Vol. 62.] IGNEOUS AND SEDIMENTARY ROCKS OE LLANGYNOG. 249 



One of the most difficult problems that confronts us is that which 

 concerns the age of the extrusive and intrusive rocks of the district 

 as a whole. There is, however, as we have seen, little doubt that 

 the whole series of extrusive rocks as developed at Coomb and 

 Bethesda may be referred to one geological horizon. The mapping of 

 surrounding districts in South "Wales has shown us that, in the 

 Ordovician System, we might expect to meet with volcanic rocks 

 on any of three horizons : (a) near the top of the zone of Didymo- 

 graptus Murchisoni ; (b) near the base of the zone of D. bifidus ; or 

 {c) low down in the Arenig Series ; for, at all these horizons, well- 

 developed ash-beds have been noticed. 



At Llangynog the first two cases may be dismissed as impossible, 

 for the quantity of ashy material is much greater on the north side 

 of the main anticline, while Llangynog is on the south side, where 

 the Didymograptus- Murchisoni ash is not developed, and the 

 D.-bifidus ash is represented by sandstones and grits. 



We are, therefore, led to the conclusion that the igneous 

 rocks may belong to the lower part of the Arenig Group, 

 or, failing that, to some much older group of rocks of which no 

 sediments exist within the district. 



East of Llangynog, in the neighbourhood of Caermarthen, there 

 seems to be perfect conformity between the Tremadoc rocks with 

 Peltur a punctata (Crosfield & Skeat) and the Arenig Series, of which 

 the lowest part consists largely of conglomeratic grits and sandstones. 

 These conglomeratic grits are those which are again brought up by 

 an anticlinal fold in the neighbourhood of Llangynog. It is 

 improbable, although not impossible, that the Tremadoc rocks should 

 have been overlapped, and so great an unconformity developed at the 

 base of the Arenig Group within a few miles : which would be the 

 case, if the igneous rocks were of pre- Arenig age. It is a significant 

 fact that the igneous masses, on one side or the other, are always 

 bounded by a member of the oldest Arenig sediments, and it seems 

 that the bulk of the evidence points to the age of the extrusive 

 rocks being that of a low horizon in the Lower Arenig Group. 



With regard to the intrusive masses, it is obvious, at Coomb, that 

 they are newer than the other igneous rocks ; while at Lambstone, 

 Bethesda, and Pentre-newydd, it may be proved that they were 

 intruded after a certain portion of the Lower Arenig beds had been 

 deposited. 



VII. Summary. 



To sum up : we find that the older sedimentary rocks of the 

 neighbourhood of Llangynog are referable to the Didymograptus- 

 bifidus Beds and Tetragrajitus-Be&s of the Arenig Series. They 

 take the form of two main anticlines due to pre-Old Bed Sandstone 

 folding, and are unconformably overlain by Lower Old Bed Sandstone. 

 The whole sedimentary series has been subsequently affected by 

 folding and faulting, which took much the same direction. 



The igneous rocks occur in three areas, which all belong to 



