﻿Yol. 64.] STRUCTUEE OF THE ST. DAVId's AEEA. 377 



Staining by the Conglomerate. 



Where the junction with the Pebidian is exposed, the rock 

 immediately underlying the conglomerate, to a depth of from 10 to 

 20 feet, has always a peculiar red coloration, not observed in the 

 Pebidian at any other point of the district. As this colour always 

 decreases in intensity, in rocks of similar composition, farther from 

 the base of the conglomerate, it is clearly due to staining. Whether 

 this is produced by water percolating through the Cambrian, or by 

 exposure during erosion, seems an open question ; but, beyond any 

 doubt, it has no connexion with a stratigraphical horizon. This 

 staining is of considerable antiquity, for it is certainly older than the 

 oldest faulting. 



Relation of the Cambrian to the Granophyre. 



The detailed examination of the boundaries of the granophyre 

 shows that most of the margins that can be traced with reasonable 

 certainty are faults, which are shown in the accompanying map 

 (PI. XLIY). It is a point of the highest importance that these faults 

 are in most cases the prolongation of faults clearly proved in the 

 adjacent Cambrian. The facts regarding the faults themselves may 

 be summarized as follows : — 



(1) Along the southern margin of the main mass, the boundary is a reversed 

 fault that crosses St. Non's Bay and dies out eastwards in a minor overfold. 

 This is clearly one of the oldest faults in the district, for it is shifted 120 yards 

 northwards by a later fault. 



(2) For a considerable part of its outcrop the granopiiyre is bounded by 

 faults that belong to a series, trending north-north-east and south-south-west, 

 forming a featu^re and an integral part of the present structure of the district. 



One of these faults bounds the eastern side of the southerly prolongation of 

 the granophyre, and is seen in the cliff-face at Ogof Llesugn, where it brings 

 the Middle Solva Beds against the basal conglomerate. A branch-fault comes 

 off here, and this fact has been noted by the late J. F. Blake, who accurately 

 described and mapped this critical section. He showed that the outcrop of 

 conglomerate seen here is at no point in contact with the granophyre, but is 

 involved in a series of interlacing injections of basic igneous rock.^ Basic 

 intrusions of this type along lines of fault are a feature of the entire district. 



(3) The western portion of the northern end of the granophyre is bounded 

 by a great fault with a general west-north-west and east-south-east direction, 

 the outcrop of which is again shown on the map (PI. XLIV) cutting the 

 Cambrian, both east and west of the city. The existence of this fault near the 

 city was clearly grasped by Dr. Hicks, who failed, however, to trace its further 

 course. 



Owing to the exceptionally clear nature of the exposures in the 

 Porth-clais area, all previous writers have focussed attention on the 

 evidencfe seen there. One side claims all the junctions as essentially 

 faults, the other as essentially intrusive. The maps given here show 

 that these junctions are for the most part prolongations of faults 

 proved by detailed mapping to cut the Cambrian; and thus Hicks's 

 contention is, in this area, substantially correct. In order to make 



^ Quart. Journ. Geol. Soc. vol. xl (1884) pp. 299 et scqq. 



