Speight. — Soda Amphibole Trachyte. 181 



Microscopical Examination. 



A microscopical examination of the rock shows it to belong 

 to the trachytes, but with characteristics connecting it with the 

 rhyolites. The phenocrysts are apparently all sanidine, clear, 

 fissured, some in Carlsbad twins, their greatest length being 

 about 4 mm. Anorthoclase was specially looked for, in order 

 to explain the fairly high percentage of soda, but no undoubted 

 crystals were detected, although some of the crystals suggested 

 the microscopic twinning of anorthoclase very faintly. In the 

 absence of decided characters I have classified them all as sani- 

 dine. No phenocrysts of plagioclase were observed. 



The only other porphyritic mineral is the soda amphibole. 

 This mineral occurs in very irregular-shaped individuals of small 

 size, re-entrant angles being extremely common. Pleochroism 

 is very strong, the maximum absorption occurring when the 

 cleavage is parallel to the short diagonal of the nicol. The 

 colours are a deep-blue, greenish-blue, and brownish-yellow. 

 The mineral is somewhat opaque, and only translucent in thin 

 sections. The angle of extinction is therefore somewhat diffi- 

 cult to determine, but it ranges up to about 10°, measured 

 from the cleavage traces in sections where they are parallel. 

 These characters show the mineral to be, in all probability, the 

 soda amphibole arfvedsonite, or a closely related variety. The 

 rock also contains aggregates of iron-ores, which are apparently 

 derived from this amphibole. 



Groundmass. 



The groundmass is noncrystalline, but the size of the in- 

 dividual crystals varies considerably in different parts of the 

 dyke. It is composed chiefly of rectangular and short lath- 

 shaped crystals of sanidine frequently twinned, with interstitial 

 matter of smaller microlites of sanidine, and sparingly plagioclase ; 

 this last is almost certainly albite. The feldspars exhibit at 

 times a rough fluxion structure, especially when the groundmass 

 is somewhat coarse in texture. Small grains of quartz are 

 commonly seen in the groundmass forming part of the inter- 

 stitial matter between the larger individuals of feldspar. The 

 most brilliant polarisation colours and the index of refraction 

 (Becke's test) show clearly that the mineral is not tridymite — 

 which might have been expected in a trachyte. The high per- 

 centage of Si0 2 in the chemical analysis shows that a consider- 

 able quantity of free silica must be present, and this must occur 

 in the groundmass, as the clear phenocrysts are apparently 

 all sanidine. The soda amphibole also occurs very plentifully 

 in the groundmass, in the form of irregular flakes. This exhibits 

 the characteristic pleochroism of the larger individuals. In 



