and harden, and this would assist in giving it that bedded
structure which is so noticeable in the shales, and which causes it to
split up into fine laminae. This uniformity of structure in the shales
over wide areas is a well ascertained characteristic of the coal-shales,
and we may therefore regard the method of their deposition as given here
with a degree of certainty.

There is a class of deposit found among the coal-beds, which is known as
the "underclay," and this is the most regular of all as to the position
in which it is found. The underclays are found beneath every bed of coal.
"Warrant," "spavin," and "gannister" are local names which are sometimes
applied to it, the last being a term used when the clay contains such a
large proportion of silicious matter as to become almost like a hard
flinty rock. Sometimes, however, it is a soft clay, at others it is mixed
with sand, but whatever the composition of the underclays may be, they
always agree in being unstratified. They also agree in this respect that
the peculiar fossils known as _stigmariae_ abound in them, and in some
cases to such an extent that the clay is one thickly-matted mass of the
filamentous rootlets of these fossils. We have seen how these gradually
came to be recognised as the roots of trees which grew in this age, and
whose remains have subsequently become metamorphosed into coal, and it is
but one step farther to come to the conclusion that these underclays are
the ancient soils in which the plants grew.

No sketch of the various beds which go to form the coal-measures would be
complete which did not take into account the enormous beds of mountain
limestone which form the basis of the whole system, and which in thinner
bands are intercalated amongst the upper portion of the system, or the
true coal-measures.

Now, limestones are not formed in the same way in which we have seen that
sandstones and shales are formed. The last two mentioned owe their origin
to their deposition as sediment in seas, estuaries or lakes, but the
masses of limestone which are found in the various geological formations
owe their origin to causes other than that of sedimentary deposition.

In carboniferous times there lived numberless creatures which we know
nowadays as _encrinites_. These, when growing, were fixed to the bed of
the ocean, and extended upward in the shape of pliant stems composed of
limestone joints or plates; the stem of each encrinite then expanded a