THE DULUTH BEDS. ree 
im many sections in the usual slender needles; epidote, which in many sec- 
tions is abundantly present in the groundmass, at times to such an extent 
as to form pseud-amygdules, besides occurring as a true amygdule in the 
amygdaloids; quartz, also as an alteration-product, associated with the epi- 
dote, and also in some of the excessively fine-grained kinds as true infil- 
trating secondary quartz.’ Olivine is absent throughout. These rocks 
then are to be called, according to their texture and degree of crystalline 
development, fine-grained diabase, porphyritic diabase, diabase-porphyrite, 
and diabase-amygdaloid. 
The two beds of gabbro above alluded to are in strong contrast with 
the rest of the rocks of this group. They show a black, rather coarse, 
highly crystalline, rough-textured rock, which in the thin section is seen to 
be made up of anorthite; diallagic augite, very coarse and abundant, partly 
fresh and partly altered to viridite and uralite; and very coarse magnetite 
or titanic iron. . 
With the exception of the last rocks described, Professor N. H. Winchell, 
if I understand him correctly, would regard all of these fine-grained rocks, 
and especially the amygdaloidal and porphyritie phases, as metamorphosed 
shales and sandstones, and as altered from the red sandstone of Fond du 
Lac, to which, as a result of alteration, he also refers the granite, granitic 
porphyry, and felsite of the Duluth gabbros, and indeed of the whole 
Minnesota coast. The usual proofs are present that the rocks now under 
description are entirely original and have flowed as lavas. These are, in 
brief, completely crystalline texture in most kinds; presence of some origi- 
nal non-polarizing base in some of the porphyritic kinds; complete absence 
in all of any traces of fragmental texture; true gas vesicles in the amygda- 
loids; elongation ot these vesicles in a common direction; flowage lines 
1QOne variety of the Duluth fine-grained rocks A. Streng has described in some detail under the 
name of ‘‘melaphyr-porphyry,” the term melaphyr being used for any older plagioclase-augite rock. 
(A. Streng und J. H. Kloos: ‘‘Ueber die Krystallinischen Gesteine von Minnesota in Nord Amerika,” 
in Neues Jahrbuch f. Mineralogie, ete., 1877, p. 42.) It is evidently the rock which is largely exposed 
near the elevator in East Daluth, N. E.} of the 8. E. ¢ Sec. 27, T. 50, R. 14 W., and again—the same 
belt—on Brewery Creek, at 250 paces north and 100 west of the southeast corner of See. 22, T. 50, R. 
14 W. It presents a very dense groundmass, with very thickly scattered porphyritic crystals of red 
feldspar, mostly triclinic. The following is the analysis given by Streng: SiO,, 50.03; A1,Os, 
15.38; Fe.O3, 11.78; FeO, 3.90; CaO, 5.39; MgO,3.60; K20, 1.14; Na.O, 5.01; H20, 2.73; COg, 0.98=99.94; 
P20,, 0.33. His analysis shows the essentially basic nature of this porphyry, which Messrs. Streng and 
Kloos class with the somewhat similar rock from Saint Croix Falls, Wisconsin. 
