ROCKS OF CASCADE RIVER. 295 
assigned in part to actual thinning, such sudden thinnings being easily 
explicable when the rock layers are nearly altogether of eruptive origin; 
but also, in part, to some non-conformity with the underlying slates. How- 
ever, the whole question of the way in which the Duluth, Lester River, 
and Agate Bay groups extend to the eastward is one which will have to 
be further studied in the interior along the various rivers entering the lake— 
especially along Brulé, Cascade, Poplar, and Temperance rivers. 
Cascade River was examined by Messrs. Chauvenet and McKinlay 
above Sec. 26, T. 62, R. 2 W., but this was not far enough down stream 
to determine the existence or non-existence here of the Agate Bay beds. 
The first rocks met with by Mr. Chauvenet on the Cascade were found just 
where he first struck the river in Sec.'26, T. 62, R.2 W. Here, for about 
half a mile, there are exposed in the bed and on the sides of the river, beds 
of a dark-gray to reddish-brown aphanitic rock with conchoidal fracture, 
much like some of the dense brown rocks of the Lester River Group. The 
only thin section made showed a diabase-porphyrite, composed chiefly of 
minute tabular plagioclases with a good deal of a matrix which shows but a 
feeble, flickering light when revolved between the crossed nicols. These 
rocks are quite plainly bedded, dipping southward at about 9°. In places 
they seem to show a subordinate structure parallel to the bedding, while 
most exposed surfaces are so weathered as to fall in showers of small frag- 
ments when struck with a hammer. One or two vesicular layers were 
noted, the vesicles smooth-surfaced, elongated, one-sixth to one-fourth inch 
in length, and either empty or lined with drusy quartz. 
Continuing the ascent of Cascade River, no more exposures were found 
until reaching the falls in section 10. In the vicinity of these falls the rocks 
succeed one another in the following descending order: (1) dark-gray very 
fine-grained rock, resembling the fine-grained diabases of the Duluth and 
Lester River groups, not examined under the microscope; (2) brick-red 
quartz-porphyry; (3) medium-grained, black gabbro or luster-mottled mel- 
aphyr. The last rock forms the barrier over which the river falls. Five 
hundred feet above the falls comes in a medium-grained, brownish-gray 
orthoclase-gabbro, the thin section of which resembles closely that of the 
peculiar orthoclase-gabbro of the Lester River Group seen largely exposed 
