THE DULUTH BEDS. PA, 
from here are belts of fine-grained ashbed-diabase and of a diabase-porphyry 
with a fine-grained to aphanitic gray matrix and large red crystals of a tri- 
clinic feldspar. Still south of these belts are others of a medium- to very 
coarse-grained gray gabbro. One of these belts, consisting of a medium- 
grained kind, with much whitened feldspar, and much magnetite, can be 
traced for several miles from the end of the lake, through a line of islands 
to the north shore. Still south of these belts, and appearing both in the 
islands and on the northeast shore, are again others of a red granitic por- 
phyry, running into a true quartz-porphyry; of fine-grained ashbed-diabase, 
and of a very coarse orthoclase-bearing gabbro like that seen farther south 
towards Eagle Mountain. 
The gabbro sheets overlying the slates of Pigeon River and Thunder 
Bay, and above alluded to as possibly belonging to the same general hori- 
zon with the Duluth gabbros, are described in connection with the slates.’ 
The Duluth Group—tThe rocks in the neighborhood of Duluth which 
I assign to this group were found exposed in the streets of the town itself, 
along the lake shore to the mouth of Chester Creek, and on the hillside in 
the triangular area between the latter creek and the lake shore. As ex- 
plained on a previous page, these rocks trend at first west of north, and then 
about north to and beyond Chester Creek, dipping at first 45° eastward, 
but near Chester Creek not more than 20°. The whole thickness displayed 
is not far short of 5,000 feet. With the exception of two thin beds of a 
moderately coarse, black gabbro, and a little interleaved detrital matter, all 
of this thickness is made up of a succession of very fine-grained to apha- 
nitic, gray to brown rocks, which are frequently porphyritically developed, 
with red and more rarely white feldspars as porphyritic ingredients; and 
which, in the upper two-thirds of the thickness often present amygdaloids 
as the upper portions of the flows. These amygdaloids have commonly a 
light-brown or reddish-brown matrix, and amygdules which are prevailingly 
epidote; but amygdules of epidote and quartz, of epidote and calcite, and 
of a green earthy substance, evidently a decomposition-product, also occur. 
A general epidotic decay is often presented in the shape of reticulated 
strings and blotches of epidote through the amygdaloids, and even, to some 
1See also Chapter VIII. 
