Spurr — Stratigraphic Position of the Thomson Slates. 161 



the folded schists lying further north, and his descriptions and 

 accompanying diagram clearly show that he included among 

 these schists the larger part, if not the whole, of what we now 

 know as Keewatin (Lower Huronian). In the fifth annual 

 report* he first confidently assigned to the Thomson (St. 

 Louis) series a place equivalent to that of the Animikie. A 

 few pages later on,f however, he states that he hesitates to 

 accept the idea of " a general unconformity between the flat- 

 lying Animikie and an older series, including the gneiss and 

 folded schists,";}: but considers that, as suggested in the third 

 annual report, there is a transition between the two, and that 

 the schists are but the folded northward continuation of the 

 Animikie. In the seventh annual report,§ he again refers to 

 the St. Louis slates as Animikie ; and here first hints as to 

 what horizon of the Huronian they were believed by him to 

 belong, i. e. the same as that of the upper slates of the Ani- 

 mikie series as represented upon the Mesabi Range. 



If the writer understands correctly what has been done, the 

 results may be briefly summed up as follows : 



Irving, Sweet, 1ST. H. Winchell,| and Hunt^f assigned the 

 Thomson (St. Louis) series to the Huronian, because of their 

 undoubted detrital nature. ~No attempt was made at first to 

 give them a definite horizon in this formation. Afterwards, 

 however, Irving was led to conclude, from an examination of 

 the Animikie rocks around Thunder Bay and Gunflint Lake 

 and westward, that the upper slate member of the Animikie 

 series of the Mesabi Range was the same as the slates along 

 the St. Louis. 



Subsequent writers have adopted this correlation, but no 

 new arguments of value have been offered, either pro or con. 

 Van Hise** has mentioned, as a circumstance tending to show 

 their Upper Huronian (Animikie) age, the occurrence of fer- 

 ruginous beds " in the northward extension of the St. Louis 

 slates." l^o ores, however, have been found in the St. Louis 

 slates ; and if by the quoted phrase the ores of the Mesabi 

 Range were meant, the occurrence of iron in the Animikie 

 can have no bearing upon the correlation between the Ani- 



* Archaean Formations of the Northwestern States, p. 196. 



f Op. cit., p. 206. 



\ That is, an unconformity at the base of the Animikie ; the Huronian uncon- 

 formity as now understood. Irving recognized perfectly well an unconformity 

 between the Huronian in general and the Laurentian, or fundamental complex of 

 granites and gneisses. 



§ Classification of Cambrian Formations, p. 422. 



|| Tenth Ann. Rep. Minnesota Geol. and Nat. Hist. Survey, p. 95 ; Eleventh 

 Ann. Rep., pp. 169 and 170. 



1 Trans. Roy. Soc. Canada, vol. i, Sec. iv, 1883, p. 250. 



** Tenth Ann. Rep. U. S. Geol. Survey, p. 461 ; Bulletin U. S. Geol. Survey, No. 

 86. Correlation papers, Archaean and Algonkian, p. 186. 



