A careful estimate of the Lancashire coal-field has been made by
Professor Hull for the Geological Survey. Of the 7000 feet of
carboniferous strata here found, spread out over an area of 217 square
miles, there are on the average eighteen seams of coal.

This is only an instance of what is to be found elsewhere. Eighteen
coal-seams! what does this mean? It means that, during carboniferous
times, on no less than eighteen occasions, separate and distinct forests
have grown on this self-same spot, and that between each of these
occasions changes have taken place which have brought it beneath the
waters of the ocean, where the sandstones and shales have been formed
which divide the coal-seams from each other. We are met here by a
wonderful demonstration of the instability of the surface of the earth,
and we have to do our best to show how the changes of level have been
brought about, which have allowed of this game of geological see-saw to
take place between sea and land. Changes of level! Many a hard geological
nut has only been overcome by the application of the principle of changes
of level in the surface of the earth, and in this we shall find a sure
explanation of the phenomena of the coal-measures.

Great changes of the level of the land are undoubtedly taking place even
now on the earth's surface, and in assuming that similar changes took
place in carboniferous times, we shall not be assuming the former
existence of an agent with which we are now unfamiliar. And when we
consider the thicknesses of sandstone and shale which intervene beneath
the coal-seams, we can realise to a certain extent the vast lapses of
years which must have taken place between the existence of each forest;
so that although now an individual passing up a coal-mine shaft may
rapidly pass through the remains of one forest after another, the rise of