brown weathering large chert nodules large eliptical chert nodules chert pebbles; large chert nodules 5Œ60% black chert bluegray black chert lenses weathering brown 50% chert weathering v. silty crin chert v. even bedding; blue gray crossbedded fissile black chert with crinoidal Is. lenses micaceous fissile dolomitic bleached white