124 C. H. GORDON — SYENITE-GNEISS (lEOPARD ROCk) FROM CANADA. 



4. The transverse arrangement of the augite and manner of crystallization of all 

 the constituents of the interstitial granular bands ; 



5. The concentric arrangement of the lumps and bands about inclosed masses of 

 apatite or pyroxenite ; 



6. The occurrence of the apatite usually in a more or less crushed condition, but 

 exceptionall}^ in large crystals imbedded in the leopard rock ; and 



7. The abrupt transition between the difterent phases in their field Velations. 



PRIMARY ORIGIN. 



Differentiation af the coarse syenite. — If, as seems probable, tbe coarse 

 fehlspathic rock represents very nearly the character of the rock at the 

 time of consolidation from the original magma, it is evident that in 

 places differentiation had proceeded to a considerable extent at the time 

 of crystallization. The crystallization of the pyroxene appears to have 

 been often well advanced before that of the feldspar began. A consid- 

 erable proportion of the ])yroxene, however, occurs in large grains and 

 aggregates in allotriomorphic development with the feldspar. In seek- 

 ing an explanation of this tendency of the pyroxene to become segre- 

 gated, two views suggest themselves : 



a. That it represents a primary differentiation of the molten magma; 

 or, 



h. That it is due to the fusion and recrystallization of included frag- 

 ments of the pyroxenite. 



In regard to the first view (a), if this tendency on the part of the con- 

 stituents toward segregration represents a primary differentiation of the 

 magma, it would seem to accord better with the more generally accepted 

 theory that differentiation has taken place when the magma was quite fluid 

 than with that which supi:)oses it to take place by crystallization, mechan- 

 ical accumulation and reliquefication. According to Professor Iddings,* 

 a study of the chemical character of rocks shows that the differentiation 

 of molten magmas is not according to stoichrometric proportions, and is 

 therefore not a mineralogic differentiation. As to the method by which 

 concentration has taken place, two views have been expressed : (1) that 

 it is due to molecular diffusion, according to Soret's principle, advocated 

 by Vogt,t Brogger X and others ; and (2) that it is the result of liquation 

 as advocated by Durocherg and Biickstrom. || 



If, according to Soret's principle, the differentiation of the magma be 

 regarded as due to differences of temperature, then, in order to explain 

 the lump\^ segregation of the coarse grained rock, it may be necessary 



*J. p. Iddings: The Origin of Igneous Rocks. Bull. Phil. See. Wash., vol. xii, p. 152. 

 f.r. H. L. Vogt: Geol. Foren, i, Stockholm Forhand., vol. 14, May, 1891, p. 476. Reviewed by J. J. 

 H. Teall in the aeol. Mag., Feb., 1892. Zeitschr. f. prakt. Geol., 1893, p. 272. 

 J W. C. Brogger: Zeitschr f. Kryst. n. Min., Leipzig, vol. 16, 1890. 

 gj. Durocher: ^.\\n. des Mines, Paris, vol. 11, 18.37, pp. 217-259. 

 II H. Biickstrom : Jour. Geol., vol, 1, 1893, p. 773. 



