256 Prof. F. Lcewinsoii-Lessing — Origin of the Igneous Rocks. 



transitional rocks (Harker's ' marscoite '), to which he gives the very 

 suggestive name of 'hybrid' rocks, showing that their abnormal 

 composition is clue to mixture and not to differentiation. The evidence 

 of admixture, the intrusion of one of the rocks into the other in 

 small veins and veinlets, the occurrence of xenoliths of the one 

 in the other, are, in my judgment, facts corroborating the original 

 independence of the two magmas. Were the granite and the gabbro 

 the result of differentiation from another original neutral magma, 

 there would not be evidence of mixture. A very instructive case of 

 identical relations between granite and gabbro is described by Philipp 

 in his recently published study on the granites of the southern 

 Schwarzwald. 1 The members of the gabbro-formation (Philipp's 

 ' gabbroides ') occur in separate intrusive bodies and also in xenolithic 

 masses of varying size included in the granitic bodies ; while between 

 the granite and the gabbro there is a transitional zone of a mixed 

 gneiss (' Amphibolmischgneiss '), produced by the mixture of the two 

 formations. These data testify to the individuality of the gabbroid 

 and the granitoid magmas, and to their different age. These facts 

 sustain the hypothesis of assimilation, and exclude the presumption 

 that the granites and the gabbroides have been formed by differentiation 

 from a common neutral magma. 



Rosenbusch cites of course, according to Lessen, the gabbros and 

 diorites of the Brocken as an example of granitic facies ; but my short 

 visit to the gabbros of Harzburg and the Badauthal leads me rather to 

 the conception that these gabbro rocks and the Brocken granites 

 belong to different intrusive bodies connected by intermediary rocks, 

 originated not by differentiation but by assimilation — a view which 

 seems to be supported by the granophyric facies of the gabbro. 



The conclusions at which we have arrived is not new. It is well 

 known that as early as 1851 Bunsen 2 was led, by the study of the 

 igneous rocks of Iceland, to the conclusion that they originated 

 through the mingling of two primordial magmas — the normal 

 trachytic (corresponding to our granitic) and the normal pyroxenitic 

 (corresponding to the basaltic). And although the hypothesis that 

 all igneous rocks must be considered as different mixtures of these 

 two is now untenable, the conception that there are two primordial 

 magmas and not one is certainly right. Michel-Levy 3 has also supported 

 the conception of two magmas. It is true that the two magmas of 

 Michel-Levy have a far more hypothetical character, and are some- 

 what indefinite and arbitrary. The alkaline magma comprises the 

 granites as well as the nepheline-syenites, rocks of very different 

 acidity ; the maguesian magma is considered to correspond to the 

 peridotites, the lamprophyres, and several other rocks. But even 

 though the exposition of the two magmas of Michel-Levy is not 



1 H. Philipp, " Studien aus dem Gebiete der Granite und umgewandelten 

 Gabbro des mittleren Wiesentales " : Mitteil. Badisch. Geol. Landesanst., vi, 

 i, 1910. 



2 E. Bunsen, " Ueber die Proeesse der vulkanischen Gesteinsbildung Islands " : 

 Pogg. Ann., lxxxiii, p. 197, 1851. 



3 A. Michel-Levy, " Sur la classification des magmas des rockes eruptives " : 

 Bull. Soc. Geol. France, xxv, 1897. 



