CLASSIFICATION OF THE PLUTONIC ROCKS 263 



the Harz l (hornblende-biotite-granite, tonalite, diorite, and 

 gabbro). 



It was Rosenbusch 2 who, by extending Durocher's 3 hypo- 

 thesis of the liquation of the original earth-magma into two 

 zones, consisting of an upper acid, and a lower basic magma, 

 first formulated the theory that the plutonic rocks have been 

 produced from a number of partial magmas which originated by 

 differentiation from a homogeneous earth-magma {(Jrmagma). 

 From the magma-basins in which these earliest separated pro- 

 ducts had accumulated, subsequent differentiations took place, 

 perhaps under the influence of the pressure due to earth 

 movements in a sinking area, as postulated by Brogger for the 

 Christiania district. 4 



According to Rosenbusch's theory, the first step in rock 

 evolution consisted in the differentiation of the original earth- 

 magma into two classes of secondary magmas : (1) those rich 

 in alkalies, which he designates the nepheline-syenite (foyai- 

 tische) and theralite magmas, and (2) those poorer in alkalies, 

 but richer in lime and magnesia, which he called the granito- 

 diorite and gabbro-peridotite magmas. This division corre- 

 sponds almost exactly to the alkali and calc-alkali rock-series, 

 and Rosenbusch applies these names to his magmas in the last 

 edition of the Mikroskopische Physiographic (vol. ii. 1907). 



The recognition of the existence of an intermediary or 

 connecting magma, giving rise to the monzonite rock-series, 

 the members of which present affinities sometimes to the 

 alkali and sometimes to the calc-alkali series, we owe to 

 Brogger's classic work in Southern Tyrol and in the Christiania 

 district. 



In conclusion, it may be claimed that there is in the classifica- 

 tion discussed in these pages a freedom from the artificiality 

 of the " quantitative system " proposed by the distinguished 

 American petrographers named in the opening sentence of this 



1 Lossen, " Ueber Augitfiihrende Gesteine aus dem Brocken Massif im Harz," 

 Zeits. d. Geo/. Gesell. vol. xxxii. 1880, p. 206. 



3 H. Rosenbusch, " Ueber die chemischen Beziehungen der Eruptivgesteine," 

 Tschermak 's Min. Pet. Mitth. vol. xi. 1890, p. 144. 



3 J. Durocher, " Essai de petrologie comparee ou recherches sur la composition 

 chimique et mineralogique des roches ignees sur les phenomenes de leur emission 

 et sur leur classification," Ann. d. Mines, ser. 5, vol. xi. 1857, p. 217. 



4 Die Triadische Eruptionsfolge bei Predazzo, loc. at., Christiania, 1895, p. 1 77t 

 and "The Basic Eruptive Rocks of Gran," Q.J.G.S. vol. 1. 1894, p. 15. 



