630 FRANK F. GROUT 
Troctolite phase—Troctolite, rich in olivine, like that described 
by Winchell, occurs from the center to the base of the mass, in 
scattered bands. See analysis number 6 in the table. The pro- 
portion of olivine to plagioclase in the several troctolites varies 
widely, so that the rocks approach peridotite on one side and 
anorthosite on the other. 
Fig. 3.—Diagram of the norms of analyses of gabbro at Duluth. Many phases 
have not yet been analyzed. 
Anorthosite phase.—At Duluth the early feldspathic dabbro 
probably contains over 80 per cent plagioclase. Large parts of it 
by a slight process of differentiation become anorthosite. Analysis 
3 of the table is a fair sample, but large masses are even purer feld- 
spar. The later more basic gabbro has less anorthosite, but thin 
bands of it are not at all rare. An outcrop in Sec. 26, T. 50 N., 
R. 15 W., resembles the famous anorthosite inclusions in diabase 
* A. N. Winchell, “‘Gabbroid Rocks of Minnesota,” Am. Geol., XXVI, 281-85. 
