450 FRANK F. GROUT 
are seen in the Yellowstone banded obsidians (27) and the flows 
of the eastern (31) and southwestern states (32). The bands in 
the Purcell sills ““approximate a position parallel to the upper and 
lower contacts of the sill’? (24). Barlow records that the strike of 
the banding is uniform over large areas in the Nipissing and Temis- 
kaming regions, and shows a “‘marked correspondence in direction 
with the line of outcrop of the neighboring stratified Huronian 
rocks”’ (8). Adams finds the banding of Mount Johnson vertical 
and clearly parallel to the walls of a volcanic plug curving around 
the mountain (g). Adams and Barlow say that the strike of the 
banding and foliation of the alkali syenites of eastern Ontario con- 
forms to that of the adjacent country rock (10). Ussing considers 
the strata of the Ilimausak mass the upper layers of a batholith, 
but records that near the walls of the chamber the bands, which 
are nearly horizontal most of the way, turn up and become parallel 
to the walls (4). Gregory mentions some dikes which are foliated 
parallel to the walls, but not parallel to the foliation of the neighbor- 
ing schists (19). Banding also appears in a dike in the Isle of 
Man, parallel to its walls (26). Harker says that the banding of the 
gabbro at Carrock Fell is parallel to “the lie of the intrusion as a 
whole” with only minor undulations (17). At the Isle of Skye the 
banding is undoubtedly related to the boundaries. Iddings says 
that it is ‘‘not locally referable to the form or boundary of the body 
of a particular igneous rock,’ but he cannot have seen as much of 
the structure as Geikie and Teall, who say that each sheet of 
gabbro “consists of many parallel layers . . . . which correspond 
in direction with the trend of the sheet itself’’(1); or as Harker, who 
says that the bands dip with the mass as a whole and are in general 
parallel with the upper and lower surfaces of the sheets (2). 
A similar disagreement may be recorded in the case of the 
Duluth gabbro, where Elftman says that the banding is irregular 
(21), and more recent data show only minor variations from the 
direction of the contacts. The agreement of strike with the 
boundaries of the mass is shown in Fig. 6. The agreement in 
vertical section is more difficult to prove, on account of the scarcity 
of exposures showing such vertical sections. A single outcrop at 
«J. P. Iddings, Igneous Rocks, I, 252. 
