738 JOURNAL OF GEOLOGY. 



contribution to the study of gneissic structure, since it showed the 

 possibility of a part, at least, of the banding in these rocks being 

 due to a primary banding of igneous masses through some pro- 

 cess of segregation or through differentiation of the magma into 

 layers. A parallel banding of igneous rocks in the neighborhood 

 of a plane of contact has been known, but its magnitude is 

 generally inconsiderable. The structure in the gabbro on the 

 Isle of Skye, however, which was described by Geikie, is on a 

 large scale, and without apparent relation to a plane of contact. 

 No attempt was made to suggest a cause for such a mode of 

 segregation, since the study of the locality where it is best 

 developed is not yet completed. 



Prof. Brogger's paper, "On the Genetic Relations of the Basic 

 Eruptive Rocks of Gran, Christiania Region," presented an array 

 of facts with regard to the differentiation of rock magmas. By 

 means of chemical analyses and field observations he showed that 

 basic magmas of like composition in neighboring localities had 

 separated into pairs of magmas, which were quite unlike one 

 another chemically ; producing dissimilar pairs of rocks. This 

 proves that a given magma may differentiate in more than one 

 manner, according to circumstances. The entire paper is to 

 appear in the Quarterly Journal of the Geological Society. 



Mr. Harker discussed the question of magmatic concentra- 

 tion, or differentiation, with reference to its probable cause, and 

 pointed out what seemed to him obstacles to the application 

 of Soret's principle. He suggested that a more probable 

 explanation would be found in Berthelot's principle, or that of 

 maximum dissipativity. The applicability of Soret's principle to 

 the differentiation of magmas is also assailed by Prof. Backstrom 

 in an article to appear in the next number of this Journal, and the 

 principle of liquation advocated. While it is quite probable that 

 all of the phenomena of segregation and differentiation may not 

 be accounted for by one law of diffusion dependent on osmotic 

 pressure, and while this law finds its most perfect realization in 

 the most dilute solutions, and while certain separations of rock 

 magma may take place near the point of saturation, still it can 



