A TYPE OF IGNEOUS DIFFERENTIATION 643 
It is not suggested for a moment that the series at Duluth does 
not include all types in complete gradation, but the gradation zone 
is small, and the separation of two types is very complete. Even 
though some samples were selected to show intermediate rocks, the 
analyses show that they belong distinctly to one side or the other 
of a break in the series. 
When these sharply divided groups are studied in detail, each 
is found to vary widely without crossing the limits of the group to 
: Salic Lime. 
Molecular Ratios, Alkalies 
Ss 1.6 1 6 33 AA 
Mineral Ratios, Salic : Femic. 
Fic. 11.—Diagram based on Duluth analyses, showing that the variation in the 
gabbro is in a different direction from the variation in the red rock. While the gabbro 
varies from salic to femic, it never becomes alkalic. While the red rock varies from 
alkalic to calcic, it nowhere becomes femic. 
approach the other. There is no way to arrange all the gabbros 
in a single linear series, but this complexity in the gabbro group is 
entirely aside from the series grading toward red rock. It is a 
gradation of a really different character (see Fig. rr). 
One series of rocks from peridotite to olivine gabbro and anor- 
thosite, with some side branches to magnetitic gabbro and troctolite, 
varies chiefly in the quantities of minerals—labradorite, augite, 
olivine, and magnetite. In the main course of variation all four 
