﻿Vol. 64.] GEOLOGICAL STEUCTIJRE OF THE ST. KATID's AEEA. 363 



21. The Geological Steucttjee of the St. David's Aeea (Pembeoke- 

 shiee). By John Feedeeick Noeman Geeen, E.A., P.G.S. 

 (Read April 15th, 1908.) 



[Plate XLIV— Map.] 



Contents. 



Page 



I. Introduction 363 



II. The Cambrian 364 



III. ThePebidian 366 



IV. The St. David's aranophvre 373 



V. Extent and Boundaries of the Dimetian 373 



VI. Quartz-Porphyry Dykes 374 



VII. Relation of the Cambrian to the Granophyre and the 



Volcanic Series 375 



VIII. Summary 381 



I. Inteoductiok. 



Theee is probably no district in Wales with regard to the geology 

 of which such serious divergencies of opinion exist as the neigh- 

 bourhood of St. David's. The only rocks upon which authorities 

 are in any wa^ agreed are the sedimentary Lower Cambrian 

 deposits, which form the geological datum-line for the area ; and 

 differences exist even here, as to whether they include contempo- 

 raneous tuffs, and as to the composition of the basal conglomerate 

 and sandstone. With regard to the age of the various acid and 

 basic tuffs, the basic igneous rocks, the quartz-porphyries, and the 

 granitoid rock of St. David's, relatively to the Cambrian and inter 

 se, entire disagreement exists throughout. 



The interpretations of the sequence fall, however, into two main 

 groups, associated respectively with the names of Dr. Hicks and Sir 

 Archibald Geikie. The former is, on the whole, supported by Prof. 

 Bonney, Prof. McKenny Hughes, and the late J. F. Blake, who were 

 all well acquainted with the district ; and the conclusions arrived at 

 by Sir Archibald Geikie and Dr. B. N. Peach have been confirmed 

 after careful examination by Prof. C. Lloyd Morgan. 



The problem may be simplified by eliminating one point of 

 disagreement. The late Dr. Hicks considered that the tuffs, sheets, 

 and dykes could be divided into two unconformable series, termed 

 by him Arvonian (earlier) and Pebidian. No other observer 

 has confirmed this hypothesis, and, in order to avoid prolixity, it 

 is proposed not to make any special reference to it. Eliminating, 

 then, this point, the two conflicting views may be summarized as 

 follows : — 



(1) Geikie and Lloyd Morgan. The tuffs form the base of, are 

 conformably overlain by, and pass up into the Cambrian rocks. The 

 St. David's granite, with associated quartz-porphyries, is intrusive in 

 and later than both tuifs and Cambrian ; the junctions exposed between 

 them are intrusive, at least in most cases. 



