Vol. 6S.'] PRE-CAMBKIAX AND CAMBRIAN OP PEMBROKESHIRE. 379 



Upwards the series seems to become still more felspathic and 

 siliceous, and the purple, pink, and green tuifs give place to fine- 

 grained felspathic rocks which form part of the upper group, and 

 on weathering are difficult to distinguish from some of the sand- 

 stones occurring low down in the Cambrian System. 



It was probably the confusion of these tuffs with the Cambrian 

 sandstones that impelled several observers to consider the Dimetian 

 rocks to be post-Cambrian intrusions. 



(ii) The Rhindaston and Gignog Group. 

 (P^ of the Map, PI. XL.) 



What appears to be the highest subdivision of the Pebidian 

 present within the district is essentially rhyolitic and keratophyric 

 in character, and comprises rhyolitic ashes, quartz-keratophyres, and 

 occasional rhyolitic breccias. It is into the various members of 

 this group that most of the Dimetian rocks have been intruded. 



The most prevalent type is the rhyolitic ash which forms much 

 -of the ground around Gignog and to the south of Hayscastle. The 

 rocks are well-bedded in character, but are in a highly altered 

 condition. They are usually pale-gre}^ blue, greenish-grey, white 

 or pink. They are of fairly fine texture, but in most cases show 

 abundant small quartz-crystals and grains, similar to the quartz of 

 the quartz-keratophyres, to be subsequently described (p. 385), with 

 which they are associated. 



These rocks break down readily, and are seldom exposed ; but 

 they are best seen in some old quarries to the west of Gignog and 

 in the slopes of the valley (Brandy Brook) east of Asheston. 



They are well represented by debris all over their outcrop, and 

 a careful study reveals the fact that with them are associated bands 

 of harder siliceous rock of felspathic character, in which minute 

 quartz-phenocrysts are visible to the unaided eye. It is possible 

 that some of these rocks may be of the nature of sills ; but their 

 identity with fragments in the tuffs, and the evidence obtainable 

 from sections on Rhindaston Mountain and in the cliffs to the 

 south of Pointz Castle proves that a great number, at any rate, are 

 iava-flows. 



Generally speaking, these quartz-keratophyres or quartz-bearing 

 soda-rhyolites are pale-grey, opaque, fine-grained rocks with minute 

 ill-formed quartz-phenocrysts. Occasionally, when fresh, they may 

 have a blue-grey colour and flinty texture, and may be translucent 

 in thin splinters. Such rocks form the greater part of the hill 

 behind Ehindaston-fawr, and are especially well represented near 

 the summit of that hill. The same blue, flinty, rocks occur on the 

 northern flank of Rhindaston Mountain, near Barch. 



A good section of quartz-keratophyres, with vesicular surfaces 

 and fluxion-structure, interbedded tuffs and rhyolitic breccia, is 

 exposed in some crags overlooking the valley of Brandy Brook, 

 half a mile south of Gignog Ford, at a point marked by a dip- 

 ■arrow on the map (PI. XL). These rocks, it may be remarked. 



