CHAPTER 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.
[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.