Prof. F. Lcewinson-Lessing — Origin of the Igneous Hocks. 293 



The hypothesis formulated ahove is the further development of the 

 syntectic-liquational hypothesis (assimilation-differentiation theory), 

 announced by me in 1898. Daly 1 has accepted this hypothesis and 

 made use of it in his paper on the origin of the alkaline rocks. 

 I highly appreciate Daly's endeavour to -apply my hypothesis to 

 concrete cases, and I am a supporter of several of his own views. 

 But it is necessary to mention that Daly is wrong in thinking that he 

 enlarges my hypothesis. He cites me as follows : "... bisweilen 

 genugt ein Einschmelzen einer unbedeutenden Menge irgend eines 

 fremden Stoffes, der im Magma oder in einem Theil desselben wenig 

 loslieb ist, um einen Anstoss zur Spaltung in mehr oder weniger 

 grossem Maastabe zu geben," 2 and concludes from this passage that 

 I mean onlj' such assimilated masses as are with difficulty, or not 

 at all, soluble in the magma. But from other passages it can be 

 easily seen that I have given to this hypothesis a wider sense, and 

 that there is no ' enlargement' of it in Daly's statements. This may 

 be seen in the whole chapter from which the above passage is cited, 

 and especially from the following passages: — " Dort, wo das 

 Einschmelzen in kleinem Maasstabe stattfindet, zeigt sieh die Resorp- 

 tion in der Zusammensetzung des Gesteins durch das Auftreten neuer 

 Minerale oder durch eine Veranderung der relativen Mengen der das 

 Gestein zusammensetzenden Minerale. Dort, hingegen, wo das 

 Einschmelzen von bedeutenden Mengen fremder Gesteinsmassen 

 stattfindet, fiihrt dasselbe ohne Zweifel zu einem Zerfall zur 

 Differentiation des Magmas bei seinem Erkalten," 3 and " dass eine 

 Uebersattigung des Magmas mit einer Gruppe von Basen sich oft 

 nicht durch ein Einfaches Ausscheiden des TJeberschusses ausgleicht, 

 sondern in vielen Fallen eine Spaltung auf irgend welche Weise 

 hervorbringen muss". 4 



And so the question standing at the head of this chapter may 

 be answered as follows : — 



1. Differentiation by crystallization is regulated by the density 

 of the minerals in process of formation. The deeper parts of the 

 magmatic basin, where the sunken minerals are dissolved, obtain 

 in this way a somewhat different composition, as has been already 

 expressed by myself 5 and developed by Schweig. 6 



2. The magmatic differentiation taking place in the liquid state 

 is induced by assimilation and governed by the tendency towards 

 the formation of definite magmas which can consolidate as rocks, 

 especially eutectic and monomineralic ones. 



3. In the intermediate zones between magmatic masses formed 

 by the processes 1 or 2 a mingling of the two may take place, and 



1 E. Daly, " Origin of the Alkaline Eocks " : Bull. Geol. Soc. Amer., xxi, 

 •p. 117, 1910. 



2 F. Lcewinson-Lessing, Studien iiber die Emiptivgesteine , p. 188. 



3 Ibid., p. 183. 



4 F. Lcewinson-Lessing, " Kritische Beitrage zur Systematik der Eruptiv- 

 gesteine : I. Zur Chemie der Magmen " : T.M.P.M., xix, p. 299, 1900. 



" Studien iiber die Eruptivgesteine." 



H M. Schweig, " Untersuchungen iiber die Differentiation der Magmen": 

 N.J., Beil. Bd., xvii, p. 516, 1903. 



