Spurr — Stratigrajphic Position of the Thomson Slates. 163 



blende schists ; but on going away from the granite area these 

 are seen to have resulted from the metamorphic influence of 

 the granite, for the rock soon changes into the common 

 " green schist." This is a rock of which the original charac- 

 ters can be suspected, but ordinarily not readily determined. 

 It is characterized by the development of sericite and kindred 

 minerals along the schistosity planes. This green schist type 

 occupies the central part of the area. In the southern part, 

 however, near the apex of the triangle, the evidence of meta- 

 morphism decreases rapidly, and the rocks become mainly 

 easily recognizable as detrital. Here are found what appear 

 to be only slightly altered siliceous and clay-slates, and gray- 

 wackes. In hand-specimen these Keewatin detritals often can 

 hardly be distinguished from specimens of the overlying Ani- 

 mikie slates. 



The Keewatin rocks possess a strongly-marked regional 

 cleavage or schistosity, not far from vertical in hade, and 

 nearly constant in trend, — averaging about ]ST. 70° E. This is 

 also nearly the direction of both the northern and southern 

 contacts of the granite belt ; and, in a general way, of the pre- 

 dominant rock-structure of a large part of the gneissic and 

 schistose terranes lying further north, in northeastern Minne- 

 sota and Canada.* In short, it indicates the direction of move- 

 ment of a regional disturbance which has affected a great area. 

 This structure is still more pronounced in some of the country 

 north of the Mesabi Range, and here granitic invasions and 

 accompanying metamorphic action have also been more com- 

 mon ; so that it seems that the seat of greatest disturbance lay 

 north, rather than south of the Mesabi. This disturbance took 

 place prior to Animikie time, for the Animikie rocks are unaf- 

 fected by it. 



The Animikie slates which overlie the Keewatin to the 

 south, in T. 57-17, disappear beneath the drift within a few 

 miles and are thence not traceable southward. On the east, 

 their southward continuation is also hid by the overlying 

 Keweenawan rocks, and on the west by the Cretaceous. 



Physical characters of the Thomson Slates at Cloquet. 



On passing south across the swamp-covered area, the first 

 rocks which appear above the thinning mantle of drift are 

 those of the Thomson series. The physical characters of the 

 rock, as it appears typically on the St Louis River at Cloquet, 

 near the place where it was first recognized, may be briefly 

 described. 



* Consult A. C. Lawson, Report on Geology of the Lake of the Woods Region, 

 p. 21 et seq., Annual Report of the Geol and Nat. Hist. Survey of Canada, 1885. 



