﻿Vol. 6 1.] BETWEEN ST. DAVId's HEAD AND STRUMBLE HEAD. 581 



the lowest beds, exposed as tuffs on the south of Abereiddy Bay, 

 were taken by Hicks as the base of the Llandeilo Series. These 

 outbursts are, therefore, of the same age as the lowest volcanic 

 series of Fishguard, as described by Mr. Cowper Reed. The higher 

 Llanrian lavas correspond with those of Goodwick, and are of 

 Upper Llandeilo and Bala age. Examination of these Llanrian 

 lavas shows them to be thoroughly acid in character, and to 

 resemble, in a general sense, those of the Fishguard and Prescelly 

 areas. By the courtesy of Mr. Parkinson, I have been able to 

 compare his slices of the Prescelly lavas with my own specimens 

 from the Llanrian district, with which (except that I have not 

 noticed very pronounced spherulitic structures at Llanrian) they 

 agree fairly well. There does not seem to be any reason to doubt 

 that, from a petrographical point of view, the eruptive rocks of the 

 Prescelly, Fishguard, and Llanrian areas are of similar type. My 

 Llanrian specimens may be briefly described as containing pheno- 

 crysts of quartz and monoclinic felspar in a microfelsitic matrix ; 

 but, in view of the very detailed descriptions of similar acid lava- 

 flows given by Mr. Cowper Eeed and Mr. Parkinson, I do not 

 think it necessary, for the purpose of the present paper, to dwell 

 further upon this portion of the subject. 



Higher in the Bala Series come the thin volcanic beds of Porth 

 Sychan and Porth Melyn on Strumble Head, of which I have 

 noticed no equivalents farther along the coast to the south-west. 



Some of the exposures mapped by the Geological Survey as 

 contemporaneous lavas, in the neighbourhood of the Pwll-Strodyr 

 Fault, are of a character totally different from these acid lava-flows. 

 I allude to certain exposures in the neighbourhood of Mathry, 

 to which reference will again be made in another section of this 

 paper (p. 599). 



III. The Intrusive Rocks of Strumble Head and the 

 Adjoining District. 



An extensive series of intrusive basic rocks breaks through both 

 the lavas and sedimentary rocks of the Fishguard district. These 

 are described in Mr. Cowper Reed's paper, and very similar rocks 

 are described by Mr. Parkinson from the Prescelly area. I need not, 

 therefore, enter into details of these rocks, more than is necessary to 

 illustrate their relation to the intrusive rocks of the district farther 

 to the south-west. The prevailing type is a normal diabase, 

 more or less ophitic, without olivine, and with an abundance 

 of chlorite and epidote. They are moderately-basic rocks, and my 

 observations tend to confirm Mr. Cowper Beed's statement that the 

 felspars generally belong to the oligoclase-andesine series. Very 

 often, however, they are too much altered for optical determination. A 

 typical development of this rock occurs at Llanwnda, near Goodwick, 

 where an extremely coarse-grained variety of the gabbro-type is 

 capriciously intermingled with the ordinary type. I shall refer to the 

 general type of these basic intrusives as the Llanwnda type, 

 in which I include both the Fishguard and the Prescelly rocks. 



