482 PROF. T. G. BONNE Y ON ROCKS IN 



The Cambrian conglomerate (D), above the admitted Bangor 

 Series on the hill pierced by the tunnel east of the station, is as 

 different as it well can be from B and C, but somewhat resembles 

 A ; from which, however, it differs in containing numerous pebbles 

 of quartz and quartzite (exceeding in number those of felsite), and 

 in possessing a more sandy matrix. 



If, then, A, B, C, and J) are the same rock, we must suppose : — 

 (1) That whenever there is an exposure along the edge of the quartz 

 felsite for more than 1-J mile, either the whole Bangor series is 

 overlapped by the Cambrian conglomerate, or the latter by faulting 

 is exactly fitted onto the igneous rock, and is composed of mate- 

 rials wholly, or almost wholly, derived from it. 



(2) That in the space of about 1J mile, measured at an angle of 

 about 45° with the general strike, the Cambrian conglomerate 

 changes from a rock consisting almost wholly of felsite to the mixed 

 but different and more brecciated types of B and C, and then returns 

 to the more similar bed D, with its well-rolled pebbles. 



The annexed section (fig. 1) represents diagrammatically the result 

 of the observations described in the above remarks ; and when we 

 regard the general agreement in dips and the lithological character 

 of the rocks, it seems more natural to consider the whole an ascend- 

 ing series, and to extend the Bangor beds of Professor Hughes down 

 to the quartz felsites. On the map (fig. 2) I have endeavoured to 

 record the same observations as well as can be done on one of so 

 small a scale. 



As regards the district between the Menai Strait and the fault 

 running S.W. from Bangor down the valley, which has frequently 

 been mentioned above, I have little to add, notwithstanding re- 

 peated examination, to what I have incorporated in the fore- 

 going remarks. Above the conglomeratic grits therein mentioned, 

 and the great conglomerate extending from the shore by Gored 

 Gith to the eastern entrance of the west tunnel, we find nothing 

 but greenish or purplish slaty beds, occasionally slightly gritty, 

 which perhaps generally agree better lithologically with the normal 

 Cambrian than with these underlying beds; but the prevalent dip 

 appears to carry them under the conglomerate. This same con- 

 glomerate, so well exposed for a considerable distance, as traced 

 by Professor Hughes (over half a mile), is a little perplexing. 

 Felsite forms a much larger proportion of its materials than is the 

 case in the conglomerate pierced by the eastern tunnel ; and it seems 

 intermediate between this and the one near Gorphwysfa *. Still, as 

 it retains its character so uniformly for so great a distance, and ter- 

 minates within less than half a mile of the Poorhouse quarry, we 

 can hardly, I think, regard it as on the same horizon as the breccia 

 (C), or, what would be simpler, identify it with that in the Hen- 



* This resemblance, and the occurrence of a patch of conglomerate between 

 the two, caused the perplexity about the position of that near Gorphwysfa, men- 

 tioned in Geol. Mag. decade 2, vol. vii. p. 302, which was not removed till 

 1880. 



