446 FRANK F. GROUT 
Superior, show an exactly similar banding. The same gabbro in 
Wisconsin shows what has been described as a “‘bedded”’ struc- 
ture (23). 
There are a number of smaller masses which also show a banding, 
possibly of similar origin. The Purcell sills gabbro has certain 
streaks of lighter color (24) in addition to the separation into 
differentiated zones. A gabbro dike near Boulder, Colorado, has 
bands of iron ore parallel to the walls (25). A dike in the Isle of 
Man is similarly banded (26). The Mt. Holmes bysmalith has a 
color banding parallel to the walls (27). Other examples will no 
LAG 73 2 iS = 7DuluthGabbro aa 
Zo8 O} hp Bins eee c 
Co aes 
dea aese (Ses) 
Cy cee a 
Gearkewecemeer) 
Animikie eS 
? Pre Animihie [SzeS] 
ag 5 os 26 miles 
SN ay (—— —— —_ ____ _} 
Fic. 6.—Sketch of the area of the Duluth gabbro showing the dip and strike of 
its internal structure. 
doubt be recalled by those who have worked in igneous rocks. A 
color banding is visible in many flows, but the difference in mineral 
content of the bands is not always clear. 
Fluxion structure—Certain igneous rocks have an abundance 
of platy or needle-like minerals, notably the feldspars and horn- 
blende. Many gabbros and syenites show a certain amount of 
parallelism of such grains (Fig. 8). Most of these rocks show 
banding of the sort just discussed. The rocks of Lizard (14) 
and the Adirondacks (12) and Laurentia (5) are ‘‘foliated.”’ 
The Ilimausak (4) rock has “primary schistose structure.” The 
Mt. Johnson rocks (9) have a “‘fluidal arrangement of grain.”” The 
