204 WATSON AND CLINE ROCKS OP THE BLUE RIDGE REGION 



normative plagioclase than normative orthoclase. Determinations of the 

 plagioclase conclusively prove it to be andesine. Based, then, on the fact 

 that the chief feldspar is andesine, with considerable but less alkali feld- 

 spar (orthoclase), the normal facies of the rock should be designated a 

 pyroxene-granodiorite. According to the usage of many petrographers, 

 the rock would be grouped as a pyroxene-quartz monzonite, with which it 

 is compared on pages 204-206. Because of the notable amount of potash 

 feldspar present, which may equal or exceed soda-lime feldspar in some 

 thin sections of the rock from some localities, but not in the norms cal- 

 culated from the five analyses in the table on page 202, the rock may also 

 be classed as a quartzose-pyroxene-andesine syenite. However the rock 

 may be grouped, it is entirely clear that it is a transitional or intermediate 

 type. 



As indicated both by microscopic study of thin sections and by the table 

 of norms on page 203 (I to V), alkali feldspar is present in notable 

 amount, being in all cases but one (IV) three-fifths of the lime-soda 

 feldspar, and therefore sodipotassic in four (I, II, III, and V) and 

 dosodic in one (IV). Considered further from the quantitative point of 

 view, three analyses (II, III, and V) of the rock place it in the alkali- 

 calcic rang tonalase and the sodipotassic subrang harzose, one (I) in the 

 alkalicalcic rang coloradase and the sodipotassic subrang amiatose, and 

 one (IV) in the domalkalic rang dacase and the dosodic subrang dacose. 



COMPARISON WITH QUARTZ MONZONITE 



Origin and application of name. — Monzonite was the name first given 

 by de Lapparent in 1864 to rocks of Monzoni, composed essentially of 

 orthoclase and andesine with subordinate pyroxene. Brogger applied the 

 term in 1895 to a transitional or intermediate group of phanerites be- 

 tween syenite and diorite having approximately equal amounts of alkali 

 feldspar and lime-soda feldspar with any kind of mafic mineral in sub- 

 ordinate amount. The name quartz monzonite 10 would apply to any 

 quartz-rich monzonite and would stand in the same relation to the granite- 

 quartz diorite groups as monzonite does to the syenite-diorite groups. 



In his discussion of the chemical composition of quartz monzonite and 

 granodiorite Iddings 11 suggests "that the name quartz monzonite be ap- 

 plied to varieties in which orthoclase exceeds the lime-soda feldspars, and 



10 The name grandiorite, given by Becker and Lindgren in 1891 to the granitic rocks 

 of the Sierra Nevada in California intermediate between granites and quartz diorites, is 

 considered by some petrographers as synonymous with quartz monzonite. Brogger uses 

 the term adamellite for acid quartz monzonites or for quartz monzonites as designated 

 by Iddings. Igneous rocks, vol. ii, 1913, p. 61. See in this connection Bull. 426, U. S. 

 Geol. Survey, 1910. 



11 J. P. Iddings : Igneous rocks, vol. ii, 1913, p. 69. 



