CLASSIFICATION OF THE PLUTONIC ROCKS 249 



orthoclase (together with nepheline) is subordinate. In kentalli- 

 nite, orthoclase and a lime-bearing felspar are equally developed. 

 The position of the essexites in the alkali-series is justified 

 by the part played by soda minerals (such as nepheline and 

 the soda-pyroxenes) in their mineral composition and the 

 consequent influence on their chemical composition. These 

 characteristics distinguish the type from gabbro in which the fels- 

 pathic element is limited to a lime-bearing felspar (labradorite). 

 The nepheline-gabbro sub-family embraces the types theralite, 

 covite, and borolanite, representing the various combinations 

 of orthoclase and plagioclase with nepheline (or leucite). Of 

 these types, the theralite is intimately related to the essexite 

 type, the covite to the shonkinite, the proportion of nepheline 

 being the determining factor in each case. 



It is to be noted that there is continuity of types both 

 vertically (in series) and horizontally (from series to series). 

 For example, not only do the granodiorites pass by decreasing 

 silica-contents into diorites and thence into gabbros, but they 

 pass also by decreasing lime and increasing alkalies into the 

 adamellites and thence into the alkali-granites or granites 

 proper. And similarly for each series and for each group. 

 The divisions are adopted for convenience, but do not represent 

 actual hard-and-fast lines existing in Nature. 



Besides the types whose names have been used to designate 

 the families in the above scheme, other points in the series have 

 been fixed by specific names. Thus the acid division of the 

 diorite family is generally known as tonalite, a name originally 

 given by vom Rath to the quartz-mica-diorite of the Adamello 

 Alps; while Brogger 1 proposes to use the name banatite for 

 the corresponding acid division (62-66 per cent. Si0 2 ) of the 

 monzonites. Again, quartz-syenite has been used for the more 

 acid members of the potash-syenite family and nord mar kite 

 (Brogger) for the corresponding acid members of the soda- 

 syenite family. 



Furthermore, varieties of the main types are distinguished in 



each family (1) by variations in the nature of the ferro-magnesian 



constituent, e.g. hypersthene-granite (charnockite), augite-syenite 



(dkerite), mica-, hornblende-, augite-, and hypersthene-diorite ; 



(2) by the presence of some characteristic constituent (as, 



for example, zircon-syenite, eudialite-syenite, or arfvedsonite- 



1 Loc. tit. p. 61. 



