160 Spurr — Stratigraphic Position of the Thomson Slates. 



areas of granite which have been referred to the Laurentian ; 

 and various post-Animikie (post-Huronian) sediments, as far up 

 as the Cretaceous. To the east lie the Keweenawan rocks, 

 chiefly igneous. To the north, however, the Thompson series 

 is hid beneath a great thickness of drift which occupies a 

 broad belt of country through which run the St. Louis and 

 Mississippi rivers. Between the southern border of this drift 

 mantle and the nearest known outcrops on the north (those of 

 the Animikie of the Mesabi Range) the distance is about 40 

 miles. 



Sketch of previous Correlations. 



The Thomson rocks have been correlated, up to the present 

 time, with the Animikie (the Upper Huronian). It is the 

 object of this paper to suggest that there are no firm grounds 

 for this correlation, and that they be referred rather to the 

 Keewatin* (Lower Huronian). It will be advisable to inquire 

 first into the reasons for which they have hitherto been as- 

 signed their place. 



Irvingf and Sweety assigned these rocks to the Huronian in 

 1880. The reasons, were mainly lithological. The Thomson 

 (St. Louis) rocks presented the characters of an undoubted 

 and little metamorphosed clastic series, especially in that part 

 which alone was definitely known, i. e. the outcrops along the 

 St. Louis River about Thomson. At that time there were 

 recognized but three general horizons to which this series 

 could be referred, — the Keweenawan, the Huronian, and the 

 Laurentian. The Thomson (St. Louis) rocks were seen to 

 underlie the Keweenawan, and to be cut by dikes of supposed 

 Keweenawan age ; and no rock of recognizable clastic charac- 

 ters could be referred to the Laurentian. 



In the third annual report of the United States Geological 

 Survey,§ Irving first hinted at the correlation of the " St. 

 Louis slates " with the Animikie of northeastern Minnesota, as 

 observed at that time around Gunflint Lake and Thunder Baj\ 

 He pointed out the general lithological resemblance between 

 the two series, and noted the difference in that the " St. Louis 

 slates" are cleaved. In the same report, however, |j he sug- 

 gested the correlation of the uncleaved Animikie slates with 



* The terms Animikie and Keewatin are to be understood here in the sense in 

 which they are employed by the Minnesota Survey ; they are provincial designa- 

 tions, both of formations and of divisions of geological time, which correspond 

 in general to the more comprehensive terms (Upper and Lower Huronian) used 

 by the United States Survey. 



f Geology of Wisconsin, III, p. 18. 



% Op. tit., p. 339. 



§ Copper-Bearing Rocks of Lake Superior, p. 162. 



|| Op. cit., p. 170. 



