V0I.51O THE COUNTRY ARO TOD -FISHGUARD. 163 



centric structures. Prof. Judd's figure of the quartz and micropeg- 

 matite groups in the granite-nucleus of a spherulite from the Cuillin 

 Hills * shows similar features. 



The cryptocrystalline type of groundmass is co-existent with 

 another type — the microlitic (which will be subsequently described) 

 — in a rock from Carn Gelli [203] amongst others. This rock in the 

 field has a pale greenish-grey, mottled appearance, and conchoidal 

 fracture, and under the microscope shows very numerous scattered 

 microlites embedded in a cryptocrystalline base. In this also lie 

 large, porphyritic, Carlsbad-twinned felspar-crystals, with their 

 angles rounded off as if corroded by the magma in which they 

 floated. Some local rise of temperature, or increase either of 

 pressure or in the quantity of water, has affected the stability of 

 the molecules of the crystal and produced this exterior corrosion. 2 

 The greenish colour of the rock is due to the swarms of minute, 

 pale-green granules or globulites. In many parts of this rock 

 beautiful perlitic structure occurs, and is brought out conspicuously 

 also by the action of weathering. The general appearance and 

 mineralogical composition of this Carn Gelli rock indicate a lower 

 percentage of silica than in the Good wick lavas, and it may reason- 

 ably be placed among the trachytoid felsites. 



The occurrence of perlitic structures among these rocks is not 

 uncommon. A good example is seen in a rock from Crincoed [4], 

 where also fluxion-structure is plainly indicated by the lines of 

 parallel microlites. Lines of flow are also beautifully shown, bending 

 round felspar-crystals in a rock from Carn Blewyn, near Llanllawer 

 [322], [325], in a cryptocrystalline groundmass. 



In all the foregoing cases devitrification of the groundmass has 

 occurred. Whether this devitrification took place at the time of 

 solidification or subsequent to it cannot be affirmed with certainty; 

 but I am inclined to think that we have here to do with subsequent 

 (secondary) devitrification, on account of the close association with 

 and passage into rocks which undoubtedly consolidated as glasses 

 (obsidians or pitchstones), as the presence of perlitic structure 

 indicates. 



Here may be mentioned two striking varieties of the felsites 

 possessing a cryptocrystalline base. The first variety is charac- 

 terized by banding, and is well shown in the immediate vicinity of 

 Goodwick, as previously described. Under the microscope [86] the 

 dark bands show numerous granules and minute crystals of magnetite 

 and a greenish-grey semi-opaque substance which is not distinguish- 

 able from the cryptocrystalline matrix with crossed nicols. In the 

 light bands these elements are absent. The ' glomero-porphyritic ' 

 groups of micropegmatite, similar to those previously described, are 

 present, and small porphyritic lath-shaped felspars with lamellar 

 twinning and orthoclase-crystals are also fairly common. Portions 

 of the groundmass have become microcrystalline. The banding is 



1 Quart. Journ. Geol. Soc. vol. xlix. (1893) p. 189. 



2 13onney, Pres. Addr., Quart. Journ. Geol. Soc. vol. xli. (1885) Proc. p. So. 



