418 THE VERMILION IRON-BEARING DISTRICT. 



Huronian sediments, for at several places on the edge of the Vermilion dis- 

 trict and just south of it are found isolated patches of the lowest part of 

 the Gruuflint formation (Animikie) included in the Keweenawan gabbro. 

 The gabbro in this part of the district may originally have been completely 

 covered by the Animikie series, but in this part of the district, also, the 

 rocks are much more closely folded than in the east near Gunflint and in 

 the west in the Mesabi district, and as a result of the erosion in this area, 

 where the rocks have been folded and fractured, all of the Upper Huronian 

 (Animikie) but the few included patches has been removed. 



In the eastern part of the Vermilion district the gabbro began to rise 

 and cut across the Upper Huronian, reaching higher and higher beds to the 

 east, and then spread out essentially along the plane between the Upper 

 Huronian (Animikie) and the base of the Keweenawan, sending sills and 

 dikes into the Rove slates of the Upper Huronian and also into the 

 Keweenawan rocks, as can be seen on Brule Lake. 



The writer is inclined to believe that the gabbro is a great basic 

 igneous mass which represents a basic differentiation product of a magma 

 from which perhaps the major portion of the Keweenawan volcanics were 

 derived, and which basic magma has, perhaps, as its complementary acid 

 rocks, the great mass "of intrusive "Red Rock" and the related rhyolite 

 flows of the Keweenawan. 



METAMORPHIC EFFECT OF GABBRO AND SILLS. 



The effect of the gabbro upon the various rocks with which it is in 

 contact has been considered under the description of those various rocks, 

 for example, under the Ely greenstone, the Rove slates, etc., but a brief 

 summary statement will be made here concerning the character of this 

 metamorphism. The most noticeable general effect of the gabbro has 

 been to produce a very large quantity of rich brown biotite in the rocks in 

 contact with it. 



Archean (Ely) greenstones. — In the meta-dolerites and meta-basalts 

 (greenstones) the general effects are much the same as in the case of the 

 sediments. The ophitic texture of those greenstones which have been 

 studied is still retained with a fair degree of distinctness. There is, how- 

 ever, a tendency to gradually destroy the texture and produce granular 

 rocks therefrom by the production of large quantities of biotite, with 



