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.)