180 ME. H. H. THOMAS OX THE [May I9I I r 



of the Skomer Volcanic Series ; but, owing to their unfossiliferous- 

 nature, due perhaps to the conditions of deposition, they afford no 

 positive evidence as to the age of the Series as a whole, and for this 

 Ave must turn to a consideration of the relations of the Series to the- 

 fossiliferous rocks which bound it on the south and on the north. 



On Skomer and the other islands neither the upper nor the lower 

 limit of the volcanic rocks is reached ; but on the mainland at 

 Wooltack a very complete section reveals the junction of the Upper 

 Llandovery rocks with the upper portion of the Volcanic Series,, 

 while at Musclewick lower horizons of that Series are seen in 

 juxtaposition to Llandeilo Plags (p. 182). 



We will first consider the relations of the Series to the Upper 

 Llandovery. The clearest section is that displayed on the northern 

 side of Renney Slip and in the bay to the north. The rocks dip- 

 steadily southwards, and as we proceed northwards the cliffs present 

 a downward succession described in the following paragraphs. 



The steep dip-slope, which forms the northern side of Kenney Slip., 

 consists of fairly thick-bedded fossiliferous quartzites and con- 

 glomerates, 1 overlain here and there by fossiliferous shales which 

 have yielded many characteristic Upper Llandovery fossils, 2 such 

 as Coelospira liemisphcerica (J. de C. Sow.), ffliynchonetta decem- 

 plicata (J. de C. Sow.), Stroplieodonta (Brachyprion) cowpressa 

 (J. de C. Sow.), Enerinurus punctatus (Briinn), and Phacops cf. 

 stohesi (M.-Edw.). 



Below these quartzites comes a basic lava-flow with thin tuffs, 

 which occupies the small promontory of the Limpet Hocks. As 

 this volcanic horizon has been noted at other localities, it has 

 proved the most useful datum upon which to base the correlation 

 of this with other sections. Below the lava-flow comes a series of 

 grits, some of which are calcareous and have yielded an Upper 

 Llandovery fauna, a large number of fossils having been collected 

 from decalcified grits at the base of the cliff in the southern corner 

 of Anvil Bay. It is thus proved that this lava is of Upper Llan- 

 dovery age. 



These beds pass down into a thick series of unfossiliferous grits 

 and dark shales, which ultimately comes to rest on a mass of 

 dolerite. The dolerite occupies Anvil Point, and is regarded as 

 forming part of the Skomer Volcanic Series. 



The complete sequence is represented by the vertical section given 

 below (fig. 2, No. I, p. 181). On the evidence of this section,. 

 it would appear that the Skomer Volcanic Series forms the lowest 

 part of. or is older than, the Upper Llandovery. 



Another section (fig. 2, No. II) on the north, separated from 

 the first by east-and-west disturbances of considerable magnitude, 

 is to be seen in the two bays known as JefFry's Haven and 

 Mouse's Haven. 



1 These quartzites and conglomerates constitute the 'Tentaculite Grit' of 

 earlier writers. 



2 Identified by Prof. O. T. Jones, and preserved in the Geological Survey 

 collections. 



