Yd. 67.] THE SK03IER VOLCANIC SERIES. 199 



They are somewhat pale grey rocks, slightly mottled, very finely 

 crystalline, and have a splintery fracture. They are compact and 

 not conspicuously vesicular, except at their surfaces. They are 

 clearly related to the skomerites, hut contain conspicuous pseudo- 

 morphs after olivine — existing as deep red or bronze-coloured 

 micaceous aggregates with a distinct sheen. These pseudomorphs 

 range from 1 or 2 millimetres to a centimetre in width. Felspars 

 are visible as narrow bright crystals, ranging up to a centimetre 

 and a half in length, but usually not more than half a centimetre. 

 The rocks weather with a pale crust, and are often streaked with 

 haematite in bands 3 to 4 millimetres wide. 



Microscopically they show a glomeroporphyritic structure, and 

 consist of olivine and acid plagioclase-phenoerysts set in a relatively 

 fine-grained ground-mass containing much augite, acid plagioclase, 

 and accessory iron-ores. The olivines, which are usually inter- 

 grown with the larger plagioclase-crystals (fig. 10, A & B, p. 198), 

 have a tendency towards idiomorphism. They are nearly always 

 represented by pseudomorphs in serpentine, ferrite, or a bright- 

 green pleochroic mineral a produced by the re-absorption of the 

 external coating of iron-ores. They are only very occasionally 

 replaced by calcite (E 7037). 



The porphyrinic felspars are albite-oligoclase ; they are usually 

 twinned on the Carlsbad and albite laws, and show incipient 

 decomposition to micaceous aggregates, which render them turbid. 

 Some of the felspars are perthitic. 



The ground-mass, of which the structure lies between inter- 

 sertal and trachytic, is composed of felspar-microlites enclosing 

 between them small but abundant subidiomorphic crystals and 

 granules of augite. Some of the latter mineral, however, as in 

 the skomerites, behaves ophitically with respect to the felspar- 

 microlites. The microlites are comparable in every respect to 

 those of the skomerites (p. 196), but occasionally may be arranged 

 in curling flows around the larger individuals. 



In these rocks there is present, in fair quantity, a brown, probably 

 soda-bearing, hornblende identical with that mentioned as occurring 

 in the olivine soda-trachytes (p. 192). It has the following pleochroic 

 scheme : — a, pale reddish-brown ; fc, pale brown ; c, pale yellowish- 

 brown. The plane of the optic axes lies in the plane of symmetry, 

 r A c' = 24° approx. ; it is optically positive, and 2E is greater than 

 120°. The birefringence is distinctly low, being considerably less 

 than that of the augite in the same rock. The mineral occurs in 

 small crystals and grains in the ground-mass, but occasionally is 

 found within some of the porphyritic felspars (E 6996). It shows 

 the characteristic hornblende-cleavages. 



The vesicles of these rocks are usually small, and filled either 

 with secondary quartz and chlorite or with interlocking prisms of 



1 This mineral is green for light vibrating parallel to the cleavages, and 

 yellow for the direction at right angles. The cleavage-traces mark the posi- 

 tive direction. 



