470 Reviews—N. L. Bowen— 
attention was first called by Lagorio, has now been generally 
abandoned, mainly owing to the criticisms of Becker, Harker, and 
others. The theories still under discussion include those based on 
assimilation, pressure phenomena, the action of vapours, liquation, 
and fractional crystallization. Loewinson-Lessing held that the 
solution of foreign material was the chief factor, a view also held by 
Johnston-Lavis and, in a modified form, by Daly. The possible 
‘influence of pressure has been indicated by Schweig, while Michel 
Lévy favoured the activity of the volatile constituents of the magma 
as the most potent operator in producing heterogeneity. The process 
of liquation, first advanced by Durocher, has received many supporters, 
including Daly, but fractional crystallization, as suggested by Becker, 
has also attracted much favour. In the paper under review this 
last hypothesis is advocated, the arguments being based on recent 
synthetic work on the equilibrium of mineral systems. 
While in the earlier investigations the systems involved were 
simple, it has now been found possible to examine some, such as 
diopside -forsterite-silica, anorthite-forsterite-silica, and diopside- 
plagioclase, which approximate to certain simple types of igneous 
rocks. Bowen summarizes the information which has been obtained 
from these synthetic investigations, and bases on them a systematic 
theory of petrogenesis, with fractional crystallization, the sinking of 
crystals, and the squeezing out of the residual magma as the chief 
factors of differentiation. A basaltic magma is taken as the starting- 
point, and it is shown that the various ways in which this can 
crystallize may give rise to a sequence from granite to peridotite. 
While the rapid cooling of such a magma produces a simple basalt, 
slow cooling, accompanied by the sinking of the early-formed 
crystals of pyroxene and lime-plagioclase, results in the enrichment 
of the upper liquid layers in alkali-felspar, and in their subsequent 
consolidation as diorite. With still slower cooling the residual 
material crystallizes as quartz-diorite, granodiorite, or granite. If 
the cooling history is such that the plagioclase crystals are zoned, 
the salic material may be found filling the interstices between the 
earlier minerals. If pyroxene alone separates out, the lower layers 
are composed of the ‘‘anchi-monomineralic”’ rock pyroxenite; if 
olivine alone sinks before resorption occurs, the resulting rock is 
dunite; if. pyroxene and olivine separate out together, or if the 
olivine is partly resorbed after sinking, various types of peridotite 
may be formed. The separation of these minerals which are poor in 
silica greatly enriches the upper layers in the latter oxide, which may, 
if the cooling has been sufficiently protracted, finally crystallize as 
uartz. 
Nevertheless, quartz is so abundant in igneous rocks that the 
early separation of olivine cannot alone be held responsible for its 
presence. The commonest ferromagnesian mineral in the acid rocks 
is biotite, and as this is relatively poor in silica Bowen holds that its 
early formation is responsible for the existence of much of the quartz. 
There is here a discrepancy between the synthetic work and the 
natural rocks, as the salic pole of the differentiation in the former is 
a pyroxene-bearing granite, and in the latter a biotite or hornblende 
