yol. 57.] ANKIYEESARY ADDRESS OF THE PRESIDENT. IxXVli 



long as they remain in the magma, they must be regarded as 

 belonging to it, and playing their part along with the other con- 

 stituents in producing the final result. 



The application of the theory of solutions to igneous rocks is com- 

 plicated in many ways. We are ignorant of the manner in which 

 the constituents revealed by analysis are distributed in the molten 

 magmas, and of the changes which take place in the molecular 

 groupings as the temperature approaches the point of saturation. 

 M. Le Chatelier has recently suggested that granite furnishes an 

 illustration of the phase-rule, and may be regarded as a stable 

 sj'stem of three phases (quartz, felspar, and mica), made up of the 

 three components — silica, alumina, and potash. Pew petrographers 

 will admit that the case can be put as simply as this. No doubt 

 the consolidation of igneous magmas is governed by the phase-rule, 

 but in the majority of cases the number of components, on any view 

 as to their nature, is too great to make the rule of much practical 

 value. Another cause of complication arises from the fact that 

 the physical conditions have often changed during the process of 

 consolidation, thus giving rise to the phenomena of resorption ; and 

 yet another from the absence of assurance that the minerals seen in 

 a rock have in all cases been developed from a magma having the 

 composition represented by the bulk analysis. 



I cannot leave this portion of the subject without calling attention 

 to the recent work of Prof. Joly on the melting-points of the rock- 

 forming minerals, and his proof of the enormous range of viscosity 

 possessed by quartz and other minerals. 



Whatever view we take as to the nature of silicate-magmas, there 

 can be no doubt that in general the process of consolidation 

 is a process of differentiation. Definite compounds separate 

 out, either successively or simultaneously, from a homogeneous 

 magma, and at the time of their formation are in equilibrium with 

 the surrounding liquid ; but owing to changes in temperature and 

 pressure the equilibrium established at one period may be destroyed 

 at another, and the igneous rock as we see it may not contain a 

 record of all the operations which have taken place during the 

 process of consolidation. So far as individual rocks are concerned, 

 we look to experiment, rather than to observation, to give precision 

 and definiteness to our ideas regarding the nature of the changes 

 which accompany solidification. 



