OLIVINE-BEARING QUARTZ-MONZONITE FROM KIANDRA. 263 
Brown, B.S;., on thin sections, giving values for 2V between 
40° and 49°. These characteristics would seem to indicate 
a diopsidic pyroxene higher than usual in MgO, approaching 
enstatite-augite in composition, although no very small 
optic axial angles were observed, as appears to be com- 
mon in the case of this mineral. 
The mineral in this rock which has in the past generally 
been regarded as hypersthene presents some unusual 
features. A goodly number of longitudinal sections show 
Straight extinction, as one would expect in ordinary hypers- 
thene, but many extinguish at angles up to and exceeding 
20°. The characters of these sections agree in other 
respects with those of hypersthene: double refraction is 
weak, the polarization colours being low ist order grey, 
with a negative sign; pleochroism is feeble except in thick 
sections, where there isa distinct colour change from pale 
greyish-green to a light apricot tint. Sections with prac- 
tically straight extinction often show a kind of irregular 
patchy undulose extinction which in some cases seems to 
be due toor accompanied by extremely fine twinning stria- 
tions. The sections extinguishing obliquely exhibit marked 
zoning betweeu crossed nicols, and a corresponding slight 
intensification of colour in the peripheral portions in ordin- 
ary light. Measurements of the optic axial angle by Miss 
Brown showed 2V to lie between 51° and 56°, which is 
lower than usual in either rhombic or monoclinic pyroxene. 
The possibility suggests itself that these sections, includ- 
ing those with straight extinction, represent clino-hypers- 
thene, a monoclinic form of the magnesia-iron pyroxene, 
corresponding to clino-enstatite, which is the stable form 
of the pure magnesia pyroxene at high temperatures in 
laboratory experiments,’ and has also been found in 
meteorites. 
1 Allen, Wright and Clement, Amer. Jour. Sci., Ser. 4, Vol. xx11, 1906, 
pp. 385 — 438. 
