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 at
the top in the shape of a gorgeous and graceful starfish, possessed of
numberless and lengthy arms. These encrinites grew in such profusion that
after death, when the plates of which their stems consisted, became
loosened and scattered over the bed of the sea, they accumulated and
formed solid beds of limestone. Besides the encrinites, there were of
course other creatures which were able to create the hard parts of their
structures by withdrawing lime from the sea, such as _foraminifera_,
shell-fish, and especially corals, so that all these assisted after death
in the accumulation of beds of limestone where they had grown and lived.

[Illustration: FIG. 20.--Encrinite.]

[Illustration: FIG. 21.--Encrinital limestone.]