380 ME. H. H. THOMAS AXD PKOF. 0. T. JO^iTES OX THE [Sept. 1912^ 



strike approximately at right-angles to tlie Cambrian basal con- 

 glomerate, which, therefore, probably overlies them with strong 

 discordance. 



Another mass of rhyolitic rocks which can be studied in detail is 

 exposed along the coast between Xewgale and Forth Mynawyd on 

 the west. The rocks forming this mass have been the subject of 

 conflicting statements : but their true nature was first detected by 

 Hicks (15, pp. 353, 354), who described them as a group of 

 rhyolites and breccias. He recognized that some of the rocks were 

 spherulitic, and that others showed fluxion-structure, while among 

 them were beds of breccia and fine ash. The greater number of the 

 rocks are white and opaque, like most of the weathered rocks of 

 this class ; but a quarry on the cliff above Cwm Bach has exposed a 

 blue flinty rock similar to that of Khindaston. Curiously vesicular 

 rocks, in which the cavities have been filled with secondary quartz, 

 are exposed in Cwm Bach. Breccias are prevalent along the coast,, 

 and fluxion-structures may be observed in the rocks forming many 

 of the crags of the upper portions of the clifl's. 



ilhyolitic breccias, presumably belonging to this group, have also 

 been detected in other parts ot the district, one beautiful purple rock 

 occurring on the path 300 yards south-west of Hayscastle Farm. 



(b) The Dimetian. (D1-D3 of the map, PL XL.) 



Considering that almost the whole of the pre-Cambrian complex 

 of Brawdy and Hayscastle was coloured as granite on the older 

 maps, it was somewhat surprising to find that granitic rocks form 

 so small a portion. A variety of intrusive rock-types, however, is- 

 represented and they may be grouped under the names quartz- 

 porphyry, biotite-soda-granite, and albite-diorite. The 

 relations of one type to another are rendered rather obscure through 

 the lack of sections, but the detailed mapping makes it apparent 

 that the granite represents the earliest plutonic phase of the 

 Dimetian in this district, and that it has been intruded into the 

 Pebidian. 



The quartz-porphyry was regarded by De la Beche and Ramsay 

 as a marginal variety of the granite ; but, although we agree in its 

 being a variety of the granite, we are not convinced that it is 

 marginal in character. 



At Brimaston Hall there appears to be a complete passage from 

 one type to another, the change being gradual and taking place 

 over a somewhat wide zone ; but in other parts of the district, as 

 at Brawdy and Silver Hill, the change of type is sudden — as if there 

 had been a slight time-interval between the intrusion of the granite 

 and that of the quartz-porphyry. 



The relation of thediorite to the other intrusive rocks is difficult 

 to establish, but it will be discussed below (p. 382). 



The hypabyssal and probably the latest pre-Cambrian intrusions 

 are, in our opinion, represented by a series of doleritic dykes 

 which we have observed to cut the Pebidian and various members 

 of the Dimetian. All the same, we have no clear evidence that these 



