Organic geochemistry of black shales, marlstones, and oils of middle Pennsylvanian rocks from the northern Denver and outheastern Powder River Basins, Wyoming, Nebraska, and Colorado / 
by J.L. Calyton... [et.al.] 
 - Washington :  U.S. Government Printing Office,  1992 
 - 44 p. :  ill. ;  28 cm. 
 - U.S. geological suvery bulletin ;  1917-K .