160 
IDDINGS. 
the effect of concentration from this cause in dilute solu¬ 
tions will be very slight for a depth of 100 meters, but 
should be more noticeable in concentrated solutions, and 
from the nature of the case would take place with great 
slowness. 
Hence it is possible that in a very large body of molten 
magma the chemical equilibrium in different parts of it 
may be sufficiently diverse to cause a shifting of the mole¬ 
cules and the consequent differentiation of the magma; 
but, while the correctness of this hypothesis is highly prob¬ 
able, we are not yet in possession of the experimental facts 
which might establish it. Moreover, the absence in nature 
of decisive evidence of the influence of pressure on the crys¬ 
tallization of igneous magmas suggests that its effect is 
relatively slight as compared with the influence of tempera¬ 
ture on crystallization, and the often noticeably localized 
character of the differentiation of igneous magmas makes it 
probable that the bodies of magma in which it has taken 
place were not large enough to have experienced great 
diversity of pressure in different parts of them; so that 
while differences of pressure may influence the chemical 
equilibrium of these magmas, their chemical differentiation 
is more likely to have been controlled by differences of tem¬ 
perature, which, it is readily imagined, may vary consider¬ 
ably in different parts of a magma which occupies an irregu¬ 
larly shaped cavity within the earth’s crust. Both the 
shape of a body of molten magma and the nature and con¬ 
ductivity of the enclosing rocks will affect the temperature 
of different parts of it to a very considerable extent. 
From the foregoing it may be concluded that the mole¬ 
cular concentration of particular constituents of molten 
magmas in the cooler parts of inclosed bodies of magma is 
a sufficient cause for their differentiation. 
THE PROCESS OF DIFFERENTIATION. 
From the independence of the elementary oxide molecules 
from any fixed combination within a molten magma and the 
