CLASSIFICATION OF THE PLUTONIC ROCKS 247 



The Granite Family. 



The Syenite Family. 



The Monzonite Family. 



The Diorite Family. 



The Alkali-gabbro Family. 



The Gabbro Family. 

 Of these, only the monzonite, diorite, and gabbro families are 

 divisions of a single series. The remaining comprise several 

 sub-families belonging to different series. Thus the granites 

 are divisible into four sub-families, the syenites into three, and 

 the alkali-gabbros into four. 



A few points require explanation. The granodiorite sub-family 

 includes granite-like rocks in which a lime-bearing plagioclase 

 largely predominates over orthoclase. Such rocks are typically 

 developed in the Andes of South America (" Andes-granite ") 1 

 and in the corresponding Cordillera of the Western States 

 of North America. 2 They occur also in the granite complexes 

 of the southern uplands of Scotland, as for instance in the 

 Criffel and Loch Dee masses of the Galloway area. 3 The 

 name "granodiorite" was first proposed in 1892 by G. F. 

 Becker in conjunction with H. W. Turner and W. Lindgren. 

 The work of these authors has defined the type as a light 

 grey granitic rock in which soda-lime felspars (basic oligoclase 

 or andesine) largely predominate over orthoclase, the remain- 

 ing essential constituents being quartz, biotite, and usually 

 hornblende, together with sphene, apatite, and magnetite. It 

 occurs in association with tonalite, diorite, and gabbro, and 

 no doubt represents the extreme acid end of the gabbro- 

 diorite series. According to Lindgren, the percentage of silica, 

 which rises as high as 73 per cent., may fall as low as 59 

 per cent., and he makes the distinction from the more acid 

 members of the diorite family depend on the orthoclase. But 

 the fact is that most, if not all, diorites contain some small 

 proportion of orthoclase ; and, therefore, in order to avoid a 



1 O. Nordenskjold, " Die krystallinische Gesteine der Magellanslander," 

 Svenska Exped. till. Magellanslandern, Stockholm, 1901, vol. i. p. 175. 



3 W. Lindgren, Amer. Jonrn. of Set. Series 4, vol. iii. 1897, and Series 4, 

 vol. ix. 1900, p. 269 ; H. W. Turner, The Rocks of the Sierra Nevada, 14th 

 Ann. Rep. U.S. Geol. Survey, 1894, pp. 478 and 482. 



3 The Silurian Rocks of Scotland, Mem. Geol. Surv., p. 610. The need of a 

 name for this type is shown by the fact that the dominant rock of these masses is 

 sometimes described as a granite, sometimes as a tonalite. 



