the water was receiving from time to time supplies of muddy and sandy
sediment.

The next class of rocks are the clays that are found beneath every
bed of coal, and which are known as _underclays_, or _warrant_, or
_spavins_. They vary very much in mineral composition. Sometimes they
are soft clay; sometimes clay mixed with a certain portion of sand; and
sometimes they contain such a large proportion of silicious matters that
they become hard, flinty rock, which many of you know under the name
of _gannister_. But all underclays agree in two points: they are all
unstratified. They differ totally from the shales and sandstones in this
respect, and instead of splitting up readily into thin flakes, they
break up into irregular lumpy masses. And they all contain a very
peculiar vegetable fossil called _Stigmaria_.

This strange fossil was for a long time a sore puzzle to fossil
botanists, and after much discussion the question was fairly solved by
Mr. Binney by the discovery of a tree embedded in the coal measures,
and standing erect just as it grew, with its roots spread out into the
stratum on which it stood. These roots were Stigmaria, and the stuff
into which they penetrated was an underclay. Sir Charles Lyell mentions
an individual sigillaria 72 feet in length found at Newcastle, and a
specimen taken from the Jarrow coal mine was more than 40 feet in length
and 13 feet in diameter near the base. It is not often these trees are
found erect, because the action of water, combined with natural decay,
has generally thrown them down. They are, however, found in very large
numbers in the roof of the coal, evidently having been tossed over, and
lying there flat and squeezed thin by the pressure of the measures that
lie above them.