sedimentary depositions of clay or sand. Now, rivers would not bring down
the dead vegetation alone; their usual burden of sediment would also be
deposited at their mouths, and thus dead plants, sand, and clay would be
mixed up together in one black shaly or sandy mass, a mixture which would
be useless for purposes of combustion. The only theory which explained
all the recognised phenomena of the coal-measures was that the plants
forming the coal actually grew where the coal was formed, and where,
indeed, we now 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.