INTERNAL STRUCTURES OF IGNEOUS ROCKS A445 
the rocks shows little trace of any crushing or recrystallization. 
Poikilitic and ophitic structures remain unaffected, and the min- 
erals are fresh. The associated earlier flows and sediments show 
no such structure as would have developed if the gabbro had been 
metamorphosed. ‘The structure is therefore a primary one. 
In general, the minerals of one band are the same as those of 
adjacent bands, and the banding is a consequence of difference 
in proportions of minerals. Textural changes are slight. Rock 
types at Duluth range from peridotite to anorthosite as extremes, 
with magnetite gabbro and troctolite as other variations from 
normal gabbro. A few measurements were made on thin sections 
of bands of gabbro, and some have been selected and presented in 
Table I to show how the bands vary. 
TABLE I 
PERCENTAGES BY WEIGHT 
y eae . Miscella- 
Plagioclase | Pyroxene Olivine Magnetite aaae 
75 ite) IO 4 I 
Commonibandsiy sss) scien 465 19 IO 5 I 
70 18 fe) 12 
Two adjacent bands such as 84 12 3 | ° 
alternate many times 62 15 12 II | fo) 
96 3 ° Tiel ° 
20 44 ° BO || ° 
Bands of extreme composition 440 48 fo) 3 fe) 
2 15 70 13 Oo 
75 2 22 I fo) 
Detailed observations of the dip and strike of gabbro struc- 
ture at Duluth show only minor irregularities. Fig. 6 shows 
the general structure of the gabbro in those townships where 
observations have been made. It gives the impression of concord- 
ance with neighboring contacts. Magnetic mapping of the por- 
tion of the gabbro far to the northeast shows the general parallelism 
of bands and contacts, as well as the probably lenticular nature of 
the bands, which in Fig. 7 represent titaniferous magnetite ore. 
It may be added that some large sills, more or less related to 
the gabbro (22) and typically exposed at Beaver Bay, on Lake 
