Vol. 62.] IGNEOUS AND SEDIMENTARY ROCKS OF LLANGYNOG. 247 



referable to two complicated anticlines ranging abont east-north- 

 east and west-south-west. Both anticlines are overfolded from the 

 north, their southern limbs being inverted. They are separated 

 one from the other by a thrust of steep dip, hading northwards 

 and cutting out the intervening syncline. 



These anticlines appear to owe their origin to a system of pre- 

 Old-Red folding, but it is evident that they have been further 

 complicated by a series of folds and faults which have affected the 

 Old Red Sandstone. 



The axis of the northern anticline 1 lies beyond the northern 

 limit of the map (PI. XXIII), and ranges along the low ground at 

 Llanllwch, traversed by the Great Western Railway west of Caer- 

 marthen, the oldest beds exposed being Tetragmptus-sh&les. Its 

 southern limb is represented by the inverted belt of Didymograptus- 

 bifidus Beds ranging east and west of Cwm Crymlyn. 



The axis of the southern anticline has the same general direction, 

 and ranges through the grit-masses of Llangynog, but has suffered 

 much fracture and displacement by later disturbances, largely 

 determined by the differential movement between the grits and the 

 shales. Its northern limb dips normally towards the north, but its 

 southern limb is generally vertical or inverted. 



The core of this southern anticline consists of the pebbly grits of 

 Llangynog, which we correlate with similar grits that form the 

 core of an anticline of like structure at Bolahaul and Cystanog, 

 south-east of Caermarthen. 



A glance at the map (PI. XXIII) shows that most of the faults 

 have a general west-south-westerly trend. They frequently throw 

 off branches, which enclose lenticular strips of ground and unite 

 again with the original line of fault. Although the faults shown 

 are numerous, it is more than likely that others remain undetected. 

 Whether the faults are normal faults or overthrusts is frequently 

 unknown, but in many cases there is ground for supposing them to 

 be thrusts. 



That much of the folding and faulting was accomplished before 

 the Lower Old Red Sandstone was deposited, is proved by the 

 striking unconformity at the base of that formation. Thus : a mile 

 north-east of Rhyd-lydan it rests on Tremadoc Beds ; at Pfynnon-wen 

 and Moelfre it overlies the Tetragraptus-shales ; at Coomb and 

 Castell Cogan it can be seen crossing the various igneous rocks of 

 the Coomb complex, and its basement-beds are made up largely of 

 detritus from the igneous rocks around which it wraps. South of 

 Pentre-newydd it can be seen resting on the Didymograptus-bifidus 

 shales, while a little farther to the south-west it spreads over the 

 Bala Beds of Llandeilo-Abercowin. v 



Neither the fault which separates the rhyolites of Castell Cogan 

 from the Didymograptus-bifidus Beds, nor that which a little farther 



1 'Summary of Progress of the Geological Survey for 1904* Mem. G-eol. 

 Surv. 1905, p. 47. 



