REPORT ON THE PETROLOGY OF OCEANIC ISLANDS. 



57 



Fio. 9. — Basalt of Red Hill. 

 Section of plagioclase perpendicular 

 to the edge P/M, with the traces of 

 two cleavages parallel P and M; on 

 the right, remains a part of an indi- 

 vidual twinned following the albite 

 law. A crossed nicols. 



olivine, and triclinic felspar appear in this mass. The triclinic felspars are prismatic, 



relatively very thick, and zonary. The zones are not numerous, as in the case of 



andesine, for example, but usually consist only of a nucleus and an outer coating. 



Extinctions of about 37° have been measured on sections parallel to h, which indicate 



a plagioclastic mixture approaching bytownite. The cut 



(fig. 9) shows a section of one of these felspars perpendicular 



to the edge PjM. The traces of two cleavages parallel to P 



and M are visible, and on the right there are the remains 



of a twinned individual following the albite law, and almost 



entirely removed by the process of polishing. The two 



individuals extinguish symmetrically at 40°, which again 



establishes the very basic nature of this plagioclase. This 



section is instructive in exhibiting clearly the form of 



the large felspars in the rock under consideration. The 



felspar is often corroded or broken, and the fragments 



scattered at a little distance from one another, separated by the ground-mass. It is 



also apparent that certain sections have been subjected to pressure ; they present traces 



of undulating extinction, which is particularly the case in the plagioclase represented in 



the figure, where this extinction is indicated by the shade in the middle towards the 



right margin. In other specimens of basalt the plagioclases have quite a simple 



structure — as in the case just spoken of : they show the Carlsbad twin and one or two 



hemitropic lamellae interposed, the somewhat small angles of extinction making them 



approach labradorite. 



Olivine appears in sharply defined sections. The decomposition of this mineral is 

 somewhat remarkable, as it changes into hematite with the simultaneous development 

 of trichites. Such an altered crystal with the curved and parallel lines of the trichites 

 invading the mineral is shown in fig. 10. Some- 

 times the little olivines of the ground-mass have 

 a quite pronounced prismatic form, which makes 

 it difficult to distinguish them from microlithic 

 augite. The augitic microliths are colourless like 

 olivine, but the sections of the latter are edged 

 with a zone of limonite which serves to distinguish 

 the species. There is little to say about the 

 large crystals of augite, which appear less often 

 in these specimens than is usual in basalts, the 

 commonest form in this case being microlithic. 



These rocks have often a vesicular appearance when in thin slices, although they 

 seem perfectly massive when looked at with a lens. The vacuoles are generally due to 



(PHTS. CHEM. CHALL. EXP. — PART VII. — 1889.) 8 



Fig. 10.— Basalt of Bed Hill. Section of olivine 

 decomposed into hematite and filled with trichites. 

 jrj crossed nicols. 



