526 THE CONTINENTAL ORIGIN OF FIJI, II., 



grey in coloiii-, and is composed of finely comminuted fragments 

 of the same rock which occurs as boulders. The latter run up 

 to 5 or 6 feet in diameter, and are all fairly angular. 



Macroscopic characters. — In hand specimen the rock is light 

 bluish-grey in colour, and very slightly vesicular. It is rendered 

 porphyritic by very numerous striated felspar crystals 5 or 6 mm. 

 long, and by less abundant but rather larger augites. As a rule, 

 no hornblende can be detected macroscopically. The rock where 

 vesicular is rendered amygdaloidal by having the cavities filled 

 with white amorphous material. In most specimens there is a 

 marked fluidal structure, but in some this structure is not apparent. 

 The rock is almost perfectly fresh and free from decomposition. 

 The specific gravity in mass is 2-60. This is a little low^ on 

 account of the vesicles, but they are so small and scattered that 

 they cannot affect the result to any great extent. 



Microscopic characters. — In section the base consists of fairly 

 abundant colourless glass crowded with microlites, thus producing 

 a hyalopilitic texture. Through the glass are scattered what 

 appear to ])e minute gas-cavities of rounded or irregular shape. 

 These areas possess \'ery dark borders, indicating a considerable 

 difference in refractive index between their contents and the 

 glassy base, but do not affect polarised light. Hair-like indeter- 

 minate microlites are very abundant, interlacing to forma "felt." 

 Amongst the individualised constituents of the base, felspar, 

 augite, and magnetite are practically all that are to be noted. 

 T'he felspar microlites are lath-shaped, square or irregular. The 

 lath-shaped sections are mostly untwinned, but their minute size 

 makes the recognition of their properties extremely difficult. 

 The extinction angle measurements are extremely unsatisfactory, 

 but appear to be about 10° to 12° from the length. If this is 

 correct it indicates either albite or andesine. The augite micro- 

 lites are prismatic in shape. They are not nearly so abundant 

 as felspars, and are easily distinguished from them by their 

 higher refractive index, stronger double refraction, and by possess- 

 ing extinction angles up to 45 '^. Magnetite is relatively plentiful 

 in idiomorphic grains. 



