REPORT ON THE PETROLOGY OF OCEANIC ISLANDS. 87 



outer zones. Sections of the zone P :h give symmetrical extinctions of 28°-27° for the 

 inner, and of 21°-17° for the external zones. The central parts thus approach 

 anorthite, while the outside comes nearer to labradorite. Notwithstanding the absence 

 of olivine in the microscopical preparations, this rock cannot be classed with the augite 

 andesites, and its structure presents fewer resemblances to that type than to the 

 dolerites. 



We may note in passing some slightly vesicular rocks, the ground-mass of which is 

 close-grained, and contains no macroscopic minerals except a few whitish grains of 

 altered felspar. The specimens resemble ordinary basalt in every respect, and show no 

 microscopic features meriting special attention. 



Related to these rocks there are some vitreous masses altered into palagonite. 

 They are scoriaceous like pumice, and are coloured yellowish by bmonite, but do not 

 show well the resinoid aspect of palagonitic rock. They contain small hetero- 

 geneous fragments, indicating the tufaceous origin of the deposit. Under the 

 microscope this substance shows, between crossed nicols, in certain parts of the 

 preparation, phenomena of polarisation like those of altered sideromelane ; the vitreous 

 mass is, however, isotropic. The base contains numerous small crystals of augite, 

 which are sometimes capillary and of a green or brown tint. Plagioclase microliths 

 are neither abundant nor well formed ; they are often hollowed out on both extremities, 

 and are usually present as skeleton crystals. Olivine is rare or altogether absent. 

 Some patches seem to be made up of heterogeneous fragments ; these lapilli are 

 characterised by an obvious difference in the texture and by their mineralogical com- 

 position, as they are formed of rather large crystals of plagioclase mixed with grains 

 of augite. The vesicles scattered through the rock contain no zeolites, remaining vacant 

 in the centre although their walls are lined with a light transparent green layer of a 

 secondary mineral. 



Having dealt with the lavas and tufa of the island, we have now to describe the 

 transversal dykes. The rocks forming these dykes are generally massive or finely 

 alveolar. The porphyritic basalt with large augite crystals, described above, is 

 traversed by a vein composed of a compact, bluish grey, slightly vesicular mass, con- 

 taining macroscopic crystals of augite and olivine. This basalt when examined 

 microscopically presents a microporphyritic appearance, produced by rather large 

 zonary crystals of augite and olivine. The ground-mass is an aggregate of minute 

 crystals of three minerals, plagioclase, augite, and magnetite, without interposition of 

 any base. Another dyke, resembling the first in colour and microscopic structure, 

 differs from it in being perfectly compact. Here also augite and olivine can be seen by 

 the naked eye, but under the microscope the ground-mass appears composed of 

 minute plagioclase and augite crystals, and contains a little vitreous matter. Large 



