264 Paleo-Geological and Geographical Maps of the British Islands. 
Nature of the Lower Silurian Beds.—The strata of this period consist of dark and 
gray slates, grits sometimes calcareous, and, rarely, bands of limestone. The fossils. 
are all of marine genera. Over the northern and western areas of the British Isles 
these strata have undergone extensive metamorphism, so that in the Highlands 
of Scotland, and of the north and west of Iveland, they consist of quartzites, 
micaceous, talcose, and chloritic schists, and crystalline limestones, sometimes 
serpentinous, with their varieties; presenting a marked contrast to their unaltered 
representatives in the south of Scotland, in Wales, and in the east of Ireland. 
Relations to adjoining Formations.—The Lower Silurian rocks are discordantly 
superimposed upon all formations older than themselves. This is the case in 
North Wales, in the east of Ireland, and in the north of Scotland Owing to this 
discordancy, and the large amount of denudation to which the upper and lower 
Cambrian beds were subjected at the close of the Cambrian period, we find 
the Lower Silurian beds resting un strata of various stratigraphical positions. 
Thus in North Wales and Salop, the Arenig beds are found resting sometimes (as 
near Bangor and Carnarvon) on the purple slates and conglomerates of the Cam- 
brian series ;* sometimes, as in Pembrokeshire and Merionethshire, on the Tremadoc 
slates. In the east of Ireland in county Wicklow, the Lower Silurian beds rest 
discordantly on the Lower Cambrian beds; and in the north of Scotland, the 
quartzites and limestones representing the Llandeilo beds, rest discordantly some- 
times on the Lower Cambrian beds, at others, on the Laurentian. 
Lower Silurian Areas.—The principal districts where the Lower Silurian rocks 
form the surface, are the north and central Highlands and the southern uplands of 
Scotland, the Lake District of the north of England, the north and centre of Wales, 
the north-west, north-east, and south-east of Ireland, and the Isle of Man. They 
also occupy portions of Normandy and Brittany,+ where they rest on Cambrian 
and Laurentian beds, and they have been proved by boring below the Tertiary 
and Cretaceous strata at Bruxelles (Brussels), Louvain, St. Tron, Menin, and 
Ostende in Belgium. They also appear at the bottoms of the valleys between 
the Sambre and the Meuse, as determined by M. Gosselet ;t they probably under- 
lie a large portion of the Paris basin, where they are concealed by Tertiary and 
Cretaceous formations. 
Beds of marine origin.—tThe fossils yielded by the Lower Silurian rocks, whether 
in the north-west of France, in Belgium, in Wales, in Ireland, or in the north of 
Scotland,§ all go to prove the marine origin of the strata themselves. ‘hey consist 
chiefly of trilobites, molluscs—cephalopods, gasteropods (lamellibranchs not 
plentiful), and brachiopods—a few corals, and graptolites. The abundance of these 
forms in the calcareous beds prove that the waters of the sea teemed with living 
* Ramsay, supra cit., p. 78. 
* Murchison ‘Siluria,” 4th edit., p. 408. 
} Quoted by Dr. Mourlon, “ Géol. de la Belgique,” p. 40. § From the Durness, or Assynt, limestone. 
