that, although we may attribute to upheaval from beneath
the fact that the bed of the sea became temporarily raised at each period
into dry land, the deposits of sand or shale would at the same time be
tending to shallow the bed, and this alone would assist the process of
upheaval by bringing the land at least very near to the surface of the
water.

Each upheaval, however, could have been but a temporary arrest of the
great movement of crust subsidence which was going on throughout the coal
period, so that, at its close, when the last coal forest grew upon the
surface of the land, there had disappeared, in the case of South Wales, a
thickness of 11,000 feet of material.

Of the many remarkable things in connection with coal-beds, not the least
is the state of purity in which coal is found. On the floor of each
forest there would be many a streamlet or even small river which would
wend its way to meet the not very distant sea, and it is surprising at
first that so little sediment found its way into the coal itself. But
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