s area extended, by the ignition of coal-dust."

We have mentioned that accidents have frequently occurred from the
falling of "coal-pipes," or, as they are also called, "bell-moulds." We
must explain what is meant by this term. They are simply what appear to
be solid trunks of trees metamorphosed into coal. If we go into a
tropical forest we find that the woody fibre of dead trees almost
invariably decays faster than the bark. The result is that what may
appear to be a sound tree is nothing but an empty cylinder of bark. This
appears to have been the case with many of the trees in coal-mines, where
they are seen to pierce the strata, and around which the miners are
excavating the coal. As the coaly mass collected around the trunk when
the coal was being formed, the interior was undergoing a process of
decomposition, while the bark assumed the form of coal. The hollow
interior then became filled with the shale or sandstone which forms the
roof of the coal, and its sole support when the coal is removed from
around it, is the thin rind of carbonised bark. When this falls to
pieces, or loses its cohesion, the sandstone trunk falls of its own
weight, often causing the death of the man that works beneath it. Sir
Charles Lyell mentions that in a colliery near Newcastle, no less than
thirty _sigillaria_ trees were standing in their natural position in an
area of fifty yards square, the interior in each case being sandstone,
which was surrounded by a bark of friable coal.

[Illustration: Fig. 33--Part of a trunk of _Sigillaria_, showing the thin
outer carbonised bark, with leaf-scars, and the seal-like impressions
where the bark is removed.]

The last great danger to which we have here to make reference, is the
explosive action of a quantity of coal-dust in a dry condition. It is
only now commencing to be fully recognised that this is really a most
dangerous explosive. As we have seen, large quantities of coal are formed
almost exclusively of _lepidodendron_ spores, and such coal is productive
of a great quantity of dust. Explosions which are always more or less
attributable to the effects of coal-dust are generally considered, in the
official statistics, to have been caused by fire-damp. The Act regulating
mines in Great Britain is scarcely up to date in this respect. There is a
regulation which provides for the watering of all dry and dusty places
within twenty yards from the spot where a shot is fired, but the
enforcement