INTERNAL STRUCTURES OF IGNEOUS ROCKS 447 
Ontario syenites have an “original foliated or schistose structure” 
(10). All of these are noted by the authors as a feature in addition 
to banding. 
The occurrence at Duluth is a particularly good example of this 
structural feature as well as of the banding. Both the early, 
relatively thin feldspathic gabbro and the later banded gabbro 
show a parallelism of plagioclase grains in many outcrops. The 
smaller sills referred to also show the fluxion structure. 
Sheet structure-—When independent of surface changes of 
temperature, this is probably related to some such feature as the 
banding and fluxion structure just described, even when they of 
0 la Imile 
———— Areas ot high magnetic attraction [ __|eabbre % x x* x MLater 
and outcrops of magnetite. x x *** *IGranite 
Fic. 7.—Map of three square miles in Cook County, Minnesota, showing in black 
the lenticular form of the outcrops of bands in the banded Duluth gabbro. In this 
case the bands carefully mapped are those rich in titaniferous magnetite. 
themselves may be inconspicuous. Platy parting is recorded in the 
Ilimausak rocks (4) and the laccoliths of Highwood Mountains (28) 
and at Tripyramid Mountain (29) and elsewhere. The Duluth 
gabbro shows such joints in many outcrops (Fig. 9). 
Combinations. —It is evident from the foregoing notes that 
several masses show two or three structural features at the same 
time. This is true for a single outcrop as well as for the mass as a 
whole.* 
1 The term “‘gneiss’’ may be extended to cover such rocks as these showing banding 
and fluxion structure, but when this is done the name should be qualified as “‘ primary 
gneiss.”’ The usage is discussed by Barlow, ‘‘ Nipissing and Temiskaming Region,”’ 
Geol. Survey of Canada, Ann. Rept., X (1897), Part I, p. 49; and Miller, Bull. Geol. Soc. 
