There is one peculiarity in connection with the habitats of the
encrinites and corals which goes some distance in supplying us with a
useful clue as to the conditions under which this portion of the
carboniferous formation was formed. These creatures find it a difficult
matter, as a rule, to live and secrete their calcareous skeleton in any
water but that which is clear, and free from muddy or sandy sediment.
They are therefore not found, generally speaking, where the other
deposits which we have considered, are forming, and, as these are always
found near the coasts, it follows that the habitats of the creatures
referred to must be far out at sea where no muddy sediments, borne by
rivers, can reach them. We can therefore safely come to the conclusion
that the large masses of encrinital limestone, which attain such an
enormous thickness in some places, especially in Ireland, have been
formed far away from the land of the period; we can at the same time draw
the conclusion that if we find the encrinites broken and snapped asunder,
and the limestone deposits becoming impure through being mingled with a
proportion of clayey or sandy deposits, that we are approaching a
coast-line where perhaps a river opened out, and where it destroyed the
growth of encrinites, mixing with their dead remains the sedimentary
dêbris of the land.

[Illustration: FIG. 22.--Encrinites: various. Mountain limestone.]

We have lightly glanced at the circumstances attending the deposition of
each of the principal rocks which form the beds amongst which coal is
found, and have now to deal with the formation of the coal itself. We
have already considered the various kinds of plants and trees which have
been discovered as contributing their remains to the formation of coal,
and have now to attempt an explanation of how it came to be formed in so