File:The Quarterly journal of the Geological Society of London (13889276030).jpg

570 C. CALLAWAY OX THE ARCH^AX AXD
Pig. 1. — Generalized Section across the Palcpozoic Roclrs of
XorLliern An-jlescy. (Scale, one incli to the mile.)
o.. Granitoid Rocks. f Conglomerate.
h. Pebidian. c. Palaeozoic . G-rir.
/. Fault. . Shale.
The blank space in the centre of the section is occupied bj black shales,
probably frequently repeated by folding and sometimes reflexed.
2. There is the clearest proof that the Palaeozoic rocks have been
thrnst 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 lie 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
Pig. 2. — Overfault in Blach Shale on Forth-y-giiichiad.
(Scale, about one inch to 50 feet.)
seen in a large quarry near the church. In another quarry to the
east of the road, a little north of Pferam-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

• ' ^lechanismus der Gebirgsbildung.' Band i. pp. 220-223. See also a paper