﻿Yol. 6 1.] BETWEEN ST. DAVID'S HEAD AND STRUMBLE HEAD. 605 



Coming next to the diabase-series-, we have evidence that the 

 Garn-Fawr Y-Garn intrusions are of a composite nature ; and it 

 appears to be certain that, between Strumble Head and St. David's 

 Head, there is an indefinite zone of enstatite-diabases, which seem 

 to graduate south-westward into the quartz-norite rocks, and 

 north-eastward into the Strumble-Head diabases, as if some amount 

 of mixing of two magmas had taken place locally. This seems to be 

 a sufficient explanation of the marked change which the gabbro- 

 diabase magma of the Fishguard area undergoes as it approaches 

 the norite-region, and, as mentioned above (p. 583), is quite in accord- 

 ance with the experimental results obtained by Prof. Yogt. This 

 phenomenon is noticeable around Trevine, in the St. Nicholas area, 

 and at Y Garn and Garn Boleh. Qf course, such a result might 

 equally be expected to occur in the case of a deep-seated magmatic 

 differentiation, as explained by Prof. Brogger ; but in either case 

 it would imply an approximately-contemporaneous origin for 

 these two types. There is need, however, of more detailed work on 

 the Garn-Fawr Y-Garn complex before its structure can be properly 

 understood. 



In the absence, therefore, of further evidence, I assume that at 

 a later period than that at which the lime-bostonites 

 were intruded, the strata were injected with more or 

 less contemporaneous laccolitic intrusions of a diabase- 

 magma from the Fishguard area, a norite-magma from 

 the St. David's area, and a mixed magma over an ill- 

 defined intermediate zone. It might be possible to gene- 

 ralize further, and to assume some connection between the enstatite- 

 bearing rocks of this district with those of other localities, such as 

 those of the Breidden Hills, 1 Carrock Fell, 2 Penmaenmawr, 3 and the 

 "Whin Sill 4 ; but in the present state of our knowledge such a 

 conclusion would be merely speculative. 



With respect to evidences of differentiation in situ, I am convinced 

 that a more detailed examination of this area will afford interesting 

 results, which, however, are beyond the scope of my present 

 researches. 



VIII. Summary and Conclusions. 



The observations recorded in the foregoing pages point to the 

 following conclusions : — 



1. The contemporaneous lavas of the Llanrian area agree generally 

 in character with the eruptive rocks of apparently-Ordovician 

 age in the Strumble-Head and Prescelly districts. These are 

 all of an essentially-acid type. 



1 W, W, Watts, Quart. Journ. Geol. Soc. vol. xli (1885) pp. 539 et seqq. 



2 A. Barker, Quart. Journ. Geol. Soc. vol. 1 (1894) p. 317. 



3 Id., ' Bala Volcanic Series of Caernarvonshire ' 1889, pp. 62 et seqq. 



4 J. J. H. Teall, Quart. Journ. Geol. Soc. vol. xl (1884) pp. 610 et seqq. 



