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



Hard and Soft Strata. 



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

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

 Carboniferous beds. 



