96 ME. H. LAPWOKTH ON THE [Feb. I9OO,- 



layers also dwindle from massive 10-foot grits to thin, impure grit- 

 flags, never exceeding 6 inches in thickness, and interstratified with 

 soft dark shales. Such phenomena would lead one to suppose that 

 the waters in which these grits were deposited were of greater 

 depth in the east than in the west ; or, in other words, as we 

 trace the group westward we are approaching ever closer to the 

 old land-surfaces. If so, we should expect the grits to become 

 coarser, and finally, perhaps, to be replaced by conglomerates. Such 

 may be the case ; but I have not yet had opportunities of satisfying 

 myself on this point. 



The Blue-black Shales that underlie these Cerig Gwynion 

 Grits in the central and western areas show no signs of passing 

 up gradually into the basement-beds of the overlying series. High 

 up above the Builth road, the bottom-grits are seen with this dark 

 shale below them ; and so sudden is the change from massive soft 

 black shales to arenaceous beds that one might imagine a fault to 

 exist between the two groups. A most interesting character of this 

 lower series is the double cleavage that can be traced in some of 

 the exposures below Cerig Gwynion. One of these cleavage-direc- 

 tions lies parallel to the normal cleavage of the Gwastaden Group, 

 that is, parallel to the strike of the rocks. The other, however, runs 

 at an acute angle to this, in a north-north-easterly and south-south- 

 westerly direction. The existence of this second cleavage suggests 

 that the shales are of much greater antiquity than the grits, being 

 cleaved before the grits were deposited. This would involve a time- 

 break between the two groups, and possibly an unconformity ; but 

 there seems to be no evidence in support of the latter hypothesis. 

 The Cerig Gwynion Grits in the Rhayader District always overlie 

 these dark shales, and if such an unconformity exists between the two 

 series it can be only of minor importance. Again, at the eastern 

 limits of the district, where the grits have dwindled down to extreme 

 tenuity, there seems evidence of a passage upward from these Blue- 

 black Shales to the Gwastaden Group, for on Llan Goch Hill it is 

 almost an impossibility, in the field, to say where the one group 

 ends and the other begins. 



No fossils have been detected in these dark shales, as developed 

 in the immediate neighbourhood of Ehayader. South and east 

 of the district, the same general type of rock extends for some 

 5 or 6 miles, — often intermixed with grits and sandstones, and 

 occasionally with beds of conglomerate. In the vicinity of JSTant- 

 mel and Hirfron, some 4 or 5 miles east of Rhayader, certain 

 dark seams in the arenaceous rocks yield Biplograptus foliaceus, 

 Murch. and other graptolites. There is a complete absence of 

 Monoprionidian forms ; a fact that points to these rocks being of 

 greater antiquity than those of the Gwastaden Group. 



(2) Dyffryn Flags. 



In addition to the areas that have been already described, these 

 flags (Ab) extend over a great part of the Elan Yalley. They make 



