382 THE GEOLOGY OP MINNESOTA. 



[Argillyte. Graywacke. 



No. 481. ARGILLYTE. (Siliceous.) 

 From the brink of Knife falls, St. Louis river. 



Meg. The piece has lines of shearing structure intersected by the slaty cleavage. 



No section. 



Age . Keewatin( ?) 



lienun-k. The structure lines in this rock might easily be mistaken for sedimen- 

 tary banding. They seem to indicate planes of movement under a shearing stress. 

 On the parting surfaces they are characterized by straight, parallel ridges and 

 depressions, which by the natural shadows give the surface a fine banding of light 

 and dark, but there is no apparent variation in the composition of the rock. Across 

 the edges of the slates produced by the prevalent slaty cleavage, are other bands and 

 ridges which do not have a close correspondence with those which mark the flat 

 surfaces of the slates. Compare a similar structm-e seen in the rock of the railroad 

 cut southeast from Carlton (Nos. 197S, 1979). N. H. w. 



No. 482. GRAYWACKE. 



At Knife falls, in the channel on the north side of the island. 

 Ref. Annual Report, x, pages 22, 23. 



Meg. Slaty graywacke, alternating with gray wacke. 



Mir. This slide, which is too thick for microscopic use, only shows the usual 

 composition of these fine graywackes, as already mentioned. It is'crossed by alter- 

 nating fine and coarser bands due to sedimentation. 



One section. 



Age. Keewatin(?) N. H. w. 



No. 483. GRAYWACKE. 



Knife falls, in the channel on the north side of the island. 

 Ref. Annual Report, x, pages 22, 23. 



Meg. Coarse and arenaceous graywacke. 

 Mic. This rock is like No. 482, except in being coarser. 

 One section. 



Age. Keewatin(?) N. H. w. 



No. 484. GRAYWACKE. 



Knife falls, in the channel on the north of the island. 

 Ref. Annual Report, x, pages 22, 23. 



Mr;/. This sample is intermediate in coarseness between Nos. 482 and 483 and 

 presents no new features. 

 One section. 

 Age. Keewatin(?) N H W- 



