Ordovician and Silurian Rocks of Lough Nafooey Area. 539 



The rocks are of Arenig, Llandeilo, and Silurian age, together 

 with intrusive felsites, bostonites, labradorite-porphyrites, and 

 dolerites. 



The Llandeilo rocks (Mweelrea Grits and Conglomerates) are 

 mainly confined to the low-lying ground along the shore of Lough 

 Nafooey, and have yielded no fossils. They clip at a high angle off 

 the Arenig rocks, which extend in a band from a third to half a 

 mile wide from end to end of the area. The Arenig rocks consist 

 in the main, as at Kilbride, of spilite -lavas associated with coarse 

 breccias, and with bands and patches of chert in which at two points 

 radiolaria were found. The Lough Nafooey area agrees with those 

 of Grlensaul and Tourrnakeady in the presence of limestone-breccias 

 which were absent at Kilbride. Unfortunately, no graptolites were 

 found in the Arenig rocks of Lough Nafooey. Pillow-lavas show a 

 splendid development, especially near the top of Bencorragh. 

 Silurian rocks form the whole southern half of the area, including 

 the highest point, Benbeg (1788 feet). They are very highly 

 inclined, being (for much of their extent) vertical or sometimes 

 slightly overfolded. They include representatives of the Llan- 

 dovery, Tarannon, and Wenlock formations, and exhibit the same 

 general succession as at Kilbride, although the relative thicknesses 

 of the several bands are not the same. The occurrence of Mono- 

 graptus galaensis on the north-eastern slopes of Benbeg confirms 

 the field-evidence as to the Tarannon age of certain grey flags. 

 The Wenlock beds, as at Kilbride, are presented b} r thick grits 

 (Doon Bock Grits), a thickness of over 800 feet being seen. Of 

 the intrusive rocks, felsites are not so prominent as at Kilbride, and, 

 while at that locality they are always associated with Arenig rocks, 

 at Lough Nafooey some are clearly of later date — occurring in the 

 Llandeilo formation, or along the line of junction of the Llandeilo 

 and other rocks. 



The bostonite-sill seen at the base of the Silurian rocks at Kilbride 

 extends almost continuously throughout the Lough Nafooey area, 

 but is much reduced in thickness. Other small intrusions of 

 bostonite occur in addition to this sill, which, as at Kilbride, is 

 associated in places with labradorite-porphyrite. The prevalence of 

 mica-dolerite intrusions in the purple shale (Tarannon) is a note- 

 worthy feature. 



The paper concludes with a table comparing the rocks of the 

 Lough Nafooey area, on the one hand with those of Kilbride, and 

 on the other with those of the Killary district described by Mr. 11. Gr. 

 Carruthers and Mr. H. B. Maufe. 



2. 'The Geology of the St. Tudwal's Peninsula (Carnarvonshire). ' 

 By Tressilian Charles Nicholas, B.A., F.G.S. 



The St. Tudwal's Peninsula is situated at the south-eastern 

 extremity of South- West Carnarvonshire (Lleyn), and forms the 

 north-western limit of Cardigan Bay ; it is underlain by Cambrian 



