642 FRANK F. GROUT 
alkali feldspar is derived from gabbro only when a similar or larger 
amount of rock has separated with a medium to acid labradorite 
or andesine. No such rock is known at Duluth, though the small 
transition zones do exhibit zoned feldspars. 
The arrangement of analyses according to the per cent of silica 
shows two well-defined and separated groups, one about 47 per cent 
silica and one about 72 per cent silica (see Fig. 9). The groups are 
evidently related to the two groups distinguished as gabbro and 
red rock. All the red rock contains more than 57 per cent silica; 
- Pw Py 
of analyses. 
oO 
1 2 3 4 
Per cent Potash 
Number of groups 
Fic. 10.—The potash content of the Duluth gabbro formation, as shown by avail- 
able analyses (in groups). 
no gabbro has as much as 57 per cent. The break appears even 
more sharply when the analyses are arranged in the order of silica 
per cent and a curve is drawn for the per cent of potash (see Fig. 10). 
All the groups of red rock analyses have more than three per cent 
of potash, while the gabbro groups all have less than 1 per cent. 
Although single analyses may show exceptional figures there is no 
real overlap." 
0.95 
x Silica Silica 
per cent Potash per cent mencent Potash per cent 
In detail By groups In detail By groups 
2.02 ° 57.98 3.44 
32.90 o+ oir, 61.09 3.05 
35.81 0.33 2 65.56 2.88 3.20 
42.24 0.27 66.36 3.05 
66.92 3.98 
45.05 I.05 
45.66 0.41 68.36 4.48 
40.45 0.34 0.56 7Z.15 2.40 
47.05 0.05 71.81 1.92 3.66% Red rock 
47.10 0.92 72.42 4.97 
73.28 4.50 
47.25 0.37 
47-79 0.53 73.70 4.56 
47.90 0.56 0.66} Gabbro 73.91 2.78 18 
48.20 0.32 74.00 4.33 3: 
49.15 1.61 75.78 1.06 
49.18 0.82 
49.39 0.10 
49.42 1.15 0.71 
49.78 0.406 
49.88 0.68 
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