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 Balscaddan Bay, on the E. side 

 of How tli Harbour, there is a coarse, breccia ted 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 formation 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 S.W., along the crest of an anticlinal 

 fold, the whole length of the narrow exposure being seven miles. 

 It contains a small characteristic assemblage of fossils. Its whole 

 thickness does not exceed 200 feet . 



