There it was, the dreaded fire-damp, in a layer above their heads. One
touch of an open flame and there would be a terrible explosion, yet the
miners were working undisturbed just beneath it with unprotected lamps
on their caps. The visitors felt suddenly like recruits under fire--they
were far from enjoying the situation but they did not want to seem
alarmed. No one made any protest, but neither did any one protest when
the Superintendent led the way to a section of the mine where there was
no gas that they might see a sight which he assured them was without
doubt wonderful.
They were glad that they had been assured that there was no fire-damp
here, for their leader lifted his lamp close to the roof. Ethel Blue
made the beginning of an exclamation as she saw his arm rising, but she
smothered her cry for her good sense told her that this experienced man
would not endanger the lives of himself or his guests. The coal had been
taken out very cleanly, and above them they saw not coal but shale.
"What is shale?" inquired Helen.
"Hardened clay," replied the Superintendent. "There were no men until
long after the carboniferous period when coal was formed, but just in
this spot it must have happened that the soil that had gathered above
the deposits of coal was very light for some reason or other. Above the
coal there was only a thin layer of soft clay.
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