576 THE GEOLOGY OF MINNESOTA. 



[Sandstone. Schist. 



around each grain, except where one grain comes directly 

 in contact with another. Under a low power this cement- 

 ing substance appears homogeneous and isotropic. Under 

 a high power each grain is seen to be surrounded by one 

 or more bands of chalcedony, and sometimes in the inter- 

 stices between the grains, and thus surrounded by the 

 chalcedony is very minutely crystallized quartz. The 

 general appearance of the rock is shown in the accompany- 

 ing figure. Whether the chalcedonic bands are distinctly fibrous or notcannot be 

 readily told from the section at hand, as it is too thick and the chalcedonic bands 

 overlap the quartz grains; thus the structure of the bands as seen in polarized light 

 is not clear. Between the grains is at. times a little iron oxide, apparently both 

 limonite and hematite. 

 One section. 



Age. Middle Cambrian, perhaps of the age .of the Puckwunge. u. s. G. 



Remark. On having made a better section the chalcedonic envelope of the 

 grains is seen to be fibrously crystallized and to have a negative elongation, the 

 fibres standing vertical on the peripheries of the grains. N. H. w. 



No. 863. SANDSTONE. 



" St. Peter sandstone, from the small island in the Minnesota bottom-lands near Fort Snelling, cemented 

 with iron and ('deposited '?) silica, so as to be hard and show different colors." 

 Kef. Annual Report, xiii, page 40. 



Meg. A medium-grained sandstone, which varies from red to yellow in color. 

 Some of the quartz grains show the development of crystal faces. On what appears 

 to be the outside of the specimens, or the side exposed to the weather, the rock is a 

 vitreous quartzyte rather than a somewhat friable sandstone. 



Mic. The section shows well rounded quartz grains in a cement of limonite. 

 Some of the quartzes show enlargements. 



One section. 



Age, St. Peter. u. s. G. 



No. 864. SCHIST. (Black.) 



South side of Jones bay, Vermilion lake; N. E. % sec. 3, T. 61-16 W. 

 Ref. Annual Report, xv, pages 276, 386; Bulletin ii, pages 45, 421. 



Meg. A hard, fine-grained, almost black, glistening, schistose rock. It contains 

 small particles, usually no larger than a pin's head, which are common, and would 

 make the term " Knoterrschief er " applicable to this rock. 



Mic. The sections show a schistose rock of fine grain, the schistose structure 

 being due to a more or less marked elongation of the component grains in one direction. 

 Scattered all through the rock, both in and between the mineral grains, is a large 



