436 LUPTON : SOME NOTES ON THE YORKSHIRE COALFIELD. 
many districts, where thick coals have been worked and exhausted, 
the old wastes will be worked over again for the sake of the slack, 
the pillars, and beds of inferior coal that have not been considered 
good enough for the market. When this time comes the average 
price of coal at the pit top will be perhaps 20s. a ton, and the smoke 
nuisance will be considerably abated. 
