Alexander Scott — Principle of Saturation. 163 



While so many obvious difficulties stand in the way of the 

 determination of the degree of saturation of the rock-minerals, there 

 are fully as many when the rocks themselves are considered. The 

 method, in common with all purely mineralogical methods, fails when, 

 not only vitreous, but all aphanitic rocks are concerned. Many 

 minerals of fundamental importance, such as nepheline and analcite, 

 are often difficult to recognize in the groundmass of such rocks and can 

 sometimes be detected only by a chemical analysis. 



Again, the operation of the principle of solid solution, which is 

 probably the most important physical-chemical factor in determining 

 what minerals crystallize from a magma, often results in the 

 development of ' hidden ' molecules. Many minerals can take up 

 free silica or other silicates in solid solution ; for example, most of the 

 extant analyses of nephelite show an excess of silica over that required 

 for the formula Na Al Si 4 ,* this excess being due to the presence of 

 an isomorphous mixture, probably of albite and nephelite. The 

 existence of such 'hidden ' molecules is of great importance when the 

 possibility of a rock reacting with the surrounding strata is considered. 

 Such reaction depends on the degree of 'normative' saturation, which 

 can in many cases only be determined by chemical means. 



If the existence of a small modal amount of an unsaturated mineral 

 is to be considered sufficient evidence to classify a rock as part- 

 saturated, certain anomalies may arise. It is conceivable that the 

 amount of free silica in solid solution might be sufficient to make up 

 the deficiency in the unsaturated mineral, thus rendering the rock 

 normatively saturated but modally part-saturated. From the 

 penological point of view, certain accessory minerals would tend to 

 assume an exaggerated importance, with the result that, not only 

 would anomalies arise in the classification of transition types, but also 

 petrological affinities would be obscured. The difficulties introduced 

 by this want of elasticity can be easily exemplified by a consideration 

 of a few such transition types. 



Some of the rocks, which have been described as pulaskite, have 

 greater affinities with the monzonites than with the nephelite 

 syenites, yet the presence of a trace of nephelite would necessitate 

 their classification with the latter. On the other hand, an umptekite 3 

 may contain no apparent nephelite and yet be more closely akin to 

 nephelite-syenite than to any other type. Some of the basaltic 

 mugearites of the Kilpatrick Hills are olivine-free, 3 yet they have 

 many affinities with the Jedburgh basalts, into which they grade by 

 the appearance of labradorite and olivine. As Iddings has pointed 

 out, 4 a minette can have the same composition as a leucite basalt, 

 olivine and leucite being the high-temperature products of a magma 

 which, if it has crystallized under plutonic conditions at a lower 

 temperature, would have resulted in a mixture of orthoclase and 



1 Cf. Foote & Bradley, Amer. Joum. Sci. (4), xxxi, pp. 25-32, 1911. 



2 Rosenbusch, Mikroskopische Physiographic, 4th ed., ii, i, pp. 151-4, 1905. 



3 Tyrrell, Trans. Glas. Geol. Soc, xiv, p. 238, 1912. 



4 Quoted by Harker, Science Progress, vol. ii, No. 6, p. 242, 1907. See also 

 Fouque & Michel-Levy, Synthase des mineraux et des roches, 1882, p. 77. 



