~ ee 
LOWER PALZHOZOIC ROCKS OF ANGLESEY. 569 
p. 586) is undistinguishable from the Cemmaes limestone *, which 
occupies so conspicuous a place amongst the Pebidian strata of the 
northern area. There are small exposures of a similar rock at 
Llanfaethlu, near the west coast, and at Cerrig Ceinwen, in the 
centre of the island, all in the Pebidian ; but since the limestone at 
Cemmaes is by much the nearest to the Byttia conglomerate, and 
occurs in great force, it is unnecessary to seek a more distant source 
of derivation. 
As the Paleozoic strata between Llanerchymedd and the northern 
area haye a general dip to the north, they present the appearance 
of a regular ascending series. If this aspect is not delusive, the 
thickness must be very great. The following facts, however, throw 
doubt upon the ordinary interpretation. 
1. The conglomerate which is in contact with the boundary fault 
from Porth-y-corwg to Byttia is underlain by grits, and the grits 
are underlain by dark shales and slates. This succession is very 
well seen in a quarry near Porth-y-corwg farm and elsewhere in 
the vicinity in natural sections. I have not found the conglomerate 
west of Byttia; but 2? miles to the west-south-west, near Gwredog, 
tne grit is seen in the road, again overlying dark shale, the Pebidian 
cropping out just beyond the grit to the north. 
I venture to make the suggestion that the conglomerate and grit 
may be the ordinary basement beds of the Paleozoic in inverted 
order. J have shown the actual succession in fig. 1, the dotted 
curve below the base-line being hypothetical. The well-known 
ascending series, as seen round the margins of the Archzean masses 
of central and north-eastern Anglesey, is (1) conglomerate ft, (2) 
grit, and (3) shale. The grit at Porth-y-corwg (No. 130, p. 587) 
and Gwredog (No. 133, p. 587) is undistinguishable from some 
varieties (Nos. 129, 132, p. 587) succeeding the conglomerates which 
underlie the black shales on the south and east. The conglomerate 
is unlike that which is admitted to be the base of the Anglesey 
Paleozoic; but this does not militate against their correlation ; for 
it has been shown by other authors that the materials of the base- 
ment conglomerates vary with the nature of the Archean rocks in 
their vicinity, and I have pointed out that the northern conglo- 
merate is in all probability derived from the northern area. That 
no other beds ever come in between this conglomerate and the 
Archean strengthens my suggestion. The fault between the two 
groups presents no difficulty ; for the Paleozoic could hardly have 
been folded back by the thrust of the Pebidian without fracture. 
On theoretical grounds, I should suspect this dislocation to be 
reversed ; but, as I have no direct proof of the hade, I have, in the 
section, made the fault vertical. 
* See Quart. Journ. Geol. Soc. vol. xxxvii. p. 236, No. 52. 
t Lat first regarded this rock as a part of the Archean, but a new reading 
of the Twt-Hill section required a reexamination of the question, and I frankly 
accept the Paleozoic age of the Nebo conglomerate and grit. The presence of 
pebbles of well-known Anglesey gneiss in the Twt-Hill conglomerate always 
appeared to me a suspicious circumstance. 
