160 Alexander Scott — Principle of Saturation. 



VI. — The Principle of Saturation. 

 By Alexander Scott, 'M. A., B.Sc. 

 N Professor Shand's reply 1 to my criticism 2 of his original paper 3 



I 



on the application of the principle of saturation with respect to 

 silica in the classification of igneous rocks, some points arise which 

 make it clear that he has misunderstood me in several particulars. 

 In the beginning of my paper, it was explicitly stated that what was 

 to be considered was (1) whether the principle, as enunciated by 

 Professor Shand, could be extended to minerals other than the silica 

 ones (quartz, tridymite, etc.), the felspars and the felspathoids, and 

 (2) whether the criterion whereby the rock minerals were divided into 

 the two groups of saturated and unsaturated was sufficiently exact. 

 The criterion used was that of "the observed facts of distribution", 

 and any mineral which was found coexisting with quartz or some 

 other form of silica in igneous rocks was said to be saturated, while 

 those which did not occur along with quartz were said to be 

 unsaturated. Corresponding^, rocks which consisted entirely of 

 minerals of the former type were classed as saturated, those consisting 

 of mixtures of the two types as part-saturated, and those consisting 

 solely of the second type as unsaturated. I endeavoured to show 

 that the rock -forming minerals cannot be satisfactorily divided into 

 two classes in this empirical fashion, without any consideration of the 

 cooling-histories of the individual rocks in which the minerals may be 

 found. 



One of the specific points discussed was the possibility of the 

 coexistence of quartz and olivine in an igneous rock, and I indicated 

 two ways in which free silica (other than xenolithic) might exist in 

 small quantities in a part-saturated or unsaturated rock. What was 

 suggested was, that under certain particular conditions metasilicates 

 might dissociate into orthosilicates and free silica ; while, under other 

 conditions, silica might be set free by the hydrolytic action of water. 

 ?$o attempt was made to suggest that the coexistence of the two 

 minerals was general or that the circumstances favouring it were 

 other than exceptional, and hence the instances of the Kilsyth-Croy 

 sill, 4 Garabal Hill, 5 etc., are not relevant to the point at issue. The 

 work of Anderson and Bo wen 6 was quoted merely as a verification of 

 the idea that dissociation was possible. These authors, however, have 

 pointed out that this dissociation, despite the fact that it takes place 

 above 1557°, has a distinct petrological bearing. Again, olivine 

 is seldom the pure magnesium orthosilicate, but generally contains 

 a considerable amount of the corresponding ferrous salt. If, as is 

 probable, the latter also undergoes dissociation near its melting-point, 

 silica will be set free at temperatures much below 1557° and well 



1 Geol. Mag., Dec. V, Vol. X, pp. 508-14, 1913. 



2 Ibid., Dec. VI, Vol. I, pp. 319-24, 1914. 



3 Ibid., Dec. VI, Vol. I, pp. 485-93, 1914. 



4 Geol. Mag., Dec. V, Vol. VI, pp. 299-309, 1909. 



5 Ibid., Dec. V, Vol. X, pp. 499-508, 1913. 



6 Amer. Joum. Set. (4), xxxvii, pp. 487-500, 1914 ; Zeit. filr Anorg. Chem., 

 lxxxvii, pp. 283-99, 1914. 



