find it. When the plants and trees died, their remains
fell to the ground of the forest, and these soon turned to a black,
pasty, vegetable mass, the layer thus formed being regularly increased
year by year by the continual accumulation of fresh carbonaceous matter.
By this means a bed would be formed with regularity over a wide area; the
coal would be almost free from an admixture of sandy or clayey sediment,
and probably the rate of formation would be no more rapid in one part of
the forest than another. Thus there would be everywhere uniformity of
thickness. The warm and humid atmosphere, which it is probable then
existed, would not only have tended towards the production of an abnormal
vegetation, but would have assisted in the decaying and disintegrating
processes which went on amongst the shed leaves and trees.

When at last it was announced as a patent fact that every bed of coal
possessed its underclay, and that trees had been discovered actually
standing upon their own roots in the clay, there was no room at all for
doubt that the correct theory had been hit upon--viz., that coal is now
found just where the trees composing it had grown in the past.

But we have more than one coal-seam to account for. We have to explain
the existence of several layers of coal which have been formed over one
another on the same spot at successive periods, divided by other periods
when shale and sandstones only have been formed.

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 geolo