“…The occurrence of oolitic ironstone within the condensed section is also of subeconomic interest, although the small size of the deposits and their high phosphorous content negate their usefulness as iron ore. The Decorah Shale has been considered as a possible source of potassium for fertilizer (Schmitt, 1924), and the Glenwood has similar potassium concentrations (Pettijohn, 1957), but it is far more economical to obtain potassium from potassium salts elsewhere.…”