94 



(JEOLOGY OF THE BLACK HILLS. 



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

 beconiin«r afterwards insoluble f 



In many i)lace8 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 followino- beds: 



W.K^: 



Arcliocan. 



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



Feet. 



15° AV. ; dip, vertical . 



W 



Potsdam. 



4 



■"u;. 11.-; 

 <iain on 

 Creek. 



.< l.l... l.i i'uls- J. 



lower Knpid «^- 



2. Massive yellow and reddish sandstone, highly cal- 

 careous, dipping X. E. 20° 



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

 lowish and reddish 



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

 stone and large quantities oi glauconitc 



Ked and pink impure shaly limestone with green glau- 

 conite grains ; sandy at bottom 



6. Reddish calcareous sandstone with glaucouite 



7. Ked and yellow sandstone with bright quartz grains, cross-stratified, and 



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 l)iaclii()])od Iragments 



10. Gray limestone, stained at base with iron 



50 



20 



80 



50 

 4 



70 



40 



35 

 SO 



