292 Prof. F. Lceivinson-Lessing — Origin of the Igneous Rocks. 



seem not to admit that foreign mineral masses must be ' assimilated ' 

 in the real sense of the word, commingled with the whole magna atic 

 mass on account of the fluidity of the magma and the gases it contains. 

 In an intrusive body of laccolitic type assimilation takes place in the 

 manner of stoping as elucidated by Daly. A third case of assimilation, 

 when parts of the solid crust are melted by rising of the isogeotherms 

 or by hot gases, will be mentioned later. 



Let us now consider what happens when assimilation has taken 

 place. The magma becomes more fluid, its composition is modified; 

 if it was at first eutectic it is now no longer so, and differentiation 

 must begin. It has been stated by Brogger 1 and myself 2 that 

 differentiation consists in the separation (Abspaltung) of definite 

 compounds of minerals, and not of separate oxides, as has been 

 formerly presumed by Iddings and others. We have no examples of 

 magmas having an abnormal composition ; even the effusive rocks, 

 which are not entirely composed of crystallized minerals like the 

 intrusives and whose glassy parts may have, as it seems, an arbitrary 

 composition, repeat in reality only the same types of magma which 

 are represented by intrusive rocks. This is a very important fact, 

 demonstrating, as I have already explained, 3 "that the effusive and 

 intrusive rocks come from one source ; the differentiation of their 

 original magmas is performed in the same waj T , namely, in the liquid 

 state." It is also important in showing that the abyssal magmas have 

 time enough and sufficient liquidity for differentiation to continue until 

 it is completed, and in such a manner that no abnormal residual 

 magmas can be formed. We may therefore conclude that every time an 

 abyssal magma has dissolved and assimilated mineral masses, modifying 

 its composition in such a way that the magma has become abnormal, 

 differentiation takes place and the original magma is divided into 

 secondary magmas. On the basis of considerations given in a former 

 paper 4 we may assume that this process of differentiation is governed 

 by the tendency of the magma to separate into magmas having the 

 composition of eutectics and into monomineralic magmas ; sometimes 

 the eutectic composition is realized directly, sometimes the original 

 magma is divided into two magmas both of which undergo further 

 differentiation. I propose to give on a subsequent occasion some 

 illustrations of this process ; two examples, however, may be cited 

 now — the formation of nepheline-syenites from granites through the 

 assimilation of limestones, according to Daly, and the formation of 

 syenitic rocks from a gabbro-noritic magma by the assimilation 

 of alkali-bearing rock-masses. 



1 W. Brogger, Die Eruptivgesteine des Kristianiagebiets. III. Das Gangge- 

 folge des Laurdalits. 1898. , 



2 F. Loewinson-Lessing, " Etudes de petrographie generale, avec un memoire 

 sur les roches eruptives d'une partie du Cauease Central," p. 178 : Trans. Soc. 

 Nat. St. Petersbourg, 1898. " Studien iiber die Eruptivgesteine," p. 187 : 

 Compt. rend. VII Congr. Geol., 1899. See also the Proceedings of the Congress 

 of St. Petersburg, p. clviii. 



3 F. Loewinson-Lessing, " Petrographical Notices. 3. Is there a difference in 

 the chemical composition of the intrusive and the effusive rocks ? " : Ann. Inst. 

 Polyt. St. Petersb., vi, p. 274, 1906. 



4 "Differentiation, Eutectics, and Entropy" : loc. cit. 





