168 A. R. Andrew—The Dolgelley Gold-belt. 
(6) The Bryntirion fault is about 2 miles long: it attains its 
maximum importance on the Clogau Hill, where it intersects the 
Clogau or St. David’s lode. 
(ce) The Afon Gain fault runs along the course of the Afon Gain, 
and extends north and south from there. Its effect is greatest in the 
north, and it dies away to the south in the Maentwrog Beds, faulting 
the Gwynfynydd lode on the way. 
(d) The Beddycoedwr fault passes down the east side of Moel 
Gwynfynydd. Like its companion, the previous one, it decreases in 
intensity towards the south, dying out finally in the Ffestiniog Beds. 
It cuts out all the Pen Rhos Beds, and a small part of the Vigra Beds. 
The eastern end of the Gwynfynydd lode terminates against it. 
Cleavage.—All the finer-grained members of the sedimentary forma-: 
tions of this Dolgelley Gold-belt are traversed by cleavage planes. 
In very few cases, however, are the resultant slates of sufficiently 
good quality to be utilized for roofing or similar purposes. The strike 
of the cleavage planes is northerly; it usually les between the limits: 
of 5° west and 10° east of the meridian ; it exceeds these limits very 
rarely. The dip of the cleavage planes is always steep, from 70° to 
90°, sometimes towards the east, sometimes towards the west. The 
direction of cleavage in the gold-belt is entirely independent of the 
geological structure, as expressed in the dip and strike of the beds as 
a whole, and also of the minor puckerings and contortions into which. 
the slates have at times been thrown. It is quite possible that there 
has been movement of the country subsequent to the development of 
the cleavage, but it cannot have been of great importance. 
The movement of the country which gave rise to the cleavage must 
have been subsequent to the intrusion of the greenstone masses. In: 
several instances we find examples in which a coarse, rude cleavage: 
has been induced in the igneous rocks themselves, in the greenstone 
above the Precipice Walk, Dolgelley, and further east towards Llan- 
fachreth. Other instances may be seen near the top of Y Garn and 
elsewhere. In these cases of cleaved greenstone, the direction of: 
cleavage is the same as that of the finer general cleavage which. 
traverses the slates. I have met with one case where the greenstone 
intrusion has an included band of shale hardened to such an extent 
that it had resisted the cleaving force and has remained uncleaved. 
It is possible that it may have been more or less protected by the 
great thickness of igneous rock in which it is enveloped, but the 
conclusion remains the same—that the greenstone was there before 
the cleavage commenced. This example is seen on the Clogau Hill, 
north of Bontddu, just where the old mine tramway crosses the 
large sill which runs from Llechfraithisaf up to the top level of the 
St. David’s Gold Mine. 
Although the intrusion of the greenstone is previous to the cleavage 
of the country, I believe that it is subsequent to the folding. In 
several cases where the rocks are puckered and folded, it is seen 
that the greenstone sills do not continue uninterruptedly round the 
folds, but feather out at the actual bend of the folds, the form of 
the intrusion being here determined by the pre-existing form of the 
folding. 
