582 C. CALLAWAY ON THE ARCHAN AND 
EK. Puystcatn GroGRAPHY OF THE ANGLESEY ReEeIon IN ORDOVICIAN 
Tovres. 
The principal Archean masses must, I think, have occupied about 
their present position during the deposition of the Ordovician rocks. 
That the northern area was dry land, is evident from the masses 
of Cemmaes limestone in the fringing conglomerates. 
The central area was also above water ; for the basement Paleozoic 
conglomerates contain not only boulders from the granitoidite, but 
also pieces of halleflinta such as we find east of Llanfaelog, and 
numerous fragments of purple and green shale, very similar to types 
characteristic of the tract west of Malldraeth Marsh. The gneissic 
rocks were probably in part covered by the Pebidian, for some of 
the fragments of shale in the conglomerate west of Llanfaelog are of 
large size and unrounded, so that they can hardly have been derived 
from the present Pebidian masses, the nearest point of which is three 
miles distant. 
The western area furnishes similar evidence. The large angular 
fragments in the Tywyn conglomerate cannot have travelled far, 
and the pieces of altered shale in the Clymwr conglomerate must 
have been derived from the neighbouring land to the west. 
But Anglesey was probably but the margin of extensive land- 
masses lying to the west or south-west. The Llanfihangel con- 
glomerate thins out rapidly towards the north-east, so that west 
of Llanerchymedd the Orthis-grits pass up into the black shales. 
Below this conglomerate is a considerable thickness of grit of similar 
composition, which must have been derived from similar rocks, but 
from a more distant source. This grit also grows thinner towards 
the north-east. 
The vanished land which once occupied what is now St. George’s 
Channel probably extended into what is now Ireland. The hill of 
Howth, north of Dublin Bay, is built up of a considerable thickness 
of Pebidian rock ; and if we extend the strike of the Anglesey gneissic 
series to the south-west, it will coincide with the strike of similar 
metamorphic rocks south of Wexford*. There is then no great 
improbability in the suggestion that the space intervening between 
Wales and Ireland was in the Ordovician period a tract of Archean 
land. 
F. Summary oF REsUtLrts. 
1. The hypometamorphic rocks of northern Anglesey are shown 
by the evidence of included fragments, of greater alteration, and of 
more intense contortion, to be older than the Paleozoic strata to 
the south. 
2. The metamorphic and hypometamorphic rocks of western 
Anglesey are proved by similar evidence to be of greater antiquity 
than the Paleozoic groups to the east. 
* See Geol. Mag. Dec. 2. vol. viii. pp. 494-498. 
