﻿580 MR. J. T. ELSDKN ON THE IGNEOUS ROCKS [Aug. I905, 



Geological Survey in 1845, under the superintendence of Sir Henry 

 do la Beche, the igneous rocks being re-surveyed in 1855-56 by 

 W. T. Aveline. 



Of the stratigraphy of this portion of Pembrokeshire there is 

 still much to learn. Hicks mapped the outcrops between St. 

 David's and Abereiddy Bay, 1 and Mr. Cowper Reed has done the 

 same for the neighbourhood of Fishguard. 2 These lines are approxi- 

 mately shown in the accompanying sketch-map (PI. XXXVIII). 

 There is, however, a good deal of uncertainty with regard to the 

 intervening area, and the scarcity of fossils increases the difficulty of 

 any attempt to fill in the stratigraphical details necessary to complete 

 the survey of this part of the county, although good exposures are 

 not lacking, both in the fine coast-section and in numerous inland 

 quarries. A prominent feature in the area under discussion is the 

 fault running in a south-easterly direction from Pwll-Strodyr on 

 the coast, about 1 mile east of Abercastle, and truncating a series of 

 narrow sills which strike east and west between Abercastle and 

 Mathry. This fault, together with the great east-and-west fault 

 extending from St. David's to near St. Edren's, isolates a triangular 

 wedge of country from the adjoining district of Strumble Head on 

 the one side, and the pre-Cambrian area of St. David's on the other. 

 In this triangle the Lingula-Flaga, Tremadoc Beds, Arenig and 

 Llandeilo Beds, form successive bands striking east and west. There 

 are other minor faults, which, however, do not need further descrip- 

 tion for the purpose of this paper. The Upper Llandeilo strata, 

 which the late Dr. Hicks recognized on the north of Abereiddy 

 Bay, seem to extend over the district of Barry, but from this point 

 northward to Strumble Head the sequence is obscure. Bala Beds 

 probably occupy the greater part of the area between Trevine and 

 Strumble Head, although the outcrops appear to have been con- 

 siderably displaced by the Pwll-Strodyr Fault. 



II. The Contemporaneous Igneous Bocks. 



The sequence of the volcanic rocks of North Pembrokeshire has 

 been already established over a considerable part of the area under 

 discussion, but to what extent Arenig lavas are represented is still 

 imperfectly known. Mr. Parkinson was not able to establish 

 definitely the age of the Prescelly lava-flows, which may possibly date 

 from Arenig time, and several bands of contemporaneous volcanic 

 rock are mapped, in strata presumably of this age, in the southern 

 portion of the triangle mentioned above. I shall, however, at a 

 later stage (p. 599), advance reasons for doubting the contempo- 

 raneous character of some, at least, of these exposures in the 

 neighbourhood of the Pwll-Strodyr Pault. 



The most conspicuous of the true lava-flows in this portion of 

 the district are seen in the neighbourhood of Llanrian, where 



Quart. Journ. Geol. Soc. vol. xxxi (1875) pp. 167 et seqq. 

 Ibid. vol. li (1895) pp. 149 et seqq. 



