﻿Vol. 64.] THE ST. DAVId's-HEAT) ' ROCK-SERIES.' 273 



14. The St. David's-Head ' Eock-Series ' (Pembrokeshire). By 

 James Vincent Elsden, B.Sc, P.G.S. (Read February 5th, 

 1908.) 



[Plates XXIX-XXXII— Microscope-Sections.] 



Contents. 



Page 



I. Introduction 273 



II. General Description 273 



III. Characters of the Eock-Types 277 



{a) The Basic Type. 



{b) The Acid, that is, Quartz-bearing, Type, 



((?) The Intermediate Type. 



{cl) The Aplite-Veins. 



IV. Petrographical Details 285 



V. Eelations of the Rock-Types 290 



VI. Age of the Intrusions 292 



VII. Summary and Conclusion 292 



I. Introduction. 



In a former paper, communicated to this Society,^ I gave a 

 general description of the St. David's-Head intrusions. On that 

 occasion I expressed the opinion that this area would repay a inore 

 detailed investigation, and I have since made a systematic examina- 

 tion of these rocks, both in the field and in the laboratory, 

 confining my attention to the two large parallel intrusions forming 

 the St. David's-Head and Carn-Llidi masses respectively. With the 

 exception of the general account given in my former paper, these 

 rocks do not appear to have been previously described in any 

 detail. 



Several distinct types of rock occur in these masses, and it is the 

 purpose of this paper to discuss their differences with the object of 

 examining the problem of their origin. Briefly, the main point to 

 be determined is whether these are to be regarded as an igneous 

 complex, or whether they constitute a ' rock-series,' derived from a 

 common magma by some process of differentiation. 



II. General Description. 



The two intrusions take the form of vertical sills intruded 

 between almost perpendicular shales of Arenig age. They are 

 composed of at least four types of rock, which, although closely 

 allied, present marked mineralogical differences, often easily 

 distinguishable in the field. There is a dark, basic variety, 

 typically seen in parts of Carn Llidi, Carn Llidi Bechan, Carn Hen, 



^ ' On the Igneous Rocks occurring between St. David's Head & Strumble 

 Head ' Quart. Journ. Geol. Soc. vol. Ixi (1905) p. 579. 



T 2 



