640 FRANK F. GROUT 
little enrichment in magnetite.‘ Weinschenk is authority for the 
statement that this is a general rule in the segregation of magnetite.” 
Fig. 8 shows the specific gravities of the-rocks of which there 
are data in their “stratigraphic”? sequence. A curve has been 
drawn to indicate in a greatly generalized way the decrease in 
Roof Diabase 2.959 
Red rock phase 
Feld- 
spathic / Gabbro phases 
Gabbro 
NN NH NHHNHW HDHD ND 
Red rock 
Granophyr diabase 
Olivine gabbro 
Olivine gabbro 
Magnetite gabbro | 
Anorthosite 
; Olivine gabbro 
Main , | Olivine gabbro 
anded Bite 
Gabbro | Olivine gabbro 
Troctolite 
Olivine gabbro 
Olivine gabbro 
Olivine gabbro 
Olivine gabbro 
Peridotite 
| Olivine gabbro 
N®wWNH ND NNW DHNHWNHHKHHKH ND HN 
Contact rock Be 
Floor { Diabase 2. 
Fic. 8.—Specific gravities of Duluth rocks in ‘‘stratigraphic” order 
specific gravity toward the top of the mass at Duluth, but when the 
data are studied in detail it is found that the curve is hardly justi- 
fied. One of the heaviest rocks found was well above the center. 
It is thought that many of the cited examples of gravitative arrange- 
ment will show in detail the same erratic curve. 
How is this irregular specific-gravity series explained? The 
idea of crystal settling has not been so stated as to cover it. The 
«'T. M. Broderick, ‘“‘The Relation of the Titaniferous Magnetites of Northeastern 
Minnesota to the Duluth Gabbro,” Econ. Geol., XII (1917), 663. 
2 Dr. E. Weinschenk, translation by A. Johannsen, ‘‘The Fundamental Principles 
of Petrology.”” McGraw-Hill Co. (1916), p. 45. 
