Illinois State Geological Survey Circular 500 

 77 p., 13 text figs., 8 tables, 15 plates, 2,500 cop., 1977 

 Urbana, Illinois 6l801 



Printed by Authority of State of Illinois, Ch . 127, IRS, Par. 58.25 

 (P.O. 0051 8-2 V2M-6/77) 



