﻿Vol. 6 I.] BETWEEN ST. DAVID's HEAD ATS T D STETJMBLE HEAD. 593 



We pass now to the series of thin parallel intrusions in the neigh- 

 bourhood of Trevine, several of which emerge on the shore at 

 Aberfelin. One of these, near the footpath leading to the beach, 

 bears a close resemblance to the variety described above. The bulk 

 of the rock consists of felspar, which, however, is associated with a 

 rather abundant groundmass of micropegmatite and quartz. The 

 extinctions and refractive index again point to albite as the 

 dominant species, the orthoclase being confined to the micro- 

 pegmatite. Some enstatite in rather fragmentary, much altered 

 shreds seems to be present, and there are irregular strings of 

 ilmenite and leucoxene, with a few specks of pyrites. Apatite is 

 still fairly abundant. Secondary patches of calcite are present, and 

 a little vermicular chlorite resembling the species which has been 

 described as helminth. 1 Augite again seems to be wanting. It 

 is possible that this may be an offshoot from the Porth-Gain mass, 

 the last-described variety of which it much resembles. 



The other intrusions, all of which seem to be narrow sills, have a 

 totally-different character, so far as I have been able to examine 

 them. They are distinctly more basic in composition. The felspars 

 have a higher refractive index than balsam, and the symmetrical 

 extinctions of twins on the albite- type correspond to a basic 

 oligoclase or andesine, in some cases approaching that of labra- 

 dorite, Ab 1 An 1 . There is much pale-brown augite, more or 

 less ophitic, and a few relics of scarcely-recognizable enstatite, 

 although some of the rather abundant chlorite may represent it. 

 Quartz is scarce, except where the felspars are much decomposed, 

 and apatite is absent or very rare. Iron-ores in the form of 

 ilmenite, more or less altered, and pyrites occur. These rocks 

 would be called diabase ; but, as enstatite undoubtedly occurs, 

 I am inclined to associate them both with the St. David's-Head 

 intrusions and with the typical diabase of the Llanwnda type. 

 It is highly probable that they represent a mixture of the two 

 types. 



I was unable to confirm the colouring on the Geological-Survey 

 maps to the east of Trevine, as the conditions did not permit of my 

 tracing the continuity of these sills into theAbercastle-Mathry area, 

 on account of the unfavourable time of year, much of the country 

 being inaccessible owing to growing crops of corn. So far as my 

 observations went, however, these diabase-sills seem to be absent 

 from the neighbourhood of the fault. 



Ynys-y-Castell is a small islet lying off the north of Abercastle 

 Bay, only accessible at low water. It consists of an igneous in- 

 trusion of fine grain and bluish-grey colour. In the hand-specimen 

 this bears a close resemblance to the rock of Porth-Gain Quarry, but 

 under the microscope it appears to be of a distinctly-basic character 



1 Otto Meyer, Zeitschr. d. Deutsch. Geol. Gesellsch. vol. xxx (1878) p. 21 ; 

 and E. Hussak, Tscherm. Min. & Petrogr. Mitth. n. s. vol. i (1878) p. 275. 



Q. J. G. S. No. 243. .2 t 



