﻿Yol. 6 1.] BETWEEN ST. DAVID'S HEAD AND STRUMBLE HEAD. 597 



Kjerulf. Some specimens of the xlbercastle rock show a similar 

 porphyritic tendency. With the exception of the occurrence in 

 Ireland, noted above, the only locality from which bostonite has 

 hitherto been described in this country is in the Orkney Islands, 1 

 the analysis of which (as given by Dr. Flett), however, so far as the 

 alkalies are concerned, diners materially from that of the Abercastle 

 rock. It is notable that the latter rock resembles the bostonites in 

 its marked tendency to decomposition, as is seen in the presence of 

 a considerable proportion of carbonic acid and combined water. I 

 have appended, for purposes of comparison, an analysis by Dr. Hatch 

 of a keratophyre from Wicklow, which, although closely agreeing 

 in the proportions of soda and potash, is in other respects a very 

 different type of rock. 



To the north of Abercastle, on the coast opposite Ynys-y-Castell, 

 is another igneous exposure. From its position, this might be 

 taken for a part of the Ynys-y-Castell diabase-intrusion. A 

 specimen, however, taken from the top of the cliff about midway 

 between Abercastle Bay and Porth Gwynion, looks very like a coarse 

 agglomerate, such as is commonly seen in the Goodwick volcanic 

 series. It contains angular greenish-grey fragments in a darker 

 matrix. Differential weathering results in the removal of the 

 angular fragments, leaving hollow spaces separated by walls of the 

 more resistant matrix, a feature which may have some bearing upon 

 the interpretation of the origin of this rock. 



Under the microscope, the angular fragments are seen to consist 

 of the bostonite-like rock, described above. The matrix is a finely- 

 granular material, through which small bostonite-fragments are 

 scattered. No fragments of other rocks are recognizable, and I am 

 inclined to think that this is a brecciated, tuff-like intrusion some- 

 what similar to those which Messrs. Kilroe & McHenry have described 

 in the South-East of Ireland. 2 Microscopic examination tends to 

 support this view (see PI. XL, fig. 5). The extreme angularity of 

 the fragments, the thin shreds of uncrushed rock which occur in 

 the matrix, and the small proportion of the matrix compared with 

 that of the coarser fragments, tend to confirm the theory that this 

 rock has been crushed in situ. If this be the case, the greater 

 resistance of the matrix to weathering can only be explained by 

 some secondary process which has indurated the powdered rock. 



It may be noted that I found a precisely-similar brecciated rock, 

 with the v same microscopic structure (PL XL, fig. 6), in a small 

 exposure at Priskilly Eawr, about 4 miles south-east of Abercastle, 

 and not far from the Pwll-Strodyr line of fault. The exposure 

 is very small, consisting of a patch only a few square yards in 



1 J. S. Flett, ' The Trap-Dykes of the Orkneys ' Trans. Eoy. Soc. Edin. 

 vol. xxxix (1900) p. 873. 



2 ' Intrusive, TutF-like, Igneous Rocks & Breccias in Ireland' Quart. Journ. 

 Geol. Soc. vol. lvii (1901) p. 479. 



