G. L. Elles — Graptolite ZoneH in Arenig Rocks. 207 



tipon the gTit, and this has yielded a few specimens of D. hirundo 

 and seems to indicate the highest bed of that zone. The grit which 

 underlies the limestones appears to rest nnconformably upon 

 calcareous flags, and in these and the beds beneath it is worthy 

 of note that the dip practically coincides with the cleavage, and 

 hence the difference in dip between the two series, while similar 

 in direction, differs to the amount of about 20°. There may also 

 be some faulting along this line. The ash of the lower series seems 

 to be a lenticular patch ; it never crosses Nant rhos ddu, and its 

 presence seems to be the factor determining the south-south-east 

 course of the stream, for with its appearance the stream turns 

 south-south-east, and with its disappearance resumes its original 

 direction. The age of the lower series seems to be sufficiently 

 determined by tlie presence of AsapJiiis JTomfrayi, Salt., Agnostus, 

 sp., and Olenus triartlirus, Wale, which have been found in several 

 localities in beds dipping 40° E.N.E. They seem to be the beds 

 designated Lower Arenig by the officers of the Geological Survey.^ 

 The Arenig beds are probably present below the ash band on the 

 slopes of Ffridd Bwlch-llestre ; they are, however, only exposed 

 at one other locality', namely, in the stream north-east of Bryn- 

 maenllifo, where they are found beneath the basal agglomerate of 

 the main volcanic group. 



On the other side ot the valley the most continuous exposure of 

 the Arenig beds is found on the north-west slopes of Arenig Fawr, 

 in the stream running down to Hafodty Milltir Cerig (Hafody 

 Filltir. Gerig). Here the succession is as follows : — 



Felspatliic ash (?) of main volcanic group. 

 / Thickly hedded ashes, ashy flags, and shales, mth Ogygia Selwyni 

 \ (Salt.), Monoholina [Obolella) pltcmbea, Machtrea, sp., Cahjmene, sp. 

 < Thinly bedded slates and flags with D. bifidus. 

 I Grits. 

 \ Finely bedded banded shales. 



Diabase intrusion. 



The thickly bedded ashes, ashy flags, and shales are very like those 

 of the Taihirion side, but somewhat thicker, and the underlying 

 slates contain D. bifidus and seem to represent the zone of D. bifidus. 

 The lower zone of D. hirundo is probably also present, since in 

 a collection made by Professor Hughes from this locality there are 

 several specimens of AzygograjAus suecicus in an impure limestone, 

 and Az. suecicus is commonly found associated with B. liirundo 

 elsewhere ; I have not myself, however, detected the zone in situ. 

 The rocks below the diabase intrusion seem to be of Tremadoc age. 



South Wales. 



The graptolite-bearing beds of South Wales, for the sake of 

 convenience, may be considered as belonging to two districts — 



(1) Western or St. Davids District. 



(2) Eastern or Haverfordwest-Carmarthen District. 



^ " Geology of Xorth "Wales," vol. iii. 



