2 GEORGE V. SESSIONAL PAPER No. 25a A. 1912 



CHAPTER XXVII. 



MAGMATIC DIFFEKENTIATION. 



Preliminary Note. — We have now arrived at the conception that the post- 

 Keewatin magmas have been of two kinds; the primary basaltic and the 

 secondary syntectic. This idea rests on a much firmer basis than does the 

 speculation that the primary acid and basaltic shells were the products of the 

 differentiation of an intermediate (andesitic) magma early in the earth's 

 history. The speculation is not important for the theory of the visible igneous 

 rock bodies, which are almost entirely of post-Keewatin age. There remains 

 the enquiry as to the extent to which differentiation has been responsible for 

 the chemical diversity of eruptive rocks other than those solidified directly 

 from primary basalt or from the syntectic magmas. 



The subject of differentiation has prompted many papers and books from 

 hundreds of geologists, who have established the reality of the process beyond 

 peradventure. They have also proved its complexity. Fortunately there have 

 appeared, during the preparation of the present chapter, two convenient 

 resumes of the subject, one by Harker, the other by Iddings.* 



By the time these pages are printed both of these works will be thoroughly 

 familiar to every serious worker in the petrology and geology of eruptive 

 masses. In each case the work is so complete on this side of petrogenesis 

 that there is no need for a discussion of differentiation in the present report. 

 It may be noted in passing that neither author gives an adequate treatment of 

 the syntectic theory which, in some respects, has been best outlined by Loewin- 

 son-Lessing in his ' Studien fiber die Eruptivgesteine.'f 



In view of the accessibility of these and other discussions of differentiation, 

 the main generally accepted principles will here be stated without detail. 



1. Kelation to Crystallization.' — The course of differentiation is, in gen- 

 eral, parallel to the order of crystallization in the parent magma. This law has 

 been discerned inductively and has become fundamental in petrology, since it 

 agrees with the recently elaborated principles of physical chemistry. As a rule 

 the ferromagnesian and cafemic (calcium-iron-magnesium) constituents separate 

 out as crystals before the salic constituents, which remain for a time as mother- 

 liquor. In fact, the formation of every crop of magnetite, titanite, augite, or 

 olivine crystals means a new magma chemically different from the one preceding. 



*A. Harker, The Natural History of the Igneous Rocks, New York, 1909; J. P. 

 Iddings, Igneous Rocks, New York, 1909. 



t Compte Rendu Congres ©ebl. Internat. Vile Session, St. Petersburg, 1899, p. 375. 



769 



