94 



GEOLOGY OF THE BLACK HILLS. 



to the formation of flint, at the time of their deposition as sandstone — the silica 

 becoming afterwards insoluble? 



In many places the Potsdam sandstone is found to contain calcareous 

 matter and also accumulations of glauconitic grains. 

 The calcareous condition of the conglomerate at the 

 base of the formation was frequently noticed and has 

 already been mentioned. It is not peculiar to any hori- 

 zon in the sandstones and does not persist for great 

 distances, but seems to be a merely local variation or 

 accident. It was observed to a slight extent on French 

 Creek, but is more marked on lower Rapid Creek and 

 on the headwaters of Box Elder Creek. 



Figure 14 gives a section of the Potsdam in the 

 canon of lower Rapid Creek, where this calcareous na- 

 ture is better exhibited than in any other place. It 

 exhibits the following beds: 



Archcean. 



1. Argillaceous slates in the bed of the creek; strike, N. 



Feet, 



15° W. ; dip, vertical . 



Potsdam. 



2. Massive yellow and reddish sandstone, highly cal- 

 careous, dipping N. E. 20° 



3. Impure shaly limestone with some clay shale, yel- 

 lowish and reddish 



4. Eed and brown shale with some thin strata of lime- 

 stone and large quantities of glauconitc .... 



5. Eed and pink impure shaly limestone with green glau- 

 conite grains ; sandy at bottom 



Eeddish calcareous sandstone with glauconite 



Eed and vellow sandstone with bright quartz grains, cross-stratified, and 



Fig. 14. — Section of Pots- 

 dam ou lower Kapid 

 Creek. 



containing at base considerable calcareous matter 



Carboniferous. 



8. Pink shaly limestone, gray at top 



9. Pinkish and yellowish limestone, thin-bedded, containing a few Carbonifer- 



ous corals and brachiopod fragments 



10. Gray limestone, stained at base with iron 



50 



20 



80 



50 

 4 



70 



40 



35 



80 



