452 FRANK F. GROUT 
THE ORIGIN OF IGNEOUS BANDING 
Former suggestions.—The field study of the structure of the 
Duluth gabbro led the writer to assume a process of convection 
during crystallization as its cause. On reference to the literature, 
it was found that Bowen recently eliminated convection from the 
list of magmatic phenomena which he considered important (33). 
No clear statement of the relation between convection and structure 
could be found, and a review of the various explanations of the 
banded structure was thought desirable. In addition to suggestions 
made in connection with specific areas already mentioned, there are 
some discussions of the phenomena in general papers G. 3) and 
textbooks (34). 
Banding is so characteristic of metamorphic gneisses that 
the structure is not rarely referred to secondary processes, but 
the papers cited above show very conclusively that much of 
it is primary. Furthermore, as igneous rocks they cannot have 
been fused in place and retained traces of earlier structure, for 
the gabbro at Duluth and banded rocks in a number of other 
places are known to be intrusive into both their roof and floor 
(Fig. 5), neither of which is much metamorphosed. Of the other 
possible causes of banding, the following tabulation includes the 
chief suggestions found: 
. Partial assimilation of inclusions, forming schlieren 
. Lit par lit, or fluidal gneiss 
. Deformation during solidification 
. Deformation just after solidification 
. Streaked differentiation, with reference to rhythmic cooling or intrusive 
action 
6. Successive intrusions: 
a) Cooling separately and successively 
b) Cooling later, all together 
7. Heterogeneous intrusion 
mM BW ND H 
The writer would add: 
8. Convection during crystallization differentiation 
Discussion.—The idea of partial assimilation of xenoliths, or 
lit par lit injection of wall rock as an explanation of banding, loses 
