572 C. CALLAWAY ON THE ARCH #AN AND 
evidently continuous with the well-known black shales north-west 
of Llanerchymedd. These shales may be traced for over four miles 
to the south-south-west, disappearing a little north of Llyn Treffwll. 
They fringe the Archean for the whole distance, usually dipping 
away from the older rocks. At their base, south of Bodnelwyn-hir, 
are some thin bands of grit. ° 
Llanfihangel Conglomerate-—North-east of Llanfihangel the 
Cwaen shales pass down through some seams of grit into a con- 
glomerate, similar to the rock at Clymwr. This conglomerate 
remains in contact with the Archean for about a mile, disappearing 
near a cottage called “‘ Harlech,” under the sandy tract which 
margins Cymmeran Bay. The rock is well exposed in the cragg 
ridge east of the farm of Tywyn, where in one place it is seen to 
dip to the east. 
The Clymwr and Llanfihangel conglomerates, with the intervening 
Cwaen shales, are thus seen to fringe the western Archzxan area for 
Fig. 3.—Sketch-Map of the Treiorwerth Syncline. 
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Shale. Grit. Conglomerate. 
