Image from page 138 of "Coal mining in Illinois" (1915 ...
White oak is generally used for entry timbering. In rooms in Illinois the variations in propping are as wideas are those in entry timbering. The roof may be limestone 138 COAL MINING INVESTIGATIONS or hard shale requiring no propping or top coal may be leftor the roof may be clod with such slight cohesion that it breaksat the prop or it may be ...