880 MR JOHN S. FLETT ON 



described, which is apparently one of the earliest minerals to crystallise out, as it 

 occurs only enclosed in brown augite or in hornblende. It encloses glass cavities, with 

 an immobile bubble, magnetite in small octahedra, and apatite. Olivine, accompanies 

 it, apparently formed at the same period. 



The innermost and oldest hornblende is of a deep brown or dark greenish-brown colour, 

 with a pleochroism f, deep greenish-brown ; b, dark brown ; a, pale brown. The 

 absorption for all the rays is more intense than usual. This is in part due to the great 

 amount of magnetite in very minute grains which are scattered throughout the mineral, 

 though not quite equally distributed. In some places they make it almost opaque. 

 They lie parallel to the prism faces, and as they run in streaks following the cleavage, 

 they give the sections a laminated appearance resembling diallage (PL II. figs. 2 and 6). 

 Higher magnification shows, however, that this is not cleavage, as in the thinnest 

 sections they are not sharply defined lines, but broadish bands along which magnetite 

 in rounded grains has been deposited, while in the centre of the bands a cleavage crack 

 may occasionally be found. The enclosures found in this form of hornblende are apatite, 

 magnetite, occasional glass cavities, and bright green augite and olivine. It has usually 

 the form of a highly corroded remnant of very irregular outlines, into which tongue- 

 shaped intrusions have penetrated from every side (PL II. figs. 4 and 6). It may indeed 

 resemble a spongy mass of hornblende, the interstices of which have been filled up by 

 later products of crystallisation. This hornblende is found only in the rocks which 

 contain large porphyritic c^stals, and in them only as a central nucleus. Still such 

 remnants may be of considerable size, measuring over \ inch in diameter. 



Around this is commonly found a thin zone of brown hornblende, clear and less 

 green in colour. It is usually free from enclosures, except scattered grains of magnetite. 

 Its external outlines follow those of the mass it surrounds, and it is never more than '01 

 mm. in breadth (PL II. fig. 4). 



The third zone in these complex crystals covers the two described, or, when they are 

 absent, forms the nucleus itself. It has always a dark appearance, owing to the abun- 

 dant grains of magnetite it encloses. These are of larger size than in the central mass, 

 where they are mere dust. It consists of an intergrowth of pale violet augite, with dark 

 hornblende, usually in about equal proportions, though often the hornblende predomin- 

 ates. In polarised light the clearer colours of the augite, owing to its smaller absorption, 

 distinguish it at once, and all the augite granules extinguish in the same position, while 

 the scattered patches of hornblende are also in optical continuity, and are moreover in 

 parallel growth with the central hornblende on which they were deposited. The struc- 

 ture is similar to that of the granophyres, where the graphic quartz and felspar of the 

 groundmass are in optical continuity with the phenocrysts. Whether this graphic 

 intergrowth does not enclose a certain number of glass cavities is not easy to establish 

 with certainty (PL II. figs. 1-6). 



The external zone varies in character. It may be brown hornblende, comparatively 

 pure, and sometimes showing zones of varying tint. It may be brown augite of the 



