454 FRANK F. GROUT 
be considered. There orientation is vertical and parallel to the 
sides of a volcanic plug as if dragged upward by eruptions through 
the channels, while in other respects the structures seem to be 
identical with those described elsewhere. It therefore seems 
necessary to adopt the customary view that the orientation of 
grains here associated with banding is a result of magma move- 
ment during crystallization in the general direction of the grains 
and of the bands, 1.e., parallel to the walls of the chamber. This 
view is so prevalent that the structure is often called “fluxion 
structure,’ even when its movement cannot otherwise be deter- 
mined. 
The question remains as to the nature of the movement. The 
common suggestions are movements of intrusion or of deformation. 
The writer is in favor of a third suggestion, viz., a circulatory 
movement. The data on which the argument is based are simple. 
It will be recalled that the banding of many rocks involves, not only 
a parallelism of grain, but an alternation—many times repeated— 
of mineralogically unlike bands. It is also known at Duluth that 
the extreme differentiates have in a large way become distributed 
in crudely gravitative positions, i.e., heavy near the bottom and 
light near the top. With these points in mind the several sug- 
gestions may be considered in detail. 
Successive intrusions of slightly varying magma are undoubtedly 
able to produce banded rocks and may even give a crudely gravita- 
tive arrangement; but the intrusion of successive layers of alter- 
nating composition, a few inches to a few feet thick, until the whole 
had a thickness of thousands of feet is inconceivable. The process 
would have to be extremely minute and often repeated in order 
to explain the detail of some outcrops. But such minute intrusion 
can hardly account for an intrusion several miles thick, where the 
intrusive action must have been on a grand scale. Even a process 
of crystal settling of each intrusive, combined with a sequence 
of intrusions, does not explain the alternations that are visible in 
some outcrops, where dozens of alternating bands appear in as many 
inches. 
Turning to heterogeneous intrusion, we find that the idea is 
accepted without any feeling of shock or surprise when attention is 
