CAMBRIAK IX CAEKXAEVOXSHIRE. 



245 



present equally well-marked characters. They are never purple, 

 but always brown or grey, and they have a habit of becoming alter- 

 nations of thin beds of grit and slate, which elsewhere shade into 

 each other, and hence a line of separation from the Bronllwyd Grit 

 cannot be drawn with accuracy. Moreover a bed of grit, often 

 attaining an immense thickness, particularly if we are justified in cor- 

 relating this with the Harlech Grits, is the more natural commence- 

 ment of a new series of rocks, in which similar grits recur, than a 

 band of slate intervening between two such grits would be. These 

 reasons alone appear to me sufficient for classing the Bronllwyd 

 Grit with the overlying rather than with the underlying- series — 

 supposing that we have to do with a conformable series of rocks. 



But on examining the junction, I think we have evidence of 

 something of the nature of an unconformity. This junction, how- 

 ever, is not often seen, since a great part of the boundary is a fault. 

 As this statement is not in accordance* with the Survey mapping, I 

 proceed to prove it. 



The boundarv on the extreme north is a curved east- and- west 



Fig. 1. — Fault in Bryn Eafod-y-Wern slate quarry {looking East). 



b. Bedded grits and brown slates. 



line, which is not a line of fault, but so soon as it begins to run 

 approximately north and south, we actually see the fault in Bryn 

 Hafod-y-Wern slate quarry (fig. 1). The fault-jDlane forms a vertical 

 boundary to the quarry on its eastern side ; and on the west of the 

 fault-plane are the massive Purple Slates, on the east the bedded 

 grits and intervening brown slates in undulating curves, which are 

 truncated abruptly at the fault-plane. A mile farther to the south, 

 in an unnamed quarry to the south-east of Achub, the faulted junc- 

 tion is shown as a convex curved surface with the massive Purple 

 Slates on one side, and on the other bands of pebbles in the grit, 

 undulating horizontally and terminating abruptly at the nearly 

 vertical face in contact with the slate. Again, in the town of 

 Bethesda the Bronllwj'd Grit is seen dipping north-east. Due north of 

 the exposures the surface of the slate is more than 100 feet higher — 

 indicating a fault between the two. South-east of Bethesda the 



