CLASSIFICATION OF IGNEOUS ROCKS 167 



(229) Granodiorite Becker. The term granodiorite was sug- 



gested by Becker^ for plu tonic rocks containing both orthoclase 

 and acid plagioclase, the latter in excess of the former. The name 

 was first published in a paper by Lindgren^ in 1893 and described in 

 more detail later.^ Lindgren"* says: "The truly characteristic 

 feature of the granodiorites is that the soda-lime feldspar, which 

 always is a calcareous oligoclase or an andesine, is at least equal to 

 double the amount of the alkali feldspar. The latter may be 

 taken to vary from 8 per cent to 20 per cent" in a rock with an 

 assumed feldspar content of 60 per cent.^ ''Below the lower limit 

 the rock becomes a quartz-diorite : above the upper a quartz- 

 monzonite." ''In the quartz-monzonite," he continues, "I would 

 give this mineral a range from 20 per cent to 40 per cent," again 

 assuming a total of 60 per cent feldspar. His divisions, therefore, 

 based on the orthoclase-plagioclase ratio are o-i3^-33|-66f for 

 quartz-diorite, granodiorite, and quartz-monzonite. In the present 

 classification the divisions are taken at 0-5-35-65, the writer 

 believing that more than 5 per cent of orthoclase changes the char- 

 acter of a quartz-diorite too much for it to retain that name. 

 Setting apart as orthogranite the rock with 95 to 100 per cent of its 

 feldspar orthoclase, and as quartz-diorite that having 95-100 per 

 cent plagioclase, the remaining 90 parts are divided into three 



'Waldemar Lindgren, "Granodiorite and Other Intermediate Rocks," Amer. 

 Jour. Sci., IX (1900), 270. 



^ Ibid., "The Auriferous Veins of Meadow Lake, California," Amer. Jour. Sci., 

 XLIV (1893), 202. 



^Sacramento Folio, U.S. Geol. Surv., No. 5, 1894; Placerville Folio, No. 3, 1894; 

 Smartsville Folio, No. 18, 1895; Nevada City Folio, No. 29, 1896; Pyramid Peak Folio, 

 No. 31, 1896; Truckee Folio, No. 39, 1897; H. W. Turner, "The Rocks of the Sierra 

 Nevada," U.S. Geol. Surv., An. 14, pp. 478, 482; Waldemar Lindgren, "The Gold- 

 Silver Veins of Ophir, California," An. 14, U.S. Geol. Surv., Part II (1894), pp. 252, 

 255-56; also "The Gold Quartz Veins of Nevada City and Grass Valley Districts, 

 California," An. 17, U.S. Geol. Surv., Part II (1896), p. 35; "The Granitic Rocks of 

 the Pyramid Peak District, Sierra Nevada, California," Amer. Jour. Sci., Ill (1897), 

 308; "The Granitic Rocks of Sierra Nevada," Abstr. in Science, New Series, V (1897), 

 361; "Granodiorite and Other Intermediate Rocks," Amer. Jour. Set., IX (1900), 

 269-82. 



1 Waldemar Lindgren, "Granodiorite and Other Intermediate Rocks," Amer. 

 Jour. Sci., IX (1900), 277. 



s Ibid., p. 279. 



