290 Prof. F. Lcewinson-Lessing — Origin of the Igneous Hocks. 



the standpoint of entropy not only a possible but a necessary 

 regulator of differentiation. Of course differentiation postulated by 

 the tendency towards eutectics can take place only in a magma 

 which is not itself an eutectic mixture but more or less different 

 from it. If granite and gabbro are really eutexia their magmas must 

 consolidate as a whole, they cannot undergo differentiation without an 

 external impulse, and such an influence is given, as will be shown 

 later, by the admixture and assimilation of foreign mineral masses. 

 And wherever a magma exists the composition of which is not 

 eutectic it must undergo differentiation, if only the cooling and 

 solidification process be not too rapid. In the case of a bimineralic 

 magma there will be generated a monomineralic and a eutectic 

 magma ; if the original magma be trimineralic, there will issue 

 a trimineralic eutexia, bimineralic and monomineralic rocks. 

 Differentiation in a eutectic magma is possible only if its composition 

 is modified by the assimilation of foreign masses. The question 

 whether a magma can dissolve and assimilate small portions or 

 considerable quantities of the surrounding rocks has been answered 

 by different geologists in different ways, some denying this possibility, 

 others admitting such a process. During recent years the number 

 of those who admit assimilation has notably increased. It is perhaps 

 sufficient to cite only a few names, as it seems to me that the wide- 

 spread occurrence of xenoliths and the marginal facies of the intrusive 

 bodies in the contact zones are eloquent illustrations of assimilation 

 on a large scale. And is not assimilation a phenomenon, that must be 

 expected even a priori in intrusive bodies, for it is difficult to imagine 

 a magmatic basin heating the rocky masses in contact with it for 

 a long period without partly dissolving them ? 



Harker ' considers ' marscoite ' to be a case of assimilation of granite 

 by a gabbro-magma. Assimilation on a large scale is admitted by Daly, 

 who has applied this conception to explain the formation of andesites 

 from basalts, 2 and to certain granites. 3 And, in a newly published 

 paper, he tries to demonstrate that all the nepheline-syenites and 

 alkaline rocks connected with them are derivates from granites owing 

 their distinctive characters to the assimilation of limestones. 4 Becke 5 

 has attributed the richness in lime and magnesia of the Pacific type of 

 lavas to the fact that there have been assimilated great quantities of 

 sediments, and Suess 6 agrees with this supposition. According to 



1 A. Harker, "The Tertiary Igneous Eocks of Skye " : Mem. Geol. Suit., 

 1904. Natural History of Igneous Bocks, p. 356. It must be remembered of 

 course that Harker considers such cases as exceptional, and is an adversary of 

 assimilation on a large scale. 



2 E. Daly, " The Origin of Augite Andesite and of related Ultra-basic Eocks " : 

 Jour. Geol., 1908, p. 401. 



3 E. Daly, " Secondary Origin of certain Granites " : Airier. Jour. Sci., xx (4), 

 p. 185, 1905. 



4 E. Daly, Cm. npira. Ha Cip. , 123. 



5 F. Becke, " Die Eruptivgesteine des bohmischen Mittelgebirges u. d. Amerik. 

 Anden. Atlantische a. pazifische Sippe der Eruptivgesteine" : T.M.P.M., xxii, 

 p. 209, 1903. 



6 E. Suess, Das Antlitz der Erde, hi, ii, 679, 1909. 





