DRED, W. R. BROWNE AND W. A. GREIG. 
It is possible to give a reasonable explanation of the 
observed mineralogical peculiarities of the quartz-mon- 
zonite. N. L. Bowen,* generalizing from the results of a 
considerable number of laboratory investigations, has 
described the probable course of crystallization during the 
slow cooling of a body of basic magma, of the composition 
of a mixture of plagioclase and a rather magnesian pyrox- 
ene. He points out that the first mineral to crystallize is. 
not pyroxene, but a member of the olivine group, which, on 
the temperature falling sufficiently to permit of the form- 
ation of pyroxene, gradually suffers resorption. He has 
also shown how in the later stages of the crystallization 
of such magmas, after the separation of plagioclase, horn- 
blende and (if potash is present) biotite may be formed, 
with some quartz in addition. 
On the basis of the principles enunciated by Bowen it is 
_ possible to some extent to trace the history of the Kiandra 
quartz-monzounite, although judging from the analyses the 
magma was not basic but intermediate in character. 
Originally, before crystallization, the magma was at a 
temperature above the crystallization-point of olivine, 
which was the first mineral to appear after cooling started. 
It continued to separate until a temperature was reached 
at which solid pyroxene, rather magnesian, was in equi- 
librium with the magma. As soon as this appeared 
resorption of the olivine commenced, and it continued with 
the concomitant formation of pyroxene. It was where the 
dissolving olivine produced magnesia-rich patches in the 
magma that the pyroxene formed, eventually enwrapping 
the remnants of olivine in many cases, and thus removing 
them from the corrosive influence of the magma. The 
olivine not protected in this way went into solution, and 
that is why the olivine in the rock invariably occurs as 


* Journ. Geol. Vol. 23, 1915, Supplement. 

