A TYPE OF IGNEOUS DIFFERENTIATION: 
FRANK F. GROUT 
University of Minnesota 
CONTENTS 
INTRODUCTION 
Tue Rocks oF DULUTH 
THE GABBRO 
Peridotite phase 
Troctolite phase 
Anorthosite phase 
Magnetite phase 
THE RED Rock 
Introduction 
Description 
Gradation and relations 
Origin of the red rock 
RELATION OF ROCK TYPES TO POSITION 
DIFFERENTIATION 
Introduction 
Processes of differentiation 
Crystal settling versus convection 
Double differentiation 
COMPARISON WITH OTHER DISTRICTS 
Scarcity of intermediate rocks 
An argument for immiscible separation 
SUMMARY 
INTRODUCTION 
This paper begins with a summary description of the rock types 
of the Duluth gabbro formation, and suggests the processes by 
which they formed. Since there are similar rock series in other 
districts, where the relations of large intrusions are not so clear, or 
where the study has been less detailed, it is suggested that the same 
processes are indicated in those districts. ‘This type of differentia- 
tion, therefore, is not at all unique. 
t Published by permission of the Directors of the Minnesota Geological and 
Natural History Survey and the United States Geological Survey. Part of a dis- 
sertation presented at Yale University. 
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