Reports and Proceedings — British Association. 475 



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 oxygen ratio or acidity, the various minerals of 

 which they consist make their appearance within fairly well-defined 

 limits. For example, in the case of the Ca Mg Fe Mn slags, which 

 contain little alumina, olivine and the melilite minerals only make 

 their appearance in the more basic slags, and the metasilicates in 

 the more acid, the limit between the two corresponding to an acidity 

 of about 1'6. 



The limit of individualisation between the various minerals is 

 supposed to correspond to their eutectic mixture. Such slags may, 

 therefore, be regarded as a mutual solution of two or more of the 

 minerals olivine, enstatite, hypersthene, augite, the gehlenite-melilite 

 group, akermanite, wollastonite, and the hexagonal metasilicate, 

 which is so characteristic of the more acid slags. The particular 

 minerals which make their appearance are practically determined by 

 the acidity of the magma and by the relative proportion of the bases 

 present, particularly by the ratio of the calcium to the magnesium- 

 iron-manganese group ; in other words, Yogt asserts that a silicate 

 magma is a mutual solution of the various crystalline compounds 

 that actually make their appearance as it solidifies, and that the 

 order of crystallisation depends upon their proportion in the magma 

 as compared with their proportion in the eutectic. The old conception 

 of a solvent and a solute ceases to have much meaning; the matter 

 which is of supreme importance is the nature of the eutectic mixture 

 when the constituents are given ; thus micropegmatite and micro- 

 felsite represent the eutectic of felspar and quartz, and correspond to 

 a mixture of about 74 parts of felspar to 26 of quartz, as indeed has 

 been stated by Teall. 



Now, if we are justified in regarding rock-magmas and fused 

 silicates as mutual solutions of certain definite compounds, and if 

 these compounds are actual minerals or other silicates which 

 crystallise out of the magma when it cools, we are also justified in 

 making use of the properties of these minerals when we apply to 

 the magma the known physico-chemical laws which govern solutions. 



The number and nature of the minerals which can be in equilibrium 

 with each other and the solution are to be determined by experiments 

 upon their solubility interpreted by the phase-rule of Willard Gibbs, 

 and especially by the laws which Roozeboom and other physical 

 chemists have deduced for components which form double salts or 

 isomorphous mixtures. Knowing the components, we ought, thei'e- 

 fore, to be able to determine their latent heat of fusion, their specific 

 heat, the lowering of the freezing-point of their mixtures, and from 

 these data to calculate the true formulge of the rock-forming minerals. 

 It will readily be understood that in a mixture of quartz and 

 orthoclase, the lowering of the freezing-point below that of either of 



