CLASSIFICATION OF IGNEOUS ROCKS 211 



"AbiAiii or more basic" by Barrell. On the other hand, Cross, 

 Iddings, Pirsson, and Washington' assume, by calculation from 

 the chemical analysis of a specimen from the Butte region, that 

 the plagioclase has AbiAui centers but more acid borclers. The 

 orthoclase in their analysis, however, was calculated as pure potash 

 feldspar, while as a matter of fact it contains considerable soda. 

 Consequently the plagioclase may be, as Barrell says, more basic, 

 and the C.I.P.W. rock would fall in the calciadamellite family 

 with Barren's rock. 



(239) Granogabbro Johannsen.^ See note under grano- 



diorite (229) and leuco-granogabbro (139). 



Rhyobasalt. To have a term analogous to granogabbro, 

 the term rhyobasalt is here used. See note under grano- 

 diorite (229). 

 (2310) Quartz-gabbro . 



Quartz-olivine-gabbro. 



Quartz-basalt. 



Quartz-oli vine-basalt. 

 (2312) Calcisyenite. No modal analysis of a plu tonic rock 

 belonging here has yet been located. 



Vulsinite Washington. Vulsinites are defined by 

 Washington^ as ''effusive rocks occupying an intermediate position 

 between the trachytes and the andesites. They are characterized 

 mineralogically by the presence of alkali feldspar with a large 

 amount of basic plagioclase (labradorite to anorthite) together 

 with augite and diopside. Hornblende and biotite are not abund- 

 ant in the t5rpe specimens, though they may be present in large 



amounts in other varieties Olivine is wanting, or if present 



is so in only accessory amounts." While the definition would 

 suggest a rock of the latite series, the modal analysis of the Bolsena 

 type'' shows it to belong to Family 12. The mineral percentages 



' Cross, Iddings, Pirsson, and Washington, Quantitative Classification of Igneous 

 Rocks (Chicago, 1903), p. 227. 



" Albert Johannsen, "Suggestions for a Quantitative Mineralogical Classification 

 of Igneous Rocks," Jour. GeoL, XXV (1917), 89. 



5 Henry S. Washington, "Italian Petrological Sketches, I: The Bolsena Region," 

 Jour. GeoL, IV (1896), 553. 



^Ibid., "The Roman Comagmatic Region," Carnegie Publication No. S7 (Wash- 

 ington, 1906), p. 65. 



