248 



MR. C. A. MATLEY ON THE 



[May 1900, 



the Llandeilo outcrops. In spite of the prevalent steep northerly 

 dip, the older rocks are forced up again and again to the surface, so 

 that even the island called the Middle Mouse (Ynys Badrig), which 

 lies beyond the extreme northern point of the coast, is formed of 

 beds not belonging, as might be expected from its geographical 

 position, to the newest group of rocks, but to the oldest or Green 

 Series. 



Although from a tectonic aspect the grouping of the faults may 



be considered as fold-forms, it 



Pig. 3.— Faulted quartz-vein in should be remembered that in 



the cliff at Trwyn Bychan this Complex actual folding is 



(inaccessible, hut apparently Q uit e subordinate to the fault- 



2 or 8 inches thick). in £- The dislocation-planes, 



both of the thrust- and the 

 S normal faults, are usually steep, 

 differing little in direction and 

 amount from the angle of dip. 

 Thrust-planes of a gentler slope 

 are in this part of Northern 

 Anglesey mostly of minor im- 

 portance, with upthrusts vary- 

 ing from a few inches to a 

 few feet. They are numerous 

 at Pig y Barcud (see PI. XIV, 

 Section 1) and in the cliffs north 

 of Porth Wen Pier ; while in 

 the Green Slates of Trwyn 

 Bychan (Porth "Wen Bay) they 

 give a false appearance of bed- 

 ding (see PI. XIV, Section 3). 

 Fig. 3 shows the effect of this 

 shear-cleavage on a quartz-vein 

 at the last-named locality. 



The sequence of the movements which have affected this part of 

 Anglesey since Llandeilo times seems to be as follows : — The strata 

 were first bent into a series of folds. A continuance of the com- 

 pression, which here acted from a direction somewhat to the east of 

 north, crushed and broke up the folds; produced an oast-south- 

 easterly stride, numerous strike-faults, and overthrusting of the 

 beds ; caused the minor shearing such as deformed the pebbles of 

 the purple conglomerate (p. 234) ; and induced the imperfect shear- 

 cleavage at Trwyn Bychan. The east-and-west cleavage which 

 obtains along part of the coast was impressed on the rocks 

 somewhat later by a change in the direction of compression. 



The transverse faults of the Hell's Mouth type may be perhaps 

 regarded as later than the strike-faults, since they appear to truncate 

 these ; but the transverse fault east of Porth Wen Bay is not easily 

 explicable on this hypothesis, as it does not pass through the planes 

 of overthrust south of Trwyn Bychan. It seems more probable 



