468 THE GEOLOGY OF THE NEW HEBRIDES, 



Macroscopic Characters. — A dark grey compact 

 rock with porph^^ritic texture and uneven fracture. Large 

 porphyritic uralites with greenish sheen are very noticeable 

 constituents; much less common are whitish areas of saussuritised 

 felspar. Pyrites is present in strings and grains in notable 

 amount. 



Microscopic Characters (PL xxiii., fig- 3) — Texture 

 hypocrystalline porphyritic. Composition: evidently a very much 

 altered pyroxene porphyry, but retaining little of its former 

 appearance. 



The grouyidmass, which composes quite 60% of the rock, is 

 almost opaque, owing to the large amount of fine magnetite 

 scattered through it ; tiny plagioclase laths of microlitic 

 dimensions are, however, discernible to some extent. There are 

 three notable porphyritic constituents : — Felspar in rectangular 

 crystals about 1 mm. in length and still retaining traces of albite 

 twinning, though now completely converted to saussurite; traces 

 of original inclusions are still retained, though now almost 

 w^ithout exception converted to some form of actinolite. Uralite 

 in beautiful crystals up to 5 mm. diam. retaining the perfect 

 idiomorphic form of the original pyroxene. Sections parallel to 

 the vertical axis are always strongly striated (PI. xxiii., iig.3). 

 Some of the sections prepared show crystals in which an outer 

 zone of uralite appears enveloping a central area still occupied by 

 the original pyroxene (PI. xxii., fig 3). The pyroxene is perfectly 

 colourless, has a low extinction angle, and in the basal section 

 shows three cleavages; it is therefore related to diallage. The 

 amphibole has an extinction angle of 12°, appears light yellowish- 

 green in section, and is strongly pleochroic : 



a = faint yellow. 

 b = olive green. 

 C = grass green. 



a < b = r. 



It therefore closely resembles actinolite, and is the variety termed 

 Smaragdite. The third notable constituent is an alteration 

 product of some mineral whose identity is yet doubtful. It 



