274 PKOJF. A. H. COX AND ME. A. K. WELLS OX THE [vol. lxxvi, 



locally. One such band occurs low down in the series, and is well exposed on 

 the path south of Waen Fechan, where it is in contact with the southern edge 

 of the diabase sill. It consists of a massive, streaky, well-laminated, ashy 

 dark-grey grit, and only giving a thin pale film on weathering. It is both 

 underlain and overlain by the normal, massive, fine-grained rhyolitic ashes. 



Higher up in the volcanic series occur two bands of felspathic grit, each 

 about 6 feet thick. These grits are greenish-grey, and closely resemble 

 some of the grits in the Bryn Brith Beds. One of the bands is seen on the 

 900-foot tump, near the granophyric marginal rocks, on the north-eastern flank 

 of Gelli-llwyd ; while the higher band is exposed immediately south-east of 

 the diabase that forms the eastern summit of Gelli-llwyd, and north-east 

 of the two small faults that cut the diabase. 



None of the three grit-bands can be traced more than a few yards, owing to 

 the presence of intrusive rocks ; but the fact that they have not been found 

 west of the Gwynant suggests that they rapidly die out in a lateral direction. 



(3) Lower Llanvirn. 



(i) The Moelyn Slates. — A band of slate succeeds the 

 ' China-stone ' Ash that, west of the Gwynant, marks the top of 

 the Lower Acid Series. The slates are well exposed near Llyn 

 Pen-Moelyn, on the northern flanks of Bryn Brith, and generally 

 speaking throughout their course hetween the Gwynant and Arthog 

 Valleys, despite the fact that the beds determine the position of 

 a marked hollow. The Moelyn Slates are of a rather darker blue 

 than any of the slates below, and they contain a good deal of 

 pyrites, wherefore scree-material is often strongly iron-stained. 

 The rocks are well cleaved, giving smooth cleavage-slabs, and they 

 exactly resemble the slates low down in the Didymograptus- 

 bifidus Beds of Pembrokeshire. They have been prospected for 

 slates on Bryn Brith, on the banks of the stream that issues from 

 Llynau Crogenen, and elsewhere ; but they have nowhere actually 

 been quarried. The band varies in thickness from 10 to 50 feet 

 in the Arthog Valley to about ]50 feet on Bryn Brith. 



East of the Gwynant the Moelyn Slates are cut out by the 

 granophyre-sill, and, except over a short stretch in the Gwynant 

 gorge, they do not reappear on the south side of the intrusions,, 

 since they (like the Pont Kings Slates below) thin out against the 

 volcanic pile of Gelli-llwyd. 



Fossils are not numerous, possibly in part owing to the strong 

 cleavage. It is, however, often the case in other districts, that 

 the lower part of the Didymograplus-bijidus Zone is not very 

 f ossiferous. The Moelyn Slates yielded I). bijidus at several 

 localities near Llyn Pen-Moelyn, and on the right bank of the 

 Gwynant immediately south of the granophyre-sill. Ortlwceras 

 cf. caereesiense Hicks was obtained from slates near Llyn Pen- 

 Moelyn. 



(ii) The Bryn Brith Grits and Ashes. — Intercalated 

 among the Bijidus Slates are the Bryn Brith Beds, a group of 

 peculiar ashy grits. The grits themselves contain much (more or 

 less silicified) argillaceotis material, distributed in such a manner 

 that the rocks often assume an agglomeratic appearance. The 



