V.

ON THE FORMATION OF COAL.


The lumps of coal in a coal-scuttle very often have a roughly cubical
form. If one of them be picked out and examined with a little care, it
will be found that its six sides are not exactly alike. Two opposite
sides are comparatively smooth and shining, while the other four are
much rougher, and are marked by lines which run parallel with the
smooth sides. The coal readily splits along these lines, and the split
surfaces thus formed are parallel with the smooth faces. In other
words, there is a sort of rough and incomplete stratification in the
lump of coal, as if it were a book, the leaves of which had stuck
together very closely.

Sometimes the faces along which the coal splits are not smooth, but
exhibit a thin layer of dull, charred-looking substance, which is
known as "mineral charcoal."

Occasionally one of the faces of a lump of coal will present
impressions, which are obviously those of the stem, or leaves, of a
plant; but though hard mineral masses of pyrites, and even fine mud,
may occur here and there, neither sand nor pebbles are met with.

When the coal burns, the chief ultimate products of its combustion