162 Spurr — Stratigraphic Position of the Thomson Slates. 



mikie and the St. Louis series. The correlation, therefore, has 

 rested entirely upon very broad lithological resemblances. 



During the summer of 1S93, the writer made an examina- 

 tion of the Western Mesabi region and south to the St. Louis 

 slates, and came to suspect that there were no valid reasons for 

 correlating the St. Louis series with the flat-lying Animikie of 

 the Mesabi district. On the other hand, the evidence seemed 

 rather in favor of correlation with the underlying schistose 

 and slaty rocks, referred to the Keewatin in the same district.* 



The Animikie Slates upon the Western Mesabi. 



The slates which constitute the uppermost recognized mem- 

 ber of the Animikie upon the Western Mesabi are usually soft, 

 black, and carbonaceous; they dip gently southward to south- 

 eastward, and disappear beneath the heavy covering of drift. 

 These rocks have been almost unaffected by altering dynamic 

 forces since their deposition. There is no sharp crumpling, no 

 cleavage or strongly-marked jointing ; and very few fissure or 

 gash-veins. 



The Keewatin of the Western Mesabi. 



The slates, with the lower members of the Animikie (the 

 iron-bearing member and the quartzite), are in abrupt uncon- 

 formity with the underlying rocks. Ordinarily, the underly- 

 ing rock appears to be the granite of the Giant's Range, which 

 has been supposed to be Laurentian ; but in places a narrow 

 belt of schists appears between the granite and the Animikie, 

 and in T. 58-17 probable faulting has brought up a triangular 

 area of schists 6 or 8 miles in its north and south extent. The 

 appearance of this area and the attendant phenomena estab- 

 lishes two important facts : first, that the rocks on which the 

 Animikie here rests are mainly schistose and slaty, instead of 

 gneissic and granitic; and second, that the granite is intrusive 

 into these cleaved rocks, and therefore cannot be regarded as 

 Laurentian. The schists have been generally recognized as 

 Keewatin. 



The triangular upthrust area, which the writer has called the 

 Virginia area,f has one side against the granite belt. ISTear the 

 contact it is made up of mainly noncrystalline mica and horn- 



* Professor 1ST. H. Winchell was at one time in the field with the writer; and 

 came to substantially the same conclusions. It should be stated, moreover, that 

 Professor Winchell had for some time previous been skeptical as to the Animikie 

 age of the Thomson series, and had even mapped a small part of the area as 

 Keewatin, in a preliminary unpublished map exhibited at the World's Fair in 

 1893. 



f Bulletin No. 10, Minn.' Geol. and Nat. Hist. Survey, p. 17. 



