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 

 unfossiliferous, 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 quadrimucronatus, Hall; Lasioqraptus Harknessi, Nich.; Retiolites 



