628 FRANK F. GROUT 
basis of mineral composition. The main gabbro types shown in 
the diagram, even those of extreme composition, occur as alternat- 
ing bands. 
The average gabbro is gray when fresh and weathers nearly 
white. The texture is medium to coarse, granitoid to ophitic (see 
Fig. 5). The order of crystallization is in most cases plagioclase, 
olivine, magnetite, and augite. Olivine is not conspicuous in the 
° ! 2 4 5 MILES 
Frc. 1.—Sketch map of the Duluth gabbro area 
average gabbro except on the weathered surface, where, being 
highly ferruginous, it turns to a bright brown, contrasting with the 
darker augite and iron ores. There is very little alteration. The 
character of the main minerals is surprisingly constant from end 
to end of the series of outcrops. All the olivine has about 30 per 
cent FeO whether in peridotite or anorthosite. There are few out- 
crops in which the feldspar differs much from Ab,An,. The 
pyroxene, with few exceptions, is augite low in lime. 
The rocks of the gabbro series may be classified as normal 
gabbro, olivine gabbro (and the corresponding diabase, Fig. 5), 
troctolite, peridotite, magnetite gabbro, and anorthosite. Several 
specimens might show some further variation, but their occurrence 
is local and there is no evident importance in the distinction. The 
