318 



mosaic of water-clear grains (see Fig. 1, Plate XLII.). The micro-flaser 

 or fluxion structure is as perfect in this rock as in the rock from 

 Kynance; it is indeed somewhat better denned under crossed nicols 

 in consequence of the distribution of white mica in wavy planes. 

 Tourmaline occurs sparingly. Prof. BONNEY mentions the occurrence of 

 garnet. This rock bears the closest possible resemblance to some speci- 

 mens of the gneissose granite of Dalhousie. Colonel MACMAHON regards 

 the micro-fluxion structure in these rocks as original and connected with 

 the intrusion of the granite. The present writer is inclined to regard 

 the corresponding structure in the Porthalla rock as the consequence 

 of the regional metamorphism which has affected the district and 

 produced allied structures in the gabbros and serpentines. 



Wales. In north Pembrokeshire there occurs, in the neighbourhood 

 of St. David's, a mass of granitic rock which has given rise to a good 

 deal of discussion. It was mapped as syenite, intrusive in Cambrian 

 rocks, by the geological surveyors. Dr. HICKS considered it to be a 

 metamorphic rock of pre-Cambrian age belonging to his Dimetian system. 

 Dr. GEIKIE maintains that it is an intrusive granite of Cambrian or 

 post-Cambrian age. Professor BLAKE holds that it is a granite, but of 

 pre-Cambrian age. The petrographical characters of the rock in question 

 have been described by Messrs. DAVIES <*> and TAWNEY, < 2) Drs. BONNEY < 3 > 

 and GEIKIE/*) and Professor BLAKE/ 5 ) In its typical development it is 

 a massive, granular holo-crystalline rock composed of quartz, orthoclase, 

 plagioclase and a green chloritic mineral of somewhat indefinite 

 character. The felspars are turbid and often contain minute crystalline 

 flakes of white mica (secondary). The quartz occurs as a rule in grains 

 but sometimes shows traces of crystalline outline. It abounds in fluid 

 inclusions which are often arranged in planes running roughly parallel 

 to each other and passing from one grain to another without change 

 of direction. Sometimes also fine cracks, occupied by a micro-crystalline 

 mosaic, traverse the slide for a greater or less distance. These 

 phenomena point to the conclusion that the rock has been more or less 

 affected by dynamic metamorphism. The relation of the quartz to the 

 felspar in this rock is interesting. Sometimes the two minerals occur 

 in irregular grains, at other times the felspar plays the role of matrix 

 to the quartz, at others the two minerals are inter-crystallized so as 

 to form micro-pegmatite. Segregation veins occur in the main mass 

 and in these the micro-pegmatitic structure is generally well developed 

 according to Dr. GEIKIE. The viridite undoubtedly represents one or 

 more of the ferro-magnesian constituents, biotite, hornblende or augite 



(1) Q.J.G.S., Vol. XXXIII. (1877), p. 231 (foot note). 



(2) Proc. Bristol Nat. Soc., Vol. II., part 2, p. 112. 



(3) Q.J.G.S., Vol. XXXIV. (1878), p. 155. 



(4) Q.J.G.S., Vol. XXXIX. (1883), p. 313. 



(5) Q.J.G.S., Vol. XL. (1884), p. 302. 



