116 REGINALD A. DALY 
by most petrologists. A similar conclusion follows if any other 
cause for initial heterogeneity be considered. 
Secondly, the term should apply to separations in hybrid 
magmas, whether formed by the mixture of two or more liquids 
(Bunsen, Harker) or by the magmatic solution of solid rock (Cotta, 
many French, Scandinavian, Russian, Australian, and Canadian 
petrologists). Bowen himself believes in a moderate amount of 
assimilation. As Loewinson-Lessing has specially insisted, a small 
degree of contamination with foreign material may change equi- 
librium in the magma, which therefore separates into strongly 
contrasted parts. It would be a pity, if it were possible, to exclude 
a change of the latter kind from the list of those properly covered 
by the name “‘differentiation.”’ 
Petrologists of the future are, indeed, likely to agree that this 
word shall be used to denote separation of phases and that its. 
definition should be kept free from any presupposition as to the 
origin of the magma which does separate into parts, liquid or solid. 
Accordingly the units of differentiation may belong to one or more 
of the following six classes: 
1. Contrasted fluid phases of an initially heterogeneous magma, 
including parts particularly rich in volatile constituents. 
2. Solid crystals (fractional crystallization). 
3. Mother-liquor left after partial crystallization. 
4. Non-consolute liquid fractions (liquid immiscibility). 
5. Material of fused country rock, not diffused into the original 
magma (ultra-metamorphism in part). 
6. Original magma locally charged with material dissolved 
from the country rock, but slowly diffusing from the source of 
supply (syntexis). 
Many field and laboratory observations suggest the control of 
gravity during the separation in any of the six cases. Given mod- 
erate viscosity for the magma, most early-formed crystals should 
rapidly sink. ‘The formation of these crystals commonly means a 
decrease of density in the adjacent liquid, which is thus made 
specifically lighter than the surrounding magma. Such locally 
generated mother-liquor should tend to rise and concentrate near 
the roof of the magma chamber. Differential densities must tend 
