GEOLOGIC HISTORY. 443 



Huronian even cut down into the Lower Huronian and Arcliean rocks. 

 From evidence in other districts of the Lake Superior region, but not from 

 evidence in the VermiHon district, the unconformity represented by this 

 period of erosion is known to have been impoi-tant. 



Following the Upper Huronian unconformity came the lava flows of 

 the Keweenawan. These probably accumulated to immense thickness 

 upon the beveled edges of the Upper Huronian series. There is no 

 evidence that these flows were not subaerial — that is, deposited upon the 

 Upper and Lower Huronian and Archean series while they were still land 

 areas. How far the process of upbuilding of the Keweenawan had 

 continued before the next great event it is impossible to say, but it is 

 probable that thousands of feet of lavas were erupted, and that sand- 

 stones and conglomerates were deposited in the upper parts of the series 

 between these lavas. If this be the case, the land must have again sub- 

 sided below the sea. However this may be, after a very considerable 

 thickness of Keweenawan lava had accumulated, there came the great 

 laccolithic intrusion of Duluth gabbro, which now has a surface area of 

 nearly 200 square miles and extends from Duluth eastward beyond the 

 eastern end of the Vermilion district, and which bounds the Vermilion dis- 

 trict on the south from the Kawishiwi River eastward. The planes between 

 the various formations would be the natural planes of easiest resistance 

 which an intrusive would follow, and these planes the Keweenawan gabbro 

 seems to have followed for the most part. In the western and central 

 portion of the district the gabbro, to judge from the way in which it 

 overlaps on the Archean and Lower Huronian rocks, and from the fact that 

 it has included in it at various places along its northern edge masses of the 

 Gunflint formation of varying size, seems to have followed along the plane 

 between the Upper Huronian and the Archean and Lower Huronian. In 

 the east the gabbro laccolith began to rise and there beveled the edges of 

 the Upper Huronian, and at one place is even found intrusive in the 

 Keweenawan lava flows. It is believed that the numerous sills, named 

 Logan sills by Lawson, which are so abundantly found in the Animikie, 

 are but part of the same magma from Avhich the gabbro came and that they 

 were introduced at the same time. Finally the great diabase dikes of 

 Beaver Bay and other places along the Minnesota coast, which intruded the 

 Keweenawan lavas, are probably connected with this great gabbro laccolith. 



