276 W. R. BROWNE AND W.-A. GREIG. 
It-would thus appear that the association of olivine and 
primary quartz in the same rock may be brought about in 
one of two ways: 
(a) As in the case of the Kiandra quartz-monzonite just 
described. 
(b) Where there has been sinking of olivine crystals ina 
magma-reservoir, an eruption of the still liquid, 
relatively acid, top portion of the magma might 
include some of the already crystallized olivine 
caught up from the lower levels. Hence might arise,. 
for example, the presence of olivine nodules, or, 
exceptionally, isolated oliyine grains in a quartz- 
dolerite: these would be cognate xenoliths and 
cognate xenocrysts respectively. An occurrence of 
olivine in a quartz-dolerite from Adélie Land, Ant- 
arctica, described by one of the authors, is probably 
to be explained in this way. 
The olivine sometimes found in intermediate volcanic 
rocks whose silica percentages would have permitted the 
crystallization of pyroxene, is possibly due to interrupted 
intratelluric crystallization. Under these circumstances. 
some or all of the olivine, which formed before eruption of 
the magma as a flow or a minor intrusion, would tend to 
persist, rapid cooling preventing complete resorption, and 
perpetuating the unstable phase. In this way may perhaps 
be explained the olivine in the dacites of Mount Shasta, 
California,’ and that in the pitchstone-porphyries of Arran, 
described by Scott.’ 
But ina plutonic magma where slow cooling obtains and 
equilibrium adjustments are possible, unstable tend to be 
replaced by stable minerals, and hence in rocks of deep- 

1 Tddings, Igneous Rocks, Vol. 1, p. 142. 
2 Geol, Mag. N.S. Dec. vi, Vol. 1, 1914, p. 319. 

