=I 
os 
OLIVINE-BEARING QUARTZ-MONZONITE FROM KIANDRA. 2 
inclusions. When the pyroxene was almost completely 
crystallized the plagioclase made its appearance and con- 
tinued to solidify until the plagioclase material in the 
Magma was nearly all exhausted. 
The continued crystallization had the effect of concen- 
trating the water and other mineralizers, so that in time 
the anhydrous pyroxene gave place to hornblende, which 
began to crystallize, partly from the small amount of ferro- 
magnesian material still present in the magma, and partly 
at the expense of the now unstable pyroxene. 
The amphibole, as is usual in such cases’ crystallized 
mostly around and in parallel circumgrowth to the pyrox- 
ene, and, judging from its colour, absorbed some of the 
albite molecule still present in the magma. At a later 
stage the concentration of mineralizers caused the partial 
dissociation of the orthoclase molecule, which united with 
the ferro-magnesian silicate molecules to form biotite. The 
latter may have formed at the expense of the solidified 
pyroxene and amphibole, but there is no evidence that this 
is so; though the biotite often clings closely to the pyroxene 
and hornblende, much of it occurs quite detached from 
these. 
The formation of hornblende, and later of biotite, instead 
of pyroxene, set free a certain amount of silica, and the 
final stage of consolidation was the crystallization of the 
residue of orthoclase, and of the remaining silica as quartz. 
The presence of olivine in the rock is therefore due to 
the richness of the magma in the hypersthene molecule, or, 
putting it another way, to the high value of the ratio 
magnesia: femic lime. The quartz is due largely to the 
relatively high silica percentage of the magma, but also, 
though in a very subordinate degree, to the formation of 

1 cf. J. A. Thomson, Geol. Surv. of W. Aust. Bull. 33. 
R- December 6, 1923. 
