CHAPTER XXX

THE CARBONIFEROUS VOLCANOES OF IRELAND

King's County--The Limerick Basin--The Volcanic Breccias of Doubtful Age in County Cork.

Although the Carboniferous system spreads over by far the larger part of the surface of Ireland, and is laid bare in many thousands of natural and artificial sections, it displays undoubtedly contemporaneous igneous rocks, so far as at present known, at only one locality--the region around Limerick. A second district, however, lies in King's County, where some vents occur which may be of Carboniferous age, and of which a description will be given in the following pages. That the relics of volcanic action should be so few, while the exposures of the Carboniferous formations are so numerous and so completely disclose the geological history of the whole system, must be regarded as good evidence that while volcanoes abounded and continued long active in Scotland and in parts of the Centre and South-west of England, they hardly appeared at all in Ireland. It is worthy of remark, also, that the Irish eruptions belong to the time of the Carboniferous Limestone--a period distinguished by volcanic activity in Scotland and England--that the nature of the materials erupted bears a close resemblance to that of the lavas and tuffs of the sister island, and that the manner of their eruption finds a close counterpart in the Puy-eruptions, already described.

1. KING'S COUNTY

In the progress of the Geological Survey several small tracts of "greenstone ash" and "greenstone" were mapped within an area of a few square miles lying to the north of Philipstown. These igneous rocks were shown to form Croghan Hill, which, rising into a conical eminence 769 feet above the sea, and some 450 feet above the general level of the great limestone plain around it, forms the only conspicuous feature in the landscape for many miles. In the maps and their accompanying Explanations, the "greenstones" are treated as intrusive masses, but the "greenstone ash" or breccia appears to have been regarded as interstratified in the Carboniferous Limestone, though the admission is made that "from the scanty exposures of the rocks and the total absence of any connected section, it has been found impossible to arrive at any definite conclusion as to the relations existing between these traps and ashes with regard to each other or to the surrounding limestone."[70]

[Footnote 70: See Sheets 109 and 110 of the Geological Survey of Ireland and Explanation to accompany Sheets 98, 99, 108 and 109, by F. J. Foote and J. O'Kelly (1865), pp. 7-18.]

In the course of a brief visit to this locality I did not succeed in obtaining any certain proof of the age of the igneous rocks, but I found their structures to be more varied and interesting than would be inferred from the way in which they have been mapped, and I came to the conclusion that the strong balance of probability was in favour of regarding them as of the age of the Carboniferous Limestone.

The first and most important fact to be announced regarding the district is that it includes a group of volcanic necks which rise through the Carboniferous Limestones. The chief of these forms Croghan Hill. It is nearly circular in ground-plan, and measures about 4000 feet in diameter from the limestone on one side to that on the other. It rises with steep grassy slopes out of the plain, the naked rock projecting here and there in crags and low cliffs. Its general outward resemblance to the Carboniferous necks of Scotland strikes the eye of the geologist as he approaches it (Fig. 192).

But Croghan Hill, though the chief, is not the only vent of the district. It forms the centre round which a group of subsidiary vents has been opened. These form smaller and lower eminences, the most distant being one and a half miles E.S.E. from the summit of Croghan Hill, and measuring approximately 1200 feet in its longest and 800 feet in its shortest diameter.

That the igneous materials of these necks really break through the limestones may be clearly seen in several sections. Thus by the roadside at Gorteen, on the south-western side of Croghan Hill, the limestones have been thrown into a highly inclined position, dipping towards the east at 60° or more, and their truncated ends abut against the side of the neck. Again, on the eastern side of the same hill the limestones have been much disturbed close to the margin of the neck, sometimes dipping towards the volcanic centre, and sometimes striking at it. Among these strata a small neck of breccia, of which only a few square yards are visible, rises close to the edge of the bog that covers the adjacent part of the great plain.

The material which chiefly forms these necks is one of the most remarkable breccias anywhere to be found in the volcanic records of the British Isles. The first feature noticeable in it is the pumiceous character of its component fragments. These consist of a pale bluish-grey basic pumice, and are generally about the size of a hazel-nut, but descend to mere microscopic dust, while sometimes exceeding a foot in length. They are angular, subangular and rounded. Occasionally they stand out as hollow shells on weathered surfaces, and in one instance I noted that the vesicles were flattened and drawn out parallel to the surfaces of the shell, as if deformed by gyration, like a true bomb.

The breccia remains singularly uniform in character throughout all the necks. Its basic pumice presents much resemblance to that so characteristic of the Carboniferous necks of Scotland, Derbyshire and the Isle of Man. The abundant vesicles are generally spherical, and as they have been filled with calcite or chlorite, they look like small seeds scattered through a grey paste. Though I broke hundreds of the lapilli, I did not notice among them any volcanic rock other than this pumice. I am not aware of any other neck so homogeneously filled up with one type of pyroclastic material, and certainly there is no other example known in the British Isles of so large and uniform a mass of fragmentary pumice.

Limestone fragments are not uncommon in this breccia. They resemble the strata around the vents. Pieces of the adjacent cherts may also be observed. In one or two cases, the limestone fragments were found by me to have an exceptionally crystalline texture, which may possibly indicate a certain degree of marmarosis, but on the whole there is little trace of alteration.

The fragments of pumice in the breccia are bound together by a cement of calcite. In fact the rock is, so to speak, saturated with calcareous material, which, besides filling up the interstices between the lapilli, has permeated the pumice and filled up such of its vesicles as are not occupied by some chloritic infiltration.

I did not observe unmistakable evidence that any part of the breccia is stratified and intercalated among the limestones, nor any vestige of ashy material in these limestones. But it is possible that traces of such interstratification may occur in the low ground to the north-west of Croghan Hill, which I did not examine.

In only two places did I notice even a semblance of the intercalation of limestone in the breccia. One of these is at Gorteen, where a band of limestone strata a few feet thick is underlain and overlain by breccia. But though the superposition of the layers of finely stratified dark limestone and chert on the breccia is well seen and thoroughly defined, no lapilli or ashy material are to be seen in the limestone. Detached pieces of similar limestone and chert occur in the breccia. The band of stratified rock, if _in situ_, may be a tongue projecting from the wall into the body of the neck, like some instances already cited from Scotland, but more probably it is really a large included mass lying within the vent itself. The breccia here as elsewhere is entirely without any trace of stratification. The second locality occurs at the most easterly neck north of Coole House, where the limestones, rapidly undulating, seem at last to plunge below the breccia, which shows a series of parallel divisional planes suggestive of bedding. But these may be only joint-structures, for there is no stratification of the component materials of the rock.

In the necks, and also through the limestone surrounding them, masses of eruptive rock have been intruded as irregular bosses and veins. The material of these intrusions presents little variety, and, so far as I could note, gives no indication of the successive protrusion of progressively different lava. It varies from a deep blue-black fine-grained basalt to a dolerite where the plagioclase is distinct. Some portions, however, are more basic and pass into limburgite. Externally there is nothing worthy of special remark in these rocks unless it be their prevalent amygdaloidal structure. The amygdales, generally of calcite, vary from small pea-like forms in the basalts up to kernels half an inch long or more in the dolerites. From a microscopic examination Mr. Watts found that some of the basalts have a base of felspar and augite rich in brown mica, and that their porphyritic felspars enclose idiomorphic crystals of augite.

Perhaps the most noticeable feature in these later parts of the volcanic series is the occurrence in them at one locality in Croghan Demesne of lumps of a highly crystalline material quite distinct from the surrounding rock. These enclosures vary from an inch or two to a foot or more in diameter. They must be regarded as blocks which have been carried up in the ascent of the basic lava. Their composition has been ascertained by Mr. Watts from microscopic examination to be somewhat singular. One specimen "contains relics of garnets, surrounded by rings of kelyphite, imbedded in a mosaic of felspar, with a mineral which may possibly be idocrase." Another specimen from the same locality (south-east from Gorteen) "contains the relics of garnets preserved as kelyphite, set in a matrix of quartz-grains, much strained, and containing a profusion of crystals of greenish-yellow or red sillimanite. This appears to be a metamorphic rock, and may be a fragment of some sediment enclosed in the igneous rocks."[71]

[Footnote 71: _Guide to the Collections of Rocks and Fossils belonging to the Geological Survey, in the Museum of Science and Art, Dublin_ (1895), pp. 38, 39.]

As regards the history of volcanic action in Britain one of the chief points of interest connected with these Irish breccias and lavas relates to their geological age. As no proof has been produced that any portion of them is contemporaneously interstratified in the Carboniferous Limestone which surrounds them, we cannot definitely affirm that the volcanic eruptions which they record took place during the accumulation of that formation. The vents must, of course, be later than that portion of the limestone which they pierce. But the evidence seems to me to be on the whole most favourable to the view that they are of Carboniferous Limestone age, for the following reasons:--

1. The breccias of Croghan Hill do not present a resemblance to any of those belonging to the Tertiary volcanic series in Antrim or the Inner Hebrides. The possibility of their being of Tertiary age may therefore be dismissed from consideration.

2. There are no known Permian volcanic rocks in Ireland. Nor does the Croghan Hill breccia show any resemblance to the ordinary material of the breccias in the Permian necks of Scotland. It is thus not likely to be of Permian age.

3. The peculiar basic pumice of these Croghan Hill vents has many points in common with the palagonite fragments so abundant among the volcanic breccias and tuffs of Carboniferous age in Scotland, Derbyshire, and the Isle of Man, and which occurs also among the Carboniferous tuffs of the Limerick basin. It differs from the general type of the material in its pale colour, in its uniformity of character, in its calcareous cement, and above all in its vast preponderance over all the other materials in the breccia.

4. The saturation of the Croghan Hill breccia with calcite is a singular feature in the composition of the rock. Had the vents been opened long subsequent to the deposition of the Carboniferous Limestone, it is difficult to understand how this calcite could have been introduced. Mere percolation of meteoric water from the adjacent limestone does not seem adequate to account for the scale and thoroughness of the permeation. But if the vents were opened on the floor of the Carboniferous Limestone sea, it is intelligible that much fine calcareous silt should have found its way down among the interstices of the breccia and into the pores of the pumice which, being caked together within the vent, did not all float away when the sea gained access to the volcanic funnel. The effect of subsequent percolation would doubtless be to carry the lime into still unfilled crevices, and to impart to the cement a crystalline structure similar to that which has been developed in the ordinary limestones.

2. THE LIMERICK BASIN

About 70 miles to the south-west of the area just described lies the most compact, and, for its size, one of the most varied and complete, of all the Carboniferous volcanic districts of Britain (Map I.). It takes the form of an oval basin in the Carboniferous Limestone series near the town of Limerick, about twelve miles long from east to west and six miles broad from north to south. Round this basin the volcanic rocks extend as a rim about a mile broad. A portion of a second or inner rim, marking a second and higher volcanic group, partially encloses a patch of Millstone Grit or Coal-measures, which lies in the heart of the limestone basin. (See the section in Fig. 196.)

But it is evident that, as the denuded edges of the volcanic sheets emerge at the surface all round the basin, the present area over which these rocks extend must be considerably less than that which they originally covered. Some indication of their greater extension is supplied by outliers of the bedded lavas and tuffs, as well as by bosses which doubtless indicate the position of some of the eruptive vents. The distance between the furthest remaining patches is 24 miles. The original tract over which the volcanic materials were spread cannot have been less than 24 miles long by 10 miles broad. If we assume its area to have been between 250 and 300 square miles we shall probably be under the truth.

This volcanic centre made its appearance on the floor of the Carboniferous Sea in the same district which had witnessed the eruptions of Upper Old Red Sandstone time. The two visible vents that crown the Knockfeerina and Ballinleeny anticlines (Chapter xxii.), are only some ten miles distant, and there may be others of the same age even under the Limerick basin. This district thus supplies another instance of that recurrence of volcanic energy in the same area, after a longer or shorter geological interval, which stands out as a conspicuous feature in the history of volcanic action in Britain. That a prolonged interval elapsed between the extinction of the Old Red Sandstone volcanoes and the outbreak of their successors during the accumulation of the Carboniferous limestone series, may be inferred from the thickness of strata which separate their respective tuffs. From the published sections of the Geological Survey there would appear to be about 500 feet of Old Red Sandstone above the volcanic series of that formation. Then comes the Lower Limestone shale, which is computed to be about the same thickness. From the scarcity of observable dip among the Lower Limestones and their variable inclination, it is not easy to form any satisfactory estimate of the depth of this group up to the base of the volcanic series. It may be as much as 800 feet,[72] and if so there would thus intervene a mass of sedimentary material nearly 2000 feet in thickness between the two volcanic platforms. Throughout this thick accumulation of stratified deposits no trace of contemporaneous volcanic activity has been detected. From the descriptions published more than thirty years ago by Jukes and his colleagues in the Geological Survey of Ireland, geologists learnt how full and interesting are the proofs of great volcanic activity contemporaneous with the deposition of the Carboniferous Limestone series in the Limerick district.[73] Nowhere, indeed, is the evidence more complete for the occurrence of a long succession of volcanic eruptions during a definite period of geological time. The officers of the Survey showed that two epochs of activity during the older part of the Carboniferous period were each marked by a group of tuffs and lavas, while the interval of quiescence between them is represented by a thousand feet of limestone. The same observers likewise mapped outside the volcanic ring a number of eruptive bosses, which they regarded as probably marking some of the actual vents of that time.

[Footnote 72: This is the thickness given in the Explanation to Sheet 144 of the Geological Survey of Ireland, p. 8. A still greater thickness is claimed in Explanation to Sheet 154, p. 8.]

[Footnote 73: See especially Explanations of Sheets 143, 144, 153 and 154, Geol. Surv. Ireland (1860, 1861). The geology of the district had been previously noticed by earlier observers, to whose writings reference is made on p. 26 of the Explanation of Sheet 144. See also Jas. Apjohn, _Journ. Geol. Soc. Dublin_, vol. i. (1832), p. 24; Prof. Hull, _Geol. Mag. for 1874_, p. 205. Jukes (_Student's Manual of Geology_, 2nd edit. 1862, p. 325) gave subsequently an excellent epitome of the volcanic history. The microscopic structure of some of the Limerick volcanic rocks has been described by Mr. Allport, _Quart. Journ. Geol. Soc._ vol. xxx. (1874), p. 552, and by Prof. Hull, _Geol. Mag. for 1873_, p. 153. See also Mr. Watts' account of these rocks in the _Guide to the Collections of Rocks and Fossils_ (Dublin, 1895), p. 93.]

The lower volcanic group, which forms a complete ring round the Upper Limestones of the Limerick basin, is estimated to reach a thickness of 1000 feet in some parts of its course.[74] Its base appears to coincide generally with the upward termination of the Lower Limestone group of this district, though here and there small patches of volcanic rocks in that group have been regarded as interstratified and contemporaneous bands.[75] It consists of a series of lavas and tuffs, the alternations and rapid incoming and dying out of which were well made out by the Geological Survey.

[Footnote 74: Explanation of Sheet 144, p. 27.]

[Footnote 75: Some of them, however, have characters that rather seem to place them with the intrusive materials of the district, and therefore not necessarily earlier than the bedded lavas and tuffs. The boundary line of the volcanic series is not consistently followed along the same horizon on the Survey maps. Thus to the east of Caherconlish, a strip of the Upper Limestone is inserted below the base of the tuffs for a distance of about four miles. Unless a different horizon has been in some places taken for the boundary between the two groups of limestones, it would appear that the eruptions had not extended over the north and north-east of the district until some time after the deposition of the Upper Limestone had begun. The division between the two limestone groups is taken at a set of chert-bands, but as these are not constant it is sometimes difficult to draw a satisfactory line of division.]

_Tuffs._--The base of the volcanic series is generally formed by a band of tuff sometimes as much as 350 feet thick,[76] which may be traced nearly continuously round the basin as well as in detached outliers even as far as Carrigogunnel overlooking the alluvial plain of the Shannon. The manner in which the bottom of this tuff is interstratified with the limestone below it may be instructively examined in many quarries around the town of Limerick. Striking evidence is there supplied that the first eruptions were comparatively feeble and spasmodic, and were separated by intervals of longer and shorter duration, during which the limestone with its fragmentary organisms was deposited, little or no volcanic detritus falling at that time. Yet even in some of the limestones the microscope reveals fine broken needles of felspar, representing doubtless the finest ejected dust.[77]

[Footnote 76: Explanation of Sheet 154, p. 21.]

[Footnote 77: For the details of the microscopic structure of the Limerick volcanic rocks I am mainly indebted to the examination of them made for me by my Survey colleague, Mr. W. W. Watts.]

As an illustration of the way in which the volcanic and organic detritus alternated over the sea-floor, the following section from a quarry in the townland of Loch Gur on the southern side of the basin is here given:[78]--

[Footnote 78: Explanation of Sheet 154, pp. 21, 22.]

Cherty limestone more than 20 feet 0 in. Decomposed green tuff 2 " 6 " Bluish-green, calcareous laminated tuff 4 " 0 " Limestone, slightly ashy 1 " 8 " Green tuff 0 " 2 " Fine-grained decomposed tuff 0 " 4 " Green tuff, obliquely laminated 1 " 7 " Fine laminated tuff 0 " 8 " Green compact tuff 1 " 8 " Obliquely laminated shaly tuff 0 " 10 " Concretionary ashy limestone 1 " 4 " Compact ashy limestone 2 " 0 " Green shaly tuff, much weathered 0 " 5 " Ashy limestone 0 " 7 " Compact green tuff more than 4 " 0 " -------------- 41 feet 9 in.

The tuffs which in the southern part of the basin underlie the less basic lavas differ in some respects from those which further north are associated with the Upper Limestones. They are green, sometimes dull purplish-red, finely granular rocks, made up in large part of andesitic debris. They are full of loose felspar crystals, minute, somewhat rounded and subangular lapilli of andesite or some less basic lava, together with bits of grit and baked shale. Though generally much decomposed, they are sometimes compact enough to be used for building-stone. Under the microscope these tuffs are seen to abound in andesite-lapilli, with a few pieces of felsitic rocks enclosed in an opaque base, through which are scattered broken felspars and occasional vesicular lapilli.

The tuffs around Limerick, interbedded with the Black (Upper) Limestone, are distinguished by a scarcity of andesitic debris, by their persistent dull greenish-grey colour, and more particularly by the abundance of minute lapilli and larger fragments of an epidote-green, finely vesicular, easily sectile basic pumice. Under the microscope much of this material is found to be an altered basic glass of the nature of palagonite. These tuffs are in evident relation with the more basic lavas that accompany them. The manner in which they alternate with the black limestone shows that the conditions for the eruption of this more basic detritus continued to be very similar to those that existed when the andesitic tuffs were ejected. As a good illustration of this feature the accompanying section (Fig. 193) is given from a quarry on the side of the high-road between Limerick and Annacotty. The total depth of strata here represented is about 15 feet. The black limestone at the bottom is a tolerably pure calcareous rock. It is divided into bands by thin partings of a fine greenish calcareous tuff, each marking a brief discharge of ashes from some neighbouring vent. Half-way up the succession of strata, the ashy material rapidly increases until it usurps the place of the limestone, though its calcareous composition shows that the accumulation of calcareous sediment had not been entirely suspended during the eruption of ash.

Among these tuffs I have noticed fragments of fine, dark, flinty felsite, grit and other rocks. The stones are for the most part small, but vary up to blocks occasionally a foot in diameter.

_Lavas._--The lavas occur in numerous sheets, sometimes separated by thin partings or thicker beds of tuff and volcanic conglomerate. On the northern rim of the basin Mr. G. H. Kinahan has described the volcanic series east of Shehan's Cross-roads as composed of six zones of tuff, each bed varying from about 50 to 250 feet in thickness, alternating with as many sheets of lava ranging from 27 to 180 feet in thickness, the total depth of tuff being estimated at nearly 500 feet and that of the lavas at about 800 feet.[79] Some of these tuffs are coarse conglomerates or agglomerates, with blocks of lava occasionally 10 feet long.

[Footnote 79: Explanation of Sheet 144, p. 28.]

Some of the lavas in the lower volcanic group are andesites quite like those of the plateau series in the Carboniferous system of Scotland. Externally they appear as dull reddish-brown or purplish-red compact rocks, with abundant porphyritic felspars scattered through the fine-grained base. They are generally much decomposed, showing on a fresh fracture pseudomorphs of chlorite, hæmatite and calcite after some of the minerals, with abundant hæmatitic staining through the body of the rock. Amygdaloidal structure is commonly developed.

These andesites, when examined microscopically, were found by Mr. Watts to present the characteristic base of minute felspar-laths with magnetite and enstatite, and with porphyritic crystals, often large, of zoned plagioclase, as well as of ilmenite and hæmatite.

But besides the andesites there occur also, and, so far as I have observed, in larger number, sheets of true basalt. This rock is typically black, exceedingly close-grained in the central portion of each sheet, but becoming highly slaggy and vesicular along the upper and lower parts. Under the microscope it is found to contain granular augite and magnetite, set in a more or less devitrified glass, with microlites of felspar, porphyritic plagioclase, serpentinized olivine, and some well-marked augite. These rocks form distinct escarpments along the northern rim of the basin as in the foregoing section east from Shehan's Cross-roads (Fig. 194). From the summit of this ridge, which is about 600 feet above the sea, the eye looks northward over the plain, across which low outliers of the volcanic series are scattered, and southwards across the basin to the corresponding line of volcanic heights forming the southern rim.

The upper volcanic group has been estimated by the officers of the Geological Survey to lie about 1000 feet higher in the Carboniferous system than the lower, the intervening strata consisting of the Upper Limestone.[80] It is possible that the interval is greater in some parts of the district than in others, and if so, the difference may be due either to greater local accumulation of volcanic materials, or to local prolongation of the eruptions into higher stratigraphical horizons. The outcrop of the upper volcanic band forms about half of a ring round the little cup of Millstone Grit or Coal-measures which lies within the volcanic basin. On the north-west side of the cup the volcanic rocks disappear. Hence the upper band has a much more restricted area than the lower. But if the tuffs immediately around Limerick are assigned to the upper group, its extent will be proportionately increased. There can be little doubt, however, that neither in thickness nor in superficial area did the lavas and tuffs of the second group equal those of the first. The volcanic energy was gradually dying out.

[Footnote 80: Explanation of Sheet 154, p. 24.]

The lavas of the second period are characteristic dull, black, compact basalts, like those of the first period, becoming here and there strongly amygdaloidal, and being occasionally separated by slaggy or conglomeratic partings. But they include also certain rocks wherein the felspar diminishes in quantity, while augite and olivine become conspicuous, together with a little enstatite. The augite occurs in large porphyritic forms, as well as of medium size and in small prisms. The olivine, as usual, is now in the condition of serpentine. These rocks are more basic than the ordinary basalts, containing only 38·66 per cent of silica, and thus approaching the limburgites. With these basic lavas are associated dull green tuffs and conglomerates, made up largely of basalt-debris, together with abundant pieces of finely vesicular basic pumice and lapilli of a palagonitic material.

The manner in which the lavas and tuffs have alternated with each other, and also with the limestones, is well seen on Nicker Hill above Pallas Grean.[81] The Survey sections show eight sheets of lava, separated by six bands of tuff and eight intercalations of limestone, the whole passing under the Coal-measures.

[Footnote 81: See Explanation of Sheet 144, p. 30, where a description with detailed map and sections of this ground will be found.]

The upper volcanic group may be as much as 600 or 800 feet thick. It appears to have been left, at the close of the eruptions, with a very uneven surface, some portions being so low as to be overspread with the Upper Limestones, other parts so high as not to be covered until the Coal-measure shales and flagstones came to be deposited.[82]

[Footnote 82: Explanation of Sheet 154, pp. 24, 35.]

_Vents._--All round the edges of the Limerick basin, where the escarpments of the volcanic groups, rising abruptly above the plain, show that these rocks once extended beyond their present limits, the progress of denudation has revealed a number of bosses which, as above stated, Jukes and his associates looked upon as marking some of the vents from which the lavas and tuffs were erupted. Especially striking is the line of these vents along the southern margin. The rocks now filling them present some unusual and rather anomalous features. They are decidedly more acid than the lavas of the basin, some of them even containing free quartz. Mr. Watts remarks that "though they have a good deal in common with the trachytes, they are crystalline throughout. They are red granite-looking rocks, which are made up chiefly of stumpy idiomorphic prisms of felspar which is mainly orthoclase. Some plagioclase also occurs, and the two felspars are imbedded in interstitial quartz. A trace of hornblende or mica is frequently present, and the rocks contain about 65 per cent of silica." These characters are specially observable in the necks furthest removed from the basin, which may possibly have been connected with the andesitic outflows. Nearer to the basin the necks "contain about 60 per cent of silica, seldom show any interstitial quartz, and stand between trachytes and porphyrites, some perhaps being bostonites."[83]

[Footnote 83: _Guide to the Collections of Rocks, etc., Geol. Survey, Ireland_, p. 93, Dublin 1895.]

A geologist, familiar with the Carboniferous and Permian necks of Scotland, has no hesitation in confirming the surmise of Jukes and his colleagues that the cones and domes around the Limerick basin mark the sites of eruptive vents. On the south side of the basin, at least nine such necks rise into view, partly from among the lavas and tuffs, but chiefly through the limestones that emerge from below these volcanic sheets. One of the most conspicuous of them, Derk Hill (Fig. 195), rises to a height of 781 feet above the sea, and comes through the bedded andesites, as represented in Fig. 196, which gives, in diagrammatic form, the general structure of the Limerick volcanic basin. Around the northern side of the basin a smaller number of necks has been observed, consisting of similar acid rocks.

A few of the necks appear to be filled with volcanic agglomerate. Here and there detached patches of fragmental volcanic material have been shown on the Survey maps, and referred to in the Explanations, as if they were outliers of the bedded tuffs; though in some cases the coarseness of their materials and the want of any distinct bedding, together with the absence of any indication of their relation to the nearest limestones, have evidently offered considerable difficulty in their mapping. One of the best examples occurs about two miles to the south-east of the village of Oola. The boundaries of this patch, as put on the map, are confessed to be "entirely speculative." It was only seen on the side of the railway where it appeared as "a very coarse brecciated purple ash."[84]

[Footnote 84: Explanation of Sheet 154, p. 25.]

On comparing the maps of the Limerick basin with those of the Carboniferous districts of Scotland, the main difference will probably be acknowledged to be the absence of any recognizable sills in the Irish ground. That no sills actually occur, I am not prepared to affirm. Indeed some of the more acid rocks, both outside the basin and among the rocks of the older volcanic group, appeared to me during my traverses of the ground to have much of the character of sills. A more critical examination of the area would not improbably detect some truly intrusive sheets which have hitherto been mapped among the interstratified lavas. Some appear to exist among the surrounding Lower Limestones.

An intrusive mass, like a sill or dyke, is represented on the Geological Survey Map as traversing the Coal-measures in the inner basin south of Ballybrood. But as the strata are on end along its southern margin, it may possibly be only a portion of the upper volcanic series which has been thrown into its present position by one or more faults.[85]

[Footnote 85: Sheet 154 and Explanation to the same, p. 24.]

3. THE VOLCANIC BRECCIAS OF DOUBTFUL AGE IN COUNTY CORK

In the south-western headlands of Ireland, from Bear Island to Dursey Island, various igneous rocks have been traced on the maps of the Geological Survey. They have been described as consisting of "greenstone," "felstone," and "ash" or "breccia," and as including both interstratified and intrusive masses.[86] If contemporaneous with the strata in which they occur, they would prove the existence of a group of volcanic rocks in the Carboniferous slate, or lowest division of the Carboniferous system. After an examination of the coast-line I came to the conclusion that while there is undoubtedly evidence of former volcanic activity in this part of Ireland, no proof has been obtained that the eruptions occurred in the Carboniferous period. The felsites and dolerites appeared to me to be all intrusive, the former having certainly been injected before the terrestrial movements that have disturbed the rocks, for some of them share very markedly in the cleavage of the region. The dolerites and diabases, on the other hand, so far as I observed, are not cleaved, and are thus probably of later date.

[Footnote 86: See Sheets 197 and 198 of the Geological Survey of Ireland, and the Explanation of these Sheets by Messrs. Jukes, Kinahan, Wilson, and O'Kelly, 1860.]

The most interesting rocks are undoubtedly the "ash" and "breccia," for they are obviously of volcanic as distinguished from plutonic origin. On the coast north of White Bull Head, a bed of volcanic breccia may be seen made up of rounded and angular fragments of different sandstones, shales and limestones, with pieces of felsite and andesite wrapped up in a dull-grey fine-grained sandy felspathic matrix. The rock weathers with a rough or rugged surface, owing to the dropping out of the more decomposable stones. This bed, about five feet thick, runs with the bedding of the strata around it, and like these dips S.S.W. at an angle of 70°. If no other evidence were obtainable, this breccia would be naturally set down as a truly interstratified deposit of volcanic detritus. A short distance from it, a second, rather thicker band of similar material occurs, specially distinguished by its abundant worn crystals of hornblende, sometimes three inches in diameter, as well as large crystals of muscovite. These minerals are not unknown elsewhere in volcanic agglomerates. The occurrence of lumps of augite in the vents of Upper Old Red Sandstone age in Caithness has been already alluded to, and a still larger series of ejected minerals will be shown in a later chapter to characterize the younger necks of Central Scotland.

In parts of its course, this second band appears to run so perfectly parallel with the bedding of the strata between which it lies that the observer would readily believe it to be a part of the same series of deposits, and might therefore regard it as affording good evidence of volcanic action contemporaneous with the formation of these deposits. A transverse section of the bed, where thus apparently conformable, is shown in Fig. 197.

Further examination, however,reveals that this seemingly regular sequence is entirely deceptive. At various points the breccia abruptly truncates the sandstones, and involves large pieces of them, as shown in Fig. 198 A. At other places, the lower side of the breccia, or what would be its base if it were a regular bed, cuts out the strata and sends veins into them (B). And the same structure is visible, on its upper side, or what would be its top (C).

It is clear that these highly-inclined bands of breccia are not contemporaneous with the deposition of the Carboniferous Slate, but have been introduced into their position at some time subsequent not only to the deposition, but to the disturbance and elevation of the strata. The peninsula of White Bull Head is crossed by several other similar bands. On Black Bull Head, also, together with abundant felsitic and doleritic intrusions, a similar breccia or agglomerate is to be seen. In some parts it is compact in texture with spheroidal flinty lumps, and weathers somewhat like a nodular felsite. This variety ends off rather abruptly to the north, but swells out southward, and then runs out into a high, narrow headland, in which it contains asbestos, as well as rounded crystals of hornblende. It has here disrupted the shales and sandstones, and near the junction is largely composed of fragments of them, the strata themselves being jumbled, bent, and broken up.

The only semblance of a neck-like mass of this volcanic fragmental material occurs on White Bull Head, where one of the bands expands about the centre of the ridge, and is there full of large blocks of grey sandstone. The breccia appears to have filled fissures which have been opened between the bedding planes of the highly tilted strata, giving rise to long narrow dyke-like intercalations. We have seen that among the Carboniferous volcanic phenomena such dyke-like masses of agglomerate occasionally present themselves in the vents both of the plateaux and the puys.

In one or two places I noticed what may be traces of cleavage in the breccia. The rock is not one that would yield easily to the rearrangements required for the production of this structure, and the doubtful cleavage may be deceptive. If we are justified in regarding the introduction of this volcanic material as having necessarily taken place after the tilting of the strata, we may not unreasonably infer further that the eruptions could only have been effected at no great distance from the surface. But the Carboniferous Slate in which these agglomerates lie is the lowest member of the Carboniferous system. As there is no known unconformability throughout this system in the south of Ireland, the whole of the rest of the pile of Carboniferous strata, amounting to a depth of several thousand feet, once probably extended over this region. It must, therefore, have been not only after the plication, but after extensive denudation of the formations that the fissures were filled with agglomerate. These geological changes no doubt occupied a vast period of time. While, therefore, no positive evidence has yet been gathered to fix the age of these volcanic eruptions of the south-west of Ireland, it is tolerably clear that they cannot be assigned to the Carboniferous period, but must belong to some later volcanic epoch. They may be of Permian age, perhaps even as late as the Tertiary volcanic series.