﻿Vol. 64.] STEFCTTJEE OF THE ST. DATID's AEEA. 375 



magma, and thus of the same general age, no evidence has been 

 found to suggest that any are apophyses of the latter. 



It is not disputed that some of them cut the granophyre, but it 

 has been contended that near Rock House, south of the city, the 

 granophyre passes gradually into a material identical with that of 

 the dykes. In the field close to (north of) Rock House there are 

 two rocks present : one the marginal modification of the granophyre, 

 the other a typical quartz-porphyry. The exposures are not clear 

 enough to justify the positive statement that the porphyry cuts the 

 marginal modification ; but the former can be followed well within 

 the main granophyre without in any way losing its identity. 

 Outside the granophyre, near St. David's Cathedral, the quartz- 

 porphyries are more abundant, but never lose their distinctive 

 character ; if they were the prolongation of the margins of the grano- 

 phyre, they should have an unbroken course towards its edge. This, 

 however, is not the case, for any connexion is severed at the surface 

 by an outcrop of the Pebidian, exposed specially well at and about 

 the Deanery. The view that these quartz-porphyries are separate 

 from the granophyre seems to be confirmed by an examination of 

 the ground-mass, which is microgranitic ; while the finer phases 

 sometimes approach the mieropoecilitic structure, and this structure 

 is maintained even when they occur well within the granophyre, as 

 at Rock House. Spherulitic modifications have been described by 

 various authors, but they do not afford any additional evidence 

 against the view that the granophyre and the dykes are approxi- 

 mately contemporaneous. . 



So far, then, it has been shown that the granophyre is almost 

 certainly intrusive in the Pebidian ; and that the dykes are so is 

 obvious, on the north side of St. David's Cathedral as well as in 

 other localities. The relation of all these rocks to the Cambrian 

 now remains to be considered. 



VII. Relation of the Cambeian to the Geanophyee 

 AND THE Volcanic Seeies. 



There are two diametrically opposite views as to the relation of 

 the Cambrian to Dr. Hicks's Pebidian and Dimetian : his view was 

 that there was a great break in time marked by a violent 

 unconformity ; whereas Sir Archibald Geikie's view is (1) that there 

 is no break of structural importance between the Cambrian and 

 the Pebidian ; and (2) that the Dimetian is an intrusion in the 

 Cambrian. As two types of relationship are involved in the latter 

 interpretation, it will be convenient to discuss each separately, 

 beginning with the first. 



Nature of the Junction between the Cambrian 

 and the Pebidian. 



In mapping the ground, it has been found that the base of the 

 Cambrian, in this particular area, cannot be proved to rest upon any 



