112 REPORTS ON THE STATE OF SCIENCE.—1914, 
shire has been spent. They have also to report that the detailed 
mapping of the island has been completed. The examination of the 
rocks and fossils which have been collected will be proceeded with. 
For the purpose of description the island can be divided conveniently 
into two areas—a northern area composed of Lingula Flags, Arenig 
mudstones and shales, Lower Llanvirn, and the intrusive mass of Carn 
Ysgubor; and a southern area of Lower Llanvirn shales with inter- 
bedded tuffs and rhyolites, and a thick mass of intrusive quartz- 
porphyry. To the latter area belongs the mass of rhyolitic and brecciated 
tuffs of Carn Llundain. 
Northern Area. 
Lingula Flags.—The Lingula Flags consist of bluish-grey flaggy, 
micaceous shales with ribs of hard grey close-grained sandstone, some 
of which reach a thickness of two feet. They occupy the headland 
of Trwyn Drain-du, and they extend eastwards to Bay Ogof Hén, while 
on the eastern side of the island they form the cliffs from the north- 
east corner to Road Uchaf. The Flags also occur in the headland to 
the south of Abermawr. They are highly fossiliferous, and yield Lingu- 
lella davisi in great abundance. 
Arenig.—All the zones of the Arenig are present. The lowest beds 
are bluish-grey sandy mudstones and shales with Ogygia selwyni, Orthis 
proava, and O. menapi@. ‘They are confined to the north-east corner 
of the island, and are faulted against the Lingula Flags. The mud- 
stones are followed by bluish-black shales belonging to the Extensus 
Zone, and are well displayed in the cliffs at Road Uchaf and Road Isaf. 
Similar shales belonging to the Hirundo Zone are present in Abermawr. 
Lower Llanvirn.—The base of the Lower Llanvirn is seen only in 
the cliffs in Abermawr, where the shales of the Hirundo Zone are 
succeeded by a thick series of hard dark- and light-coloured tuffs of fine 
texture, which yield Didymograptus bifidus in their highest beds. The 
tuffs are followed by fossiliferous blue-black shales, but their full 
thickness is not seen in the northern area. 
Intrusive Rocks.—Carn Ysgubor is formed of an intrusive mass of 
quartz-albite-diabase, which has invaded the sediments of Lower Llan- 
virn, Arenig, and Lingula Flags. A small intrusion occurs south of 
Abermawr, where Lingula Flags are in contact with a quartz-kerato- 
phyre. 
Southern Area. 
This area was described in the first report, in which it was shown to 
be composed of D. bifidus shales which had been invaded by a thick 
mass of quartz-porphyry. The shales, well displayed in the cliffs 
of Porth Llauog and Foel Fawr, are highly fossiliferous, and a large 
collection of graptolites has been made from them. They contain 
layers of coarse agglomeratic tuff, and at Foel Fawr pass upwards 
into thick beds of tuff which are conformably overlain by grey rhyolites. 
The tuffs and conglomerate on Carn Llundain belong to the same period 
of eruption. 
