ROCKS IN N.W. CAERNARVONSHIRE. 285 



rounded, and cannot have come from far ; and no such larue slate- 

 fragments, as far as I have been able to discover, are to be found 

 in an}' of the conglomerates of Bangor, which must bo nearer to 

 their source if they come from Anglesey. The quartz- and jasper- 

 pebbles at Moel Tryfaen, on the contrary, are rounded and not so 

 large ; and vrhen we remember the largo quartz-pebbles at Bangor 

 and the jasper-fragments at Tairffynon, and also at Llandwrog, we 

 may well believe that these pebbles have come from their original 

 home by two stages, and that their previous resting-place was in 

 the earlier Cambrian conglomerates. It is true that the majority 

 of these " slates " are so indurated and compact that they are more 

 comparable to the lower beds near Bangor, which are regarded by 

 Dr. Hicks as Pre-Cambrian ; but among them there is a minority 

 showing that peculiar banding of purple and green which can scarcely 

 be matched in any bed near Bangor, but only further up the series. 

 Thus, in any case the pebbles are consonant with, and they may be 

 considered to confirm the Mid-Cambrian age of, the conglomerate, 

 and therefore the possibly still Cambrian age of the Llyn-Padarn and 

 Moel-Tryfaen porphyry. But on the ground of a supposed uncon- 

 formity, a long interval is claimed to have elapsed between the two 

 rocks : and this idea is likely to militate against the view of these 

 porphyries and their companions being mere incidents in the Cam- 

 brian succession. As, however, the underlying beds are volcanic 

 breccias, they are naturally irregular, and I will therefore quote 

 the words of Prof. Green in describing them : — " The unconformity 

 . . . does not necessarily indicate any great difference in age . . . 

 The breccias are of volcanic origin, and the irregular and restricted 

 upheavals and disturbances, which are always liable to occur where 

 volcanic activity is going on, are quite competent to bring about 

 unconformities quite as marked as those of the present section." 

 I may quote also the words of Prof. Hughes, which exactly express 

 my view of the matter : — " In the Llyn-Padarn section the frag- 

 mants in the agglomerates are much rounded, so as to suggest that 

 towards the close of the period of volcanic activity a larger and 

 larger proportion of the volcanic ejectamenta got worked up by the 

 action of the sea, until, at a subsequent (but perhaps not long sub- 

 sequent) period, they were all sea-washed and rolled, forming, with 

 the waste of rocky shores, the coarse conglomerate which we have 

 taken for the base of the Cambrian." These words are just as true, 

 whether we take the conglomerate as the base of the Cambrian, or 

 as a bed some distance up in the series. 



But, again, if the beds, as we find them, are approximately in 

 chronological order, the series to the west of the porphyry ought to 

 be distinct from the series on the east. As to this there is a differ- 

 ence that must at once strike every one ; on the west side there is 

 not that great mass of workable slate which is so remarkable on the 

 east, and which clings so closely to this old porphyry, coming in 

 with it at Penrhyn and going out with it at JSTant-y-llef. To go 

 further it is necessary to enter into some detailed stratigraphy, espe- 

 cially as I regret to find myself in disaccord with the distinct words 



