Stj-atigraphical Geology. yi 



Along the southern shores of Belfast Lough occur repre- 

 sentatives of the Llandeilo and Lower Bala series ; that 

 is, the representatives of the Lower Silurian or Ordovician. 

 Between Orlock Point and Donaghadee the rocks pass 

 into the Llandovery series of the Silurian system ; while still 

 further south, near the extremity of the Ards peninsula, 

 occur representatives of the Tarannon shales. 



The Ordovician rocks of County Down and County 

 Cavan are of the Moffat type, while to the north-west, in 

 County Tyrone, they are of the Girvan type. In County 

 Dublin and further south they are of the Lake District type. 



As in the earlier rocks we trace a general line of strike 

 for similar beds, so in the Ordovicians we may correlate the 

 various rocks occurring in Ireland by tracing a similar north- 

 east and south-west series of strikes, and comparing the 

 Irish rocks with those of the neighbouring land of Great 

 Britain. The Silurian rocks of County Down, as found at 

 Coalpit Bay and at Tieveshilly, near Portaferry, correspond 

 with the Birkhill and Gala beds of the south of Scotland in 

 the graptolites which they yield. 



In Swanston and Lapworth's paper above referred to, the 

 conclusions there arrived at are summarized as follows : 



"The Silurian rocks of County Down are, from the fore- 

 going, proved to belong to several distinct divisions of the 

 system. 



" I St. The lowest, exposed at Coalpit Bay, the shales of 

 Ballygrot, Craigavad, etc., are the representatives of the 

 Glenkiln shales, the equivalents of the Upper Llandeilo of 

 Wales. 



" 2nd. The black shales of Carnalea and the barren mud- 

 stones at Coalpit Bay represent respectively the Lower and 

 Upper Hartfell series, the equivalents of the Bala or Caradoc. 



" 3rd. The black shales at Coalpit Bay, characterized by 

 the zones of D. acumi>iatus^ D. vesicitlosus, M. gregaritis, 

 and D. come fa, represent in part both the Lower and 

 Upper Birkhill shales, which are paralleled with the Lower 

 Llandoveries. 



"4th. The grits and conglomerates which immediately 

 succeed the latter are of the same age as the Bala series, 

 and occupy a high place in the Middle Silurians. 



