INTERNAL STRUCTURES OF IGNEOUS ROCKS 451 
Duluth (in the NW. Cor. Sec. 22, T. 50. N., R. 14 W.) reveals the 
upper contact of the gabbro with a clear exposure of dip. The 
roof here dips east a trifle irregularly at an angle of about 15°. At 
Lincoln Park it may be seen further that the later banded gabbro 
dips east under the earlier feldspathic gabbro. At the base of the 
gabbro, where one might search for the exposures of the floor, 
the relations are confused by pegmatitic and aplitic emanations 
and differentiates of great variety. At the Paulson mine in Cook 
County the floor apparently consists of eroded iron formation. 
Though the dip of the contact is not well exposed, drilling was con- 
ducted on the assumption that the bedding of the sediment and the 
banding of the gabbro indicated the direction of the contact. As far 
as exploration went, this proved to be true.’ A floor under the 
gabbro, conforming to the position of the banding, is also indi- 
cated by the constancy of the horizon of the gabbro intrusion, 
and by the arrangement of differentiates; some heavy segrega- 
tions are on the northwest, as if a floor dipped under them on 
that side. 
A review of literature and suggestions to be presented later with 
regard to the origin of these structures has no reference to any 
process which would tend to develop a banding independent of the 
boundaries. The favored theories involve movement during 
crystallization, and it would be expected that such movement 
would be more or less controlled by the boundaries of the magma 
chamber. 
These results are sufficiently uniform—only one apparent 
exception—to warrant the assumption that in a large way the 
fluxion and banded structures, as well as sheet jointing (when not 
referable to surface weathering), may be a guide to the position of 
the boundaries of igneous masses, and therefore of great value in 
mapping igneous forms and interpreting their position. The 
occurrences include flows, dikes, sills, a plug, a bysmalith, and 
laccoliths. Exceptions may be found, but even if the idea proves 
untrustworthy it is worth stating for the sake of stimulating 
accurate observations and records, which are at present not very 
numerous. 
«E. C. Harder, personal communication. 
