142 PROP. T. M f KENNY HUGHES ON THE 



green slates of the Lake District we not unfrequently see three or 

 four small faults shown on one slate. 



Professor Bonney has kindly made a microscopic examination of 

 some selected specimens, and given full notes of the results, which 

 are appended, and quite confirm the view of the volcanic origin of 

 the beds (Appendix, Nos. VII.-IX.). 



Below the grit and conglomerate of Llanddeiniolen we do not find 

 this great series of slates and agglomerates ; but at a short distance 

 below them we come upon the massive quartz felsite which forms 

 the greater part of the ridge, and at the S.W. end we find this pass- 

 ing down into the coarser crystalline quartz-felspar rock of Caer- 

 narvon, which, as Prof. Eamsay remarked, but for the absence of 

 mica, looks like a true granite (Mem. Geol. Surv. vol. iii. p. 162). 

 There are, however, divisional planes and successive variations of 

 lithological character, which make it highly probable that it is only 

 an altered rock, though perhaps originally in part of volcanic origin ; 

 and the microscopic investigation of Professor Bonney appended 

 (Wo. III.) makes this inference more sure. 



As to the relations of the Cambrian, the Bangor volcanic series, 

 and the Caernarvon quartz-felspar rock, to one another, there is much 

 in the behaviour of the beds on Bryniau-Bangor to suggest that the 

 conglomerate is transgressive across the edges of the volcanic series ; 

 and near Llanddeiniolen it appears to have overlapped the whole of 

 the Bangor beds, and to rest on the upper Caernarvon beds, or quartz 

 felsites ; but whenever we get the two series in close juxtaposition, 

 there is such a coincidence of dip and strike as to render the infer- 

 ence of unconformity from the mapping only, in that faulted district, 

 at present unsafe ; and, moreover, if the Bangor beds are a volcanic 

 series, we must not depend upon this kind of evidence in proof of 

 an unconformity, as we might if we had to do with ordinary marine 

 sediment, like the Lingula Flags, for instance. 



In the Llyn-Padarn section, especially, the fragments in the ag- 

 glomerates 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 subsequent) period they 

 were all sea-washed and rolled, forming, with the waste of rocky 

 shores, the coarse conglomerate we have taken as the base of the 

 Cambrian. 



This must be left for the present a doubtful interval, just as there 

 is some doubt as to the relations of the Coniston or Bala limestone 

 series to the old volcanic green slates and equivalent rocks in North 

 "Wales ; but, notwithstanding all these difficulties and other possible 

 explanations, it does seem to me that the balance of evidence is in 

 favour of a considerable break between the Cambrian conglomerate 

 and underlying Bangor series. 



There is nothing to suggest any discordancy between the Bangor 

 and Caernarvon beds ; but on this point I have not much evidence to 

 offer. 



