Vol. 67.] 



SKOMER VOLCANIC SEEIES. 



1-7 



In the finely banded rocks, like those of the Basin, the bands are 

 often not more than a millimetre in width, and appear as alternating 

 light and dark layers. The light bands consist either of quartz, 

 often with slender needles of apatite projecting inwards from the 

 sides of the band, or of patches of alkaline felspar or coalesced 

 spherules. The darker bands are composed of iron-stained erypto- 

 crystalline felsitic material, almost isotropic, but with small patches 

 of secondary felspar which appears to represent spherulites. 



The coarsely spherulitic rocks of Skomer have received a fair 

 share of attention since they were first described by Eutley in 

 1885, and are by far the best-known members of the Skomer 



Fig. 4. — Nodular rliyolite of the Basin, Skomer Island, 



Series. They are magnificently displayed on the coast between 

 Tom's House and the Basin. The smaller spherules, those about 

 the size of a pea or less, have a concentric structure towards the 

 exterior, but the internal structure is fibrous and radiate. There is 

 seldom any recognizable nucleus, and the felspar fibres radiate 

 either from a central point or from a short line. The felspar has 

 optically negative elongation, practically straight extinction, and 

 low refractive indices. 



The spherules are usually white or grey, and are often collected 

 in bands to form large axiolites, while a mammillated or botryoidal 

 structure is sometimes observable on their surfaces. The big 

 spherules are mostly solid spherulites, and are generally paler than 

 the rock in which they lie. They range up to 9 or 10 inches in 



