INTERNAL STRUCTURES OF IGNEOUS ROCKS 443 
In Maine some gabbro masses show alternating bands about two 
inches thick, in some of which segregation of feldspar produces light 
colors (13). 
The rocks of Lizard show a linear structure and an occasional 
distinct banding which is said to have nothing to do with dynamo- 
metamorphism (14). Such banded rocks are reported from the 
Himalayas (15), the Kola Peninsula (16), and the British Isles 
(17, 18). In the “‘Cottian sequence” (19) the banding has been 
supposed to be metamorphic, but there are some dikes with a folia- 
Fic. 5.—Apophyses of feldspathic Duluth gabbro into its traprock roof, east of 
Duluth Heights. 
tion parallel to their walls and at a high angle to the structure of the 
schist. 
The banding of the Duluth gabbro was long ago mentioned 
(20, 21), but new work has recently been done on the area by the 
geologists of the Minnesota Geological Survey. The structure is 
exposed in typical, as well as in some exceptional, conditions at the 
city of Duluth. The gabbro intrusion (Fig. 5) occurred after the 
accumulation of a great thickness of diabase and other. flows of 
the Keweenawan. It spread at or near the base of the flows, and 
along the unconformity at the base of the Keweenawan sediments a 
little below the flows. While the roof and floor are not an exactly 
continuous horizon, the transgression of a few hundred feet in a mass 
