CHAP. XXX 
THE LIMERICK BASIN 
45 
bouring vent. Half-way up the succession of strata, the ashy material 
rapidly increases until it usurps the place of the limestone, though its cal- 
careous composition shows that the accumulation of calcareous sediment 
had not been entirely suspended during the eruption ot ash. 
Among these tuffs I have noticed fragments of fine, dark, flinty felsite, 
grit and other rocks. The stones are for the most part small, but vary up 
to blocks occasionally a foot in diameter. 
Lavas . — The lavas occur in numerous sheets, sometimes separated b) tlim 
partings or thicker beds of tuff and volcanic conglomerate. On the northern 
rim of the basin Mr. 0. H. Kinahan has described the volcanic series east 
of Shehan’s Cross-roads as composed of six zones of tuft, eacli bed \aiying^ 
from about 50 to 250 feet in thickness, alternating with as many sheets ot 
lava ranging from 27 to 180 feet in thickness, the total depth of tuff being 
estimated at nearly 500 feet and that of the lavas at about 80 0 feet. 
Some of these tuffs are coarse conglomerates or agglomerates, with blocks ot 
lava occasionally 10 feet long. 
Some of the lavas in the lower volcanic group are andesites quite like 
those of the plateau series in the Carboniferous system of Scotland. 
Externally they appear as dull reddish-brown or purplish-red compact rocks, 
with abundant porphyritic felspars scattered through the fine-grained base. 
They are generally much decomposed, showing on a fresh fracture pseudo- 
niorphs of chlorite, haematite and calcite after some oi the minerals with 
abundant lnematitic staining through the body of the rock. Amygdaloidal 
structure is commonly developed. , , . , 
These andesites, when examined microscopically, were found by Mr. 
Watts to present the characteristic base of minute felspar-laths with 
magnetite and enstatite, and with porphyritic crystals, often large, ot zonei 
plagioclase, as well as of ilmenite and hiematite. 
But besides the andesites there occur also, and, so far as I have observed, 
ill larger number, sheets of true basalt. This rock is typica y iacv. 
Fig. 194. Section of tlie volcanic escarpment, east of Slieliau’s Cross-roads, sontli of Limerick. 
1. Limestone ; 2 2. Tuffs ; 3 3. Lavas. 
exceedingly close-grained in the central portion ol each sheet, hut becoming 
highly slaggy and vesicular along the upper and lower parts. Under the 
microscope it is found to contain granular augite and magnetite, set in 
a more or less devitrified glass, with microlites of felspar, porphyritic 
plagioclase, serpentinized olivine, and some well-marked augite. These 
rocks form distinct escarpments along the northern rim of the basin as in 
1 Explanation of Sheet 144, p. 28. 
