Prof. F. Loewinson-Lessing — Origin of the Igneous Hocks. 251 



TABLE III. 



Molecular Eatios. 

 Si0 2 . • • 1-013 

 A1,0 3 . . . 0-165 l n . 1Rn 

 Fe 2 3 . . . 0-015/° 180 

 FeO . . . 0-056 

 MgO . . . 0-112 V0275^ 

 CaO . . . 0-107 J L.„-, 



Na,0. . . 0-049\ n . ft7fi | Urf5i 

 K 2 . . . 0-027/ UWbj 

 3-5 RO 1-8 R 2 3 10-1 Si0 2 or 1'95 RO R 2 3 5'62 Si0 2 

 a = 2-27 = 52 7 = 1-90. 

 R 2 0:RO = 1:3-6. 

 Syenite: T8 RO R 2 3 5-6 Si0 2 . 



a = 2-4 R 2 0:RO = 1:2-2. 



Diorite: 1*5 RO R 2 3 4 Si0 2 . 



a = 1-77 R 2 0:RO = 1 : 4'3. 



Thus the average composition of the igneous rocks (probably of the 

 entire outer part of the crust) corresponds nearly to the syenitic 

 magma. Is this to be interpreted as meaning that the primordial 

 magma is in reality a syenitic one ? Such a conception exists, but we 

 can oppose to it several geological data and petrogenetic considerations. 

 If the syenitic magma were really the fundamental magma, from 

 which were derived and are still derived all the other types of 

 igneous rocks, we should expect that this magma would have a great 

 development, that it would be represented by definite plutonic 

 and volcanic regions, and that the eruption of granites, gabbros, and 

 other ' derived ' rocks would be preceded by an eruption of the 

 fundamental magma. Observation does not confirm these suppositions. 

 The syenitic intrusive bodies are generally of far smaller dimensions 

 than the masses of granitic or gabbro-peridotitic-pyroxenitic formations ; 

 the syenites are often only a marginal facies (Rafidfacies) of these 

 two formations. In regard to the well-known syenite of Elauen this 

 connexion with the Meissen granite, and the subordinate position 

 of the syenite, have been pointed out by Rosenbusch, and, as far as 

 I can see, all syenites are only local facies of a granitic or a gabbroidal 

 formation. 



Were the syenitic formation the fundamental magma, and the 

 other rocks derived from it, we should expect it to afford proofs 

 of differentiation which we do not find, because the syenite is itself 

 a derived magma, generated by one of the two above-mentioned 

 formations, which show various processes of differentiation. The 

 figures on Tables I and II demonstrate also that Rosenbusch was 

 not right in considering that the monzonites and essexites represent 

 the average composition of the terrestrial magma. Rosenbusch says, 1 

 speaking of the monzonites : " . . . dass sie zusammen mit den 

 Essexiten ziemlich genau mit dem terrestrischen Gesammtmagma 

 chemisch ubereinstimmen," and further: ' : . . . Diese beiden 

 Gesteinsfamilien [the monzonites and the essexites] stellen, urn die 



1 H. Rosenbusch, Mikroshopische Physiographic der massigen Gesteine, 

 4th ed., pp. 142, 395, 1907. 



