Vol. 67.] 



SKOMER VOLCANIC SERIES. 



I'll 



The phenocrysts in the finer-grained varieties (fig. 6, B, below) 

 reach 2 mm. in length by about 0*7 in width, but the majority 

 are smaller. The microlites in the samo rock measure about 1 mm. 

 in length by "01 mm. in width; they are arranged in well- 

 defined flow-lines, and enclose between them minute specks of 

 chlorite and grains of iron-ore. In these rocks the rare vesicles 

 (for instance, E 7029, 7285) are filled with a mosaic of quartz, 

 and lined with a little chlorite ; while the groundmass contains 

 many granules of secondary sphene. In the rock figured below 

 (fig. 6, A) the microlites are somewhat larger than usual, but the 



Fis. 6. 



A=Albite-trachyte from Captain Kite's Eock (Skomer). Slide E 7029, ordinaiy 



light. X 25 diameters. 

 B = Albite-trachyte, from low down on the North Cliff of Skomer, opposite 



the Garland Stone. Slide E 7122, ordinary light. X 25 diameters. 

 [Both these rocks show small phenocrysts of acid plagioclase-felspar set. in 



a fine mass of acid plagioclase-microlites. The rock A shows well-shaped 



apatite-prisms.] 



flow-structure is still beautifully preserved. Occasionally, as in 

 this rock (E 7029), there occur small though uudoubted pseudo- 

 morphs after augite ; but the percentage of ferromagnesian mineral 

 is always small. Apatite is not an uncommon accessory (fig. 6, A) 

 and the iron-ore is titaniferous. These soda-trachytes show a 

 family connexion with the keratophyres, in the restricted sense — 

 into which they would pass by an increase in the size and basicity 

 Q. J. G. S. No. 266. * 



