In the south of Ireland the Upper Silurian strata are followed upwards conformably by the great series of red sandstones and conglomerates known as the "Dingle Beds." Lithologically these rocks present the closest resemblance to the Lower Old Red Sandstone of Central Scotland. They occupy a similar stratigraphical position, and though they have not yielded any palæontological data for comparison, there can, I think, be no hesitation in classing them with the Scottish Lower Old Red Sandstone, and regarding them as having been deposited under similar geographical conditions. They offer one feature of special interest for the purpose of the present inquiry, since they contain a well-marked group of contemporaneous volcanic rocks, including nodular felsites, like those so characteristic of the Silurian period.
The area where this remote and isolated volcanic group is best developed forms a range of high rugged ground along the northern front of the hills that stretch eastward from the Lakes of Killarney. Their general distribution is shown on Sheets 184 and 185 of the Geological Survey of Ireland;[390] though I may again remark that petrography has made great strides during the thirty years and more that have passed since these maps and their accompanying Memoirs were published, and that, were the district to be surveyed now, probably a considerable tract of ground coloured as ash would be marked as felsite. At the same time the existence of both these rocks here cannot be gainsaid.
[390] See the Memoir (by J. B. Jukes and G. V. Du Noyer) on Sheet 184, p. 15. Other volcanic rocks have been mapped at Valentia Harbour in the Dingle Beds, but these I have not had an opportunity of personally examining.
The felsite was long ago brought into notice by Dr. Haughton, who published an analysis of it.[391] It is also referred to by Mr. Teall for its spherulitic structure.[392] Seen on the ground it appears as a pale greenish-grey close-grained rock, sometimes exhibiting flow-structure in a remarkably clear manner, the laminæ of devitrification following each other in wavy lines, sometimes twisted and delicately puckered or frilled, as in some schists. Portions of the rock are strongly nodular, the nodules varying in size from less than a pea to that of a hen's egg.
[391] Trans. Roy. Irish Acad. vol. xxiii. (1859), p. 615.
[392] British Petrography, p. 349.
The close resemblance of this rock to many of the Lower Silurian nodular felsites of Wales cannot but strike the geologist. It presents analogies also to the Upper Silurian felsites of Dingle. But its chief interest arises from the geological horizon on which it occurs. Lying in the so-called "Dingle-Beds," which may be regarded as the equivalents of the Lower Old Red Sandstone of England and Scotland, it is, so far as my observations go, the only example of such a nodular felsite of later date than the Silurian period. We recognize in it a survival, as it were, of the peculiar Silurian type of acid lava, the last preceding eruption of which took place not many miles to the west, in the Dingle promontory. But elsewhere this type does not appear to have survived the end of the Silurian period.
The detrital rocks accompanying the felsite, in the district east of Killarney, vary from such closed-grained felsitic material as cannot readily be distinguished from the felsite itself to unmistakable felsitic breccias. Even in the finest parts of them, occasional rounded quartz-pebbles may be detected, while here and there a reddish shaly band, or a layer of fine pebbly conglomerate with quartz-pebbles an inch in length, shows at once the bedding and the dip. Mr. W. W. Watts, who, with Mr. A. M'Henry of the Irish Staff of the Geological Survey, accompanied me over this ground, found that a microscopic examination of the slides which were prepared from the specimens we collected completely confirmed the conclusions reached from inspection of the rocks in the field.[393] He detected among the angular grains slightly damaged crystals of felspar, chiefly orthoclase. Many portions of these felspathic grits much resemble the detrital Cambrian rocks which in the Vale of Llanberis have been made out of the pale felsite of that locality.
[393] Mr. Watts also examined the microscopic structure of the felsite of Benaun More. He found that the spherulites appear to have a micropegmatitic structure, owing to the intergrowth of quartz and felspar. In some parts of the rock the spherulites, from ·02 to ·01 inch in diameter, are surrounded by exceedingly minute green needles, possibly of hornblende, while inside some of them are small quartz-grains. Larger porphyritic felspars occur outside the spherulites, some being of plagioclase, but most of orthoclase. The spherulitic structure is not so well developed near the felspars. A few of the large nodules are hollow and lined with crystals, while some of them show a finely concentric lamination like the successive layers of an agate.