£14 MESSES. P. LA.KE AND S. H. KETKOLDS ON THE [Aug. 1 896, 



direction from the beds below. It was, however, visible only in the 

 stream. Probably better exposure would disclose other small 

 complications ; but the general structure of the slope is clear. 



Bryn-rhug. — The chief bed of lava just mentioned can be clearly 

 traced over the fields both S.E. to the diabase of Mynydd Gader, and 

 N.W. to the stream at Bryn-rhug ; and it affords a good datum-line 

 for correlating the section in Nant Ceunant with that at Bryn-rhug. 



At Bryn-rhug the lava forms the top of a waterfall of considerable 

 height, and the beds immediately below it have been quarried, so 

 that the conditions are more favourable for examination than in 

 Nant Ceunant. The beds in the quarry are dark hard slates, con- 

 taining numerous fossils referable to the following species: — Orthis 

 lenticularis, Dalm., Agnostus, Leptoplastus, and Parabolina spinulosa, 

 Wahl. (one specimen). They belong to Belt's ' Dolgelly Group.' 



Above the lava we get the same black fissile slates as in Nant 

 Ceunant. 



Tan-y-gader. — When we reach the next stream on the west, the 

 one which flows past Tan-y-gader, the lava has sunk down into the 

 level alluvial plain, and the first rocks which are visible, close to the 

 farm, are the black fissile slates. Slaty beds continue in the stream 

 up to the diabase ; but they gradually become paler in colour and 

 firmer in texture. A fragment of a trilobite was found some 

 distance above Tan-y-gader ; and near one of the small tributaries 

 close to the diabase, Lingulella lejois, Salt., was discovered, not 

 actually in situ, but in blocks which clearly belonged to the 

 neighbouring rocks. This is a Tremadoc form. 



Gilfachwydd, etc. — Similar slates occupy most of the lower slopes 

 of Mynydd Gader west of this stream, and a little south of 

 Gilfachwydd they have yielded Dictyograptusflabelliformis, showing 

 that the higher beds of this slaty series belong to the Tremadoc. 



In the upper part of one of the streams between Tan-y-gader and 

 Gilfachwydd, the slates are succeeded by a band of rhyolite, which 

 may be traced westward as far as the small diabase-mass south of 

 'Gilfachwydd. 



Similarly in the stream which flows past Gilfachwydd, the slates 

 which form most of its bed, and which near the farm have yielded 

 Dictyograptus, are succeeded on the south by a mass of rhyolitic 

 lavas and ashes. 



General sequence. — The general sequence of the beds, therefore, 

 on the northern slopes of Mynydd Gader, from the Nant Ceunant 

 westward, is as follows (in ascending order) : — 



LlNQULA- r Ffestiniog ( Bluish slates with grit-bands, containing Lingulella 

 -p, ' \ Davisii and passing up into 



,t. t l i fDark slates, with Orthis lenticularis, Leptoplastus, 



A Sk)| I etc. (at Bryn-rhug). 



and upper;. 1 Do lgelly. ./ Andesitic lava. 



I Black fissile pyritous slates, which appear to pasa 

 [ up into 



(Dark grey slates, with Lingulella lepis, Dictyo- 

 graptus flabelliformis. 

 Rhyolitic lava. 



