part 2] ITS STRTJCTURE AE-D EOCK-SUCCESSIO^. 159 



east and south-west trend-lines of the volcanic masses of the Arans 

 and Arenig Bach that appear to control the trend of the thrust- 

 planes, the rocks above the volcanics being apparently thrust over 

 the more resisting volcanic rocks underneath ; hence the major 

 lines of displacement have a general north-east and south-west 

 trend, and in the central area cross the lines of folding obliquely. 

 The differential resistance to the thrusting force in different parts 

 of the thrust-masses shows itself in the development of numerous 

 tear-faults. The facts seem to indicate that the rocks must first 

 have been thrown into a series of major folds with minor folds and 

 ripples, the trend of which at the present time varies in different 

 parts of the area between north-east and south-west and north and 

 south, with a gentle southward pitch (fig. 9). Among these folds 

 were developed an important anticline and syncline trending 

 parallel to the line of the Central Wales Syncline ; as pressure 

 continued, the folds as a whole were packed with increasing close- 

 ness against the Harlech Dome, and, the resistance of the volcanic 

 belt becoming more pronounced, the anticline gave way, and its 

 south-eastern limb Avas driven over on to the syncline by means of 

 a series of compressional faults, of which the Bala Fault is one. 

 The existence of this synclinal fold is still indicated in the westward 

 di^^s of the rocks along the north-western shore of the lake : as, 

 for example, where the Frondderw Ash is brought up again half a 

 mile south-west of Llan-j^-cil, farther south 1 mile and li miles 

 south-west of Llanwchllyn, and perhaps still more clearly north of 

 the lake north-west of Moel-Emoel, where the apex of the fold, 

 much torn by faults, is distinctly visible with the line of the Bala 

 Fault lying to the east of it. Simultaneously with this squeezing- 

 out of the anticline and the further packing or thrusting on to the 

 Harlech Dome, owing to the unequal resistance of different portions 

 of the thrust-masses there were developed the striking series of 

 tear-faults which are so marked a feature of the district. 



That these really are tear-faults and not merely normal faults, as 

 they were regarded by the earlier workers on this ground, is clear 

 from the horizontal displacement of the beds affected, which is 

 entirely irrespective of the amount of dip. 



The country appears to be affected by the following major lines 

 of displacement (fig. 1, p. 136) : — 



(1) The Llyn-Tegid Line. 



(2) The Bala-Lake Line. 



(3) The Llangower Line. 



(4) The Cefn-ddwy-graig Line. 



(5) The Moel-fryn Line. 



(6) The Ffridd Defaid Line. 



There is almost certainl}^ at least one other line to the north- 

 west within the Dicranograptus Shales ; but this unfortunately 

 I have been unable to map, owing to the nature of the ground 

 and the character of the rocks occupying it : for the black shales 

 afford no evidence of their age, and the lines of break that can 

 be seen in the section visible in the Nant-Hir cannot be traced for 



