness. In several genera the bone was coated with a layer of glittering enamel. Even now, after the vast lapse of time since their day, the cuirasses and scale-armour of these fishes keep their bright sheen in the hardened sand and mud from which they are disinterred.
A difference is observable between the faunas of the different water-basins. Even where the same genus occurs in two adjacent areas, the species are often distinct. Two large lakes, separated by the tract of the Scottish Highlands, had each its own assemblage of fishes, not a single genus being common to the two basins. Such contrasts, whether the two lakes were geologically contemporaneous, or the northern arose later than the southern, undoubtedly indicate long-continued isolation and the gradual evolution of new forms under different conditions of environment.[310]
[Footnote 310: In my memoir "On the Old Red Sandstone of Western Europe" (Trans. Roy. Soc. Edin. vol. xxviii. 1878), I argued for the probable geological contemporaneity of the conglomerates, sandstones and flagstones on either side of the Grampian chain, even although their organic contents were so unlike. The stratigraphical evidence favours this view. In each case a thick series of strata is covered unconformably by Upper Old Red Sandstone, containing Holoptychius nobilissimus and other fishes. The question cannot perhaps be definitely settled by the data available in Scotland. It is quite possible that the basin on the northern side of the Grampians, which I have termed "Lake Orcadie," came into existence after that on the southern side. But I do not think the differences in their respective faunas are to be accounted for simply by lapse of time and the gradual organic evolution in progress over one continuous region. The more the Old Red Sandstone is studied, the more local do its various fish-faunas appear to have been. These strongly-marked diversities appear to me rather to point to prolonged isolation of the basins from each other, as stated above. Dr. Traquair has drawn attention to the remarkable fact that, even in what appears to be one continuous series of strata of no great thickness forming the Upper Old Red Sandstone of the Moray Firth basin, the fishes found about Nairn are entirely different from those met with in the rest of the region.]
Such, in brief, were the aspects of the physical geography of the time on the further consideration of which we are now to enter. The subterranean disturbances, so characteristic of the period, were accompanied by a display of volcanic activity more widespread, perhaps, than any which had yet taken place in the geological history of Britain. Nevertheless, it is worthy of remark that this manifestation of underground energy did not begin with the commencement of these displacements of the crust. The earliest eruptions only took place after the geography of the region had been completely changed; at least no trace of them is to be found in the earliest portions of the Old Red Sandstone. After the last lingering Silurian volcanoes in the west of Ireland had died out, a protracted quiescence of the subterranean fires ensued. In the latest ages of Silurian time there was not in Britain, so far as at present known, a single volcanic eruption. Not until after the inauguration of the Old Red Sandstone topography, when the lakes had taken shape and had begun to be filled with sediment from the surrounding hills, did a series of new volcanoes burst into activity over the northern half of Britain. Rising in the midst of the lakes in groups of separate cones, these vents poured out floods of lava, together with clouds of ashes and stones. Their sites, the history of their eruptions, and the piles of material ejected by them, can still be ascertained, and I shall now proceed to give some account of them.
The thick mass of sedimentary material known as the Old Red Sandstone, lying between the top of the Silurian and the base of the Carboniferous system, has been divided into two sections, which, however, are of unequal dimensions, and doubtless represent very unequal periods of time. The older series, or Lower Old Red Sandstone, is by far the more important and interesting in its extent, thickness, palæontological riches, and, what specially concerns us in the present inquiry, in its volcanic records. Wherever its true base can be seen, this series passes down conformably into Upper Silurian strata. It sometimes reaches a thickness of 15,000 and even 20,000 feet. There is generally a marked break between its highest visible strata and all younger formations. Even the upper division of the Old Red Sandstone rests unconformably upon the lower.[311] Such a hiatus undoubtedly points to a considerable lapse of geological time, and to the advent of important geographical changes that considerably modified the remarkable topography of the older part of the period.
[Footnote 311: Quart. Journ. Geol. Soc. vol. xvi. (1860), p. 312. In Wales no break has actually been discovered between the two divisions of the Old Red Sandstone, though it is suspected to exist there also.]
The younger division or Upper Old Red Sandstone passes upward conformably into the base of the Carboniferous system. Its red and yellow sandstones, conglomerates and breccias, covering much more restricted areas, and attaining a much less thickness than those of the lower division, indicate the diminution and gradual effacement of the lakes of the older time, and the eventual return of the sea to the tracts from which it had been so long excluded. So vast an interval elapsed between the time recorded in the deposits respectively of the two sections of the Old Red Sandstone that the characteristic forms of animal life in the earlier ages had entirely passed away, and their places had been taken by other types when the diminished lake-basins of the second period began to be filled up. Volcanic action also dwindled to such a degree that in contrast to the abundant vents of the older period, only one or two widely scattered groups of vents are known to have existed in the area of the British Isles during the later period, and these, after a feeble activity, gave way to a prolonged volcanic quiescence, which lasted until the earlier ages of the succeeding or Carboniferous period.
Although geologists are in the habit of grouping the Old Red Sandstone and the Devonian rocks as equivalent or homotaxial formations, deposited in distinct areas under considerably different conditions of sedimentation, the attempt to follow out the sequence of strata in Devonshire, and to trace some analogy between the Devonian succession and that of the Old Red Sandstone, presents many difficulties for which no obvious solution suggests itself. Into these problems it is not needful to enter further than was done in the last chapter. We may assume that not improbably some of the eruptions now to be described were coeval with those of Devonian time in the south-west of England, though we may hesitate to decide which of them should be brought into parallelism.
As we trace the shore-lines of the ancient basins of the Lower Old Red Sandstone, and walk over the shingle of their beaches, or as we examine the silt of their deeper gulfs, and exhume the remains of the plants that shaded their borders, and of the fishes that swarmed in their waters, we gradually learn that although the sediments which accumulated in some of these basins amount to many thousand feet in thickness; yet from bottom to top they abound in evidence of shallow-water conditions of deposit. The terrestrial disturbances above referred to continued for a vast interval, and while, as already suggested, the floors of the basins sank, and the intervening tracts were ridged up, as the results of one great movement of the earth's crust, the denudation of the surface of the land contributed to the basins such a constant influx of sediment as, on the whole, compensated for the gradual depression of their bottoms.
We need not suppose that these movements of subsidence and upheaval were uninterrupted and uniform. Indeed, the abundant coarse conglomerates, which play so prominent a part in the materials thrown into the basins, suggest that at various intervals during the prolonged sedimentation subterranean disturbances were specially vigorous. But the occurrence of strong unconformabilities among the deposits of the basins sets this question at rest, by proving that the terrestrial movements were so great as sometimes to break up the floor of a lake, and to place its older sediments on end, in which position they were covered up and deeply buried by the succeeding deposits.[312]
[Footnote 312: An unconformability of this kind occurs between the south end of the Pentland Hills and Tinto in Lanarkshire, and another in Ayrshire.]
It is not surprising to discover, among these evidences of great terrestrial disturbance, that eventually groups of volcanoes rose in long lines from the waters of most of the lakes, and threw out enormous quantities of lava and ashes over tracts hundreds of square miles in extent. So vast, indeed, were these discharges, across what is now the Midland Valley of Scotland, that the portions of sheets of lava and tuff visible at the surface form some of the most conspicuous ranges of hills in that district, stretching continuously for 40 or 50 miles and reaching heights of more than 2000 feet above the sea. Exposed in hundreds of ravines and escarpments, and dissected by the waves along both the eastern and western coasts of the country, these volcanic records may be studied with a fulness of detail which cannot be found among earlier Palæozoic formations.
It might have been supposed that a series of rocks so well displayed and so full of interest, would long ere this have been fully examined and described. But they can hardly be said to have yet received, as a whole, the attention they deserve. Without enumerating all the writers who, each in his own measure, have added to the sum of our knowledge of the subject, I may refer to the labours of Jameson,[313] Macknight[314] and Fleming,[315] among the observers who began the investigation. But of the early pioneers, by far the most important in regard to the igneous rocks of the Old Red Sandstone was Ami Boué. While attending the University of Edinburgh, where he took the degree of M.D. in the year 1816, he imbibed from Jameson a love of mineralogy and geognosy, and for several years spent his leisure time in personally visiting many parts of Scotland, in order to study the geological structure of the country. Probably in 1820 he published in French his now classic Essai.[316] The value of this work as an original contribution to the geology of the British Isles has probably never been adequately acknowledged. For this want of due recognition the author himself was no doubt in some measure to blame. He refers distinctly enough to various previous writers, notably to Jameson and Macculloch, but he mingles the results of his own personal examinations with theirs in such a way that it is hardly possible to ascertain what portions are the outcome of his own original observations. Less credit has accordingly been given to him than he could fairly have claimed for solid additions to the subjects of which he treated. In the later years of his life I had opportunities of learning personally from him how extensive had been his early peregrinations in Scotland, and how vivid were the recollections which, after the lapse of half a century, he still retained of them. Judged simply as a well-ordered summary of all the known facts regarding the geology of Scotland, his Essai must be regarded as a work of very great value. Especially important is his arrangement of the volcanic phenomena of the country, which stands far in advance of anything of the kind previously attempted. Under the head of the "Terrain Volcanique," he treats of the basaltic formations, distinguishing them as sheets (nappes, coulées) and dykes; and of the felspathic or trachytic formations, which he subdivides into phonolites, trachytes, porphyries (forming mountains and also sheets) and felspathic or trachytic dykes. In the details supplied under each of these sections he gives facts and deductions which were obviously the result of his own independent examination of the ground, and he likewise marshals the data accumulated by Jameson, Macculloch and others, in such a way as to present a more comprehensive and definite picture of the volcanic phenomena of Scotland than any previous writer had ventured to give.
[Footnote 313: Memoirs of the Wernerian Society, vol. ii. (1811), pp. 178, 217, 618; vol. iii. (1820), p. 220, 225.]
[Footnote 314: Op. cit. vol. ii. pp. 123, 461.]
[Footnote 315: Op. cit. vol. i. (1808), p. 162; vol. ii. (1811), pp. 138, 339.]
[Footnote 316: Essai géologique sur l'Écosse (Paris; no date, but probably about 1820). He acknowledges his indebtedness to Jameson, whose demonstrations of the geology of the Edinburgh district he partly reproduced in his book. Jameson's early writings in the Wernerian Memoirs and in separate works were mere mineralogical or "geognostical" descriptions. His later lectures became more valuable but were never published, save indirectly in so far as they influenced the opinions of his pupils who published writings on the same subjects. See, for instance, Hay Cunningham's Geology of the Lothians, p. 59, footnote. Compare an article on Boué, Edinburgh Review for May 1823 (vol. xxxviii. p. 413).]
The account which Boué wrote of the Old Red Sandstone and its associated igneous rocks marked the first great forward step in the investigation of this section of the geological record. He was the earliest observer to divide what he calls the "roches feldspathiques et trappéennes" into groups according to their geological position and mineralogical character, and to regard them as of igneous origin and of the age, or nearly of the age, of the red sandstone of Central Scotland.
Of later writers who have treated of the volcanic rocks of the Old Red Sandstone, my old friend Charles Maclaren deserves special recognition. His survey and description of the Pentland Hills embodied the first detailed and accurate investigation of any portion of these rocks, and his Geology of Fife and the Lothians may still be read with pleasure and instruction.[317] Boué had indicated roughly on the little sketch-map accompanying his Essai the chief bands of his felspathic and trappean rocks of the Old Red Sandstone, but their position and limits were more precisely defined in Macculloch's "Geological Map of Scotland," which was published in 1840, five years after the sudden and tragic death of its author. The observers who have more recently studied these rocks have been chiefly members of the Geological Survey, and to some of the more important results obtained by them I shall refer in the sequel.
[Footnote 317: Geology of Fife and the Lothians, 1839. More detailed reference will be made in later pages to this classic.]
For many years I have devoted much time to the investigation of the Old Red Sandstone and its volcanic rocks. In the year 1859 I ascertained the existence of the great hiatus between the Lower and Upper divisions of the system.[318] A first sketch of the volcanic history of the Old Red Sandstone was given by me in 1861,[319] which was subsequently enlarged and filled in with more detail in 1879.[320] But it was not until 1892 that I published a somewhat detailed outline of the whole subject, tracing the history of volcanic action during the period of the Old Red Sandstone, the distribution of the volcanoes, and the character of the materials erupted by them.[321] This outline I now proceed to amplify, filling in details that were necessarily omitted before, though there are still several districts regarding which information is scanty.
[Footnote 318: "On the Old Red Sandstone of the South of Scotland," Quart. Journ. Geol. Soc. xvi. (1860), p. 312.]
[Footnote 319: "On the Chronology of the Trap-Rocks of Scotland," Trans. Roy. Soc. Edin. vol. xxii. (1861), p. 63.]
[Footnote 320: Article "Geology," in Ninth Edition of the Encyclopædia Britannica, vol. x. (1879), p. 343. Reprinted in my Text-Book of Geology, of which the first edition appeared in 1882.]
[Footnote 321: "Presidential Address to the Geological Society," Quart. Journ. Geol. Soc. vol. xlviii. (1892).]
In arranging the treatment of the subject I shall divide the record into two main sections, the first and much the more important being devoted to the Lower and the second to the Upper Old Red Sandstone. In the first of these divisions it will be convenient to begin by taking note of the distribution of the various districts over which the geological evidence is spread. We may then proceed to consider the general character of the volcanic rocks and the manner in which they are arranged in the stratigraphy of the country, taking in consecutive order (1) the superficial lavas and tuffs; (2) the vents; (3) the dykes and sills. From these general considerations we may pass to the detailed history of events in each of the separate volcanic areas, and thus obtain, as far as the evidence at present permits, a broad view of the progress of volcanic action during the time of the Lower Old Red Sandstone in Britain.
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]
[Footnote 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 (see Map III.).
[Footnote 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.
[Footnote 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.
[Footnote 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.
[Footnote 326: The only examples known to me are those of Benaun More and other hills in County Kerry.]
(a) The Diabase-lavas are 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]--
[Footnote 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.]
+------------+-----------+---------+-----------+---------+-----------+ | | Carnethy | Buiselaw. | Warklaw Hill. | | | Hill[328]| Sp. grav. 2·80. | Sp. grav. 2·77. | +------------+-----------+---------+-----------+---------+-----------+ | | | Soluble | Insoluble | Soluble | Insoluble | | | | in HCl | in HCl | in HCl | in HCl | +------------+-----------+---------+-----------+---------+-----------+ | SiO_{2} | 51·16 | ... | 52·00 | ... | 47·77 | | Al_{2}O_{3}| 22·27 | 1·30 | 17·46 | 5·23 | 13·08 | | Fe_{2}O_{3}| 2·94 | 1·53 | 7·85 | 7·32 | 0·84 | | FeO | 4·02 | 1·14 | ... | ... | ... | | CaO | 5·61 | 2·43 | 6·80 | 7·88 | 4·07 | | MgO | 3·46 | 0·98 | 1·06 | 3·65 | 0·30 | | K_{2}O | 2·42 | ... | 1·66 | ... | 1·17 | | Na_{2}O | 2·58 | ... | 4·17 | ... | 2·30 | | H_{2}O | 3·42 | ... | 2·68 | ... | 2·48 | | P_{2}O_{5} | 0·48 | 0·32 | ... | 0·12 | ... | | CO_{2} | 1·28 | ... | ... | 5·01 | ... | +------------+-----------+---------+-----------+---------+-----------+
[Footnote 328: There was a trace of manganous oxide in this specimen.]
(b) The Andesites, 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 the verde antique of 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]
[Footnote 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).]
[Footnote 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 on the 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.
[Footnote 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:--
+-----------+-----+-------+-------+--------+-------+--------+-----------+-------+ | |Scawd|Rennie-|Cunrie-|Duncan's|Whitton|Cuddies'| Cocklaw- |More- | | | Law | ston | ston | Dubs | Hill | Tops | foot | battle| |-----------+-----+-------+-------+--------+-------+--------+-----------+-------+ |SiO_{2} |59·29| 62·81 | 63·38 | 59·44 | 60·70 | 60·58 | 62·29 | 59·82 | |Al_{2}O_{3}|16·30| 16·40 | 15·77 | 16·15 | 17·98 | 12·25 | 17·03 | 16·96 | |Fe_{2}O_{3}| 1·77| ·55 | ·73 | 1·05 | ·66 | 1·01 | ·93 | ·20 | |FeO | 3·70| 3·27 | 2·65 | 2·83 | 2·58 | 4·13 | 2·44 | 6·57 | |MnO | ·41| ·81 | ·08 | ·37 | ·20 | ·15 | ·21 | ·15 | |CaO | 4·81| 4·46 | 4·44 | 6·70 | 7·07 | 4·40 | 3·92 | 4·73 | |MgO | 3·15| 1·64 | 1·88 | 2·46 | 2·20 | 2·86 | 2·71 | 2·84 | |K_{2}O | 4·19| 3·60 | 1·88 | 3·18 | 3·57 | 2·19 | 1·14 | 2·63 | |Na_{2}O | 3·44| 3·02 | 4·54 | 3·70 | 2·95 | 3·61 | 3·20 | 3·04 | |H_{2}O | 3·84| 4·04 | 4·69 | 3·35 | 3·45 | ... | ·29[332]| ... | |H_{2}SO_{4}| ...| ... | ... | ... | ... | ·55 | ·37 | trace | |Loss. | ...| ... | ... | ... | ... | 2·15 | 4·81 | 1·98 | +-----------+-----+-------+-------+--------+-------+--------+-----------+-------+
[Footnote 332: This is CO_{2}.]
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 be inferred 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]
[Footnote 333: Geol. Mag. for 1883, pp. 100, 145, 252.]
[Footnote 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.]
[Footnote 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 as Trachytes offer 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:--
+-----------------------+ | N.W. of Whitton Hill, | | Jedburgh (No. 1938) | | Sp. gr. 2·55. | +-------------+---------+ | SiO_{2} | 62·44 | | Al_{2}O_{3} | 18·99 | | Fe_{2}O_{3} | 3·35 | | FeO | 1·8 | | MnO | ·25 | | CaO | 1·84 | | MgO | 1·37 | | K_{2}O | 5·02 | | Na_{2}O | 2·65 | | H_{2}O | 2·48 | +-------------+---------+ | Total. | 100·19 | +-------------+---------+
(d) Acid rocks such as Felsites and Rhyolites are 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 in Fig. 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]
[Footnote 336: Geol. Mag. for 1883, pp. 100, 145, 252; and British Petrography, pp. 272, 278.]
[Footnote 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]
[Footnote 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's British 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 be shown 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]--
[Footnote 339: Two analyses of rhyolites from Shetland by Mr. Tatlock will be found in Trans. 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.]
+-----------+------------+------------+-------------+-----------+ | | | |Tinto, | | | |"Hornstone."| | Lanarkshire:| | | | Torgeith |"Hornstone."| Soluble in | | | | Knowe, | Braid | hydrochloric|Insoluble | | | Pentlands | Hills[340] | acid | in ditto. | +-----------+------------+------------+-------------+-----------+ |SiO_{2} | 73·91 | 64·73 | ·04 | 70·28 | |Al_{2}O_{3}| 14·41 | 17·01 | 1·01 | 12·54 | |Fe_{2}O_{3}| ·76 | 2·35 | 1·24 | ·43 | |MnO | ·07 | ·24 | ... | ... | |CaO | 1·21 | 4·19 | ·92 | ·91 | |MgO | 4·90 | ·66 | ·52 | ... | |K_{2}O | 3·36 | 3·27 | ... | 3·92 | |Na_{2}O | 1·57 | 3·75 | ... | 5·84 | |P_{2}O_{5} | ... | ·26 | ·16 | ... | |H_{2}O | ·90 | 2·78 | ... | 1·99 | +-----------+------------+------------+-------------+-----------+
[Footnote 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] (see Fig. 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.
[Footnote 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. Watts to "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:--
Silica percentage.
1. Quarry above Woodhouselee 63·3 2. South-west side of Castlelaw Hill 73·15 3. Quarry on road, ½ mile N.E. of Swanston (Braid Hill vent) 74·1 4. South-west side of Castlelaw Hill 75·0 5. Castlelaw Hill 76·00 6. South side of White Hill Plantation 90·00
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.
Chapter XVIII
STRUCTURE AND ARRANGEMENT OF THE LOWER OLD RED SANDSTONE VOLCANIC ROCKS IN THE FIELD
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.
[Footnote 342: See Jukes' Manual of Geology, 3rd edit. (1872), Fig. 111, p. 276.]
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.
[Footnote 343: An abundant organism in some of these deposits, named Parka, was first regarded as a plant, was afterwards believed to be the egg-packets of crustacea, and is now pronounced by competent authorities to belong to an aquatic plant with creeping stems, linear leaves and sessile sporocarps.]
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]
[Footnote 344: I suggested this possible connection many years ago in Trans. Geol. Soc. Edin. vol. ii. (1874) p. 21.]
[Footnote 345: The volcanic geology of the Cheviot Hills is described by Mr. Teall, Geol. Mag. for 1883, p. 106; and by Mr. Clough, Mem. Geol. Survey, "Geology of the Cheviot Hills," Sheet 108 N.E., 1888, p. 24.]
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.
[Footnote 346: See the descriptions of the Shap granite by Messrs. Marr and Harker, Quart. Journ. Geol. Soc. xlvii. (1891) p. 266, and xlix. (1893) p. 359.]
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]
[Footnote 347: Messrs. B. N. Peach and J. Horne, _Trans. Roy. Soc. Edin._ xxxii. (1884), p. 359.]
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.
[Footnote 348: See Sheet 56 of the Geological Survey of Scotland.]
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.
XIX. VOLCANOES OF THE LOWER OLD RED SANDSTONE OF "LAKE CALEDONIA"
Description of the several Volcanic Districts: "Lake Caledonia," its Chains of Volcanoes--The Northern Chain: Montrose Group, Ochil and Sidlaw Hills, the Arran and Cantyre Centre, the Ulster Centre.
I now propose to give some account of each of the districts which have been separate areas of volcanic action during the time of the Lower Old Red Sandstone, tracing its general structure, the arrangement and sequence of its volcanic rocks and the history of its eruptions. As by far the most varied development of the Old Red Sandstone is to be found in the great Midland Valley of Scotland, and as it is there that the remarkable volcanic phenomena of the system have been most abundantly displayed and are most clearly recorded, I shall begin my description of the volcanic eruptions of the Lower Old Red Sandstone with a detailed account of the different centres of volcanic activity in that region. The phenomena are so fully displayed there that a more summary treatment of the subject will suffice for the other regions.
Under the designation of "Lake Caledonia," as already remarked, I include the whole of the Midland Valley of Scotland between the Highlands and the Southern Uplands, likewise the continuation of the same ancient hollow by Arran and the south of Cantyre across the north of Ireland to Lough Erne.[349] Throughout most of the area thus defined, the present limits of the Lower Old Red Sandstone are sharply marked off by large parallel faults. On the north-west side one, or rather a parallel series, of such dislocations runs from Stonehaven along the flank of the Highland mountains to the Clyde, thus traversing the whole breadth of the island. On the south-east side another similar series of faults, which there skirts the edge of the Silurian tableland, has nearly the same effect in precisely defining the margin of the Old Red Sandstone. As thus limited, the tract has a breadth of about 50 miles in Scotland, while the portion of it now visible in the British Isles has an extreme length of about 280 miles (Map III.).
[Footnote 349: My own investigations of this region have been continued over an interval of forty years. Besides personally traversing every portion of it, I have mapped in detail, for the Geological Survey, many hundreds of square miles of its area from the outskirts of Edinburgh south-westwards into Lanarkshire, in Ayrshire, and in the counties of Fife, Perth and Kinross. The Geological Survey maps of the volcanic tracts of the Sidlaw Hills have been prepared by my brother, Prof. James Geikie, and Messrs. H. M. Skae and D. R. Irvine. The Western Ochils were mapped chiefly by Mr. B. N. Peach, partly by Prof. J. Young, Mr. R. L. Jack and myself; the Eastern Ochils were surveyed mainly by Mr. H. H. Howell; while the volcanic belt between the tracts mapped by me in Lanarkshire and in Ayrshire was chiefly traced out by Mr. Peach. As a rule, each of these geologists has described in the Survey Memoirs the portions of country surveyed by him.]
But though the boundary-faults determine, on the whole, the present limits of the tract of Old Red Sandstone, they do not necessarily indicate the shore-lines of the sheet of water in which that great series of deposits was laid down. They point to an enormous subsidence of the tract between them--a prolonged and extensive sagging of the strip of country that stretches across the Midland Valley of Scotland into the north of Ireland.[350] This downward movement began as far back as the close of the Silurian period, but the marginal fractures and the disruption and plication of the thick masses of sandstone and conglomerate which were accumulated in the lake chiefly took place after the close of the period of the Lower Old Red Sandstone. I think we may reasonably connect these movements with the general sinking of the area consequent upon the enormous outpouring of volcanic materials during that period.
[Footnote 350: In some of the dislocations along the Highland border, the Old Red Sandstone is bent back upon itself, and the older schists are thus made to recline upon it, as if there had been a push over from the Highland area.]
Along both the northern and southern margins of the basin there occur, on the farther side of the boundary faults, outlying patches of Lower Old Red Sandstone that rest unconformably on the rocks forming the flanks of the hills. These areas possess a peculiar interest, inasmuch as they reveal some parts of the shore-line of the lake, and show the relation between the earlier rocks and the sediments of the Old Red Sandstone. We learn from them that the shore-line was indented with wide bays, but nevertheless ran in a general north-easterly direction. It thus corresponded in trend with the present Midland Valley, with the axes of plication among the schists of the Highlands as well as among the Silurian rocks of the Southern Uplands, and with the subsequent faulting and folding of the Old Red Sandstone.
I may remark in passing that the conglomerates and other associated materials which have been preserved in these bays and hollows beyond the lines of the great faults, though they lie unconformably on the rocks beneath, are not the basement portions of the Old Red Sandstone. On the contrary, where their probable stratigraphical horizons can be recognized or inferred, they are found to belong to parts of the series considerably above the base of the whole. They point to the gradual sinking of the basin and the creeping of the waters with their littoral shingles further and further up the slopes of the hills on either side (Fig. 73).
But this is not all the evidence that can be adduced to show that the limits of the lake extended considerably beyond the lines of dislocation between which the present area of Old Red Sandstone mainly lies. No one can look at the noble escarpments of the Braes of Doune on the one side (Fig. 74), or walk over the upturned conglomerates and andesites which flank the Lanarkshire uplands on the other, without being convinced that if the effects of the boundary faults could be undone, so as to restore the original structure of the ground, the prolongations of the rocks, now removed by denudation, would be found sweeping far into the Highlands on the north and into the Silurian Uplands on the south.
If the area of "Lake Caledonia" were taken to be defined by the boundary faults, it covered a space of about 10,000 square miles. But, as we know that it certainly stretched beyond the limits marked by these faults, it must have been of still greater extent. We shall probably not exaggerate if we regard it as somewhat larger than the present Lake Erie, the superficies of which is about 9900 square miles. In this long narrow basin the remarkable volcanic history was enacted of which I now proceed to give some account.
The Lower Old Red Sandstone of Central Scotland may be conveniently divided into three great groups, each of which marks a distinct epoch in the history of the basin wherein they were successively accumulated. The lowest of these groups indicates a time of quiet sedimentation during which the basin was defined by plication of the terrestrial crust, and when, by the same subterranean movements, some parts of the floor of the lake were pushed upward above water, and were then denuded and buried. The middle group consists largely of volcanic rocks. It points to the existence of lines of active volcanic cones situated along the length of the lake. The uppermost group records the extinction of volcanic action and the gradual obliteration of the lake, partly by the pouring of sediment into it, and partly no doubt by the continued terrestrial movements which had originally produced the basin.
It is evident from these records that though volcanic activity continued vigorous for a vast period of time, it had entirely ceased in "Lake Caledonia" long before the last sediments of the Lower Old Red Sandstone were laid down. The great cones of the Ochil Hills, for example, sank below the waters of the lake in which they had long been a conspicuous feature, and so protracted was the subsidence of the lake-bottom that the site of these volcanoes was buried under 8000 or 9000 feet of sandstones and conglomerates, among which no trace of any volcanic eruptions has yet been found. The sagging of the terrestrial crust over an area from which such an enormous amount of volcanic products had been discharged would doubtless be a protracted process. Long after the subsidence of the lake-bottom and the accumulation of its thick mass of sediments, after even the entire effacement of the topography and the deposition of the thick Carboniferous formations over its site, the downward movement showed itself in the production of gigantic north-east faults, and the sinking of the Carboniferous rocks for several thousand feet. These dislocations, as was natural, have run through the heart of some of the volcanic groups, carrying much of the evidence of the ancient volcanoes out of sight, and leaving us only fragments from which to piece together the records of a volcanic period which is by no means the least interesting in the geological history of this country.
Confining our attention for the present to the records of the middle or volcanic group, we find evidence of a number of distinct clusters of volcanoes ranged along the whole length of the basin. The independence of these volcanic districts may be inferred from the following facts:--1st, The actual vents of discharge may in some cases be recognized; 2nd, Even where these vents have been buried, we may often observe, as we approach their probable sites, a marked increase in the thickness of the volcanic accumulations, as well as a great development of agglomerates and tuffs; 3rd, Traced in opposite directions, the volcanic materials are found to thin away or even to disappear. Those from one centre of discharge may be observed now and then to overlap those from another, but the two series remain distinct.
Reasoning from these data and studying the distribution of the various volcanic areas, we are led to recognize the former existence of two parallel chains of vents, running along the length of the lake at a distance from each other of somewhere about twenty miles. They may be conveniently distinguished as the northern and the southern chain.
The northern band runs from the coast-line near Stonehaven south-westward through the Sidlaw and Ochil Hills. It is then abruptly truncated by a large fault and by the unconformable superposition of the Carboniferous formations. But 60 miles further to the south-west, where the Old Red Sandstone comes out on the west side of the Firth of Clyde, a continuation of the volcanic band has recently been detected by Mr. W. Gunn of the Geological Survey in the Island of Arran. Twenty-five miles still further in the same direction a much ampler development of the volcanic rocks occurs to the south of Campbeltown in Cantyre. If we cross the 22 miles of sea that separate the Argyllshire coast-sections from those of Red Bay in Ireland, we find near Cushendall a repetition of the Scottish volcanic conglomerates, while still further along the same persistent line, some 50 miles into the interior, the hills of Tyrone include sheets of lava precisely like those of Central Scotland. The total length of this northern chain of volcanoes is thus not much less than 250 miles, and as its north-eastern end is now cut off by the North Sea it must have been still longer. It ran parallel to the north-western coast-line of the lake, at a distance which, over the site of the Midland Valley of Scotland, seems to have varied from 10 to 20 miles, but which greatly lessened further to the south-west.
At a distance of some twenty miles to the south of the northern belt, the second parallel chain of volcanoes ran in a nearly straight line, which is now traceable from the southern suburbs of Edinburgh to the coast of Ayrshire, a distance of about 75 miles, but as its north-eastern end is concealed by Carboniferous formations, and its south-western passes under the sea, its true length is probably considerably more.
If the areas which present evidence of distinct and independent vents are grouped according to their positions on these two lines, they naturally arrange themselves as in the following list:--
I. Northern Chain of Volcanoes
1. The Montrose Centre. 2. The Sidlaw and Ochil Group. 3. The Arran and Cantyre Centre. 4. The Ulster Centres.
II. Southern Chain of Volcanoes
5. The Pentland Volcano. 6. The Biggar Centre. 7. The Duneaton Centre. 8. The Ayrshire Group.
The distribution of these various volcanic areas will be most easily understood from an examination of Map III. accompanying this volume.
I. THE NORTHERN CHAIN OF VOLCANOES IN "LAKE CALEDONIA"
1. The Montrose Centre
Beginning at the north-eastern end of the area, we first encounter a series of volcanic rocks which attain their maximum thickness in Forfarshire around the town of Montrose. The main vents probably lay somewhere to the east of the present coast, under the floor of the North Sea; at least no clear indication of their existence either on the coast or inland has been detected. From Montrose, both to the north-east and south-west, the lavas thin away, becoming intercalated among the sandstones, flagstones and conglomerates, and gradually dying out. The total length of the volcanic belt is about 18 miles, that is nine miles from the central thick mass in a north-easterly and the same distance in a south-westerly direction.[351] The volcanic pile must be several thousand feet thick, but owing to the prolongation of the great Ochil anticline, the lavas roll over and do not allow their base to be seen. The axis of the fold must pass out to sea, through the hollow on which the town of Montrose stands. The volcanic series consists of andesite-sheets with volcanic conglomerates. It contains little ordinary tuff, but the conglomerates no doubt partly represent ejected fragmental material, as well as the waste of exposed lavas. A section across the anticlinal fold from Forfar to Panbride, a little to the south-west of Montrose, would reveal the structure shown in Fig. 67.
[Footnote 351: The south-western part of this area from Arbroath to Johnshaven was mapped for the Geological Survey by the late Mr. H. M. Skae, the north-eastern part by Mr. D. R. Irvine. My account of it is mainly taken from notes made by myself on the ground preliminary to the commencement of the mapping of the Survey.]
In the north-eastern prolongation of the volcanic series from the Montrose centre, successively lower members are exposed along the coast-line. But the lavas are dying out in that direction, and sometimes many hundreds of feet of ordinary sediment intervene between two successive flows. It was in one of these long pauses near the top of the whole pile of lavas that the strata of Canterland were deposited, to which reference has already been made. South-west from Montrose the thick volcanic mass rapidly diminishes, and is prolonged to the end only by three or four bands separated by sandstones and flagstones. It is in these intercalated groups of sedimentary material that the "Forfarshire flags" occur.
Nowhere can the details of the Old Red Sandstone volcanic rocks be more conveniently studied than along the coast-section in this district from the Red Head to Stonehaven. The rocks have not only been cut into vertical cliffs, but along many parts of the shore they have been also laid bare in ground-plan, so that a complete dissection of them is presented to the geologist. At the south end, the top of the volcanic series appears at the bold promontory of the Red Head. There, at the base of the cliffs of red sandstone, the accompanying section may be seen. Beneath the red false-bedded and sometimes pebbly sandstones (e), which form nearly the whole precipice, lies a band of dull purplish ashy conglomerate (d), composed almost wholly of fragments of different andesites, imbedded in a paste of the same comminuted material. Towards the south, this rock rapidly becomes coarser, until it passes into a kind of agglomerate, in which the andesite blocks are sometimes a yard or more in diameter. It includes bands of sandstone, which increase in number and thickness towards the north, and sometimes intervene underneath the conglomerate. The lowest rocks here visible are sheets of andesite or "porphyrite" (a), separated from each other by irregular bright red layers of tufaceous sand and agglomerate. These lavas are dull purplish-grey to green, some of them being tolerably compact, others highly amygdaloidal, with large steam-cavities often drawn out in the direction of flow.
One of the most striking features in the andesites of this coast is the remarkable manner in which they include the veinings of pale green and red sandstone already described (see Figs. 65, 66). Some of the sheets have in cooling cracked into rude polygons. They are likewise traversed by large cavernous spaces and intricate fissures or steam-cavities. Into all these openings the sand has been washed, filling them up and solidifying into well-stratified sandstone, the bedding of which is generally parallel with that of the rocks that enclose it, the dip of the whole series of strata being gently seawards. But a still more intimate mixture of the sand with the lava-sheets is to be remarked where these rocks assume their most slaggy character. In some of them the upper part, to a depth of ten or twelve feet, consists of mere rugged lumps of slag which, while the mass was in motion, were probably in large measure loose, and rolled over each other as they were borne onward. The sand has found its way into all the interstices of these clinker-beds, and now binds the whole mass firmly together. At first sight, these bands might be taken for agglomerates of ejected blocks, and as already suggested, some of the slags may have been thrown out as loose pieces, but a little examination will show that in the main the rough scoriaceous lumps are pieces of the lava underneath. In these instances, also, it is clear that the blocks were in position before the fine sand was sifted into their interspaces, for the pale green sandstone is horizontally stratified through its intricate ramifications among the pile of dark clinkers.
The seaward inclination of the rocks allows the succession of lavas to be seen as the coast is followed westward into Lunan Bay. On the further side of that inlet, after passing over a group of sandstones that underlie the volcanic series of the Red Head, the observer meets with a second and lower succession of lavas which in the five miles northward to Montrose Harbour are admirably exposed both along coast-cliffs and on the beach. They resemble those of the Red Head, being made up of alternations of highly vesicular andesite with more compact varieties, and showing similar sandstone veinings. Here and there, as at Fishtown of Usar, the sea has cut them down into a platform from which the harder parts rise as fantastic half-tide stacks. In some cases, the more durable rock consists of the slaggy upper portions of the flows, and in one case this material stands up as a rude pillar twelve feet high, composed of clinkers firmly cemented with veinings of sandstone. The geologist who wanders over this coast-line is arrested at every turn by the marvellously fresh volcanic aspect of many of the lavas. Their upper parts are so cellular that if the calcite, chalcedony and other infiltrated minerals were removed from their vesicles, they would be transformed into surfaces of mere slag. In one respect would their antiquity still be evident. These slaggy bands are generally a good deal reddened, as if they had been long exposed to oxidation before being covered by the overlying sheets of lava--a feature already cited, as probably indicating the lapse of some considerable interval of time between successive outflows.
Along this coast-section the absence of intercalated tuffs is soon remarked. The volcanic ejections seem to have consisted almost entirely of andesitic lavas, though it is possible that here and there the very slaggy bands between the more solid parts of the sheets may include a little pyroclastic material. The lowest portion of the volcanic group here visible is reached at Montrose Harbour, where, in the flagstones and shales of Ferryden, the late Rev. Hugh Mitchell obtained some of the fossil-fishes of the formation.
A space of more than three miles now intervenes where the rocks are concealed by blown sand and other superficial accumulations. It is through this hollow, as already stated, that the great Ochil anticline runs out to sea. On the north side of the North Esk River, we again come upon the same band of lavas as to the south of Montrose, but with a dip to the north-west. This inclination, however, soon bends round more westerly, and the result of the change is to expose a slowly descending section all the way to the Highland fault at Stonehaven.
A picturesque line of high inland cliff, running northwards beyond St. Cyrus, reveals with great clearness the bedded structure of the andesites. But as one moves northward, owing to the change in the direction of dip, one finally passes out of this volcanic belt and begins gradually to descend into the thick Kincardineshire Old Red Sandstone. The amount of conglomerate exposed along this part of the coast-line probably considerably surpasses in thickness any other conglomerate series in the Lower Old Red Sandstone of Britain. Throughout the enormous depth of sedimentary material, the conglomerates are well-bedded, consisting of a dull green paste, composed in large degree of comminuted andesitic debris, and interstratified with green felspathic sandstones. They are often remarkably coarse, the pebbles sometimes measuring three feet in length. Interposed among them are some ten or twelve bands, probably often single outflows of andesite, sometimes compact and porphyritic, at other times highly amygdaloidal. Such is the succession of rocks for many miles along the shore; and as the inclination varies from a little north of west to west, or even west by south, the observer gradually passes over a thickness of rather more than 2000 feet from the base of the St. Cyrus andesites to Gourdon. In this accumulation of coarse, well water-worn material, with abundant intercalations of finer sandstone and occasional sheets of lava, there is the record of prolonged and powerful denudation with intermittent volcanic activity. Dykes of a quartziferous porphyry cut the conglomerates, and at Gourdon they are pierced by the intrusion of serpentine above referred to.
The proportion of andesite fragments in the conglomerates of this part of the coast varies, but is generally much lower than that of the rocks from the Highlands. Thus at Johnshaven, out of 100 blocks, broken promiscuously from the conglomerate, I found that only 8 per cent were of andesite, while 44 per cent were of quartzite, and the remainder consisted of various quartz-porphyries, granites and schists. It is evident, therefore, that some area of crystalline rocks was subjected to enormous waste, and that its detritus was strewn over the floor of Lake Caledonia, at the same time that from the Montrose volcanic vents many streams of andesitic lava were poured forth.
A vast mass of coarse conglomerate intervenes between Gourdon and Dunnottar, and forms a nearly continuous line of precipices which in some places rise 200 feet above the waves. The bedding is everywhere distinctly marked, so that there is no difficulty in following the succession of the strata, and estimating their thickness. From the last of the lavas at Gourdon to the base of the conglomerates near Stonehaven, there lies an accumulation of conglomerate at least 8000 feet thick. The boulders and pebbles in these deposits are generally well-rounded, and vary up to four feet or more in length. I observed one of quartz-porphyry at Kinneff which measured seven feet long and six feet broad. The proportion of andesite fragments in these conglomerates continues to be small. I ascertained that in the coarsest mass at Kinneff they numbered only 14 per cent; at Todhead Point, a mile and a half to the north, 20 per cent, and at Caterline, three quarters of a mile further in the same direction, 21 per cent.
In the midst of this gigantic accumulation of the very coarsest water-worn detritus, there are still records of contemporaneous volcanic action. Near Kinneff the beautiful andesite, with large tabular crystals of plagioclase, alluded to on p. 274, occurs in the conglomerate.[352] South of Caterline two flows, lying still lower in the system, project into the sea. One of these presents a section of much interest. It shows a central solid portion, jointed into rudely prismatic blocks, with an indefinite platy structure, which gives it a roughly-bedded aspect. Its upper ten or twelve feet are sharply marked off by their slaggy structure, ending upwards in a wavy surface like that of the Vesuvian lava of 1858. Into its fissures, steam-cavities and irregular hollows, fine sand has been washed from above, as at Red Head, while immediately above it comes a coarse conglomerate of the usual character (Fig. 76). Still lower down, beneath some 900 feet of remarkably coarse conglomerate, another group of sheets of andesite abuts at Crawton upon the coast, with which, at a short distance inland, it runs parallel for more than two miles, coming back to the sea at Thornyhive Bay and at Maidenkaim. We have then to pass over about 5000 feet of similar conglomerates, until, after having crossed several intercalated sheets of andesite, we meet with the last and lowest of the whole volcanic series of this region in the form of some bands of porphyrite at the Bellman's Head, Stonehaven. The peculiar geographical conditions that led to the formation of the coarse conglomerates appear to have been established at the same time that the volcanic eruptions began, for as we descend in the long coast section, we find that the coarse sediment and the intercalated lavas cease on the same general horizon. Below that platform lie some 5000 feet of red sandstones and red shales, yet the base of the series is not seen, for the lowest visible strata have been faulted against the schists of the Highlands. It is thus obvious that more than 5000 feet of sediment had been laid down over this part of the floor of Lake Caledonia before the first lavas were here erupted.
[Footnote 352: For an analysis of the felspar in this rock, see Prof. Heddle's paper, Trans. Roy. Soc. Edin. xxviii. (1879), p. 257.]
2. The Sidlaw and Ochil Group
The volcanoes which poured out the masses of material that now form the chain of the Ochil and Sidlaw Hills appear to have been among the most vigorous in the whole region of Lake Caledonia. Their chief vents probably lay towards the south-west in the neighbourhood of Stirling, where the lavas, agglomerates and tuffs discharged from them reach a thickness of not less than 6500 feet, without revealing their bottom. From that centre the lavas range continuously for nearly fifty miles to the north-east, until they reach the sea at Tayport; but they are prolonged on the north side of the Firth of Tay from Broughty Ferry to near Arbroath, so as to overlap those of the Montrose group. They thus attain a total length of nearly sixty miles in a north-easterly line. How far they stretched south-west cannot now be ascertained, for they have been dislocated and buried in that direction under the Carboniferous formations of the Midland Valley.
It will be observed from the map (No. III.) that the great volcanic ridge of the Ochil Hills continues unbroken for twenty-two miles, from Stirling to Bridge of Earn. Thereafter it branches into two divergent portions, one of which runs on through the north of Fife to the southern promontory of the estuary of the Tay, while the other, after sinking below the alluvial plains of the Earn and the Tay, mounts once more into a high ridge near Perth, and thence stretches eastward into Forfarshire as the chain of the Sidlaw Hills. This bifurcation is due to the opening out and denudation of the great anticlinal fold above mentioned. The rocks in the northern limb dip north-westward, those in the southern limb dip south-eastward. The lower members of the Old Red Sandstone, underlying the volcanic series, ought to be seen beneath them along the crest of the anticline. Unfortunately, however, partly by the action of faults along the boundaries of the volcanic bands, but chiefly from the unconformable overspread of Upper Old Red Sandstone and Lower Carboniferous rocks across the plains of the Carse of Gowrie and of the Earn, the lower parts of the system are there concealed (see Fig. 78). As already remarked, this important anticlinal fold runs to the north-east across Forfarshire, and passes out to sea north of Montrose.
Through the Ochil chain the fold runs obliquely in a south-westerly direction, until it is truncated by the great fault which lets down the Clackmannan coal-field. The total traceable length of this anticline is thus about sixty miles. It flattens down towards the south-west; consequently the rocks in the western part of the Ochil Hills are so gently inclined that the same bands may be followed winding round the sides of the valleys, and giving to the steep declivities the terraced contours to which allusion has already been made (see Fig. 68). Another result of this structure is that the base of the volcanic series is entirely concealed by its higher portions.
From an examination of the map it will be further obvious that the whole wide plain of Strathmore--that is the great hollow, more than 80 miles long and about ten or twelve miles broad, which stretches between the base of the Highland mountains and the north-western slopes of the Ochil and Sidlaw chain--is underlain with volcanic rocks of Lower Old Red Sandstone age. This plain lies on a broad synclinal fold, along the south-east side of which the lavas, tuffs and conglomerates of the Ochil and Sidlaw Hills dip under a thick accumulation of red sandstone and flagstone. On the north-west side similar lavas and tuffs rise again to the surface, both on the southern side of the great boundary faults, and also in the little bays which here and there survive on the northern side of the dislocations (Fig. 77). I have already alluded to these interesting relics of the shore-line of Lake Caledonia, and to the fact that though they lie unconformably on the Highland schists, they do not belong to the actual basement members of the Old Red Sandstone (ante, p. 295, and Fig. 73). We have seen that below the bottom of the volcanic series a thickness of 5000 feet of sandstones and shales emerges on the Stonehaven coast, and yet that even there the base of the whole system is not visible, owing to the effect of the Highland boundary fault.
It is thus evident that over the bottom of Lake Caledonia a very thick deposit of tolerably fine sedimentary material was spread before the commencement of the Ochil and Sidlaw eruptions,--that when the lavas were poured out and the coarse conglomerates began to be formed, these materials overlapped the older deposits and gradually encroached upon the subsiding area of the Highlands. The lavas rolled across the floor of the lake and entered the successive bays of the northern coast-line, where their outlying patches may still be seen.
From these facts it is clear that to the actually visible area of volcanic material in the Ochil and Sidlaw region, and to the anticlinal tract whence the andesites have been removed by denudation, we have to add the area that lies under the plain of Strathmore, which may be computed to be at least 800 square miles, making a total of probably not less than 1300 square miles. But it will be remembered that practically only one side of the anticlinal fold is accessible to observation. We cannot tell how far in a southerly direction the lavas of the Ochil Hills may extend. It is quite possible that not a half of the total area covered by the eruptions of this volcanic group is now within reach, either of observation or of well-founded inference.
One further general characteristic of this volcanic district will be obvious from an inspection of the map. While the thickest mass of lavas and tuffs, lying towards the south-west, points to the existence of the most active vents in that part of the area, the actual positions of these vents have not been detected. Probably they lie somewhere to the south of the edge of the Ochil chain, under the tract which is overspread with the coal-field. But other and possibly minor orifices of eruption appear to have risen at irregular intervals towards the north-east along the length of the lake. Thus there are numerous bosses of felsitic and andesitic rocks among the central Ochils, some of which may mark the positions of active vents. For some miles to the east of that area an interval occurs, marked by the presence of only a few small intrusive masses. But as the broad anticline of the Firth of Tay opens out and allows the lower or pre-volcanic members of the Old Red Sandstone to approach the surface, another group of bosses emerges from the lower sandstones and flagstones. Some of these cover a considerable space at the surface, though a portion of their visible area may be due to lateral extravasation from adjacent pipes, the true dimensions of which are thereby obscured. Some of the masses are undoubtedly sills. In the case of Dundee Law we probably see both the pipe and the sill which proceeded from it; the prominent, well-defined hill marking the former, while the band of rock which stretches from it south-westwards to the shore belongs to the latter. The material that forms the bosses and sills in this neighbourhood is generally a dark compact andesite. The rock of Dundee Law was found by Dr. Hatch to show under the microscope "striped lath-shaped felspars abundantly imbedded in a finely granular groundmass, speckled with granules of magnetite, but showing no unaltered ferro-magnesian constituents." Here and there in the same district a solitary neck may be observed filled with agglomerate (Fig. 78).
The variations in the structure of the Ochil and Sidlaw volcanic group will be most easily understood from a series of parallel sections. Beginning on the north-eastern or Sidlaw branch of the volcanic band, we find the arrangement of the rocks to be as is shown in the accompanying figure[353] (Fig. 78). As is usually the case in this region, the base of the volcanic series is here concealed by the fault which brings down the Upper Old Red Sandstone under the alluvial deposits of the Carse of Gowrie. The total thickness of the series in this section is about 2500 feet. The rocks consist of successive sheets of andesite of the familiar types, varying in colour through shades of blue, purple and red, and in texture from a dull compact almost felsitic character to more coarsely crystalline varieties. They are often amygdaloidal, especially in the upper and lower portions of the individual flows. They are not infrequently separated from each other by courses of conglomerate or ashy sandstone and grit. Of these intercalations four are of sufficient thickness and persistence to be mapped, and are shown on the Geological Survey Sheet 48. The stones in the conglomerates vary up to blocks two feet in diameter, and consist chiefly of andesites, but include also some pink felsites and pieces of greenish hardened sandstone. Generally they are more or less well-rounded; but occasionally they become angular like those of volcanic agglomerates.
[Footnote 353: This section and the notes accompanying it have been supplied by Prof. James Geikie, who mapped the western half of the Sidlaw range for the Geological Survey. The eastern half was mapped by the late Mr. H. M. Skae.]
One of the most interesting features in this section is the neck which at Over Durdie rises through the volcanic series. Oval in form, it measures 630 yards in one diameter and 350 in another, and is filled with pinkish granular tuff, full of andesitic lapilli and blocks. A much smaller neck of similar material lies about 100 yards further to the south-west. There seems no reason to doubt that these necks mark two of the volcanic vents belonging to a late part of the volcanic history of the district.
The structure of the Sidlaw range is repeated among the hills of east Fife on the southern side of the great anticlinal fold.[354] Thus a section from near Newburgh on the Firth of Tay southward to near Auchtermuchty in Stratheden gives the arrangement of rocks shown in Fig. 79. In this traverse a thick mass of fragmental material occurs in the higher part of the series of volcanic rocks. Though on the whole stratified and forming a group of conglomerate-beds between the lavas, the material is in places an amorphous agglomerate of volcanic blocks varying in size up to two feet in diameter. These portions show abundant angular and subangular blocks, many of which, after having undergone some attrition, have been finally broken across before reaching their present resting-places. Sharply fractured surfaces can be picked out of the felspathic ashy matrix. The stones are chiefly varieties of andesite, but they include also pink felsites and pieces of some older fine-grained tuff.
[Footnote 354: The eastern part of the Ochils was mapped for the Geological Survey by Mr. H. H. Howell and Mr. B. N. Peach.]
These fragmental materials form a local deposit about nine miles long, and probably not less than 1700 feet thick. They are partly interstratified with flows of andesite. Though, from the rounded forms of some of the pebbles, wave-action may be inferred to have been concerned in their accumulation, they seem to be mainly due to volcanic explosions. No trace, however, has been found of the vent from which the eruptions took place. Not improbably its site lies somewhere to the south in the area now concealed under the Upper Old Red Sandstone and Carboniferous formations. The large size of many of the blocks suggests that they do not lie far from their parent focus of discharge. It is impossible to tell how much of the volcanic series is here concealed by the unconformable overlap of the younger formations.
A section across the centre of the Ochil chain,[355] from Dunning in Strathearn to the Crook of Devon and the Fife Coal-field, gives the structure which is generalized in Fig. 80. At the north end the volcanic series is found to be gradually split up into separate lava-sheets until it dips under the red sandstones of Strathearn. Traced southwards the rocks become entirely volcanic. Some of their most conspicuous and interesting members are pale felsitic tuffs, which occupy a considerable tract of ground about Craig Rossie, south-east of Auchterarder. As the dip gradually lessens the harder lavas are able to spread over wider tracts of ground, capping the hills and ridges, while underneath them thick masses of tuff and conglomerate are laid bare in the valleys. A number of bosses of orthophyre rise through these rocks and are accompanied by many veins and dykes of similar material. It is not improbable that some of these bosses, as already suggested, may represent vents. They are especially prominent among the hills due south of Auchterarder. One of these eminences, known as the Black Maller, is composed of a typical orthoclase-felsite without mica. Another, about four and a half miles further south, forms the conspicuous summit of Ben Shee overlooking Glen Devon, and consists of a similar rock with a characteristic platy structure.
[Footnote 355: The central portion of the Ochils was mapped for the Geological Survey by Mr. B. N. Peach, Prof. James Geikie, Prof. J. Young, Mr. R. L. Jack and myself.]
No necks of agglomerate have been observed in this part of the chain. It will be seen from the section that the lowest visible parts of the Ochil volcanic series are here truncated by a fault which brings in the lower part of the Carboniferous system. By a curious conjuncture, immediately on the south side of this fault, a band of tuff appears, lying on the platform of the Carboniferous "plateau-lavas," to be hereafter considered, and passing below the well-known Hurlet seam of the Carboniferous Limestone, while through these strata rises one of the puys belonging to the second phase of volcanic activity in Carboniferous time in Scotland.
The best sections to show the nature and sequence of the volcanic series of the Ochil Hills are to be observed at the west end of the chain. But as the whole succession of rocks cannot conveniently be obtained along one line, it is better to make several traverses, starting in each case from a known horizon. In this way, by means of three parallel sections, we may obtain the whole series of lavas and tuffs in continuous order. The first line of section starts in the lowest part of the tuffs represented at the bottom of the group in Fig. 80, and runs up to the first thick ashy intercalation among the lavas. Following this bed south-westward to the Burn of Sorrow, we make from that horizon a second traverse across the strike to the summit of King's Seat Hill (2111 feet above the sea), where we meet with a well-marked lava which can be traced south-westwards, gradually descending the southern escarpment of the hills until it reaches the boundary fault near the village of Menstrie. Starting again from this definite horizon, we take a third line across the top of Dumyat (1373 feet) to the plain of Sheriffmuir, and there pass beyond the volcanic series into the overlying red sandstones. Arranged thus in continuous vertical sequence the succession is found to be as represented in Fig. 81. The total thickness of volcanic material amounts to more than 6500 feet.
In this vast pile of volcanic ejections the lavas are almost entirely andesites of the usual characters. They include many slaggy and amygdaloidal varieties, some beautiful porphyries with large tabular felspars, likewise the resinous or glassy variety already referred to as occurring above Airthrey Castle. Their upper and under surfaces show the same structure as already described in those of the coast-sections in the Montrose tract. They include also more acid lavas, like the pale pink decomposing felsites of the Pentland Hills.
The tuffs and conglomerates occur on many platforms throughout the succession of lava-sheets. They form the lowest visible part of the whole volcanic series, but they are most abundant towards the top, and are best displayed at the western end of the hills. In Dumyat they form a conspicuous feature. The whole of that hill consists of a constant alternation of lavas (chiefly slaggy andesites, but including also one felsitic flow) with bands of coarse and finer tuff and volcanic conglomerate. The greatest continuous mass of this fragmental material is 600 or 700 feet thick. From the extraordinary size of its included blocks it obviously must have been formed of ashes, stones and huge pieces of lava ejected from some vent in the near neighbourhood. Some of the individual blocks in this mass are as large as a Highland crofter's cottage.
The uppermost lavas of Dumyat dip under a still higher series of coarse volcanic conglomerates entirely made up of andesitic debris and reaching a thickness of about 1000 feet. This enormous accumulation was probably due partly to the abrasion of exposed cones and lava-ridges, and partly to volcanic discharges of fragmentary materials. Yet it is worthy of note that even amidst these evidences of the most vigorous volcanic activity we have also proofs of quiet sedimentation and traces of the fishes that lived in the waters of the lake. This particular zone of coarse conglomerate as it extends in a south-westerly direction becomes finer, and its upper part passes into a chocolate-coloured sandstone which has been quarried at Wolfe's Hole, Westerton, Bridge of Allan, at a distance of about three miles from where the line of section runs, which is embodied in the diagram, Fig. 81. It was from this locality that the specimens of Eucephalaspis, Pteraspis and Scaphaspis were obtained which were described by Professor Ray Lankester.[356]
[Footnote 356: Palæontographical Society, vols. xxi. (1867) and xxiii. (1869).]
Above the last-named thick group of coarse volcanic conglomerates a solitary sheet of dark slaggy andesite may be observed. This lava is then overlain by the great depth of chocolate-coloured and red sandstones and marls of the plain of Strathmore (c in Fig. 81). Nevertheless a few hundred feet up in these sedimentary deposits we meet with yet one further thin sheet of lava--the last known eruption of the long volcanic history of this district.
Before quitting the Ochil range I may refer to the evidence there obtainable as to the horizontal extent of separate sheets of lava. The western end of this range affords great facilities for following out individual beds of andesite along the bare terraced front of the great escarpment. Thus, the easily recognizable porphyrite which caps King's Seat Hill, above Tillicoultry (see Fig. 68), can be traced winding along the hill-slopes until it descends to the plain, and is then lost under the great fault, at the foot of Dumyat--a distance of more than six miles. There is, therefore, no difficulty in supposing that from the Ochil line of vents streams of lava should have rolled along the floor of the lake across to the base of the Highland slopes, 10 or 12 miles distant. We cannot tell, of course, whether any buried vents lie below the plain of Strathmore, but certainly no unquestionable trace of vents has yet been found among the crystalline rocks along the borders of the Highlands.[357]
[Footnote 357: Allusion has already been made to the possible connection of the younger Highland granites with the volcanic series of the north-eastern part of Lake Caledonia; also to the occurrence of isolated masses of breccia piercing the crystalline schists near Loch Lomond (ante, p. 272).]
Reference has already been made to the comparative scarcity of sills in this region, and to the occurrence of the acid group of Lintrathen porphyry and the more basic sheets between the Firth of Tay and Forfar. This scarcity no doubt arises in part from the extent to which the rocks that underlie the volcanic series are concealed. Yet it is noteworthy that along the coast-section of these rocks near Stonehaven hardly any intrusive sheets are to be seen.
3. The Arran and Cantyre Centre
It is unfortunate that the Ochil chain should be broken across and buried under younger formations at the very place where some of the most interesting vents in the whole area of the Old Red Sandstone might have been looked for.[358] We have to pass westwards across the Firth of Clyde to the Isle of Arran before we again meet with rocks of the same age and character.
[Footnote 358: The Ochil area is not the only example of the abrupt termination of a volcanic band near its centre owing to faults or overlaps. The sudden disappearance of the Pentland lavas and tuffs on the northern side of the Braid Hills is another striking illustration.]
In the course of the recent work of the Geological Survey in that island, Mr. W. Gunn has discovered that the Lower Old Red Sandstone includes some interstratified volcanic rocks on the north side of North Glen Sannox, and he has supplied me with the following notes regarding them. "The area in which the volcanic intercalations occur is much faulted and only a part of it has been mapped in detail, but the position of the interbedded igneous rocks is quite clear. The Old Red Sandstone here consists of three distinct members, the lowest of which is made up of coarse, well-rounded conglomerates, alternating with sandstones and purple mudstones. Above this, and apparently unconformable to it, is a middle series of light coloured conglomerates and sandstones, the pebbles in which are mainly of quartz. Finally comes an upper series of red sandstones and conglomerates, which occupy nearly the whole of the coast section, and it is this series which has generally been taken as the typical Old Red Sandstone of the island. The volcanic series is intercalated between the middle and upper divisions given above, and may be seen in several places on the hillside between the shepherd's house at North Sannox and Laggan. It consists mainly of old lava-beds of a dull reddish or purplish colour, often soft, and in places much decomposed. It seems basic in character. A specimen from near the Fallen Rocks, examined by Mr. Teall, was found to be too much altered for precise determination, but was probably a basalt originally. These rocks do not occur on the coast."
In the southern extremity of Cantyre some important relics of the volcanic rocks of the Lower Old Red Sandstone have been recently detected and mapped for the Geological Survey by Mr. R. G. Symes.[359] This division of the system has been ascertained by him to be extensively developed to the south of Campbeltown, and to include some small but interesting remains of the volcanic action which was so marked a feature in the areas of Lake Caledonia, lying further to the east. To the student of volcanic geology, indeed, this small tract at the extreme southern end of Argyllshire has a peculiar interest, for in no other part of the British Isles have the phenomena of the eruptive vents of the Lower Old Red Sandstone been more admirably laid bare. Not only are there necks in the interior like that represented in Fig. 82; but others have been dissected by the waves along the southern shore, and their relations to the deposits of fragmentary material showered over the bottom of the lake have been more or less clearly exposed.
[Footnote 359: The late Prof. James Nicol published in 1852 an account of the geology of the southern portion of Cantyre. He grouped all the igneous rocks of the district as one series, which he regarded as later than the Coal-formation and possibly of the same age as those of the north-east of Ireland. He made no distinction between the Lower Old Red Sandstone and the younger unconformable conglomerates (Quart. Journ. Geol. Soc. vol. viii. (1852), p. 406).]
At Keil Point, a little to the east of the most southerly headland of the Mull of Cantyre, some reddish and purplish highly felspathic sandstones (a in Fig. 83) dipping towards the east are found to pass upward into coarse volcanic breccias (b), which, followed eastwards, lose almost all trace of stratification, and are then abruptly succeeded by a neck of coarse agglomerate (c) measuring 25 yards from north to south, where its limits can be seen, and at least 12 yards from west to east. It is hardly possible to distinguish between the breccias to the west and the agglomerate of the neck, except by the rude bedding of the former which pass down into the well-bedded sandstones.
The agglomerate is a thoroughly volcanic rock. The materials consist chiefly of angular blocks of a pale purplish or lilac highly porphyritic mica-porphyrite, with large white felspars and hexagonal tables of black mica. These blocks might sometimes be mistaken for slags from their cavernous, weathered surfaces, but this rough aspect is found on examination to be due to the decay of their felspars.
Perhaps the most singular feature among the contents of this neck is the number of well-rounded and smoothed pebbles and boulders of quartzite. These are dispersed at random through the mass, and are often placed on end. There can be no doubt that they are water-worn stones, but the contrast of their smooth surfaces and rounded forms with the rough angular blocks of igneous material is so striking as to lead at once to the conclusion that they cannot have acquired their water-worn character in the deposit where they now lie. Their positions and their occurrence with ejected volcanic blocks suggest that they too were discharged by volcanic explosions. They so exactly resemble the quartzite boulders and pebbles in the neighbouring Old Red Conglomerates that there can be little hesitation in regarding them as derived from these conglomerates. They seem to me to have come from a lower part of the Old Red Sandstone, which was shattered by volcanic energy either before the conglomerates were firmly consolidated or afterwards by such violent explosions as served to separate the pebbles from the matrix of the rock.
There occur also in the agglomerate blocks of fine tuff and ashy sandstone sometimes four feet long, and often stuck on end, showing that the deposits of earlier eruptions were broken up during the drilling of this little vent.
A few hundred yards further east a larger neck rises on the beach, immediately to the south of the old Celtic chapel of St. Columba. It consists also of exceedingly coarse agglomerate, with andesite blocks three and four yards in diameter. It is about 125 yards broad from east to west, on which sides it is seen to be flanked by coarse volcanic breccias and conglomerates, resembling in composition the materials of the neck, but showing an increasingly definite stratification as they are traced eastward in the ascending succession of deposits. Following the section in still the same easterly direction along the coast, we find that bands of fine felspathic sandstone, marking probably intervals of quiescence, are again and again succeeded by coarse brecciated conglomerates of igneous materials, which may be inferred to have been due to a renewal of violent eruptions. By degrees the evidence of stratification and of attrition among the volcanic materials becomes more pronounced as the ascending section is followed; blocks of andesite, even 18 inches or two feet in diameter, assume well-rolled, rounded, water-worn forms, like the pebbles of quartzite associated with them, and eventually the strata return to the usual aspect of the conglomerates of the district.
I have never seen anywhere better proofs of volcanic explosions, contemporaneous with a group of strata, and of the distribution of volcanic fragmentary material round the vents. A further point of much interest is the additional evidence furnished by this shore-section of considerable wave-action during the accumulation of the coarse conglomerates. To give to blocks of porphyrite two feet in diameter a smoothed and rounded form must have required the action of water in considerable agitation.
4. The Ulster Centres
From the volcanic breccias and conglomerates of the Mull of Cantyre to the coast of Antrim in a straight line is a distance of little more than twenty miles. On a clear day the Old Red Sandstone of Cross Slieve, and the range of cliffs in which it abruptly descends to the sea between Cushendall and Cushendun, can be distinctly seen from the Argyllshire shore. The geologist who passes from the Scottish to the Irish sections cannot fail to be impressed with the resemblance of the rocks in the two countries, and with the persistence of the types of conglomerate in Lake Caledonia.
A picturesque section has been laid bare between the Coastguard Station south of Cushendall and Cushendun Bay.[360] At the south side of the little inlet of Cushendall, a compact dull quartz-porphyry is exposed in crags along the shore. This rock ranges in colour from dark brown and purple to pale-green and buff. Its texture also varies, as well as the proportion of its felspar-crystals and quartz-blebs. Some parts have a cavernous structure, like that of an amygdaloid, the small globular cavities being filled with green decomposition products.
[Footnote 360: For descriptions of this district see J. Bryce, Proc. Geol. Soc. i. (1834) p. 396, v. (1837) p. 69; J. Kelly, Proc. Roy. Irish Acad. x. (1868), p. 239. The area is contained in Sheet 14 of the Geological Survey of Ireland, and was mapped by Mr. A. M'Henry and described by him in the accompanying Explanatory Memoir (1886), pp. 12, 25.]
The stratigraphical relations of this rock are not quite clear, but it is certainly older than the Old Red conglomerates which lie to the north of it, for these are largely made up of its fragments. The matrix of these detrital masses consists mainly of the comminuted debris of the porphyry. The pebbles include all the varieties of that rock, and are tolerably well-rounded. There is no distinct evidence of volcanic action among these conglomerates. They resemble, however, many of the conglomerates in the Midland Valley of Scotland, which, as in the case of those on the Forfarshire and Kincardineshire coast, are in great part made of the detritus of andesitic lavas. The Cushendall rocks become coarser as they are traced northwards into lower members of the series, while at the same time the proportion of porphyry-debris in their constitution diminishes, and materials from the metamorphic series take its place. Thus at Cushendun the percentage of quartz-pebbles rises to 70 or 80. These blocks, of all sizes up to two feet or more in diameter, are admirably rounded and smoothed, like those in the Stonehaven section and those among the conglomerates at the south end of Cantyre. Fragments of the porphyry, however, still continue to appear, and the matrix shows an admixture of the finer detritus of that rock. I may remark in passing that no conglomerates of the Old Red Sandstone show more strikingly than these at Cushendun the effects of mechanical crushing subsequent to deposition and consolidation. In many parts of the rock it is hardly possible to find a rounded block that has not been fractured. Some of them, indeed, may be seen cut into half a dozen slices, which have been pushed over each other under the strain of strong lateral or vertical pressure.
In the interior of the country, after passing over the broad Tertiary basaltic plateau of Antrim, we come upon a large area of Lower Old Red Sandstone in Tyrone. It stretches from Pomeroy to Loch Erne, a distance of about 30 miles, and is about 12 miles broad. In lithological character the strata of this tract exactly resemble parts of the deposits of Lake Caledonia in Central Scotland. They include also a volcanic series which, down to the smallest points of detail, may be paralleled in the sister island.[361] This interesting westward prolongation of the volcanic record consists of a number of outlying patches confined to the eastern part of the district.
[Footnote 361: This area of Old Red Sandstone is represented on Sheets 33, 34, 45 and 46 of the Geological Survey of Ireland, and the igneous rocks are described in the Memoirs on Sheets 33 (1886, p. 17) by Mr. J. R. Kilroe, and 34 (1878, p. 16) by Mr. J. Nolan.]
The largest of these patches lies to the south of Pomeroy, where it forms a line of hills about four miles long, and covers an area of some five square miles. The rocks consist of successive sheets of andesite-lavas. These, as a rule, are not markedly cellular, though they include some characteristic amygdaloids. A distinguishing feature of some of the sheets is their remarkably well-developed flow-structure. Thus on Sentry Box, at the north-western end of the ridge, the fissility resulting from this structure so perfectly divides the rock into parallel flags that the material might easily be mistaken for a bedded rock. Where this structure has been produced in a cellular lava, the cavities have been drawn out and flattened in the direction of flow.
I have not observed true tuffs in any of the sections traversed by me in this district. But the conglomerates furnish abundant evidence of the contemporaneous outpouring of the lavas. Thus, in a brook a little west of Reclain, five miles south of Pomeroy, the section shown in Fig. 84 may be seen. At the base lies a coarse conglomerate (a) largely composed of andesite-debris, the stones being here, as elsewhere in the district, well rounded. Then comes a series of green and reddish highly-felspathic sandstones (b), followed by an exceedingly coarse conglomerate (c), formed mainly of the debris of andesites, especially lumps of slag. Some of the stones measure 18 inches in diameter, and all are well water-worn. Immediately over this mass of detritus lies the lowest sheet of andesite-lava (d).
Some sections visible in the neighbourhood of Omagh afford further evidence of volcanic action at the time of the deposition of the Old Red Sandstone of this region. At Farm Hill, a little to the east of the town, felspathic sandstones and breccias enclose angular and subangular pieces of various andesites, and occasionally even pieces of tuff. Near these strata a decayed andesite occurs in the bed of a stream, and a fresher variety is quarried at Farm Hill. A little further south another variety of andesite is exposed in two quarries at Recarson Meeting-House--a fine granular purplish-grey rock, with abundantly-diffused hæmatite pseudomorphs, probably after a pyroxene, and sometimes strongly amygdaloidal.
There can thus be no doubt that this region of Ulster included several centres of volcanic activity during the deposition of the red sandstones and conglomerates, and that the lavas and volcanic conglomerates belonged to precisely the same types as those of the same geological age which occur so abundantly in Scotland.
Further south-west, near Boyle, in the county of Roscommon, certain curious felspathic breccias in the Old Red Sandstone have been mapped as "felstone."[362] So far as I have been able to examine them, however, they are entirely of fragmental origin. They contain pieces of andesitic and felsitic rocks, with fragments of devitrified glass, which undoubtedly point to the occurrence of volcanic eruptions during their deposition, though no tuffs and lavas appear to crop out in the narrow strip of the formation there exposed.
[Footnote 362: See Sheet 66 Geological Survey of Ireland, and Explanation to that sheet (1878), p. 15. The rocks were previously described by Jukes and Foot, Journ. Roy. Geol. Soc. Ireland, vol. i. (1866), p. 249.]
The accompanying section (Fig. 85) may be seen on the hills to the north-east of Boyle. Where quarried on the road-side to the north of Boyle, the series of deposits here represented contains a bed of coarse and exceedingly compact breccia, similar to that just referred to, but containing angular and subangular fragments six or eight inches long. The joints of these compact strata are remarkably sharp and clean cut, so that where the fragmentary character is not very distinct the rocks might easily be mistaken on casual inspection for felsites.