LOWER PALHOZOIC ROCKS OF ANGLESEY. 571 
rocks are homogeneous, we find, as we traverse the section, similar 
varieties of the shale reappearing again and again. If these are 
folded repetitions, there must be folding back towards the south, 
since the dip is almost uniformly to the north. 
The facts here adduced appear to me most easily explicable on the 
supposition that the Palozvic rocks of northern Anglesey originally 
formed a trough, which was fringed on both the north and south by 
conglomerates, and that, by thrust from the north, the strata on the 
northern side were folded back. Whether or not this hypothesis can 
be sustained, there is no doubt that the Porth-y-corwg conglomerate 
was derived, at least in part, from the altered rocks of the northern 
area, which are thus shown to be of greater antiquity than the 
strata which they apparently overlie. ‘The close lithological resem- 
blances between this older series and the Pebidian rocks of central 
Anglesey place their correlation beyond reasonable question. 
B. RELATIONS BETWEEN THE HotynHEap ARCHHZAN AREA AND 
THE Pataozoic Rocks to tHe Kast. 
The Clymwr Fault.—A little south of Llanbabo, the strike of the 
Paleozoic rocks abruptly shifts from about east and west to south- 
south-west or north and south, and the deposits change from black 
shale, with basement bands of grit and breccia, to massive conglo- 
merate. We must therefore place a fault immediately to the north 
of Clymwr. This dislocation coincides in direction, and may be 
actually continuous with, the Porth-y-defaid fault*, which brings 
the ashy Pebidian slates and shales of the north-west against the 
Holyhead schists. The Clymwr fault separates the Paleozoic area 
just described from the one about to be discussed. 
Clymwr Conglomerate—South of the Clymwr fault is a mass of 
conglomerate and grit, one mile and a half in length from north to 
south, by about one mile broad. Commencing on the north at Clymwr, 
it terminates on the south about 100 yards south of Bodnolwyn-hir, 
A small hollow intervenes between these rocks and the nearest out- 
crop of the Archean. The dip is steadily to the east, that is, away 
from the Archean, at moderate angles. The strata have weathered 
in several parallel escarpments, and, unless there is repetition, the 
thickness must be several hundred feet. The included fragments of 
the conglomerate are chiefly rounded pebbles of granitoidite and 
angular fragments of altered slate (Nos. 117, 118, p. 585) undis- 
tinguishable from the Pebidian (No. 112, p. 584) of the area to the 
west. Associated with the coarser strata, are beds of grit, apparently 
derived from granitoid rocks. There appears to be no definite order 
in this series, but I may observe that the bands with the largest 
fragments do not occur at the base. 
Cwaen Shales.—Where the conglomerate disappears, south of 
Bodnolwyn-hir, black shales come in, and are well seen around 
Cwaen-wen, almost in contact with the Archean. They are also 
exposed on the strike to the north, near Cwaen-goch, and are 
* Quart. Journ. Geol. Soc. vol. xxxvii. p. 224. 
