58 THE GEOLOGY OF MINNESOTA. 



[Extension of the gabbro toward t-hn went . 

 Southern limit of the gabbro. 



wholly conjectural. It is simply the boundary of the most western line of out- 

 cropping. The rock disappears beneath the drift. The character of the rock at 

 these western outcrops indicates that they are not " peripheral." The rock is 

 coarse and rough. The gabbro may extend many miles further, underlying the 

 whole Cloquet valley, which is also deeply buried under drift, and crossing the St. 

 Louis valley north from Knife falls. It may even extend as far west as to include 

 the gabbro which is seen in the northwestern part of Morrison county, for there 

 is but one outcrop of the underlying formations between the Cloquet river and 

 the Long Prairie river. That is a quartzyte, seen in Aitkin county, as described 

 by Mr. Upham, and is probably of later date than the gabbro. If thus extended the 

 gabbro belt would pass north of Knife falls, and the hills of gabbro seen at Short 

 Line park would fall into the same category as the Sawteeth mountains, and could 

 be classed with the Beaver Bay diabase, to which they show a general petrographic 

 alliance, having characters indicative of surface flow. However, even with such a 

 westward extension, the gabbro probably dies out in that direction in the same 

 manner as toward the east, /. c-., by a diminishing series of dikes through the older 

 rocks. 



The southern limit of the i/abbro. This has thus been described by Mr. Elftman:* 

 " The southern boundary of the gabbro is irregular on account of the invasions of 

 other members of the Keweenawan series. From the south side of East Greenwood 

 lake the boundary passes westward- for thirty miles and turns south and east through 

 Brule lake along the Brule River valley to the east side of T. 63 N., R. 1 W. In the 

 vicinity of Brule mountain and Eagle mountain, the limit of the gabbro zigzags and 

 finally follows a southwesterly course through sec. 6, T. 62 N., R. 2 W., and through sec. 

 15, T. 62 N., R. 4 W., on the east branch of the Temperance river; continuing 

 westward it passes through the central part of T. 60 N., R. 6 W., between lakes 

 Harriet and Bellissima; thence through the southeastern part of T. 60 N., R. 7 W., 

 and between West Greenwood lake and Greenwood mountain, in T. 58 N., R. 10 W. 

 At the last locality it turns sharply toward the south, passes near the northwest 

 corner of sec. 19, T. 55 N., R. 11 W., and from there continues in a southwesterly 

 direction to Duluth. These boundaries give the widest areal distribution of the 

 gabbro. Within this area are other rocks, some of which are quite extensive and 

 nearly all of later geologic age. The chief area of this kind is the region west and 

 southwest of Brule lake.'' 



Along the northern and northwestern side of the great gabbro mass, the gabbro 

 is plainly intrusive on the older formations, from the Animikie downward to the 

 oldest of the Archean. On the southern and eastern border it is penetrated by and 



* American Geologist, vol. xxii, p. KfJ. I 



