Fig. 27. — Diagram to exhibit the relation of the Drainage System of the Black Hills to the Zones of 



Hard and Soft Strata. 

 The central dotted space shows the Archaean area of soft rocks. The annular dotted zone shows the 

 outcrop of the soft Red Beds and Jura. The intervening white zone shows the outcrop of the hard 

 Carboniferous beds. 



