258 KEV. J. F. BLAKE ON THE ROCKS MAPPED AS 



Minffordd to the Pooriiouse, forming a synclinal with the axis along 

 the high ground. This is consonant with the apparent running of 

 the Tairffynnon Conglomerate below, but it requires a fault to run 

 from the east side of Minffordd to the station, an arrangement which 

 a reversed dip in the road north of Minffordd renders probable. If 

 this be correct, then both Prof. Bonnej^'s map and my own previous 

 map must be wrong in this respect, as neither recognizes more than 

 one breccia. Eut, however they run, there they are with 

 associated Laminated Grits and hard slates. There is indeed a 

 breccia at a higher level at Bangor which has been noticed by 

 previous authors, though it is of a different character, being more 

 earthy, and in any case is quite exceptional, so that we may say 

 that this type of breccia is characteristic of the lower part of the 

 series. 



Crossing the faulted Silurians, we find this breccia well shown 

 in the cutting half-way between Felin Hen and Tregarth (fig. 7, 

 p. 253), with a well-marked easterly dip. Following the strike 

 thus indicated we find it again in numerous exposures leading by 

 "Waen-hir to Craig-llwyd on the 6-inch Ordnance map, where 

 it meets a fault, on the other side of which is Purple Slate dipping 

 towards it. It occupies, therefore, in this locality exactly the same 

 position as at Bangor. Farther south, below Moel-y-Ci, there are 

 numerous crags ranged in a N.E. and S.W. direction consonant with 

 the bedding of the overlying banded series, which show this peculiar 

 breccia to perfection, here again in its expected place (see fig. 6, 

 p. 253). Finally, to the west of Dinas Mawr we find identical rock 

 crossing the road in many spots round Pen Isa'r Waen as we 

 should now expect if the overlying beds have been rightly interpreted. 

 The fragments of slate are here rather more rounded than usual. 



In the case of grits, banded slates, and conglomerates, we are 

 always liable to confusion between one example of the kind and 

 another, though we may keep oursehes fairly straight by attention 

 to stratigraphy. But with a rock like this a mistake seems scarcely 

 possible ; thus we have at least one fixed point, in a correlation of 

 the beds above and below in the different areas. The fixed point is 

 the top of these Bangor Breccias, for they may be repeated below, 

 and the interval which separates them from other noted beds may 

 vary considerably. 



§ 9. The Blue Lamii^ated Grits and Tatrfpynnon Conglomerate. 



The typical Bangor Breccias are separated in the Bangor district 

 by a considerable mass of Laminated Grits from the Tairffynnon 

 Conglomerate, below which somewhat similar rocks come on in force 

 and in places become more irregular and compact, like one of the 

 Anglesey pelites. These rocks are well known from Prof. Bouney's 

 description,^ so they need not be dwelt upon here, yet these Blue 

 Laminated Grits are exceedingly characteristic and merit special 



1 Quart. Journ. Geol. Soc. vol. xxxv. (1879) pp. 317 & 319 (2). 



