Reports and Proceedings — British Association. 427 



In this matter 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 

 differentiation, the nature and order of the minerals which crystallise 

 when a cooling magma becomes a solid rock-mass. 



It has been laid down by Eosenbusch, 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 consequent 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 constituents 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 afiinities of the bases, the silica, and the alumina. Con- 

 cerning future research he ventured to predict that the next 

 advances were to be made by experiment controlled by the modern 

 theory of solutions. 



Thirteen years earlier Teall had himself contributed a valuable 

 suggestion based upon 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 



