Spurr — Stratigraphic Position of the Thomson Slates. 165 



Grounds for Correlation. 

 We may now suggest grounds for correlating the Thomson 

 series with the Keewatin of the Mesabi Range rather than 

 with the Animikie of that district. 



1. Lithological. — Since the basis on which the Thomson 

 slates have been correlated with the Animikie was purely 

 lithological, we will first present the same argument for corre- 

 lation with the Keewatin ; and it appears that the grounds for 

 the latter are much stronger. 



The slates as seen about Cloquet resemble very closely the 

 slates of the southern part of the Virginia area. Almost 

 every phase of the Cloquet rocks can be duplicated in this 

 Keewatin district. They differ only in that the St. Louis 

 rocks are traversed by a minor transverse cleavage. But the 

 resemblance of the northern part of the known St. Louis rocks, 

 those in T. 51-19, described above, to those of the Virginia 

 area is complete. Here not only the little altered detrital 

 phases are duplicated, but to a certain extent some of the less 

 altered schistose rocks, such as the sericitic slate already de- 

 scribed. Moreover, all the specimens have the peculiar 

 greenish tint which is usual in the Keewatin rocks (the "green 

 schists ") and is a distinctive mark as compared with the dead 

 black of the unaltered carbonaceous Animikie slates. 



In the region further west, especially in the vicinity of the 

 Mississippi River, the Thomson series becomes in part crys- 

 talline, having changed into sericitic, micaceous, hornblendic, 

 staurolitic, or garnetiferous schists. These phases, which 

 occupy a considerable portion of the series, correspond exactly 

 to the "green schists" and crystalline schists of the Keewatin 

 in the Mesabi district. 



2. Dynamical. — One of the greatest differences between the 

 least altered Keewatin rock of the Virginia area and the Ani- 

 mikie slates in the vicinity is the presence of the steeply dip- 

 ping regional cleavage, which has an average trend of about 

 1ST. 70° E. This, as has already been noted, marks a distinc- 

 tively pre-Animikie disturbance. In the Cloquet rocks there 

 is a strongly developed cleavage, nearly constant, running a 

 little north of east (averaging perhaps N. 85° E.) and so hav- 

 ing approximately the same trend as that of the Mesabi dis- 

 trict. In T. 51-19, ten miles further north, the same strong 

 cleavage, with nearly the same direction, was found, and the 

 transverse cleavage which is. seen at Cloquet was not observed. 

 From here to the first rock exposures, upon the Mesabi Range, 

 the distance is about 40 miles ; and here, as before observed, 

 there is no sign of disturbance in the Animikie rocks, but the 

 Keewatin exhibits the same cleavage, as well as the same litho- 

 logical peculiarities. 



