126 Chap'inan and Tltiele: 



of the exposure. The rock has probably solidified at or near the 

 surface, as there are no particularly large crystals in the 

 magma, the phenocrysts being of very moderate size. The rock 

 is decomposed in the superficial layer, but in the interior ii? 

 dense and of a blue-black colour. There are a few vesicles in the 

 mass, but many of the cavernous parts of the surface-material 

 are seen to be due to the decomposition of the olivines. It was 

 found impossible to discover a clean, exposed section of the 

 lava, but at one spot we were able to see the jointed, rubbly 

 rock lying on decomposed lava to some depth. Numerous joint- 

 planes have coanpletely broken up the rock, and the form of the 

 fragments is platy rather than cuboidal or prismatic, as in 

 ordinary basalt. Some of the less dense pieces, in contrast to 

 the dark, rusty brown colour of the majority of the lava-slabs, 

 are of an earthy yellow to sage-green colour ; a difference due 

 to the presence of a larger quantity of decomposed olivine. 



Detailed Descpiption of the Balwyn Lava. 



Megascopic Chara^-ters. — Typical rock specimens are dense, 

 and dark blue to black in the decomposed parts. The wea- 

 thered surface is yellowish to dark bro^^^l, and forms a thin 

 crust on the exposed portion. Very frequently the surface 

 shows a pitted structure which is very conspicuous, and due to 

 the removal of olivine phenocrysts. Fractured surfaces reveal 

 a few glassy phenocrysts of olivine. Some of the rock frag- 

 ments picked up at the excavations are rich in alteration pro- 

 ducts of the ferro-magnesian minerals, and of a dull green 

 colour. 



Microscopic Characters of the Blacl- Lava. — Ground-mass 

 with hyalopilitic structure ; also indications of local fluxion 

 structure, with numerous phenocrysts of olivine, large and 

 small, and an occasional pale green augite ( = diopside). The 

 ground-mass consists of closely textured crystals of minute 

 augites, a moderate proportion of felspar laths, ^ numerous small 

 crystals of magnetite, and some larger platy ilmenites, together 



1 In a slide of the rock analysed for the present piper, the felspars are more numerons ; 

 nevertheless the strong basic character of that particular specimen is shown by its high 

 specific gravity (2.994). 



