REPORT ON THE PETROLOGY OF OCEANIC ISLANDS. 31 



slightly waxy lustre. With the naked eye only some crystals of sanidine, from 

 2 to 3 millimetres in length on the average, can be made out among the constituent 

 minerals ; cleavage lainellse parallel to M are seen gleaming. The rock yields some 

 water on heating in the closed tube ; when attacked by acids it gelatinises readily. 

 Its specific gravity is 2 "635. The specimens of massive phonolite from the summit of 

 the mountain do not differ in any essential manner from that of which the macroscopical 

 characters have just been given. 



When examined with the microscope, this phonolite shows a microporphyritic 

 texture ; embedded in a very close-grained ground-mass, in which one notices only 

 small, somewhat irregular microliths of augite showing fluidal structure, there are 

 seen, as minerals of the first generation, sections of nepheline, sanidine, augite, horn- 

 blende, magnetite, titanite, and nosean. We shall now consider the characters of these 

 minerals of first generation. 



Sections of nepheline are very common ; they are distinguished at a glance from 

 the other constituent minerals by the sharpness of their contours, and by their com- 

 pleteness. Comparison of the form of sections, cut in various directions, show that 

 the nepheline in this rock takes the form of crystals slightly tabular, parallel to OP, 

 with the faces of the prism somewhat shortened. The commonest sections are 

 equilateral hexagons with an angle of 1 20" ; they are remarkably limpid, and very 

 slightly blue or almost colourless in tint. This mineral has no inclusions except titanite ; 

 it is perfectly homogeneous. The lines of cleavage which traverse it are distinct ; they 

 have the appearance of regular blackish strokes, parallel to three alternate sides of the 

 hexagonal sections. In parallel polarised light these sections remain constantly 

 obscured throughout a complete rotation ; in convergent light it is rather rare to 

 be able to observe the usual black cross of monaxial crystals. This mineral also 

 presents rectangular sections with a similar physical aspect. They show two 

 cleavages : the more distinct of the two is indicated by streaks parallel to the 

 traces of the prism ; the other, perpendicular to the first, is less marked, and is 

 parallel to the pinacoid OP. These sections always extinguish parallel to the sides. 

 Nepheline often occurs in this phonolite in aggregates of several crystals grouped 

 parallel to the vertical axis ; these aggregates are recognised by the outlines forming 

 reentrant angles, and, between crossed nicols, these adherent crystals are distinguished 

 one from another by different shades of the same tint. The tints of chromatic 

 polarisation are feeble, and generally clear blue. An alteration is sometimes seen 

 between crossed nicols, which has akeady been pointed out as occurring in 

 nepheline ; we refer to a more or less complete zeolitisation. In polarised light 

 several sections assume a darker tint than usual, at the same time they look as if 

 stumped ; but sometimes certain patches are almost colourless, and a kind of 

 marbling is produced by this want of homogeneity. On examining these sections 



