gical
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
the strata above each forest-bed then was tremendously slow, and the
period between the growth of each forest must represent the passing away
of countless ages. Perhaps it would not be too much to say that the
strata between some of the coal-seams would represent a period not less
than that between the formation of the few tertiary coals with which we
are acquainted, and a time which is still to us in the far-away future.

The actual seams of coal themselves will not yield much information, from
which it will be possible to judge of the contour of the landmasses at
this ancient period. Of one thing we are sure, namely, that at the time
each seam was formed, the spot where it accumulated was dry land. If,
therefore, the seams which appear one above the other coincide fairly
well as to their superficial extent, we can conclude that each time the
land was raised above the sea and the forest again grew, the contour of
the land was very similar. This conclusion will be very useful to go
upon, since whatever decision may be come to as an explanation of one
successive land-period and sea-period on the same spot, will be
applicable to the eighteen or more periods necessary for the completion
of some of the coal-fields.

We will therefore look at one of the sandstone masses which occur between
the coal-seams, and learn what lessons these have to teach us. In
considering the formation of strata of sand in the seas around our
river-mouths, it was seen that, owing to the greater weight of the
particles of the sand over those of clay, the former the more readily
sank to the bottom, and formed banks not very far