246 THE VERMILION IRON-BEARING DISTRICT. 



belts for manj years, no additional deposits have yet been developed. 

 However, it is by no means proved that some of these belts may not yield 

 runs of ore. But the iron-bearing formation and the Ely greenstone are 

 so intimately mixed that some of the belts which seem to be largely iron 

 formation may be really to a very large extent composed of greenstone and 

 jasper. Under these conditions, the systematic exploration of even those 

 belts which appear to be the most promising is a matter of extraordinary 

 difficulty. Certainly the Vermilion district is the most difficult to explore 

 of any of the iron-bearing districts within that part of the Lake Superior 

 region in the United States. 



SECTION IV —GRANITES. 

 geistsratj statement. 

 At a great number of places throughout the Vermilion district acid 

 rocks of various kinds have been found. Their macroscopic and microscopic 

 features demonstrate their igneous character without possibility of question, 

 and their relations to the adjacent rocks give further proof of this, as they 

 are found cutting through both the Ely greenstones and the iron-bearing 

 Soudan formation of the Archean. Tliese rocks vary from fine- to coarse- 

 grained granites and from porphyries with very fine-grained groundmass to 

 granite-porphyries. The normal granites predominate. They are known 

 from the topographic featiu-es with Avhich they are associated as: (1) The 

 granites of Vermilion Lake; (2) the granites of Trout, Burntside, and 

 Basswood lakes; (3) the granite between Moose Lake and Kawishiwi 

 Eiver; (4) the granite of Saganaga Lake. These granites will be considered 

 in detail in this section. 



THE AGE OF THE ACID INTBUSIYBS. 



All of these rocks are younger than the Ely greenstone, for they 

 occur in it as dikes. A number of the dikes are also found in the Soudan 

 formation, which is itself of more recent origin than the greater part of the 

 Ely greenstone. That these intrusives are older than the next sedimentary 

 formation of the district — the Ogishke conglomerate, of Lower Huronian 

 age, which succeeds the Soudan formation — is shown positively by 

 the fact that they occur as pebbles in this conglomerate, and that their 

 detritus largely constitutes the rocks of this formation. Speaking broadly, 



i 



