LOWER PALAOZOIC ROCKS OF ANGLESEY. 573 
its entire distance from north to south, and where seen, they almost 
invariably dip away from it. 
The Treiorwerth Syncline (see map, fig. 3).—The recognition of 
the three groups just described will help us to work out the stra- 
tigraphy of the band of Paleozoic rocks between the central axis 
and the western (Holyhead) area. A few sections will throw some 
light upon its structure. As the beds lie in a basin, and Treior- 
werth is situated about midway between the two extremities of 
the zone, we may call it the Treiorwerth syncline. 
Section (1) from Llanfaelog to Tywyn* (fig. 4).—Leaving the 
granitoid axis, we first come to grit, which, a little higher up, 
near Ty-ceryg, yields Ortlis, and, within 200 yards, passes gra- 
dually up into green conglomerate +, with pebbles of granitoidite 
and unrounded fragments of green shale, the pebbles and fragments 
being sometimes of great size. A little to the south, on the west 
side of Llanfaelog, this conglomerate is overlain by black shales, 
which are also well exposed along the shore of the bay to the south- 
west. All the dips hitherto seen are north-westerly. Rock is 
Fig. 4.—Secirion from Tywyn to Llanfaelog. 
(Scale, two inches to one mile.) 
Ty- Ridge of Orsedd-y- Llan- 
wyn. figs. 7-9. person. faelog. 
: t t 
i 
1 
j 
' 
{ 
b. Pebidian. 
a. Granitoid rock. c. Palgeozoic. 
jf. Fault. 
covered for some distance to the west, but reappears at the railway, 
near “ Orsedd-y-person.” South of the line, there is a clear dip 
to the south-east, so that we seem to have come to the west 
side of a syncline. The rocks, too, are very similar to the grits and 
conglomerates overlying the granitoid axis; and in a quarry to 
the north of the railway we find the same Orthis in a green grit 
overlain by conglomerate, the rocks dipping to the north-east. 
From here we soon come to the Llanfihangel conglomerate in the 
ridge at Tywyn. This rock, on the Survey map, is lettered as 
Carboniferous (d), with a queryt. There is not, however, the 
* The numbers (1) to (6) in this and the following sections correspond with 
the nuinbers on the map. . 
+ This end of the section agrees with part of one by Dr. Hicks, Quart. Journ. 
Geol. Soc. vol. xl. p. 193. 
{ This may have been suggested by the apparent derivation of the conglo- 
merate from the metamorphic rocks to the west, originally supposed to be 
Cambrian. 
Q.J.G.8. No, 159. 2@ 
