BY D. MAWSON. 407 



microcrystalline granular consisting of felspar, greenish-yellow 

 pyroxene and magnetite; it appears as a cementing material 

 between the coarser individuals. 



2. Olivine Basalt Porphyrite outcrops at the timber-shoot 

 about 1 mile inland. 



Macroscopic Character s. — A light grey-coloured rock 

 in which large pyroxenes and clear felspars appear in a micro- 

 crystalline base. 



Microscopic Character s. — The rock is seen to be com- 

 posed of two distinct factors — porphyritic constituents of idio- 

 morphic felspar, pyroxene and olivine individuals to the extent 

 of about 50 % of the rock scattered through a typical pilotaxitic 

 base. 



Of the porphyritic constituents the felspars^ which represent 

 about 25 % of the rock, are large clear crystals, often 7 mm. in 

 length, with extinction angle corresponding to basic labradorite. 

 Another 15 % is represented h^^ pyroxene crystals, more or less 

 granular, and averaging 2 mm. diameter; they are of a very light 

 lemon-yellow colour, only faintly pleochroic, and are free from 

 inclusions. Large pinkish olivines, about 1 mm. diameter, com- 

 pose another 8 %, and magnetite grains, 0-5 mm. diameter, 

 another 2 %. 



The microcrystalline pilotaxitic groundmass of the rock is a 

 meshwork of lath-shaped plagioclase crystals O'l to 02 mm. in 

 length, with interstices occupied by tiny light greenish-yellow 

 granular pyroxenes and magnetic grains. 



Both this rock, and the one described preceding it, have certain 

 of the characters of a dolerite, but are in the main more nearly 

 related to the basalts. They differ from each other not in magma 

 composition, only in relative development and size of the mineral 

 constituents. The darker colour of the first-described type is 

 attributable to the darker appearance of the felspars, due to very 

 numerous inclusions. 



Tanna has always attracted much attention from travellers on 

 account of its containing the active volcano of Yasowa.* 



* For details see Section 2 (p. 429). 



