chap. XXX 
THE LIMERICK BASIN 
4i 
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 111 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 
°f 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 tu s 
of the Limerick basin. It differs from the general type of the material m 
its pale colour, in its uniformity of character, in its calcareous cement, and 
above all in its vast preponderance over all the other materials 111 ie 
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. Meie 1 
lation 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 i s intelligible that much fine calcareous silt should have found 1 a 
w ay down among the interstices of the breccia and into the pores o ie 
pumice which, being caked together within the vent, did not all float away 
when the sea gained access to the volcanic funnel. The effect ot subse- 
quent percolation would doubtless be to carry the lime into still unfilled 
devices, and to impart to the cement a crystalline structure similar 0 
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 111 I ig- • ■) 
