INTERNAL STRUCTURES OF IGNEOUS ROCKS; THEIR 
SIGNIFICANCE AND ORIGIN; WITH SPECIAL 
REFERENCE TO THE DULUTH GABBRO: 
FRANK F. GROUT 
University of Minnesota 
INTRODUCTION 
- It is commonly said that igneous rocks are structureless, or of 
massive structure, as distinct from stratified or banded rocks of 
other origin. When considered in detail, however, they are known 
to show a number of characteristic structures. Under special 
conditions igneous rocks develop lithophysae, orbicules, bunchy 
segregations, spherulites, etc. But besides these there are a num- 
ber of rock masses which show a banded structure. It is this band- 
ing which is the main subject of this paper, first as to its relation to 
the form of the rock mass, and later as to its origin. 
Three somewhat distinguishable features give a plane structure 
to.an igneous rock unaffected by metamorphism; they will be dis- 
cussed here as banding, sheeting, and fluxion structure. Various 
geologists have noted these structures and combinations of them 
under the terms bedded, stratiform, gneissic, laminated, foliated, 
trachytoid, schistose, linear, streaked, platy, schlieren, layers, 
benches, etc. 
Banding.—The banding noted in many igneous rocks is an 
alternation of mineralogically unlike layers or flat lenses (Figs. 1, 
2,3, and 4). The dip and strike of the bands can be estimated in 
many cases, but may show minor undulations and bunches. In 
some cases the layers are all thin, but in others they range more 
widely, up to a hundred feet. The line of division between bands 
may be sharp or gradual. The texture of one band is in most cases 
very little different from the textures of adjacent bands, and the 
t Published by permission of the directors of the United States Geological Survey 
and the Minnesota Geological Survey. Appeared first as part of a thesis presented at 
Yale University. 
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