746 PRE-CAMBRIAN NORTH' AMERICAN LITER A TURE 



ascribed to igneous rocks in other localities, and it yet remains to be 

 proved that the series is sedimentary. Even if sedimentary, no satis- 

 factory evidence is given that either the schistosity or banding cor- 

 responds with bedding. Using the classification of the United States 

 Geological Survey, the Keewatin or a part of it would be regarded 

 as Huronian, and the Coutchiching or a part of it would be regarded 

 as Archean. It is rather probable that in the Thunder Bay district 

 of Ontario and in northeastern Minnesota parts of two series have 

 been included under each of these terms. 



Smyth and Finlay ' describe the western part of the Vermilion 

 range. The sedimentary rocks fall into two divisions. The older is 

 a fragmental slate formation, while the younger is an iron-bearing 

 formation lithologically identical with certain phases of the lower iron- 

 bearing formation of the Marquette district. To all appearances it is 

 devoid of clastic material. It is believed, from analogies with other 

 iron-bearing districts of the Lake Superior region, that the jasper of 

 the Vermilion district is derived from a cherty iron carbonate or from 

 a glauconitic greensand, or both. However, as the jasper is a final 

 product of the alterations, it is not possible to show this. 



Intrusive igneous rocks are very abundant, cutting or being inter- 

 leaved with the sedimentary rocks in masses running from the thickness 

 of a knife blade to those loo feet across. In quantity the igneous 

 rocks exceed, perhaps several times the sedimentary rocks. The old- 

 est igneous rocks are greenstones. These vary from massive to 

 schistose, and into conglomerate-breccias. The acid rocks were 

 intruded later than the basic rocks. They were originally for the most 

 part quartz-porphyries, but these have been extensively changed to 

 sericite-schists and conglomerate-breccias, and to rocks intermediate 

 between these and the original form. Within the larger masses of the 

 igneous rocks, both basic and acid, are frequently included fragments 

 from both the slate and iron formations, from those of small size to 

 masses more than loo feet long. 



The conglomerate-breccias are of dynamic origin. The first step 

 in the development of the breccias was the formation of two intersect- 

 ing sets of planes of fracture, dividing the originally massive rocks 



' The Geological Structure of the Western Part of the Vermilion Range, Minn., 

 by H. L. Smyth and ]. Ralph Finlay. Trans. Am. Inst, of Min. Engineers, Vol. 



XXV, 1895, pp- 595-645- 



I 



