On the Physical Geology of the Neighbourhood of Dublin. 147 
~ The largest exposure of these rocks is at Portrane and Dona- 
bate, where an upheaval has enabled the subsequent denudation 
to lay them bare between the Silurian and the Carboniferous 
Limestone. The whole thickness of these beds in this place does 
not exceed 350 feet. 
There is another outcrop of these strata near Lyons, 14 miles 
W.S.W. of Dublin, where they have been exposed by the removal 
of the Limestone, for a very small length and width. The 
remnant patches of these beds to be seen lying on the upturned 
edges of the Lower Silurian rocks on Shenick’s Island (Skerries) 
and on Lambay Island have been already mentioned. 
Along the base of the cliffs at Balseaddan Bay, on the E. side 
of Howth Harbour, there is a coarse, brecciated conglomerate of 
quartz-rock materials, red and yellow stained, which, from its 
proximity to the Lower Carboniferous Limestone, doubtless 
belongs to these underlying beds. 
Just outside the railway at Blackrock Station there is the 
still exposed part of a rock, the rest of which has been covered 
by the railway embankment. It consists of a remarkable firmly 
compacted pure granite breccia. The granite from which it was 
formed and the remains of the limestone which must have 
covered it are both visible close by in the People’s Park. But as 
the latter is Upper Limestone, this breccia cannot belong to, nor 
lie beneath, the base of the Carboniferous formation which could 
be overlapped only by Lower Limestone. 
CARBONIFEROUS LimesTONE.—This forniation is one of the 
salient features of the geology of Ireland. It extends continuously 
from the E. to the W. coast of our island, occupying the greater part 
of the central plain and its ramifications. Its greatest thickness is 
from 2,500 to 3,000 feet. From want of suitable continuous sections 
its thickness near Dublin cannot be determined, although the 
bottom of the formation occurs at Donabate, and the top of it 
only three miles northward of that, a little beyond Rush, 
Lower Limestone Shale——This consists of dark shales and 
thin flaggy limestones. It surrounds the exposure of Old Red 
Sandstone or of basal Carboniferous conglomerate at Donabate 
and runs thence towards the 8.W., along the crest of an anticlinal 
fold, the whole length of the narrow exposure being seven miles. 
Tt contains a gal characteristic assemblage of fossils. Its whole 
thickness does not exceed 200 feet. 
