336 



Some of the felsitic tuffs of the St. David's area have been 

 converted into sericitic schists by the dynamic metamorphism which 

 has affected the district. 



Felsitic rocks have been described by Professor BLAKE as occurring 

 in Ramsey Island. One of these exhibits perlitic structure in great 

 perfection although the rock is now devitrified. 



Felsites are known to occur in many other localities in South 

 Wales. On the west side of Goodwie Bay, near Fishguard, there is 

 a fine exposure of banded and brecciated felsites associated with 

 felsitic agglomerates. Nodular and banded felsites are seen on the 

 south side of Skomer Island. The nodules in these rocks vary in 

 size from minute globules, no larger than small peas, to spherical 

 masses measuring several inches in diameter. They are sometimes solid 

 to the core, at other times they contain a hollow cavity on the 

 surface of which quartz-crystals have been formed. A radial structure 

 may occasionally be observed, but as a rule it is absent. Bands in 

 the felsite may often be followed through the nodules. Under the 

 microscope the matrix in which these nodules lie occasionally shows 

 perlitic structure although it is now completely devitrified. There can 

 be no doubt that we have here a group of ancient (probably 

 Ordovician) devitrified glassy rocks/ 1 ) 



North Wales. Felsites and felsophyres occur in North Wales. They 

 belong to two distinct periods the Cambrian or pre-Cambrian and the Or- 

 dovician. Those of Cambrian or pre-Cambrian age have been described by 

 Professor BONNEY ^ as devitrified rhyolites. They are well exposed near 

 Llyn Padarn and between Caernarvon and Bangor. They consist of por- 

 phyritic crystals and crystal -fragments of quartz, orthoclase and plagio- 

 clase embedded in a compact grey or purple matrix. Under the 

 microscope they frequently show the fluxion structure of rhyolites. The 

 ground-mass is usually crypto-crystalline. A ferro-magnesian constituent 

 is not definitely recognizable. An analysis of one of these rocks from 

 Brithdir near Bangor is quoted below. The specific gravity of a speci- 

 men from Cwm-y-glo near Llanberis is 2*61. 



The Ordovician felsites and felsophyres are extensively developed 

 both in Merionethshire and Caernarvonshire. They have been described 

 by Messrs. WARD/ 3 ) RuTLEY/ 4 > BONNEY^) and COLE. (C) 



They are generally compact, dark grey, greenish grey or bluish 

 grey rocks which exhibit a marked tendency to weather white. Small 

 felspars may frequently be detected, but the rocks rarely become con- 

 spicuously porphyritic. Quartz is certainly rare as a porphyritic 

 constituent a fact which appears somewhat remarkable when we take 



(1) RIJTLEY. Felsitic Lavas. Mem. Geol. Survey, 1885. 



(2) Q.J.G.S., Vol. XXXV., p. 309 ; Vol. XXXIV., p. 137 ; and Vol. XXXIX., p. -178. 



(3) Q.J.G.S., Vol. XXXI. (1875), p. 388. 



(4) Q.J.G.S., Vol. XXXV. (1879), p. 508; and Vol. XXXVII.(1881), p. 403. 



(5) Q.J.G.S., Vol. XXXVIII. (1882), p. 289. 



(6) Q.J.G.S., Vol. XLI. (1885), p. 162 ; and Vol. XLII. (1886), p. 183. 



