444 THE GEOLOGY OF MINNESOTA. 



[Pebbles. Shale. 



No. 599A. PEBBLES (from No. .','.>'.>). 



Ref. Annual Report, x, page 56. 



Meg. The pebbles are red and hard. 



Mic. Two sections made from different pebbles are alike in consisting essentially 

 of fine-grained quartz, but they differ in microscopic aspect by reason of the distribu- 

 tion of the coloring matter. They are both originally composed of grains of sand, 

 such sand being itself a fine-grained quartzyte. About these grains grew secondary 

 quartz in coarser grains, filling the interstices completely. In one of these sections 

 that would conclude the description, but in the other there is a concentric banding 

 of the coloring matter, making the secondary quartz bands appear oolitic, the coloring 

 matter forming entire circuits about the original nuclei and passing independently 

 through all the coarser secondary quartzes without deviation from the contours of 

 the original muclei, although the quartzes embraced in the encircling band have 

 various orientation. In general the width of the band is less than the diameter of 

 the contained grain, but in a few instances it is equal to it, thus increasing the 

 apparent pebble 200 per cent in its shorter diameter. The belts of impurities that 

 can be counted are sometimes six or even ten, depending on the minuteness with 

 which they are differentiated. In a few instances the nuclei of these concentric 

 bands are of other substances, calcite or magnetite, but the quartzyte nuclei are 

 always of the same character. They are themselves made up of secondary quartz 

 growths, which took place at an earlier date, and may be referred to aporhyolyte, 

 from which they cannot be distinguished, except that they are not known to show a 

 fluidal structure, or to taconyte, which is also made up, in its last phase, of minute 

 secondary quartz grains. 



Age. Puckwunge(?) (Basal Potsdam). N. H. w. 



No. 600. SLATE. 



Silver Islet landing, a short distance north of the lake. 

 Ref. Annual Report, x, page 56. 



Meg. Greenish or grayish, aluminous, but rather coarse and quartzose slate. 

 From the lower portion of the beds exposed. The higher beds are softer. 



Mic. The quartz grains are angular and the cementing material is opaque. 



One section (too thick). 



Age. Upper part of the Animikie (Grand Portage graywacke). 



It cma,rk. In the fall of 1896 a considerable thickness of this slate, or what is 

 presumed to be at this horizon, was noted in the Puckwunge valley (compare Nos. 

 2070, 2071, 2073), and was named subsequently the Grand Portage graywacke. So far 

 as could be learned from the exposures near Silver Islet landing there is a gradual 

 transition from hard quartzyte upward into this rather fragile and slaty graywacke. 



N, H. w. 



