Yol. 67.] BKOMEB VOLCANIC SERIES. 189 



rhyolites 1 might have been written for those of Skomer, with 

 which he himself compares them. I agree with him in regarding 

 these nodules, now often devoid of structure, as having been derived 

 from simple spherulites by a process of alteration and substitution. 



Phenocrysts arc never present in large numbers in these rhyolitic 

 rocks, and are often entirely wanting. When present they consist 

 either of minute crystals of an acid plagioclase-felspar (between albite 

 and oligoclase), or of microperthitic intergrowths. No potash-felspar 

 has been detected, although the analysis of a rock from the Table 

 (Analysis I, below) shows that potash is present in small quantity ; 

 it probably exists, therefore, in the grouudmass, in the perthite, or 

 in the phenocrj'sts in the albite-molecule, which may contain potash 

 up to 16 per cent, of the total alkalies. 



The albite-oligoclase crystals are usually twinned according to 

 the Carlsbad and albite laws, but pericline lamellae are rarely seen. 

 Occasionally these rocks contain scarce and small chloritic and 

 serpentinous pseudomorphs after some ferromagnesian mineral, 

 such as those in the rocks of Manies Island and the island south- 

 west of the Garland Stone, which appear to be after biotite ; and 

 those in the uppermost portion of the Mewstone mass, which 

 appear to have the form of augite (fig. 5 B, p. 188). Sometimes, 

 in rocks which have undergone secondary silicification, a mosaic 

 of secondary quartz and chlorite occupies the cavity left by the 

 decaying mineral (fig. 5 A, p. 188). 



The groundmass of the devitrified obsidians is usually crypto- 

 crystalline felsitic material, but in the more felspathic varieties it 

 breaks up into irregular ill-defined patches of felspar, representing a 

 tvpe of devitrification common to many of the older rhyolitic lavas. 



I. II. III. 



SiO, 79-64 77-8 77-29 



TiO; 050 



AlA 11 -t-fc 13-2 1462 



Fe" 2 3 Oil 0-2 trace 



FeO 0-30 0-7 



MnO 008 



CaO 071 trace 



MgO 0-15 trace 038 



K,0 038 21 016 



Na.,0 6-40 5-1 760 



H ; at 105° C 0-16 



H;0 above 105° C... 0"30 



PA 008 



CO., 002 



Loss on ignition ... ... 0*6 057 



Totals 100-27 997 10062 



Anal E. G. Kadley. F. H. Hatch. F. 11. Hatch. 



I = Soda-rhyolite, near the Table, top of cliff, east of the Spit (Skomer). 



[Anal. No. 345, Slide E 7768.] 

 II = Soda-felsite, south-east of cairn on Castletituon Hill. Co. Wicklow (Ire- 

 land). Geol. Mag. 1889, p. 546. 

 III = Soda-felsite, Brittas Bridge, Co. Wicklow (Ireland). DM. p. 72. 



1 ' Bala Volcanic Series of Carnarvonshire ' 1889, pp. 29 eiseqq. 



