On the Physical Geology of the Neighbourhood of Dublin . 149 



quarry-pit near Carlow, whence stone was being taken to build 

 a new church, some years ago, the workmen, to their great incon- 

 venience, came down upon the granite. 



Sometimes masses of the Limestone rock have been more or 

 less highly dolomiuzed, so that the bedding sometimes becomes 

 obliterated. Examples occur in Howth harbour, and one mile "VV. 

 thereof close by the railway, and one mile S. of this latter place, 

 also near Mill town bridge and on the shore S.E. of Malahide, 

 and near Loughshinny. Anthracite, probably of animal origin, 

 or perhaps derived from marine algae, has been found in the Lime- 

 stone at Castleknock ; and sometimes bed faces are covered with 

 a thin film of black carbonaceous matter. 



The Limestone, occasionally, contains large and small frag- 

 ments of granite, both rounded and angular, sometimes associated 

 with granite sand, as also slabs of mica schist, as near Crumlin 

 and at Milltown. These extraneous masses are sometimes quite 

 isolated in the limestone matrix ; as though they had been carried 

 out from the land into comparatively deep water in the Carbo- 

 niferous sea by some unusual means of transport, as, for instance, 

 by being floated by plants to whose roots they may have been 

 attached. Fragments of (unmetamorphosed) Silurian material 

 are found in some of the Limestone beds at Kilsallaghan and 

 Lispopple, eight miles W. by S. of Portrane. In the ravine of 

 the Delvin river, near the Naul, and on the shore, both near Rush, 

 and near Baldungan, two and a half miles northward thereof, there 

 are beds of conglomerate of which the blocks, pebbles, and frag- 

 ments, both rounded and angular, and layers of sand are of 

 Silurian origin ; these were evidently shore bels. 



It seems most probable that the Calp, or dark earthy limestone 

 so prevalent around Dublin, was largely formed of fine mud 

 derived from the black Lower Silurian slates. The small 

 proportion of lime in the Calp may account for the fact that 

 there are, apparently, no underground streams in this vicinity. 

 The percolating water, not being able to dissolve the Calp, could 

 not make subterranean passages, as it has done to such an extent 

 in the Limestone of the W. and S. of Ireland. There is a strono- 

 spring of fresh water rising through the sea water in Howth 

 Harbour. This comes doubtless from a subterraneous passage in 

 the Lower Limestone of that locality, which is pure, excepting the 

 very local dolomitization near the spot. 



M 



