536 PROF. W. J. PTIGH ON THE GEOLOGY OP THE [vol. Ixxix, 



Faulting. 



Strike -faults. — The northern margin of the area described is 

 bounded by the great Tal-y-llyn or Bala Fault, which follows the 

 deep valley of Tal-y-llyn ; but no investigations have been carried 

 out on this fault itself. 



However, hj mapping the upper limit of the volcanic rocks on 

 Craig y Llam, the presence of several small strike-faults has been 

 proved. They trend approximately parallel to the main Tal-y- 

 llyn Fault, and are in all probability closely related to it. These 

 relatively small faults make conspicuous features when the junction 

 of the Craig y Llam and Hengae Croups is followed. Each of 

 them affects the outcrop of the volcanic rocks in the same way, 

 only with slightly differing results. The}' act as repeating faults, 

 and the rocks are thrown up on their south-eastern side. As to 

 whether the movement is mainly a vertical or a lateral displacement, 

 it is difficult to decide definitely. 



The presence of these parallel strike-faults leads one to believe 

 that the Tal-y-llyn Fault will be found in this region, not to consist 

 of a single fracture, but rather more probablv of a belt of parallel 

 faults ('shatter-belt'). 



The Waun Fault. — The western margin of the volcanic rocks 

 on the north-western slopes of Mynydd y Waun is defined by a 

 well-marked strike-fault which is shown on the 1-inch Geological 

 Survey Map. Old Series, No. 59 N.E. The fault runs along a 

 marshy hollow between Mynydd y Waun and Craig y Llam. In 

 the middle of this depression the crags of volcanic rocks end off 

 very abruptly. When seen north-eastwards from Mynyddd Fron- 

 fraith, this line of crags with abrupt slopes facing north-westwards 

 presents a very characteristic topographical feature. 



This fault acts in the same way as the small strike-faults 

 described on Craig y Llam. 



The Aberllefenni Fault. — At Aberllefenni there is a well- 

 marked fault, which affects the outcrops of the Abercovris Group 

 and the Pont Erwyd Stage. As a result, the Narrow Vein occurs 

 twice on the south side of the Llefenni Valley. Its effects may be 

 compared with those of the strike-faults mentioned above. It is 

 of the nature of a repeating fault, and the rocks are thrown up 

 on the south-eastern side. The fault-plane probably is steeply 

 inclined south-eastwards. 



It may be the south-western extension of this fault that causes 

 the comparatively narrow outcrop of the Abercwmeiddaw Croup, 

 north-west of Corris. 



The transverse faults. — In the extreme north of the area 

 the margin of the volcanic rocks is interrupted by two relatively 

 unimportant dip-faults. They make conspicuous notches on the 

 boundary between the volcanic rocks and the Ceiswvn Mudstones. 



