this was cleverly explained by Sir Charles Lyell, who noticed, on one of
his visits to America, that the water of the Mississippi, around the rank
growths of cypress which form the "cypress swamps" at the mouths of that
river, was highly charged with sediment, but that, having passed through
the close undergrowth of the swamps, it issued in almost a pure state,
the sediment which it bore having been filtered out of it and
precipitated. This very satisfactorily explained how in some places
carbonaceous matter might be deposited in a perfectly pure state, whilst
in others, where sandstone or shale was actually forming, it might be
impregnated by coaly matter in such a way as to cause it to be stained
black. In times of flood sediment would be brought in, even where pure
coal had been forming, and then we should have a thin "parting" of
sandstone or shale, which was formed when the flood was at its height. Or
a slight sinking of the land might occur, in which case also the
formation of coal would temporarily cease, and a parting of foreign
matter would be formed, which, on further upheaval taking place, would
again give way to another forest growth. Some of the thicker beds have
been found presenting this aspect, such as the South Staffordshire
ten-yard coal, which in some parts splits up into a dozen or so smaller
beds, with partings of sediment between them.

In the face of the stupendous movements which must have happened in order
to bring about the successive growth of forests one above another on the
same spot, the question at once arises as to how these movements of the
solid earth came about, and what was the cause which operated in such a
manner. We can only judge that, in some way or other, heat, or the
withdrawal of heat, has been the prime motive power. We can perceive,
from what is now going on in some parts of the earth, how great an
influence it has had in shaping the land, for volcanoes owe their
activity to the hidden heat in the earth's interior, and afford us an