THE NEIGHBOURHOOD OF BANGOR. 479 



stand him — all, in short, but the greenish breccia which occurs on 

 Bangor mountain, Professor Hughes considers to be only the Cam- 

 brian conglomerate repeated by faulting, so that the series sepa- 

 rating the quartz felsites from the conglomerate at the base of the 

 Cambrian is reduced to the above-named green breccias and the 

 gritty slates in association with them — that is to say, rather less 

 than one half of the thickness which I should assign to it. Of the 

 faults which are needed for this repetition I have not been able to 

 find satisfactory evidence in the field. The principal argument for 

 their existence appears to be the hypothesis that one conglomerate 

 is equivalent to another, an argument which it is obvious stands or 

 falls with the hypothesis. 



I commence, then, my revision of the district with the eastern 

 edge of the great mass of quartz felsite near Brithdir Farm. In 

 apparent succession to this (I could not discover any evidence for 

 the intermediate band of slate inserted on Prof. Hughes's map) we 

 find (on both sides of the lane which mounts in a S.S.W. direction 

 from the main road along the bed of the valley) a grit mainly (if 

 not wholly) composed of quartz, felspar, and felsite fragments, occa- 

 sionally becoming conglomeratic and containing pebbles of felsite, 

 which can be traced pretty continuously round the brow of the hill 

 near Wern, until, on its northern scarp, we again come to the main 

 mass of felsite. We pick it up by the high road in the valley 

 below (on the other side of the fault), still in apparent sequence 

 with the felsite ; we follow it by Taffarn newydd to Beulah Chapel, 

 where it is still in sequence, and find it thus in the field-way 

 leading to a farm opposite to the lodge of Gorphwysfa, and it is 

 seen for the last time just above the west entrance of the western 

 tunnel. This felsite grit, the constituents of which certainly seem 

 to have been derived mainly, if not wholly, from the adjacent 

 felsite, appears to be succeeded near the turning to Wern Farm by 

 greenish gri^s and a little purplish slate, over which comes a rock 

 rather like that at Tairffynnon quarry, varying from a green grit 

 with reddish spots to a coarsish breccia or conglomerate more like 

 that in the above quarry. It is also worth notice that about 80 yards 

 to the east of the above-named tunnel-entrance is a small outcrop 

 of a similar rock, and many fragments are to be seen in a spoil- 

 bank consisting of materials which have doubtless been obtained in 

 making the tunnel. 



It seems, then, not unreasonable to suppose that this grit and 

 conglomerate, which, for purposes of reference, I will call A, thus 

 seen at intervals between two points about If mile apart in a 

 straight line, is in true sequence with the felsite, and is not brought 

 by a fault into its present position. 



The next breccia which I have described (we will call it B) is well 

 exposed in a pit in the wood at Tairffynnon, and some overlying 

 beds of a more slaty character are seen in the lane. As mentioned 

 in my paper in the ' Geological Magazine,' I have since traced it on 

 the upper parts of the ridge to the N.N.W. (roughly) of that pit ; 

 and in 1 880 I found that a new pit opened by the Caernarvon road 



