REPORT ON THE PETROLOGY OF OCEANIC ISLANDS. 33 



furrowed by rather indistinct cleavages parallel to the axis c, and by fractures more or 

 less nearly parallel to the pinacoid OP. It is somewhat rare to find the outlines well 

 shown in these various sections : they are usually bordered by a rim of small augite 

 prisms, which belong to the second phase of consolidation. In the sections more or 

 less parallel to the clinopinacoid, extinctions are observed that exceed 20° and some- 

 times amount to 30°. The pleochroism and absorption are — 



a > j3 > y 



yellowish brown. greenish yellow. green. 



In exceptional cases it is found that the green tint of this augite changes to the reddish 

 coloration so common in the pyroxene of basalt. The inclusions seen in these augite 

 sections are microscopic prisms of apatite, and a few granules of magnetite. 



The hornblende of this phonolite ought to be regarded as an accessory mineral ; 

 it occurs only in some few individual crystals, these being for the greater part trans- 

 formed into magnetite. It has the shape of very deformed hexagonal sections, at the 

 centre of which are brown patches markedly dichroic in brownish shades, the differ- 

 ences, however, arising generally from differences of absorption. These sections 

 are bordered by a broad zone of magnetite, which tends to encroach upon the 

 centre of the crystal, where nothing but a round nucleus usually remains. The 

 black opaque girdle which surrounds it is homogeneous, and is not formed of an 

 aggregation of isolated grains as is often the case, — in the amphibolic andesites, for 

 example. This zone of magnetite is frequently larger than the amphibolic centre ; in 

 some instances the hornblende is entirely displaced, the contours of the section being 

 the only indications of the pseudomorphosis. We always find around these sections of 

 hornblende bordered by magnetite a second zone made up of small augite microliths 

 crowded one over another, indeed, such a girdle of small green augite crystals is 

 almost always found round all the microporphyritic crystals of this rock. 



Of the accessory minerals titanite is that which next to hornblende is best repre- 

 sented in this phonolite. Its sections are in general smaller than those of the latter 

 mineral, descending even to the dimensions of the microliths in the ground-mass. This 

 mineral has crystallised most perfectly, and has best preserved the entirety of its 

 forms. Sections with rhombic outlines are the commonest, — they can be set down 

 as sections of the zone Px, for the extinctions are parallel to the diagonals, and 

 the angular values correspond to those of the prism (129°-133°). One might conclude 

 from the abundance of these sections that titanite has crystallised in this rock in the 

 tabular form, and that the dominant face is more or less nearly perpendicular to the 

 zone of the prisms. Besides these sections we find some of the same mineral that are 

 more elongated, and, finally, others that have reentrant angles and appear twinned in 

 polarised light. The surface of the sections is fretted ; they show a very pronounced 



(PHYS. CHEM. CHALL. EXP. — PART VII. — 1889.) 5 



