﻿Vol. 6 1.] BETWEEN ST. DAVID'S HEAD AND STRUMBLE HEAD. 607 



Fig. 6. Enstatite-diabase, Tresseysilt, near St. Nicholas. Two crystals of 

 almost unaltered enstatite are seen enclosed in augite. The crystals 

 are similarly oriented, and, lying nearly at right angles one to the 

 other, sbow simultaneously two axial colours with the single nicol. 

 X 30 diams. [The clear spaces in this figure are merely holes in the 

 rock-slice.] (See p. 594.) 



Plate XL. 



Fig. 1. Lime-bostonite, from Long House, near Abercastle. X 80 diams. 

 (See pp. 594-05.) 



2. Oligoclase-porphyrite, from Carnachen Wen, near Abercastle. X 30 



diams. (See p. 598.) 



3. Pyroxene-porphyrite, Cwm-y-Grraig, near Mathry. The large crystal 



has the appearance of a rhombic pyroxene. X 30 diams. (See 

 pp. 598-99.) 



4. Lime-bostonite, 1 mile north of Mathry. X 20 diams. (See p. 598.) 



5. Brecciated lime-bostonite, from the cliff north of Abercastle. x 30 



diams. (See p. 597.) 



6. Brecciated lime-bostonite, from Priskilly Fawr, 3 miles south-east of 



Mathry. X 30 diams. (See p. 597.) 



Discussion. 



Prof. Watts referred to the fact that the Author had dealt with 

 the igneous rocks associated with the Ordovician in greater detail 

 than hitherto. Many of the Author's types seemed to occur in 

 Shropshire and Montgomeryshire. He wished to know whether 

 the Author had succeeded in ascertaining the age of the intrusive 

 rocks. The sudden transitions between gabbro and diabase observed 

 in this district were also to be seen in Shropshire, and even in Skye. 



Prof. Judd congratulated the Author upon the accomplishment 

 of a very valuable piece of detailed petrographical work. He 

 stated that a parallel for the apparently-capricious association 

 of fine-grained and coarse-grained varieties of rock remarked 

 upon by the Author could be found in many intrusive masses. 



Mr. J. Lomas joined in the congratulations which had been 

 accorded to the Author, and agreed with Prof. Watts that rocks of 

 similar types were found in the Berwyn Hills and other parts 

 of North Wales. He further asked whether any of these rocks 

 were really undoubted interbedded lavas, as work in North Wales 

 had tended to show that many rocks mapped as such were intrusive. 



The Author said, in reply, that he was not able to bring forward 

 any evidence as to the precise age of these intrusions, but those of 

 the South-East of Ireland appeared to agree generally in time with 

 those of the Breidden Hills. W T ith regard to the sudden and 

 apparently-irregular transition from rocks of coarse to those of 

 finer grain, this seemed to point to the conclusion that the size 

 of crystals was determined by various factors and not alone by the 

 rate of cooling. As to contemporaneous igneous rocks, the Llanrian 

 acidic lavas were obviously of this nature ; but whether these 

 Llanrian sheets were continued to the east into the Mathry district, 

 he could not definitely state. He thanked the Fellows present for 

 the manner in which his communication had been received. 



