The St. David's-Head ‘Rock-Series’, Pembrokeshire

Abstract

1. I ntroduction . 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 more detailed investigation, and I have since made a systematic examination 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. 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 Cam Llidi, Carn Llidi Bechan, Carn Hen, and occasionally at Pen Lledwen and in parts of the St. David's-Head mass. This melanocratic type is moderately coarse in grain. It stands in conspicuous contrast to a leucocratic