CHAP. XIV 
LLANDEILO AND BALA ERUPTIONS OF IRELAND 
253 
volcanic rocks.^ Near Shaiigort 1 noticed in one of these breccias one block 
measuring 12 feet, another 20 feet in length and 3 or 4 feet thick, composed 
of alternating bands of grit and slate. It is interesting to note that these 
strata had abeady undergone cleavage before disruption, the bands ol slate 
being strongly cleaved obliquely to the bedding. None of the Llandeilo or 
other rocks in the neighbourliood display this structure. The blocks seem 
to have been derived from some deeper group of strata. They are laid 
down parallel with the rude bedding of the breccia in which tliey lie. 
The fine tuffs and thin asliy limestones associated with the thicker 
band of limestone show the renewal of volcanic explosions after the “iterva 
marked by the calcareous deposit which is sometimes 30 or 40 leet thick. 
Ill many places this limestone is brecciated and much mingled with volcanic 
dust and lapilli. At Sliangort, for example, the thick tolerably pure 
limestone is truncated on the west and north sides by a coarse agglomerate 
probably filling a volcanic vent. A few hundred yards further north, 
beyond the interrupting agglomerate, the limestone reappears on the same 
line of strike, but is then found to be nodular and brecciated and much 
mingled with volcanic detritus. It lies among ashy grits and tuffs. 
The general structure of the ground occupied by the Lough Mask volcanic 
rocks is 'diagrammatically represented in Tig. 64. The thickness of the 
Fig 64.-Diagi-aui of the general relations of the diftereiit groups of ro^ in the Lower Silurian volcanic 
flistrict along the western shore of Lough Alask. 
<!, Llanihiilo shales, cherts anil silts ; u,fe'^a,la‘th1nhiusls of asliy ^ i 
wards from these to overlie an ohler senes of schists ; , I'uult. 
volcanic series must amount to many hundred feet, but it lias not been 
precisely determined. The uppermost parts of the series pass under a great 
thickness of coarse conglomerates and pebbly grits winch lorm the ridge 
of Formnamore, and stretch thence westwards along Killary Harhoiir and 
throimh the Mweelrea mountains. These strata are classed as the Upper 
Silurian on the Geological Survey map. Since, however, they coniormahly 
•overlie rocks containing Bala fossils, and in the Killary district include 
m-een shales which have yielded fossils of the same age, they doubtless 
Lloim in lanm part to the Lower Silurian division. The remarkable 
coarseness of "these , conglomerates towards the south, and tlieir rapid 
passage into much finer grits and shales towards the noith, probaiy 
1 In re-examining this region, Mr. Kilroe has fouinl in the stream west of the monastery, 
Tourmakeady, an uprise of graptolitic black shale containing foims belonging to the very lowest 
Llandeilo or Upper Arenig strata, and a similar hand above Lcenane, Killary Harbour. 
