396 N. H. DARTON— JURASSIC FORMATIONS OF THE BLACK HILLS 



Shallow and strong currents prevailed over a broad area of the West 

 These currents carried and deposited a series of widespread sheets ot 

 coarse sands, with occasional intercalations of pebbly admixture. There 

 were at this period a number of intervals of deeper or quieter wa ers 

 with clay deposition and local uplifts, giving rise to shallow surface 

 basins in which carbonaceous deposits were laid down. 



