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.
[Illustration: Fig. 192.--Croghan Hill, King's County, from S.S.W.]
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 (