384 REPORT—1905. 
simple principle as the mere relative proportions of the various constituents to 
each other and to their eutectic mixture; the order of crystallisation must be 
determined by a number of factors, such as temperature, velocity of crystallisation, 
the interval between the softening and fusing of each mineral (which he finds to be 
considerable), viscosity, capillarity, the presence of water and mineralising agents, 
and the absorption of adjacent rocks. 
To choose a simple example: minerals such as zircon, corundum, and titanite 
separate for the most part early, because they are less soluble. On the other hand, 
magnetite is one of the more soluble minerals, and yet it is one of the first to 
separate; the same is to a certain extent true of augite, but not always. It is 
possible that ina magma which still contains the iron of the magnetite in solution 
plagioclase and augite may be comparatively soluble and magnetite comparatively 
insoluble, but that when magnetite has already crystallised out from the magma 
the plagioclase and augite may be comparatively insoluble ; the experiments which 
are wanted are experiments upon the solubility of certain minerals in magmas of 
known composition under known conditions; in these and similar instances the 
order of separation is that of the solubility, but such physical factors as the velocity 
of crystallisation (which varies very considerably with the temperature), and the 
viscosity, may completely invert the order. 
Direct experiments made by Barus and Iddings upon the electric conductivity 
of silicate magmas afford evidence that such magmas contain dissociated as well as 
undissociated molecules, so that they cannot be regarded as merely fused mixtures of 
certain minerals. If two or more rock-forming minerals be fused together it may 
happen that they form new compounds and crystallise out as different minerals, 
or if one or the other remains unchanged it may crystallise out in a different pro- 
portion. All this shows that double decomposition goes on in the liquid. We 
cannot therefore expect, without knowing the degree of dissociation, tomake much 
use of the lowering of the freezing-point in order to calculate the other factors in 
the process of rock-formation. 
Doelter concludes that upon the whole the normal order of crystallisation in 
rocks is in the main that laid down by Rosenbusch long ago, namely, an order of 
increasing acidity, but that it is determined by the mutual affinities of the mole- 
cules in the magma, and by the relative power of crystallisation of the components 
into which they unite themselves, and that the physical factors which I have 
already enumerated play a very important part in the proces3. No one has 
endeavoured more systematically than Doelter to determine for the rock-forming 
minerals the melting-points and the solubilities, without which it is impossible to 
make much progress in our reconstruction of the history of rocks. He has recently 
shown us how the microscope may be used in the study of fused silicates at high 
temperatures, and has so opened up a new field of research. 
Vogt’s Applications of the Laws of Solutions. 
The work of Vogt has extended over many years, and is now summarised in 
two remarkable memoirs recently published by him, in which are expressed his 
mature opinions upon silicate magmas; the reasoning is based upon ,his own 
experiments, upon those of Doelter, and upon the classic researches of Akerman. 
It is now generally conceded that the particular minerals produced in a silicate 
magma depend much more upon the chemical composition of the magma than 
upon temperature and pressure ; Lagorio and Morozewicz were led to this con- 
clusion by their own experiments upon fused silicates. Experiments upon slags 
at ordinary temperatures and pressures may, therefore, be invoked to elucidate 
the formation of rocks. 
In 1902 Vogt stated his conviction that the laws of solutions may be applied 
to igneous rocks, and his two recent memoirs are, in fact, an attempt to explain 
the experiments upon slags and fused silicates as examples of the operation of 
these laws. 
All important, according to him, is the composition of the eutectic mixture ; 
he finds that if the analyses of silicate magmas be arranged according to their 
