﻿292 ME. J. Y. ELSDEX OX THE [MaV I908. 



The range of specific gravities of these rock-types accords well 

 with the hypothesis of a differentiated magma-basin. This is shown 

 by the following table : — 



Specific gravity. 



Aphte 2-62 



Quartz-enstatite-diorite 2*87 



Enstatite-diabase 2'92 



Biotite-norite 2-96 



The extreme basic phase of this series would be a pyroxenite, 

 which might be expected to occur in connexion with these intrusions. 

 Although none of my specimens can be referred to this type, it is 

 approximately represented by the dark streaks in the banded 

 varieties. 



YI. A&E OF THE IXTRrSIOXS. 



It remains to consider whether any conclusions can be drawn 

 as to the age of these intrusions. In connexion with this point 

 it is significant that, although the Arenig strata have been sharply 

 folded, the igneous masses show no sign of movement since their 

 consolidation. In the field there is a conspicuous absence of 

 shear-planes and other signs of fracture. Under the microscope 

 the constituent minerals are remarkably free from any of the 

 usual evidences of dynamic metamorphism. I take this to be an 

 indication that no profound earth-movements have affected this 

 region since the consolidation of the injected rocks. The key, 

 therefore, to the age of these intrusions seems to lie in the 

 period of the last great disturbance which com23leted the tilting 

 of the Cambrian and Ordovician rocks of Pembrokeshire, and 

 compressed them against the pre-Cambrian ridge of St. David's, to 

 the axis of which their strike is approximately parallel. ISTow, the 

 Llanrian lavas, lying a few miles farther north, are of Llundeilo 

 and Bala age, and these rocks have been affected by this com- 

 pression. The St. David's-Head intrusions are, therefore, evidently 

 younger than the Bala period, and may have been injected during 

 the post-Bala disturbances. At the same time, the possibility that 

 they may be of post-Carboniferous, or even of still later age, 

 cannot be altogether excluded. 



'&'- 



YII. SUMMAET AXn CoXCLUSIOX. 



I will now briefly recapitulate the conclusions suggested by the 

 preceding investigation : — 



(1) The St. David's-Head and Carn-Llidi intrusions are of 

 complex composition, ranging from a basic biotite-norite to an acid 

 quartz-enstatite-diorite, and finally soda-aplite. Throughout all 

 the types, except the aplite- veins, there is a high magnesia- 

 percentage. 



(2) The extreme types sometimes pass sharply one into the other, 

 at other times are mixed in various proportions. 



