PEE-CAMBRIAN ROCKS OF BANGOR. 141 



phwysfa the beds, which seem to pass under the conglomerate, are 

 first pale altered slates, cut by a greenstone dyke, and below them a 

 crystalline brecciated conglomerate. Over this area the balance of 

 evidence is rather against there being any visible unconformity at 

 the base of the Cambrian conglomerate, as it seems to be succeeded 

 always in descending order by similar beds. 



It is highly probable, to say the least, that a considerable 

 fault runs from near Bangor Station almost in a straight line S.W. 

 to where the railway crosses the more southerly of the two faults 

 drawn in the Survey map. This will explain the N.E. and S.W. 

 trend of the ridges of agglomerate seen near Traws Canol and 

 Penrhos Garnedd, and would explain away some difficulties near 

 Penychwynfan. 



On the S.E. of the Bangor and Caernarvon road, that runs along 

 the valley by the station at Hendre Wen, about half a mile S.W. of 

 Bangor, there is a large quarry in which, following a greenstone 

 dyke, they have exposed a thin bed of crystalline conglomerate, 

 dipping N.W. About three quarters of a mile further to the S.W., 

 near Tai'rfiynnon, lenticular beds of pale altered slate occur in a 

 purple brecciated conglomerate, dipping N.E. Hornstones and vari- 

 ously altered slates crop out near Tafarn Newydd ; and beds of fine 

 brecciated and ash-like deposits and grey or speckly granular altered 

 rock are seen along the road near Tyddyn du ; and near Brithdir 

 there is a purple porphyry associated with a breccia. The relation 

 of this to the associated beds I have not yet made out. 



The bedding of the Pre-Cambrian rocks is so far well defined, and 

 near Bangor, generally in an easterly direction, sometimes a little 

 S., sometimes a little N. of E., but always oblique to or across the 

 trend of the hills. When we get about a mile S.W. of Bangor, ex- 

 cept in the bit between the two faults S.W. of the Station, the pre- 

 valent dips are N.E., i. e. quite across the ridge, and so persistent 

 and high that we must infer a very considerable thickness of rocks, 

 while the variations in their lithological character are such as to 

 lead us to believe that it is a fair descending series, and not a repe- 

 tition of the same rocks by folds. I will not, however, on such in- 

 complete evidence, offer any more exact estimate of thickness. 



This series can be matched almost bed for bed among the green 

 slates and porphyries of the Lake District, and, like these, may be 

 referred to an original volcanic origin and some subsequent meta- 

 morphism ; but they do not present a sequence like the lowest part 

 of the Cambrian, among which we find no beds that could by any 

 process be changed into the alternating agglomerates and ashes of 

 the Bangor volcanic series. 



The included fragments are chiefly bits of the associated rocks, 

 just as old lavas and other volcanic rocks at the present time get 

 broken up and ejected in later eruptions. The finer slates are 

 traversed by small faults, such as might be caused by shocks 

 soon after their deposition, but which have been healed by the 

 pressure which long afterwards produced the cleavage. So in the 



