378 THE GEOLOGY OF MINNESOTA. 



(Argillyte. Graywacke. Slate. 



In the feldspar extinction on 010 is 25A, showing the usual result for these 

 dikes, viz., labradorite-bijtoicnite. It is pierced by long and large microscopic needles 

 of apatite. 



< '/ilorite, Hot He and boirlingite, as alteration products, are dispersed scantily in the 

 section, while magnetite plays its customary role. 



Two sections. 



Age. (Cabotian? ) dike in the Animikie( ?) N. H. w. 



No. 468. ARGILLYTE (?) ( Metamorphic. ) 



Rock adjoining the dike at the Thomson bridge; a part of the argillyte hardened. 

 Ref. Annual Report, x, page 16; Annual Report, xxiii, page 239. 



Meg. Dense, black, apparently a fine-grained basalt. 



Mk. The thick section shows between the nicols that an incipient crystal- 

 lization has been started. This is evinced by the existence of roundish and oblong 

 areas which thickly stud the slide, which are much lighter than the rest. These 

 areas occupy more than one-half of the whole space. They are specked with scales 

 of muscocite( ?), which lie in all positions, and which cause the areas to remain light 

 continually on rotation, although separately each scale has its extinctions. 



One section. 



Age. Animikie(?) 



Remark. This may be from the selvedge of the dike itself. N. H. w. 



No. 469. GRAYWACKE. 



Near Miller's mill, a quarter of a mile north of Thomson; probably in N. E. % S. W. % sec. 5, T. 48-16 W. 

 This bed is eleven feet thick, and in places it is conglomeratic; it has slaty layers on each side of it. 



Ref. Annual Report, ix, page 82; Annual Report, x, pages 16-18, 20, 21, 23, 29; Annual Report, xxiii, 

 page 239. 



Meg. A light-gray rock, hard and siliceous. Small quartz and feldspar grains 

 occur in a fine-grained matrix. 



Mic. The section shows numerous rounded to angular quartz and feldspar grains 

 in a dirty greenish groundmass. The section is too thick to enable this groundmass 

 to be carefully studied, but it is evidently composed essentially of chlorite, iron ore, 

 and a fine-grained aggregate of quartz a,n&feltlx)>ar. 



One section. 



Age. Keewatin(?) u. s. G. 



No. 470. SLATE. (Black.) 



Near the same locality as No. 469. Quarried for roofing slate. 

 Ref. Annual Report, x, page 17. 



Mic. A very fine-grained, black, clay slate, with a rather rough cleavage. 

 No section. 



- 



Age. Keewatin(?) u. s. G. 



