514 THE CONTINENTAL ORIGIN OF FIJI, II. 



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Q u a r t z i t e (Togicedra). Plate xxxv., fii 



In hand specimen very little of the structure of the rock can 

 be made out, as it ver)^ fine-grained and ver\' uniform in texture. 

 The colour is almost black, and the stone is intensely hard. As 

 stated in the first part of tliis paper, the rock is very considerably 

 jointed, but the bedding planes are in many cases obscure. 

 When they are visible, the dip is at very high angles, but variable 

 in direction and amount. 



In thin section the rock is seen to consist of a ver}' fine-grained 

 aggregate, the base of which is a very fine mosaic of quartz and 

 one or more other colourless minerals. INIany of the colourless 

 granules which have broken away from the edge of the section 

 have a refractive index lower than that of Canada l)alsam. This 

 material cannot be quartz. It is perfectl}^ clear and colourless, 

 shows no microscopic structures (cleavage, twinning, &.C.), and 

 possesses very weak double refraction. An acid felspar, probably 

 albite, answers to this description. The principal coloured con- 

 stituent is referable to amphibole. It occurs in minute prismatic 

 sections whose minor limit of size is submicroscopic. The larger 

 ones are greenish-3'ellow in colour, and noticeably pleochroic. 

 The extinction angle is within about 10" of their direction of 

 elongation. Kefractive index is high, and double refraction 

 quite strong. The colour for vibrations perpendicular to the 

 length is light yellow ; for those parallel to the length it is 

 slightly darker greenish-yellow. The largest prisms are distinct!}' 

 fibrous, and are frayed out at the end; the smaller individuals 

 occur independentl}^ or clustered together to form tufts. The 

 arrangement is on the whole irregular, but a more or less marked 

 grouping in parallel layers imparts a foliated structure to the 

 rock. In cross section the outline is rounded to irregular. No 

 mica can be perceived. A little apatite in minute prisms is 

 recognisable. Iron ore in rounded grains is exceedingl}- abundant. 

 It is almost exclusivel}' referable to magnetite, though an occa- 

 sional speck of pyrites is present. 



The rock is traversed by numerous irregular quartz veins, in 

 which the quartz assumes the character of a mosaic. 



