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 dolomitized, so that the bedding sometimes becomes 
obliterated. Examples occur in Howth harbour, and one mile W. 
thereof close by the railway, and one mile 8. of this latter place, 
also near Milltown bridge and on the shore 8.E. of Malahide, 
and near Loughshinny. Anthracite, probably of animal origin, 
or perhaps derived from marine algz, 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 <éng 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 8. of Portrane. In the ravine of 
the Delvin river, near the Naul,and on the shore, both near Rush, 
and near Baldungan, two anda 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 bes. 
It seems most probable that the Calp, or dark earthy limestone 
so prevalent around Dubiin, 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 8S. of Ireland. There is a strong 
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 
