Vol. 65.] SUCCESSION AROUND PLTNLXM.ON AND PONT EEWYD. 527 



Faults. 



Strike-faults. — Few strike-faults of any importance have 

 been detected, but it is quite possible that their number is greater 

 than I suspected. Several may be seen in the Bheidol gorge near 

 Bryn-chwith, where they are generally overthrusts from the west, 

 but the throw only amounts to a few feet (see PI. XXV). Tbe 

 most powerful is the one near the main anticline in the gorge ; it 

 brings the acuminatus beds against the upper part of the atavus beds. 

 The throw at that point must be about 50 yards to the east ; but, 

 as the fault-plane is nearly vertical, it is impossible to say whether 

 it is a normal fault or an overthrust. So far as I have observed, 

 none of the strike-faults carry ore. 



Transverse faults. — These are' normal faults, usually with 

 a nearly vertical plane of fracture, and they generally carry one 

 or more of the ores of lead, zinc, copper, and sometimes manganese 

 (as on Drosgol). Most of them have a nearly constant direction 

 of east-north-east and west-south-west, and cut cleanly across the 

 folds, behaving as if they were of posterior date. The throw varies 

 considerably, not only from one fault to another, but also in the 

 same fault in different parts of its course, as might be expected 

 from the folding of the rocks which it traverses. Several veins 

 are marked on the Geological Survey Map (Old Series) which I could 

 not detect as faults by mapping ; but two of them, the Castell 

 Fault and the Camddwr Fault, can be traced with ease, and have 

 a considerable downthrow : in the former varying from about 30 

 to about 100 yards south-eastwards, and in the latter probably 

 reaching 300 yards or more towards the north-west. Their course 

 is usually marked on the ground by a conspicuous notch, or by an 

 abrupt fall towards the downthrow side. 



VII. Comparison with the Deposits of other British Areas. 



In Table I, facing p. 530, the equivalents in other parts of Britain 

 of the Pont Erwyd deposits are shown. A much closer comparison 

 might, however, be instituted in many cases, as may be gathere'i 

 from Table II ; but it is considered unnecessary to do more than 

 call attention to certain points of interest, especially the resem- 

 blances to and the differences from the deposits of other parts of 

 Central Wales as described by Dr. Herbert Lapworth in the neigh- 

 bourhood of Bhayader, and by Mrs. E. M. B. Shakespear in the 

 Tarannon district. 



The Bryn-glas Mudstones are undoubtedly the equivalents of the 

 cleaved blue-black shales which underlie the Cerig Gwynion Grits 

 of Bhayader. They agree not only in their lithological characters, 

 but also in their peculiar ' double cleavage ' which gives them the 

 appearance of much greater antiquity than those grits. I have, 

 however, given reasons for supposing that this feature is due to 



Q. J. G. S. No. 260. 2 o 



