BY W. H. TWELVETREES AXD W. F. PETTBRD. 11 



prisri'iS have been dynamically bent. They carry abun- 

 dant inclusions of haliyne (nosean) decomposed to zeolitic 

 matter of a light yellow colour, giving mottled grey and 

 white interference appearances under crossed nicols., 

 Augite is in prismatic elongated forms, and is a green 

 pleochroic variety with extinction angle of 40°, though as 

 the colour becomes yellowish the angle seems to decrease. 

 a = light green, t == dark green. Bleaching is common. 

 Grains and rods of augite border crystals of apatite.. 

 Large decomposed haliyne crystals are abundant as diver- 

 gent and fan-shaped zeolites of a pale yellow colour in 

 plain light. Allotriomorphic orthoclase felspar forms a 

 cement uniting the above elements, and this gives the 

 aplitic character to the rock. Yellow crystals of sphene in 

 prismatic and acute rhombic sections are plentiful. Zircon 

 is less common ; magnetite in grains. 



Soda Aplite, a Sanidixe-Augite-Mica Dyke Rock.. 



Sp. gr. 2 'So. 



Localitij — On the beach, at Port Cygnet, south of the Regatta Ground.. 



Meter osco}') icet I characters . 



A hard granular dark glistening rock resembling a 

 minette (mica-trap). Numerous little brilliant faces of 

 dark mica visible. The felspar looks granular and sugary. 

 The green augite is too minute for satisfactory identifica- 

 tion. 



Jlinerrd constituents. 



Sanidine, augite, biotite, apatite, soda decomposition 

 products and pyrite. 



3Iic) 'oscoj) ica I cli ai xicte) 's. 



The first thing which strikes one on looking at a slide 

 is the panidiomorphic structure of the rock, reminding 

 one at once of aplite. At the same time the prisms of 

 augite and plates of mica follow linear directions like the 

 minerals of a foliated rock. 



The felspar is granular and imperfectly prismatic, with 

 its boundaries abridged by neighbouring prisms. Carlsbad 

 twinning is frequent. Where elongated sections are avail- 

 able with some approach to a prismatic character, the 

 extinction is straight. The crystals are pellucid and con- 

 tain numerous microliths, rods, ovoid and circular grains,, 

 perfectly transparent, colourless, and without any definite 

 action on polarised light. Some are light green ; these 

 are augite. The inclusions, which are of large size, are a 

 constant feature. 



