INTERNAL STRUCTURES OF IGNEOUS ROCKS 457 
geneous material probably occur, and crystal settling, differentia- 
tion, and deformation all leave their mark; but these things are 
apparently not sufficient to produce the structures seen. Convec- 
tion currents during crystallization result in bands and aid in the 
differentiation. Such a circulation would drag into parallel posi- 
tion any crystal formed near the wall of the chamber just as it 
became lodged in the viscous matrix and was removed from circula- 
tion. Rhythmic effects in the way of cooling, intrusive action, or 
gas emanation (all of which are known to be rhythmic) might 
rhythmically change the mineral composition of the crystals growing 
along the walls, and thus result in banding. Other features also are 
favorable and the writer does not find the mechanics of the process 
at all difficult. 
SUMMARY 
A review of the descriptions of banding in igneous rocks and a 
detailed study of the Duluth gabbro show that the alternation of 
mineralogically unlike bands is commonly accompanied by a 
fluxion structure and in some places by a sheet jointing. 
These structures are found to be parallel to the bounding surfaces 
of the igneous masses in nearly every case. Exceptions should be 
carefully studied and the facts in all cases noted, because such 
a relation of form and structure would be of great value in mapping 
and economic work. 
The banding and related structures probably develop during 
crystallization, while the magma is in convection circulation. 
SELECTED BIBLIOGRAPHY 
t. Geikie, A., and Teall, J. J. H., ‘‘On the Banded Structure of the Gabbro 
in the Isle of Skye,” Quart. Jour. Geol. Soc., L, 648. 
2. Harker, A., ‘‘Tertiary Igneous Rocks of Skye,’”’ Mem. Geol. Survey of the 
United Kingdom, 1904. 
3. Hoégbom, A. G., “Zur Petrographie von Orné, Hufvud,” Bull. Geol. Inst. 
Upsala, X, 150. 
4. Ussing, N. V., Geology of Julianehaab, Greenland (Copenhagen, 1911), 
p. 318. 
5. Wilson, M. E., ‘‘ Banded Gneisses of the Laurentian Highlands,” Am. Jour. 
SY eiig POOsQVA Ly sere} 
6. Lawson, A. C., Geol. Survey of Canada, III (1887-88), Part I, pp. 130 f. 
7. Adams, F. D., Problems of American Geology (1915), p. 80. 
