278 COPPER-BEARING ROCKS OF LAKE SUPERIOR. 
and ropy texture in the amygdaloids; and division of the beds into lower 
compact and upper vesicular portions. To these we need only add that 
should we look on them as of detrital origin, we must do the same for all 
the amygdaloids of the Lake Superior basin, and for all of the crystalline 
rocks with which they occur. This would leave us only a few insignificant 
dikes to regard as of eruptive origin. Between the Keweenaw Point dia- 
bases and amygdaloids and those of Duluth there is no difference as to the 
general nature—-the origin of the one is the origin of the other. 
True detrital material was, however, observed interbedded in the Du- 
luth series at two points; one of these was low down in the series, on the 
hillside above the Catholic church at Duluth, where a small exposure of a 
very finely laminated argillaceous slaty rock is to be seen; the other is on 
the lake shore between Brewery and Chester Creeks, where may be seen a 
light-brownish, quite plainly bedded fine-grained sandstone composed of a 
mixture of basic and acidic detritus, the former predominating. 
Several small dikes were observed cutting the beds of this mem- 
ber on the lake shore between Chester Creek and Duluth. These dikes 
trend with the strata, but cut across them at right angles to the bedding. 
They are composed of a fine-grained black rock, which, near the middle of 
the dike, is plainly crystalline, while towards the sides it is aphanitic. This 
rock has not been examined under the microscope, but is precisely the same 
macroscopically as that of some similar dikes occurring five miles further 
down the coast, below the mouth of Lester River, which is a very highly 
augitic diabase, without olivine, and with but little magnetite. 
Since these beds make so large an angle with the coast near Duluth, 
they depart rapidly inland. So far as our limited explorations went inland, 
the belt of country under which they are supposed to lie is largely low and 
without exposures, lying back of the first or lake range of bold hills. Fol- 
lowing the range line between ranges 11 and 12 north seven miles from the 
crossing of Knife River in the 8. W. 4, Sec. 6, T. 52, R.11 W., Mr. McKinlay 
found no exposures, but eastward from the last point three miles, in the N.W.4 
of the 8. W. 4, Sec. 4, T. 53, R.11 W., he found a large exposure of a very 
fine-grained massive gray rock, with porphyritic triclinic feldspars, which 
both to the naked eye and under the microscope resembles closely the gray 
