57 C. CALLAWAY ON THE ARCH EAN AND 
Fig. 1.—Generalized Section across the Paleozoic Rocks of 
Northern Anglesey. (Scale, one inch to the mile.) 
a. Granitoid Rocks. Conglomerate. 
6. Pebidian. ce. Paleozoic } Grit. 
f. Fault. | Shale. 
The blank space in the centre of the section is occupied by black shales, 
probably frequently repeated by folding and sometimes refiexed. 
2. There is the clearest proof that the Palzozoic rocks have been 
thrust laterally by a powerful force from the north. On the north 
side of Porth-y-gwichiad (fig. 2) the shales have been fractured 
by a reversed fault with a very low hade to the north, and the 
beds of the upper mass le with a low northerly dip upon almost 
vertical strata, which at the top are bent over towards the south. 
This appears to be a case of an overfold converted into an overfault 
by a continuance of the lateral pressure, according to the principle 
expounded by Prof. Heim *. At Llanbabo similar evidence is to be 
Fig. 2.—Overfault in Black Shale on Porth-y-qwichiad. 
(Scale, about one inch to 50 feet.) 
Ss. N. 
fF 
seen in a large quarry near the church. In another quarry to the 
east of the road, a little north of Fferam-gyd, the strata are 
repeatedly broken by small horizontal faults, the upper masses being 
pushed a few inches to the south. The presence of thin quartzose 
seams renders the dislocations very clear. 
3. In the black-shale section between Porth-y-gwichiad and 
Porth-y-corwg, there are suggestions, though not actual proofs, of 
repetitions by folding. In high cliffs the beds are sometimes seen 
to form part of a sigmoid curve; and though, on the whole, the 
* ‘Mechanismus der Gebirgsbildung,’ Band i. pp. 220-223. See also a paper 
by Prof. Lapworth, F.G.S., in the ‘ Geological Magazine,’ August 1883. 
