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]
[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.
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]).