136 Reports & Proceedings — Geological Society of London. 



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. Thej^ dip 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 liadiolaria were 

 found. The Lough Nafooey area agrees with those of Glensaul and 

 Tourmakeady in the presence of limestone breccias whicli were absent 

 at Kilbride. Unfortunately, no graptolites were found in the Arenig 

 rocks of Lough j!^afooey. 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 

 (1,788 feet). They are very highly inclined, being (for much of their 

 extent) vertical or sometimes slightly overfolded. They include 

 representatives of the Llandovery, 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 Monograptus 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 represented 

 by thick grits (Doon Rock 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 noteworthy 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. E,. G. 

 Carruthers and Mr. H. B. Maufe. 



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

 By Tressilian Charles JS^icholas, B.A., P.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 

 and Ordovician rocks. In the southern part of the peninsula the 

 structure is relatively simple, and the succession very plainly 

 displayed in numerous cliff-sections; Cambrian rocks very similar in 

 character to those of Merionethshire form most of the coast, but the 

 interior is mainly occupied by Arenig beds, which rest with a marked 

 unconformity on every local member of the Cambrian in turn. The 



