Geological Society of London. 93 



Pehtdian, is that they belong to one volcanic series, whose members 

 are those usually recognized in eruptive areas, and whose age is 

 anterior to, and independent of the true Cambrian epoch. 



The independence of this series and the Cambrian is shown by the 

 nature of the junction at all points of the circuit that have been seen. 

 In a bay west of Nun's Chapel the junction on the beach is actually 

 a faulted one, the conglomerate being cut out, and tlie amount of 

 ashy rock seen comparatively small. To the west of this all ashes 

 have been cut out, granite and slate being in contact to the east : at 

 Caei'bwdy there is a quarter of a mile across the ashes. This shows 

 discordance. In the Solva valley the beds beneath the conglomerate 

 are again dilFerent, and up the higher reach the series on the north 

 and south side are quite distinct, showing a fault. At Trehenliw 

 the conglomerate is absent, at Ogof Goldfa there is a forked fault. 

 South of Castell the conglomerates and slates strike directly at the 

 consolidated ashes, and at Carn-ar-wig the conglomerate is actually 

 seen overlying unconformably green ashes and agglomerates, silky 

 schists being in the neighbourhood, but nowhere near the visible 

 junction. At Ogof Llesugn the appearances are due to the intrusion 

 along a fault of a diabase dyke which has caught up large fragments 

 of granite, felsite, and conglomerate, and cemented them in its sub- 

 stance ; but the granite scarcely anywhere comes in contact with the 

 conglomerate, and is nowhere intrusive. The junctions in the Allan 

 valley are all faulted with forked faults, some reversed, others 

 normal, the intervening mass often decaying. 



The supposed isocline west of the granitic mass cannot be verified 

 on an examination of the coast-section, there being great irregularity 

 and gentle synclinals not far from where the apex of the isocline 

 should be. 



With regard to the nature of the rocks which thus antedate the 

 Cambrian, the author was unable to recognize any true alternations 

 in the materials of the granitic axis, though the rock is a peculiar 

 one in the arrangement of its constituents. The felsitic rocks are 

 not independent of the granite, as they surround it on all sides, the 

 line along the north and south being specially traced. They are 

 also often intrusive into the ashes, and hence can have no definite 

 strike. The general features of these rocks are therefore most 

 easily to be matched in such volcanic districts as that of Mull. 



These results are confirmed by the structure of two outlying 

 masses, one at Ramsey Island, the other south of Points Castle. In 

 the former, at Perth Hayog, quartz-porphyry is succeeded by a band 

 of rhyolite showing flow-structure, and this by ashes and agglo- 

 merates. On the South Carn is a mass showing peiditic structure 

 and contorted lines of flow, and on the north, at Pwll Heudre, large 

 masses of banded spherulite of somewhat doubtful character, and an 

 apparent strike of E. and W. The conglomerate in this area has 

 many pebbles of the associated rocks. In the latter ai'ea we have a 

 similar series separated by a fault from the Cambrian rocks, and 

 consisting of banded felsites, weathering into apparent beds of very 

 irregular lie, and followed by an ashy series. 



