DURHAM AND NORTHUMBERLAND. 77

removed or piled up in walls for the supportof the mines. This produces many curiousspecimens of natural roofs and domes, accord-ing to the fall of the upper stratum. In someparts there are large areas propped up by greatnumbers of wooden piles, which not unfre-quently appear bent or cracked in the middlefrom the great pressure of the upper strata.

The ordinary mode of working a mine isin parallel passages called boards, about fouryards wide and eight yards asunder. These areconnected by headways, which are smaller pas-sages, at right angles to the former and twentyyards apart, thus leaving strong pillars of coalto support the roof of twenty yards by eight.The headways and boards are drawn at rightlines through the whole extent of the mine.