Lastly, we come to coal itself--a rock which constitutes a small portion
of the whole bulk of the carboniferous deposits, but which may be fairly
looked upon as the most important member of that group, both on account
of its intrinsic value and also from the interest that attaches to its
history. That coal is little else but mineralized vegetable matter is a
point on which there has for a long time been but small doubt. The
more minute investigations of recent years have not only placed this
completely beyond question, but have also enabled us to say what the
plants were which contributed to the formation of coal, and in some
cases even to decide what portions of those plants enter into its
composition. It is a thing so universally admitted on all hands, that I
shall take it for granted you are all perfectly convinced that coal has
been nothing in the world but a great mass of vegetable matter. The only
question is: How were these great masses of vegetable matter brought
together? And you must realize that they were very large masses indeed.
Just to take one instance. The Yorkshire and Derbyshire coal field is
somewhere about 700 to 800 square miles in area, and Lancashire about
200. Well, in both these coal fields you have a great number of beds of
coal that spread over the whole of them with tolerable regularity and
thickness, and very often with scarcely any break whatever. And this is
only a very small portion of what must have been the original sheet of
coal, so that you see we have to account for a mass of vegetable matter
perfectly free from any admixture of sand, mud, or dirt, and laid down
with tolerably uniform thickness over many hundreds of square miles.

At one time it was supposed that coal was formed out of dead trees and
plants which were swept down by rivers into the sea, just in the same
way as shales and sandstones were formed out of mud and sand so swept
down. The fatal objection to this theory, however, is that rivers would
not bring down dead wood alone, but they would bring down sand and mud,