n8 Swanston— Silurian Rocks of Co. Down. 
As before stated, the section at Coalpit Bay is the best exposure of these 
fossil-bearing rocks in the district. The lowest are barren mudstones, which 
form the axis of a sharp anticlinal, and are followed by black flinty shales con- 
taining II species of Graptolites. The same species, associated with 15 others, 
are also found at Ballygrot, where a wider area of rock is exposed, thus indi- 
cating that the rocks of these widely separated localities are but parts of the 
same bed. This group of 26 species enables us at once to correlate these beds 
with those of South Scotland ; and it is interesting to find that they agree al- 
most fossil for fossil with those from the representative localities of Berrybush 
Burn and Black Linn, which are the upper beds of the Glenkiln Shales. The 
small exposure in the railway cutting near Craigavad has also yielded fossils of 
this age, and is doubtless but a south-westerly extension of the Ballygrot Beds. 
The same may be said of the black shales still farther to the south-west, in a 
quarry near Cultra, although I have not been able to procure any recognisable 
fossils from the greatly crushed rocks of this latter locality. 
The lower Glenkiln Shales, which in South Scotland are comparatively 
unfossil iferous, may be represented in Ireland by the grey mudstones which 
form the centre of the anticlinal at Coalpit Bay, and by the series of unfossilife- 
rous grey and purple shales associated with the black bands at Ballygrot. 
The next beds in ascending order at Coalpit Bay are the barren mudstones 
noted in the detailed account of that locality at paragraph 5. The only fossils 
found associated with them were obtained from a fragment of black shale 
folded up in their broken strata. This fragment had doubtless been derived 
from some bed intimately associated with the mudstones, and which may yet 
be detected in situ among the shattered black bands at their base. The fossils 
which it has yielded, though few in variety, include Dicellograptus Forchammeri, 
Geinitz ; Climacograptus, sp. ? and Diplograptus truncatus , Lapw. species 
highly characteristic of a thin, but persistent, band of shale occurring near the 
base of the barren mudstones of the Upper Division of the Hartfell Series (1). 
This circumstance, coupled with the identity in lithological character of the 
barren beds in the two countries, leaves no doubt but that these Coalpit Bay 
Mudstones are the representative of that sub-division. This being admitted, it 
is evident that the rich beds of the Lower Hartfell Series are absent here, 
having either thinned out or been lost by faulting. The latter view is more 
probable, as we find them undoubtedly represented by the fossiliferous black- 
shales of Carnalea, which have yielded ten species of Graptolites, the following 
six species being peculiar to that sub-division:— Diplograptus truncatus , Lapw.; 
Diplograptus qua dr im ucronatus, Hall; Lasioqraptus Harknessi, Nich.; Retiolites 
