398 MR. H. H. THOMAS AND PKOF. 0. T. JONES ON THE [Sept. I912, 



Valley there is evidence of folding of the Cambrian rocks along 

 axes roughly parallel to the main direction of faulting ; the folds 

 have a general easterly pitch, and therefore the outcrops meander 

 widely in a north-and-south direction. An anticlinal axis passes 

 near Stone Hall, and another to the south of jS^ewton East. 

 A faulted syncline occurs to the south of Stone Hall, and a wide 

 syncline can be traced to the east of Brimaston. 



Insomuch as the trend of the outcrops in this region departs 

 widely from the direction of the main faults, the original relation 

 of the Cambrian to the underlying rocks is likely to be unchanged ; 

 it is in such a district, therefore, that one might hope to discover 

 unfaulted junctions of the base of the Cambrian. 



The assumption previously made that the rocks of Pont-yr-hafod 

 are brought to the surface by an anticlinal fold is in accordance 

 with the structure revealed farther north-east, for the well-marked 

 anticline near Stone Hall appears to range directly for Pont-yr- 

 hafod. 



The age of the faults. — It is clear that some of the faults 

 which traverse the horst are post-Carboniferous in age, as the Coal 

 Measures and the Millstone Grit are involved in the movements. 

 It is probable, however, that the greater part of the faulting is duo 

 to movements which took place in pre-Carboniferous times : the 

 southern boundary-fault, for instance, which must be of con- 

 siderable throw in the older rocks, abuts near Eoch Bridge against 

 a post-Carboniferous north-and-south fault, but no fault of corre- 

 sponding importance can be traced in the Coal Measures along the 

 same line. 



The existence of great pre-Carboniferous movements, and the 

 consequent overstep of the Carboniferous rocks on to the older 

 formations, seems the most reasonable way of accounting for the 

 association in this district of Carboniferous strata with rocks of 

 much greater antiquity. Any other view involves the existence 

 of faulting on a prodigious scale, and is contrary to the evidence 

 obtainable in other parts of Pembrokcvshire. 



Y. Summary and Conclusions. 



About 8 or 10 miles to the east of St. David's lies an extensive 

 area of plutonic and volcanic rocks surrounded by and' intimately 

 associated with sedimentary rocks of the Cambrian System. 



The igneous and pyroclastic rocks are brought to the surface along 

 an axis, ranging in an east-north-easterly and west-south-westerly 

 direction, which is a] .proximately parallel to that of the ancient 

 ridge of St. David's. They are divisible into two classes : an older 

 volcanic series and a newer plutonic and hypabyssal series, for 

 which Hicks's names ofPebidian and Dimetian are respectively 

 retained. 



The Pebidian is separable into several types, the lower exposed 



