516 MESSES. P. LAKE AND S. H. KEYNOLDS ON THE [Aug. 1 896, 



almost continuously up to Bron-y-gader. The only fossil which we 

 discovered was a fragment of a trilobite ; but Belt states that he 

 found ' Dictyonema fenestrata, Salt.,' below Bron-y-gader. 



Above Bron-y-gader these black slates are succeeded by a band 

 of lava ; and beyond this no further exposures are visible in the 

 stream until we reach the diabase. 



But the band of lava is seen again in the hill on the west, in the 

 path near Bryn-y-gwin farm ; and farther west still it is exposed for 

 a considerable distance in the woods of Bryn-y-gwin. Everywhere 

 it is underlain by black slates similar to those in the stream, and 

 overlain by a second band of black slates, in which we discovered 

 Obolella Sabrince, Call., at two localities. The upper part of the 

 ■second band close to the diabase sometimes contains fragments of 

 ash. 



The band of lava which is shown upon the map is, like that in 

 Nant Ceunant, an andesite ; and, as in Nant Ceunant, we have 

 •slight indications of the existence of a smaller volcanic band below 

 it. About 350 yards above Pandy'r-odyn a small ashy looking 

 band crosses the stream ; and a somewhat similar band is rather 

 badly exposed at the farm of Bryn-y-gwin. 



Although in the absence of better palseontological evidence, there 

 is no absolute proof that the beds in this area are the same as those 

 in Nant Ceunant, yet the correspondence between the two sequences 

 is so close as to leave little room for doubt ; and we are justified 

 in considering the Bryn-y-gwin lava to be the continuation of that 

 in Nant Ceunant. 



West of the Derwas fault. — West of the little valley where the 

 lava and black slates of the Bryn-y-gwin woods abruptly cease, the 

 predominant rocks are bluish slates with bands of grit. East of 

 Derwas they dip S.E. or S.S.E., while near Gwern-y-barcud they 

 dip due south. Close to the latter farm we found Lingulella 

 Davisii. 



These beds are succeeded, as in Nant Ceunant, by darker coloured 

 slates, which lie along the northern margin of the diabase. In one 

 of the small woods a little west of Maes-angharad they yielded 

 Parabolina spinulosa in abundance, and one specimen which looks 

 like Arionellus. 



The slates with Lingulella Davisii are clearly the equivalents of 

 those near Tan-y-fedw, and the beds with Parabolina spinulosa 

 probably lie at a somewhat lower horizon than the fossiliferous 

 slates of Bryn-rhug. The latter are close to the lava-band, and 

 seem to be separated from the L. Davisii-be&s by a greater thick- 

 ness of deposit. 



(c) Upper Volcanic Series. 



On the slopes of Mynydd Gader the beds which contain Lingu- 

 lella lejpis and Dictyograptus fiabelliformis are succeeded, as already 

 shown, by a band of rhyolite ; and towards the western end of the 

 hill this is followed by a considerable area of volcanic rock which 

 we have not yet examined in detail. 



