ACID ROCKS IN THE BRULE LAKE COUNTRY. 145 
position, as do also thin seams of a material midway between true granite 
and granitic porphyry. 
Closely similar occurrences, both as to the containing and cutting rocks, 
obtain at Duluth, save that here we have none of the‘true granite, a brick- 
red augite-syenite being the principal intrusive rock. As seen at Duluth, 
this rock often verges closely on granite on the one hand, and on the other 
can be traced through finer kinds into felsite and true quartziferous por- 
phyry. It intersects the gabbro in the most irregular manner, forming 
great irregularly outlined patches in it, often many hundred square feet in 
area, and again occurring in sharply defined veins, a few feet, or even 
inches, in thickness. Some three or four miles north and west from the 
lake shore at Duluth, N. H. Winchell reports that most of the hills are com- 
posed of a crystalline red rock, similar to that of Duluth. 
Mr. W. M. Chauvenet has carried the Duluth gabbro, with its accom- 
panying red rocks, westward for some ten miles and northward for thirty- 
five miles, to the Cloquet River, where the gabbro appears in large expo- 
sures. ‘To the northeast N. H. Winchell reports a similar rock with a similar 
accompaniment at the headwaters of Poplar River, some 20 miles back from 
the lake, while Mr. Chauvenet has examined a large area of like character 
at the headwaters of the Brulé and Cascade rivers, and about Brulé Lake, 
in townships 62 and 63, ranges 1, 2, and 3 W., where the red rock rises into 
mountain masses, a prominent instance of which is Eagle Mountain. This lies 
near the middle of T. 63, R. 1 W., and rises, a bald mass of red rock, toa 
height of 450 feet above the small lake at its western foot and 1,500 or 
1,600 feet above Lake Superior. Similar granitoid rocks occur at higher 
horizons along the Minnesota coast, and are always in intersecting masses 
except when tending towards the quartziferous porphyries in character, when 
they appear to form flows, of a similar nature to those of the latter rock. 
Intrusive augite-syenites are also met with in the Bohemian Range of Ke- 
weenaw Point. 
Quartziferous porphyries, in association with bedded diabases and mela- 
'The Building Stones of Minnesota, 1880, p. 7; also Annual Report of the Geological Survey of 
Minnesota for 1879, p. 24. 
10LSs 
