MAGMATIC DIFFERENTIATION OF IGNEOUS ROCKS 665 



igneous rocks, relatively much rarer with basic igneous rocks, and 

 they are, so far as I know, entirely lacking in the often very large 

 massives of anorthosite. As more fully treated in Beyschlag- 

 Krusch-Vogt (Die Erzlagerstdtten,Y ol.ll, 2d ed. [i92i],pp. 564-65), 

 this may be explained by the relatively high content of volatile com- 

 pounds in the granitic magmas. And as far as the granite-pegmatite 

 dikes are concerned, these are, as is known, frequently character- 

 ized by a very considerable supply of pneumatolytic (or magmatic- 

 pneumatolytic) minerals. 



We further get very instructive information in investigating the 

 relations between orthorhombic and monoclinic pyroxene on the one 

 hand, and hornblende, biotite and Muscovite on the other, in the 

 various rocks. 



As is known, muscovite is noted by Tschermak with the formula 

 KH2Al3(Si04)3 and biotite (meroxene) with K2HAl3(SiO)4-nMg2Si04, where 

 n is 3 or at times somewhat lower. Some of K usually is replaced by Na, and 

 especially in biotite some of AI by Fe and some Mg by Fe. Further some F 

 commonly enters into the mica, replacing O (or HO?). The relation between 

 K and H moreover, is subject to certain variations. These standard formulas 

 prove that the muscovite contains considerably more HjO^ than the biotite. 

 Primary muscovite from granite-pegmatite dikes (and granite) contains, accord- 

 ing to the analyses at hand, mostly 5-8 per cent H2O and 11-9 per cent K3O 

 -fNaaO, and the biotite from granite and other acid igneous rock, mostly 

 1-2 a 2.5 per cent H2O and 9-7 per cent K^O+NaaO. 



Also the hornblende commonly carries some H2O (or HO), viz., in the 

 igneous rocks at most 2 per cent, usually considerably less, and tremolite 

 up to 2.5 per cent. 



As to the conditions for the formation of biotite in the igneous 

 rocks, we refer especially to the account given by N. L. Bowen* 

 and to the treatise of P. Niggli,^ "Die gasformigen Mineralisationen 

 im Magma." 



In the previously (pp. 430-35) described orbicular quartz-norite 

 from Romsaas, consisting of ca. 63 per cent hypersthene, 8 per cent 

 biotite, 24 per cent plagioclase (on an average Ab420r3Ans5), 4 per 



' Here and in the following I do not enter upon the question on whether it is 

 hydroxyl, HO, or H2O, which appears. 



2 "The Later Stages of the Evolution of the Igneous Rocks," Jour, of Geol. (1915). 



3 Geol. Rundschau, HI (191 2). 



