QUARTZ-BEARING HYPERSTHENE-ANDESINE SYENITE 



205 



that granodiorite be used for those rocks in which lime-soda feldspar ex- 

 ceeds orthoclase." Concerning their quantitative relations, he says : "Dis- 

 tinguishing quartz monzonites from granodiorites by the relative amounts 

 of orthoclase and lime-soda feldspar, it will be convenient to limit the 

 term quartz monzonite to those rocks of this group in which orthoclase 

 exceeds lime-soda feldspar up to the ratio of 5 :3, and to confine the term 

 granodiorite to those in which lime-soda feldspar exceeds orthoclase up 

 to the ratio of 5 :3." 



Chemical composition. — There are given in the table below five analyses 

 of quartz monzonites (one each from Idaho, Colorado, California, and 

 Montana, and an averaged analysis of 20 analyses from various localities) 

 for comparison with the five analyses of the Blue Eidge, Virginia, type 

 on page 202. The rocks are all sodipotassic ; and I, II, and III correspond 

 to the subrang name toscanose, and IV to amiatose. 12 Each one (I, II, 

 III, and IV) has been described as quartz monzonite, but, following Idd- 

 ings, they would belong chemically with granodiorite rather than with 

 quartz monzonite. Tbe correspondence nevertheless with the Virginia 

 type is very close, except IV, page 202, which is clearly a quartz diorite. 

 In each case the rocks are characterized by considerable alkali feldspar 

 and a larger amount of lime-soda feldspar of andesine composition. Ac- 

 cording to the grouping by Iddings, the Virginia type corresponds more 

 closely to granodiorite than to quartz monzonit<\ 



Analyses of Quartz Monzonites 



Si0 2 68.42 



Al 2 O s 15.01 



Fe 2 3 



FeO 



MgO 



OaO 



Na 2 



K 2 



H 2 0+ 



H 2 0— 



Ti0 2 



P 2 5 



MnO 



BaO 



Incl 



I 



II 



III 



IV 13 



V 



68.42 



66.83 



65.70 



63.88 



67.41 



15.01 



15.24 



15 . 31 



15.84 



15.76 



.97 



2.73 



2.54 



2.11 



1.93 



1.93 



1.66 



1.62 



2.50 



1.96 



1.21 



1.63 



1.62 



2.13 



1.43 



2.60 



3.59 



2.56 



3.97 



3.54 



3.23 



3.10 



3.62 



2.81 



3.45 



4.25 



4.46 



4.62 



4.23 



3.74 



.73 



.56 



.42 



.66 





.54 





.17 



.22 





.50 



.54 



.72 



.65 



.51 



.13 



.18 



.33 



.21 



.19 



.06 



.10 



trace 



.07 



.06 



.12 



.11 



.12 



. .09 





.25 



.09 



.18 



.36 





99.95 100.82 



99.53 



99 . 82 100 . 00 



12 Near harzose. 



13 Two other analyses of the quartz monzonite (granite) from Butte correspond to the 

 subrang name harzose, as do three of the five analyses of the Virginia type. Compare 

 this analysis with the analyses of the Virginia type on page 202. 



