REPORT ON THE PETROLOGY OF OCEANIC ISLANDS. 93 



cleavage which run parallel to the short sides of the hexagon. The trace of TjM is 

 indicated by its parallelism with the prismatic cleavage, which is rather less distinct 

 than that just spoken of. It frequently happens that all the outlines of these sections 

 are not equally distinct, or only appear clearly for part of the section, the rest being 

 terminated by a fracture nearly parallel to the prism. These hexagonal sections show 

 that the plagioclase is zonary, and that the extinction is negative. The angular value 

 is greater for the centre than for the outer zone, being from 14° to 10° for the former, 

 and 10° to 3° for the latter. This felspar is thus probably composed of mixtures 

 intermediate between oligoclase and labradorite. Sections following M show the 

 lamellae after the pericline law almost parallel to the edge PjM; this, according to M. 

 Schuster, is the case for plagioclases approaching andesine. These sections are full of 

 vitreous inclusions, which have no definite arrangement, but are specially numerous 

 near the centre of the crystal, and sometimes follow the external outline and planes of 

 cohesion. The included vitreous matter, which also occurs in the large crystals of 

 augite and hornblende, is of a less deep brown colour, and sometimes contains microliths 

 similar to those of the ground-mass. This fact, taken together with the corrosion of 

 the crystals containing these inclusions, proves that they have been penetrated by the 

 magma in which they were immersed. 



Hornblende plays an important part in this rock. Its crystals are prismatic, much 

 elongated, corroded, and fragmentary; this mineral is generally decomposed, its cleavages 

 being as a rule indistinct. Magnetite often encircles the sections as an external zone ; 

 probably these small opaque crystals were attracted around the hornblende even before 

 alteration commenced. Inclusions of apatite sometimes occur. The pleochroism is — 



a < /8 < y 



yellow. brown. dark brown. 



Augite of the first generation appears in corroded crystals of the ordinary form and 

 pleochroic — /3, bright yellow; a and 7, green. The polarisation colours are whitish yellow, 

 and the tints are brilliant in sections more or less perpendicular to the vertical axes. 

 The mineral is often twinned following the orthopinacoid. It is zonary, and gives larger 

 extinctions for the central parts than for the peripheral layers (34° for the former, 30° 

 for the latter). Patches of augite formed of agglomerated grains are also sometimes 

 seen. Biotite is not common in the rock, but small crystals of magnetic iron are 

 extremely abundant. 



The ground-mass embedding the minerals described above is composed of an almost 

 colourless base containing minute lamellse of plagioclase extinguishing at very small 

 angles, and nearly colourless augite microliths distinguished sharply from the felspar 

 by their more brilliant polarisation colours. 



Some specimens of rock forming the floor of " Bromley's Cave " — one of the cliff- 



