CHAP. III.] OEDOVICIAN PERIOD. 35 



ment derived from the land was mingled with the calcareous 

 and organic matter." 



" After the deposition of the Serpulite grit hardly any 

 sediment derived from the land entered into the composi- 

 tion of the overlying limestones, and eventually nothing 

 seems to have fallen on the sea-floor but the remains of 

 minute organisms, whose calcareous and siliceous skeletons 

 slowly built up the great mass of limestone and chert so 

 conspicuously displayed at Durness." 



Ireland. There are five principal districts in Ireland 

 where Ordovician rocks occur : (1) Wicklow, Wexford, and 

 Waterford; (2) Clare and Tipperary; (3) South Ulster- 

 Down, Armagh, &c. ; (4) North Ulster Donegal and Lon- 

 donderry ; (5) G-alway and Mayo. Between 1 and 3 

 are two small but important exposures, that on the Dublin 

 coast at Portraine, and the ridge known as the Chair of 

 Kildare. 



No Irish rocks have yet been identified by their fossils as 

 contemporaneous with the Welsh Arenig. Wherever the 

 base is seen in the east of Ireland, beds with Llandeilo 

 fossils rest unconformably on metamorphic rocks, which 

 are classed as Cambrian. Coast sections near Bannow 

 and Greenore Point in Wexford expose the basement beds, 

 which are purple conglomerates and sandstones overlain 

 by black shales with Llandeilo fossils, a succession which 

 recalls that of the Girvan district in Ayrshire. The Dark 

 Shale series (Llandeilo) may have a thickness of 2,000 or 

 3,000 feet, and it is surmounted by an equal thickness of 

 grey and greenish shales with interstratified igneous rocks, 

 the slates containing fossils of Bala types. 



The Portraine section is remarkable for the resemblance 

 of the rocks to the upper part of the Cumberland Ordovi- 

 cian system, i.e. the Coniston Limestone and the underlying 

 Borrowdale .group, with its lavas and ash-beds. There 

 can be little doubt that this area was the centre of similar 



