TRANSACTIONS OF SECTION C. 383 
arm represents the temperature and constitution of the liquid which can be in 
equilibrium with a, and the other that of the liquid which can be in equilibrium 
with 4; and the lowest point corresponds to the eutectic, which is in contact with 
both. 
Let a point above the curve represent the temperature and constitution of the 
liquid magma containing excess of b; as the magma cools this point descends to 
the 4 branch and travels along it while 4 is crystallising out, until the eutectic 
oint is reached, when a and 6 both crystallise out together at a temperature 
low the melting-point of either. The order of crystallisation is therefore deter- 
mined solely by the composition of the magma as compared with that of the 
eutectic. If, however, the liquid be cooled slowly, crystallisation may be postponed 
until it has become supersaturated with regard to one constituent or the other, or 
both ; a state of affairs represented by a prolongation of the arms of the Y below its 
lowest point, and then the order of the crystallisation may be inverted. 
In a rock-magma there are of course many other factors to be taken into account 
as determining the order in which the minerals separate ; for example, the. forma- 
tion of both double salts and isomorphous mixtures, the possible production of 
unstable solid compounds which may become converted into stable compounds or 
may be redissolved soon after they have come into existence ; and also the relative 
velocities of crystallisation, changes of temperature and pressure, action of steam, 
&e.; but the principle laid down by Meyerhoffer must be that which controls 
the process. 
It might be objected that on this hypothesis the consolidation of every rock- 
mass ought to terminate with a eutectic mixture, whereas this appears to be by 
no means the case; in fact, it is only among some acid rocks that structures much 
resembling the eutectic mixtures of alloys are to be found. On the other hand, if 
the conditions of cooling are such that the magma becomes supersaturated with 
one mineral after another, it will overshoot the eutectic composition before each 
crystallises, and the final consolidation may be a well-marked sequence instead of 
a simultaneous crystallisation. 
The controversies which have ragrd concerning the classification of rocks and 
their nomenclature appear to me to contribute little to the real advancement of 
knowledge. There are, I think, two more profitable lines of research which should 
accompany each other. We may take the facts as we find them and endeavour to 
explain them by the known laws of solutions aided by the phase-rule, provided that 
we have good reason to believe that rock-magmas behave like solutions, and we 
may make experiments upon slags and fused silicates and ascertain how far they 
resemble natural rocks in their behaviour and their mineral constitution. Some 
of the workers in this field have been led to regard rock-magmas as undoubtedly 
similar to ordinary solutions; others hesitate to seek an explanation for their 
features in the laws which govern the solutions studied in the laboratory. The 
two views are represented in the persons of the two men whose names are most 
mee identified with recent experiments, Vogt of Christiania and Doelter of 
raz. 
Doelter’s Work on Melting-Points and Solubilities. 
The labours of Doelter and his pupils have been largely devoted to the melting- 
points of the rock-forming minerals and their solubility in silicate magmas. From 
experiments upon these minerals and their mixtures they have come to the conclu- 
sion that in many cases the melting-point of the mixture is about the mean of the 
melting-points of the constituents, and that in such cases, therefore, there is no 
evidence that the freezing: point is lowered, or that a eutectic mixture is formed ; 
so that it is not safe to apply the theory of cryo-hydrates to fused mixtures of 
silicates. b 
Doelter is therefore led to regard the silicate-magmas rather as mixtures of 
various constituents which may be dissolved in each other, but which are not by 
any means necessarily identical with the minerals which separate on cooling. The 
whole process seems to him to be far too complicated to be explained by any such 
