and other matters, and that in the bottom of the sea the dead wood would
be mixed with these matters, and instead of getting a perfectly unmixed
mass of vegetable matter, we should get a mixture of dead plants, sand,
mud, and other things, which would give rise to something like coal, but
something very different, as any one who tries to burn such coal will
soon find out, from really good, pure house coal. So that this theory,
which is generally known as the "drift" theory, was totally inadequate
to account for the facts as we know them.

The other theory was that coal was formed out of plants and trees that
grew on the spot where we now find coal itself. On this supposition it
is easy to account for the absence of foreign admixtures of sand, mud,
and clay in the coal; and we can also understand very much better than
by the aid of the drift theory how the coal had accumulated with such
wonderful uniformity of thickness over such very large areas. This
theory was for some time but poorly received; but after the discovery
of Sir William Logan, that every bed of coal had a bed of underclay
beneath, and the discovery of Mr. Binney, that these underclays were
true soils on which plants had undoubtedly grown, there was no doubt
whatever that this was the real and true explanation of the matter.

I dare say many of you have had occasion to walk across peat bogs.
The peat bog is a great mass of vegetable matter, which is every year
growing thicker and thicker; and underneath it there is almost always a
bed of thin clay, in look very much like the underclays, and this thin
clay is penetrated by the rootlets of the moss forming the peat, exactly
the same way as the underclays of the coal measures are penetrated by
the stigmaria and its rootlets. But you must not suppose that the plants
out of which coal was formed were exactly the same low type of moss
which forms our present peat bogs. However, it is pretty certain that