THE BASIC MASSIVE ROCKS, ETC. 69 1 



zons, together with acid eruptives of all kinds common to the 

 group, as quartz-porphyries, quartzless-porphyries, and fine- 

 grained red granites ; (3) olivine-free diabases and other basic 

 rocks with amygdaloidal upper and lower surfaces ; and (4) 

 detrital beds, chiefly porphyry conglomerates and sandstones, 

 rare in the lower third of the series, but increasing in thickness 

 and frequency towards the top. These various subordinate 

 divisions have been separated into smaller sub-divisions, and 

 their sequence, where possible, has been carefully detailed, but 

 since a discussion of this classification is not necessary to our 

 present purpose it need not be entered upon. 



The lowest of the divisions of rocks belonging in Irving's 

 Keweenawan has been said to consist of a succession of heavily 

 bedded coarse-grained olivine and orthoclase gabbros. The best 

 exhibition of these gabbros is found in north-eastern Minnesota, 

 where the area underlain by them occupies about 2100 miles of 

 the surface of the state, extending from the east line of Range I, 

 E., to about the middle of Range 15, W. The general shape 

 of the area is crescentic with the concave side turned toward 

 Lake Superior and its convex side facing the north-west. In its 

 widest part the crescent measures about twenty-two miles from 

 south-east to north-west. The chord connecting its two horns is 

 about 125 miles in length. The eastern extremity forms a nar- 

 row point about three miles north-west of Greenwood Lake, from 

 which point the area extends westward, widening gradually until 

 it reaches its broadest expanse, and then gradually contracting 

 until it finally abuts against the north shore of St. Louis Bay 

 west of Duluth, where it appears as a band forming the shore 

 line for ten or twelve miles, beginning in the western portion of 

 the city of Duluth and ending four miles east of Fond du Lac. 



A second 1 area of basal gabbro is in the Bad River region in 

 Wisconsin. Here the rock forms a narrow belt about forty-eight 

 miles in length and from two to five miles in width, stretching 

 from the Gogogashugun river south-westward to near Numakagon 

 lake, in T. 43 N., R. 6 W., Wis. 



1 Cf. pi. XXII., Copper-Bearing Rocks. 



