94 



Macroscopic CJtaracters. 

 The rock is dark-browii in colour, but wlien freshly 

 broken is practically black, possessing many vesicules of small 

 dimensions. The rock has picked up foreign particles during 

 its period of flow, as is indicated by the j)resence of rounded 

 grains of quartz. There are a few visible phenocrysts of oli- 

 vine scatt-ered throughout. The rock shows lines of flow, 

 possesses a very rough and slaggy surface, and is tachylytic 

 in character. 



Micioscopic Churacttrs. 



Not unlike rock 2, but very much more vesicular. The 

 most distinguishing feature is the predominance of augite in 

 the base, over the felspar microlites, which ai'e very jfew in 

 number. Flow structure is not developed to the extent that 

 it is in rock 2. The glass is dark-brown in colour, containing 

 a great amount of magnetite dust. The microlitic compo- 

 nents are, in order of abundance — augite, both with crystal- 

 line outline and in grains ; olivine occurring principally in 

 grains ; and a few scattered microlites of plagioclase felspar. 

 The augite, which is light-green in colour, exists in the form 

 of short tabular ci'ystals possessing a noticeable cleavage. 

 Small rosettes occur less frequently, besides a few cross twins. 

 The principal inclusion is magnetite, in the form of small 

 grains, and also a few long irregular inclusions of glassy 

 base. 



The olivine of the base can be recognized only by its 

 high polai'ization tints, the grains being very small. 



Of the few scattered plagioclase microlites only one or 

 two gave symmetrical extinctions up to 26°, the others being 

 unsuitable for determination. In one or two cases a distinct 

 ophitic structure is noticeable, the augite including the plagio- 

 clase microlites. 



The phenocrysts of the rock are olivine and augite. The 

 olivine is by far the most abundant, although a great deal 

 of it has been absorbed by the magma. A great deal 

 of the olivine possesses no sign of idiomorphism, being 

 usually in curved and irregular crystals, which has probably 

 been due to extraneous causes, together with diffusion. They 

 contain a great many inclusions of magnetite of fairly large 

 dimensions, and more rarely glassy base. Cracking and de- 

 composition have gone on to a small extent. 



The augite phenocrysts are rare, and in the majority of 

 cases have undergone decomposition to serpentine. They con- 

 tain many inclusions of magnetite and glass, and is ophiti- 

 cally intergrown with the plagioclase felspar. Glomero- 

 porphyritic aggregates of augite and olivine are to be seen 

 in some sections, but this structure is rare. 



