INTEHMEDIATE CHAUAC'TER OF APPALACHIAN GliANlTES 



39 



Tlie above analyses correspond to normative quartz and feldspar as 

 follows : 



Quartz and feldspar composition of quartz monzonites. 



Quartz 



Orthoclase 



Albite 



Anorthite 



Total plagioclase. 



Total feldspar 



AbnAn™ ratio 



Or — plag. ratio 



19.1 

 27.2 

 30.4 

 12.1 

 42.5 

 69.7 

 2.5:1 

 1:1.56 



The position of these three rocks in the quantitative system of classifica- 

 tion has been computed by Washington^, and may be represented in tabular 

 form as follows : 



Quartz monzonites of Western United States. 



They are sodipotassic, and under the old system of classification they 

 may be regarded on the basis of the potash-soda ratio (1:1) in quartz- 

 feldspar plutonic igneous rocks, as typical quartz monzonites, and were so 

 described by Lindgren, Cross, and Turner. 



It is a noteworthy fact that of the large number of analyses of so-called 

 granites, represented in the three areas, only two- (western United States 

 1, N"ew Jersey 1), on the basis of potassic feldspar content, fall under the 

 old system of classification within the limits ascribed by Lindgren for a 

 granite. "The definition of granodiorite would give it, say from 8 per 

 cent, to 20 per cent, orthoclase. In the quartz monzonites I would give 

 this mineral a range from 20 per cent, to 40 per cent., all to an assumed 

 total of 60 per cent, feldspars. The rocks containing more than 40 per 

 cent, orthoclase would then be classed as granites, . . ."^ 



'Washington, H. S. Chemical Analyses of Igneous Rocks. Professional Paper, 

 U. S. Geol. Survey, No. 14, 1903. 



The maximum percentages of orthoclase computed from the cliemical aanlyses 

 of rocks from the various sections are: Three from the Western United States 

 47.3, 38.9, and 38.4, respectively; one from New Jersey 51.15; one from New Eng- 

 land 34.05 ; and one from the central and southern Appalachians 33.36. 



=Lindgren, W. Op. eit., p. 279. 



