RESUME OF LITERATURE. 123 



the New Ulin quartzite in Cottonwood County, and the quartzite in Pipes- 

 tone County. 



The igneous rocks of the Keweenawan vary in age from the late 

 Animikie time to the top of the Keweenawan series. They are divided 

 into two groups, the Cabotian or Lower Keweenawan, and the Mfin'::!;i or 

 UpjDer Keweenawan. 



The Cabotian division inchides gabbro and contemporaneous red rock 

 and their surface lavas, and all other dikes and sills which are associated 

 with, but younger than, the Animikie clastic rocks, and which are older 

 than the Puckwunge conglomerate. The lower member of the Cabotian is 

 the gabbro, which covers an enormous area. It extends on the east to East 

 Greenwood Lake, in T. 64 N., R. 2 W. On the north it is bounded by the 

 Animikie strata of the Mesabi iron range. Its westernmost exposure is in 

 the vicinity of Short Line Park, Duluth. The southern limit is irregular, 

 swinging from East Greenwood Lake in a zigzag manner through T. 63 N., 

 R 1 W.; T. 62 N., R. 2 W.; T. 62 N , R. 4 W.; T. 60 N., R. 6 W.; T. 60 N., 

 R. 7 W.; T. 58 N., R. 10 W.; and T. 55 N., R. 11 W., to Duluth. 



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

 the gabbro is plainly intrusive on the older formations, Animikie and 

 Keewatin. 



From the northern border of the gabbro many sills offshoot and pene- 

 trate the Animikie strata parallel to the bedding. These are known as the 

 Logan sills. 



Near its contact with the underlying rocks, both the Animikie and 

 Keewatin series, there are various altered rocks which can be connected in 

 places with the gabbro and in places with the underlying rocks. To these 

 altered rocks the term muscovadyte has been applied. It includes the 

 various so-called peripheral phases of the gabbro. 



On the southern and eastern border the gabbro is penetrated by and 

 penetrates in a confused manner the red rock, with which it alternates both 

 structurally and areally. It is believed to have resulted from the meta- 

 morphism by the gabbro of the Animikie, and perhaps earlier fragmentals. 



As the granites of the Archean are believed to have resulted from the 

 softening of acid fragmentals, so the gabbro may probably have been the 

 result of the metamorphism or refusion of the Keewatin greenstones. 



The anorthosite masses of the Beaver Bay diabase, supposed by Lawson 



