382 REPORT—1905. 
In this maiter of magmatic differentiation, then, there has been during the last 
few years a large accumulation of geological evidence, a little new speculation, but 
practically no new experimental work, and scarcely any progress, 
Mineral Differentiation and Eutectics. 
Let us pass to the second petrographical problem, that of mineral differen- 
tiation, the nature and order of the minerals which crystallise when a cooling 
magma becomes a solid rock mass. 
It has been laid down by Rosenbusch, and is accepted as a general rule 
(in spite of many exceptions), that the order in which the various minerals 
crystallise is one of increasing acidity, ores and oxides and so-called accessory 
minerals first, then those minerals which are comparatively poor in silica, then 
those which are richer, and finally, if it be present in excess, the silica itself. It 
has also been supposed that the order may be one of the fusibility of the various 
minerals under the conditions of their formation; the least fusible minerals 
being the earliest to crystallise, and the most fusible the latest. Interesting 
speculations concerning the melting-point of quartz at high pressures, and its con- 
sequent order of crystallisation, have, for example, been published recently by 
Stromeyer and Cunningham. 
It is not necessary, however, to regard the molten magma as a mere mixture 
of fused minerals which solidify more or less independently and consecutively ; it 
is more reasonable to regard the whole magma as a solution in which the various 
minerals are dissolved, and from which they crystallise as it cools. Now the 
temperature at which a substance separates from solution is generally far below 
its melting-point, and the order in which the constituents of a mixed solution will 
crystallise is the order of their solubility in it, and bears no direct relation to their 
fusibility or to their chemical composition. 
Teall in 1901, after discussing the controversies and the evidence on which 
they are based, came to the conclusion that rock-magmas are solutions, and that 
the order in which the minerals consolidate depends upon the nature of the ecn- 
stituents and their properties, and is not by any means the order of their freezing- 
points. As to the particular minerals which crystallise, he thought that the 
molecular grouping in the magma is determined by mass action and by the mutual 
affinities of the bases, the silica, and the alumina. Concerning future research he 
ventured to predict that the next advances were to be made by experiment con- 
trolled by the modern theory of solutions. 
Thirteen years earlier Teall had himself contributed a valuable suggestion 
based tipon Guthrie’s work on cryo-hydrates. When a mixture of nitrate of lead 
and nitre is fused and allowed to cool, the constituent which is in excess will 
crystallise out as from a solvent until the proportions left in the liquid state 
are 47 of lead-nitrate to 53 of nitre, and this mixture will then solidify at 
207°, not as a uniform compound, but as an intimate mixture of the two salts, 
the eutectic, which crystallises at the lowest possible temperature, and is the only 
mixture which has exactly the same composition as the liquid from which it 
solidifies. Teall made the illuminating suggestion that micropegmatite is an 
eutectic consisting of quartz and felspar, and represents in certain rocks the 
final mother-liquor from which the other minerals have crystallised out. Eutectics 
in metallic alloys have been much studied during recent years: in the Address 
of 1901 Teall was able to strengthen his case by showing that spherulitic and 
micropegmatitic structures found in obsidian and other acid rocks are paralleled 
by similar structures developed in eutectic alloys, according as they have been 
rapidly or slowly cooled. 
In the following year appeared a theoretical paper by Meyerhoffer concerning 
the ideal case of a molten mixture of two substances, a and 4, which do not suffer 
donble decomposition, nor form a double salt, nor an isomorphous mixture. 
{Let a diagram be constructed, with temperatures as ordinates and composition 
of the magma as abscisse, giving Ly a curve the nature of the magma which is in 
equilibrium with either solii a or solid 6. The curve has the form of a VY; one 
