THE POTATO RIVER SECTION. 251 
46, R. 2 E., the coarse rocks appear more plentifully, and by the time the 
Potato River is reached the finer rocks have nearly or quite disappeared. 
These coarse rocks, as shown elsewhere, are much like the coarse gabbros 
of Duluth and the Saint Louis and Cloquet rivers, which they resemble, 
moreover, in being cut by masses of brick-red granitic porphyry, a large 
exposure of which rock is to be met with, for instance, on the old Ironton 
trail in the northern part of Sec. 8, T. 45, R. 1 W. 
For the rest, the Potato section is chiefly made up of ordinary types 
of diabase and melaphyr. At least two bands of felsite and quartziferous 
porphyry are included. One of these, in the southern part of T. 46, R. 1 
W., is evidently the same as that noted on the Gogogashugun, See. 5, T. 46, 
R. 2 E., while the other much broader band, which is exposed on the river in 
sections 14 and 15 of T. 46, R. 1 W., appears to belong with the porphyry 
of the Porcupine Mountains. ‘The rock of the latter belt is largely a true 
quartziferous porphyry, with a lilac-tinted matrix, in which are thickly scat- 
tered minute black quartzes, one-twentieth inch in diameter, and whitish 
kaolinized feldspars, one-tenth to two-tenths inch across. Faint white 
lines are occasionally seen, and the whole aspect of the rock is very much 
that of the rock at the Great Palisades on the Minnesota coast. 
Ten miles farther southwest, along the strike, Bad River crosses the 
series, and here the changes are carried to a yet greater extreme. The 
plack shale and underlying conglomerate have thinned respectively to 125 
and 350 feet, while the entire width of the Lower Division to the underlying 
Huronian is only some 17,000 feet, of which 12,000 feet are taken up by 
the coarse gabbro. The remaining 5,000 feet are made up chiefly of the 
typical fine-grained diabase and diabase-amygdaloid; but two beds of 
quartzose porphyry are included, the lower one of which is evidently the 
same as the broad belt of the Potato River section. One porphyry-con- 
glomerate has been noted in this thickness. 
Beyond Bad River the general trend of the formation changes abruptly 
to a westerly direction, but otherwise the conditions remain much as ob- 
served on that stream, as far as the Brunschweiler River, save that the gab- 
bro below expands to a great width. In the townships between Bad and 
Brunschweiler rivers the gabbro makes such frequent exposures—those 
