242 SCIENCE PROGRESS 



above that point. Thirdly, we may expect that chemical 

 equilibrium will be attained in a rock-magma, as between the 

 several compounds present, and this equilibrium will in the 

 most general case depend upon the temperature. It is probable 

 that in some cases reactions take place, involving important 

 mineral rearrangements, as a consequence of falling temperature 

 in the still fluid magma. Thus it appears that the association of 

 orthoclase and biotite, which is stable at low temperatures, and 

 is found so frequently in plutonic rocks, is unstable at high 

 temperatures, and there gives place to the association of leucite 

 and olivine, which is of common occurrence in volcanic rocks. 

 This, which might be inferred from the nearly identical analyses 

 of some minettes and some leucite-basalts, has been illustrated 

 experimentally by Fouque and Levy. Again, Arrhenius 1 has 

 given some reasons for supposing that, at magmatic temperatures, 

 water is an acid capable of decomposing silicates, giving rise to 

 acid silicates and free silicic acid on the one hand and basic 

 hydrates and oxides on the other. Although this hypothesis 

 rests on a rather doubtful inference from the properties of water 

 at relatively low temperatures, it may be entertained as a 

 possibility, which would imply a partition of acids and bases in 

 highly heated magmas different from that which is found in the 

 products of crystallisation. 



Although these considerations may modify our conception of 

 the nature of rock-magmas at high temperatures, they do not 

 touch the point with which we are directly concerned, viz. that 

 a magma in which crystallisation is imminent is a mixture of 

 silicate and other compounds identical with those which appear 

 when crystallisation takes place. The case of isomorphous 

 series is no exception, since the intermediate members of such 

 a series are not compounds, but mixtures of two (or more) 

 compounds. Further, if every body of uniform igneous rock 

 represents a magma which had an identical mineralogical com- 

 position, the great diversity actually found in different rock- 

 bodies shows that the principal rock-forming minerals must be, 

 in the fluid state, freely miscible with one another in all 

 proportions. An igneous rock-magma is thus seen to be closely 

 comparable with a mixture of salts in mutual solution with one 

 another or with water. 



1 "Zur Physik des Vulkanismus," Geol. Fbren. Fork. Stockholm, vol. xxii.(i9oo), 

 pp. 395-419- 



