93 



The olivine of the base exists in the form of isolated 

 grains, with practically no idiomorphic outline. The propor- 

 tion of olivine to augite is very small. 



The inclusions in the augite and olivine are magnetic, 

 and a few gas cavities. The principal inclusion in the felspar 

 is apatite, in the form of lath-like crystals. 



The phenocrysts of the rock are olivine and augite, the 

 former being by far the most abundant. The olivine, in 

 part, possesses an idiomorphic outline, showing traces of the 

 (010), (Oil), q.nd (100), but a great deal of the crystal has 

 been absorbed by the base. A fair cleavage is developed 

 parallel to the (001), but is often obliterated by irregular 

 cracks, which have been partly opened and filled with the 

 glassy base. Slight decomposition has gone on, owing to the 

 presence of a small proportion of serpentine. Some sections, 

 particulai-ly those parallel to the (010), show spheroidal 

 cracking. The olivine, which is perfectly colourless, contains 

 numerous inclusions of magnetite and probably some ilmenite, 

 there being present a few idiomorphic grains of a mineral, 

 not so glistening as the magnetite; glass is also included. 



The augite phenocrysts vary greatly in size and shape. 

 They show traces of the "^(100), (010), and (111). The colour 

 is a light-greenish-brown, and the pleochrium is noticeable 

 but very weak, and therefore the absorption cannot be indi- 

 cated. Twinning has gone on principally in sections parallel 

 to the (100), but owing to the small proportion of augite pre- 

 sent there is not very much to choose from. They are very 

 much smaller than the olivine phenocrysts, and show a 

 gradual diminution in size, even down to the dimensions of 

 the microlitic components of the base, so that the consolida- 

 tion of the augite may have started after the olivine, and 

 kept on crystallizing after the felspars were formed. 



Magnetite occurs usually as small grains or dust, and in 

 a few cases as larger idiomorphic grains round the edges of 

 the olivine. 



Order of Consolidation. 



Magnetite . . . 



Olivine ... ... ... ... 



Augite ... ... ... — 



Labradorite ... ... 



Glass 



VI, — Vesicular Olivine Basalt (slagfgy type). 



Rock 3. 



On the ridge of the flow in the north-western corner of 

 Brown Lake. 



