276 LOWER PALEOZOIC OF AKTHOG-DOLGELLEY. [vol. lxXVl. 



occasionally to give rise to distinct slaty beds. Examples have been observed, 

 however, in which the maximum elongation of the slaty patches followed 

 the cleavage, and cut across the bedding quite obliquely. Such cases appear 

 to be exceptional, and their present position is due to distortion under the 

 cleavage stresses. 



The silicified ' pebbles ' continue for a foot or two, up into the overlying 

 Crogenen Slates, in which, however, the pebbles are set in a matrix of normal 

 slaty material, instead of in the ashy gritty material characteristic of the 

 Bryn Brith Beds themselves. 



Coming between two slate-bands, tbe ashy grits of the Bryn 

 Brith Group give rise to a distinct feature, which often develops 

 into a prominent escarpment. This escarpment forms, on Bryn 

 Brith itself, at an altitude of 1259 feet, the highest point within 

 the area mapped. Along the narrow scarp-ridge which culminates 

 in Bryn Brith, the dip-slope, as well as the scarp-slope, is largely 

 bare rock, owing to the steepness of the dip, which is about 60°. 



The actual sequence among the Bryn Brith Beds as seen on Bryn 

 Brith itself, remains remarkably constant all the way from the 

 Arthog Valley to a point immediately east of Llyn Ben-Moelyn. 

 There is then a break in the outcrop, owing to the transgression of 

 the gran ophyre- sill. The actual passage of the Bryn Brith Beds 

 under the granophyre is not visible, owing to some minor but com- 

 plicated faulting, due to the fact that the strike suddenly changes 

 from approximately north-east and south-west to more nearly east- 

 and-west. The uppermost agglomerate-like beds reappear, however, 

 on the south side of the granophyre near the farm of Glasdir Mawr, 

 and from this point to Gelli-llwyd they give rise to a marked 

 eseai'pment overlooking the hollow occupied by the sill. Another 

 sadden change in the strike then brings the beds on to the steep 

 dip-slope that runs down to Llyn Gwernan, and so eventually 

 under the alluvium of the fault- valley. 



As was the case in the Lower Acid Series, so likewise in the 

 Bryn Brith Beds certain differences are visible between the develop- 

 ments west and east of the Gwjmant respectively, differences that 

 assert themselves both in the actual composition of the strata and 

 in their relations to the rock-groups above and below. 



On G-elli-llwyd, fo«r example, there is a more gradual upward 

 transition into Crogenen Slates than on Bryn Brith, where the 

 change is quite sudden. More important is the fact that on Gelli- 

 llwyd the Bryn Brith Beds rest directly on the ashes of the Lower 

 Acid Series, without the intervention of any slates comparable with 

 the Moelyn Slates. In fact, there appears to be at this locality a 

 complete transition between the topmost ashes of the volcanic 

 series and the lowest ashy beds of the Bryn Brith Group, and hence 

 it is difficult to fix the exact point at which the dividing-line 

 should be drawn. The Bryn Brith Beds then on the east 

 rest directly upon and pass down into, clastic volcanic 

 rocks, from which, on the west, they are separated by 

 a considerable thickness of slate (fig. 2). The forma- 

 tion was evidently deposited during a period when a 

 large volcanic pile was undergoing rapid erosion. It 



