A TYPE OF IGNEOUS DIFFERENTIATION 629 
pegmatitic phases have been discussed elsewhere.* The analyses 
tabulated below show the composition of the abundant types. 
Peridotite phase-——Bands of peridotite occur in the banded 
gabbro a few yards above the base. The large bands are over 
Gabbro sill 
SSAASSAY 
SS [Se SSP BI gS SN 
aS Sa TI ga SR NT 
SNS SI ye PN CUPRA WT 
BS SANSA AAAAAAN [isc pT 
SASAASY [Sap apeS aM Dan SOR SPO ST CAN TNT 
= QA NAS AN 
SNA ES 
ES = KRNSASASAI 
TN NANA AN 
KN 
Banded gabbro 
Biotite Magnetite Olivine Augite Plegioclase Apatite. 
Fic. 2.—Diagram of measured modes of Duluth gabbro 
15 feet thick and somewhat variable. They are nearly black, 
weathering rapidly to a crumbling brown soil. Analysis number 2 
in Table I shows the chemical composition, and the diagram (Fig. 2) 
indicates the mineral composition. 
t Frank F. Grout, ‘‘The Pegmatites of the Duluth Gabbro,”’ Econ. Geol., XIII 
(1918), 185. 
