319 



In the tendency to a micro-pegmatitic structure this rock differs from 

 the granites of the West of England, the south-east of Ireland, and the 

 central and eastern parts of Scotland; but resembles the granites 

 (granophyres) of Mull and the Mourne Mountains. Mr. DAVIES was 

 struck with the resemblance of some portions of the St. David's rock 

 to the Mull granophyres. It is worthy of note that in these 

 granophyres augite is frequently the only ferro-magncsian constituent. 



The rock above described is associated with felsites in which a 

 spherulitic or pseudo-spherulitic structure is well developed. These 

 will be referred to later on. They are supposed by Dr. GEIKIE to 

 represent the marginal portions and the apophyses of the granitic rock. 

 Dr. HICKS, however, considers them to be of independent origin. 



In the ridge of crystalline rocks running more or less continuously 

 from Langwm to Talbenny, south of Haverfordwest, granite occurs in 

 association with quartz-diorite ; the granite veins the diorite. In the quarry 

 at Targate both rocks are massive. Near Talbenny they become gneissose, 

 as has been pointed out by Dr. HICKS and Mr. DAVIES. W 



Granitic rocks are exposed in many localities in North Wales. 

 At Moel-tan-y-grisiau near Ffestiniog there occurs a rock, mapped 

 as syenite, which bears the closest resemblance both macroscopically 

 and microscopically to the rock of Bryn-y-garn near St. David's. It is, 

 however, decidedly fresher. The quartz is tolerably free from inclusions. 

 The felspar is turbid but frequently shows sufficient individual action 

 to enable one to recognize its character. It is partly striated after the 

 fashion of normal plagioclase, partly unstriated and partly cross-hatched 

 in such a way as to suggest microcline. The felspar and quartz are 

 related to each other as in the St. David's rock. The viridite of the 

 St. David's rock is represented by brown or green mica which occurs 

 in aggregates, not in individual crystals. Hornblende occurs sparingly 

 in ragged grains. Grains of zircon are present as inclusions in the 

 other constituents and there are a few grains of iron-ore. 



The rock of Twt Hill near Caernarvon, for which Professor BONNEY W 

 proposed the term granitoidite, is allied to the rock above described. The 

 relation of the quartz to the felspar is similar and there is the same 

 tendency to a micro-pegmatitic or granophyric structure. Ferro-magnesian 

 constituents are, however, almost entirely absent. Similar rocks occur 

 at Tymawr and Rhos Hirwain in the Lleyn peninsula and at Ty Croes, 

 Llanfaelog and other localities in Anglesea. 



A rock of this type from Meillionydd House, about three miles N.E. 



(1) Q.J.G-.S., Vol. XL. (1884), p. 507. The district in which these rocks occur has been 

 profoundly affected by post -carboniferous earth-movements. An interesting question arises as to 

 whether the gneissose structures in the igneous rocks may not be the result of these movements. 



(2) Q.J.G.S., Vol. XXXV. (1879), p. 321. 



(3) See HICKS & BONNEY, Q.J.G.S., Vol. XXXV. ; also Vol. XL. (1884), p. 200 ; 

 TAWNEY, G.M., Decade II., Vol. X. (1883), p. 65. 



