190 MR. H. H. THOMAS ON THE [May. 1911, 



The accessory minerals are zircon, apatite, titaniferous iron-ore, 

 magnetite, and pyrite, while common secondary minerals are 

 granular sphene and chlorite. In some rocks (as, for instance, the 

 Table felsite) the sphene is intensely pleochroic, like that of some 

 metamorphic rocks, and has a almost colourless, h pale greenish, 

 and t deep reddish with 2 E about 63°. The chlorite is distributed 

 through the base of the rocks, partly replacing felspars and in- 

 filling occasional vesicles. 



All these rhyolitic rocks are evidently rich in silica and soda, 

 and the analysis set forth on p. 189, of a rock forming a white and 

 little altered band above the Table, is representative of the class. 

 The analysis is of a rock which evidently consists almost entirely of 

 quartz and albite ; for its theoretical percentage composition would 

 be orthoclase 2'2, albite 53*9, anorthite 1*1, and quartz 27*6. 

 These rocks are closely allied to the soda-felsites of the South-Eust 

 of Ireland, described by Dr. Hatch, 1 of which two analyses are 

 tabulated for comparison. In the nomenclature of the American 

 classification this rock would be designated westphalose, and 

 placed in the persodic division of a las k as e, a division containing 

 the quartz-keratophyres of some authors. 



The soda-rhyolites, with the assumption of a trachytic micro- 

 scopic structure and an increased percentage of combined or free 

 orthoclase, pass gradually into the albite - oligoclase trachytes 

 described below. 



(b) The Soda-Trachytes. 



These rocks may be divided into three types : — 



(1) Soda-trachyte, with little or no augite or hypersthene (for instance, 



Slides E 7029, 7122). 



(2) Olivine-soda tracbj^te, with pseudomorphs after olivine (for instance, 



Slide E 7759). 



(3) Hypersthene-soda trachyte, with pseudomorphs after hypersthene (for 



instance, Slide E 7148). 



The first type is common and well represented on Skomer in the 

 central belt of trachytic lavas which form a clearly-marked band 

 across the island. The olivine-bearing rocks are best seen on 

 Skomer and in the cliffs to the east of Martin's Haven. 



The hypersthene-bearing type is represented on the maixiland at 

 Crabhall, north of Dale, where a large quarry has been opened for 

 road-metal. In the hand-specimen all these rocks are somewhat 

 splintery, grey in colour, and it is but exceptionally that they 

 show any marked vesicular habit (for instance, Slide E 7485). They 

 have only a few phenocrysts of sufficient size to be distinguished 

 by the unaided eye. These rocks weather with a pale buff-coloured 

 crust, which may extend for about half a centimetre into the rock. 



Under the microscope they are seen to consist of a few well-formed 

 phenocrysts of albite, usually twinned, set in a mass of microlites of 

 similar composition. The structure is typically trachytic. 



1 Geol. Mag. dec. iii, vol. vi (1889) pp. 70 & 54.5. 



