150 THE VERMILION IRON-BEARING DISTRICT. 



from the Micliipicoteii irou-bearing district on the east side of Lake 

 Superior by A. B. Wilhnott,* and Dr. S. Weidmau, of the Wisconsin Greolog- 

 ical and Natural History Survey, states that it occurs in greenstones of 

 su]oposed Huronian age in the vicinity of Wausau, Wis. It has been 

 observed in the Archean greenstones on Lake Nipigon in Ontario, Canada. 

 As the result of field studies of the Keweenawan volcanics of the north 

 shore of Lake Superior in 1900, the writer knows that it occurs also in 

 them. Although so very common throughout the Lake Superior region in 

 the rocks of pre-Cambrian age, it appears to be relatively rare in the petro- 

 graphically similar rocks of later age found elsewhere in North America. 

 This structure has been found to be so common throughout the Lake 

 Superior region that it is now considered characteristic of the pre-Cambrian 

 greenstones of the region. It is not, however, confined to any one of the 

 divisions of the pre-Cambrian rocks. The rocks in which it occurs range 

 from the Archean of the Vermilion district of Minnesota and the adjacent 

 Canadian districts and the Marquette district of Michigan, to the Keween- 

 awan. It occurs within the greatest surficial areas of the Archean. This 

 same structure has been found by Greikie in the lavas of Grreat Britain.'' 

 In a letter to the writer Geikie says: "This remarkable structure appears to 

 be far more common in lavas of all ages than I supposed. It is admirably 

 developed in our Arenig lavas, and I have lately found it in those of the 

 Old Red sandstones and Carboniferous system." 



MICROSCOPIC CHARACTERS. 



The rocks composing the Ely greenstone have been divided according 

 to their macroscopic characters into the porphyritic and non-porphjaitic 

 ■varieties, the normal diabasic or ophitic textured forms, the amygdaloidal, 

 spherulitic, and ellipsoidal forms. Stress has been laid upon some of the 

 principal macroscopic characters, and these divisions have been made 

 merely for the purpose of aiding in the study of the rocks and not because 

 the varieties were distinguished by important differences in microscopic 

 characters, except in a few cases. As the reader would infer from their age 

 and from the use of the name greenstone in connection with them, these 



"The Blichipicoten Huronian area, by A. B. Willmott: Am. Geologist, Vol. XXVIII, 1901, p. 14. 

 The nomenclature of the Lake Superior formations, by A. B. Wilmott: Jour. Geol., Vol. X, 1902, 

 p. 71. 



b Ancient Volcanics of Great Britain, by Sir Archibald Geikie, Vol I, 1898, pp. 184 and 193. 



