Swanston— Silurian Rocks of Co. Down. 123 
they are certainly subsequent to the deposition of the Silurians, which have 
been forced up and highly metamorphosed by them. The north-western 
boundary of the Silurian area has been subjected to the greatest amount of 
upheaval, and as denudation has reduced all to a somewhat uniform elevation, 
the lowest beds are here consequently most exposed. The Ballygrot beds form 
the axis of what was the most north-westerly anticlinal, the black shales of 
Crawfordsburn, Camalea, and Orlock Point possibly represent waves of minor 
importance; while that at Coalpit Bay seems to have been one of considerable 
elevation. 
Southward of this no exposures of the underlying shales are seen in Down ; 
on the contrary, they and the overlying grits and conglomerates sink into a 
trough and are covered by higher beds at Portaferry in which the dark 
fossiliferous shales of Tieveshilly occur. Still further south, near Ardee, m 
County Louth, the underlying beds again come to the surface, the black 
Graptolite shales of the Hartfell series being well represented, in association 
with arenaceous bands containing other fossils of Bala or Caradoc age. 
The foregoing exposition of the Silurian rocks of the County Down has 
been very much facilitated by the aid so kindly afforded by Charles Lapworth, 
Esq., F.G.S., of St. Andrews, who undertook the revision of the Graptolithina 
of our rocks ; to his acuteness and critical judgment it is owing that our list 
of species is so extensive, many of the forms would otherwise have escaped 
detection. The monograph, with figures of the Graptolites of the County Down, 
which accompanies this paper is the most complete hitherto published in 
Britain, and I cheerfully acknowledge the obligations under which Irish 
geologists are placed for this synopsis of a most difficult group of fossils. 
Through the kindness of William Bullock, Esq., of Donaghadee, I have 
had access to the fine collection of Coalpit Bay fossils, made by his father 
the late William Bullock, Esq. In addition to enriching my collection with a 
number of good specimens, Mr. Bullock has also placed a valuable series 
from the above locality in the Belfast Museum. The set has been completed, 
as far as possible, by a selection from my own collection, embracing forms 
representing the various beds, and including several type specimens, all of 
which will be available for future reference. 
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