He will find layers in those beds, of several feet in thickness, as
hard as flint, but as porous as sponge. On examining their cavities
he will find them to be simply hollow casts of innumerable joints of
Crinoids, so exquisitely preserved, even to their most delicate
markings, that it is plain they were never washed about upon a beach,
but have grown where, or nearly where, they lie. What then, has
happened to them? They have been killed by the sand. The soft parts
of the animals have decayed, letting the 140,000 joints (more or
less) belonging to each animal fall into a heap, and be imbedded in
the growing sand-rock; and then, it may be long years after, water
filtering through the porous sand has removed the lime of which the
joints were made, and left their perfect casts behind.
So much for the millstone grits. How long the deposition of sand
went on, how long after it that second deposition of sands took
place, which goes by the name of the "gannister," or lower coal-
measures, we cannot tell. But it is clear, at least, that parts of
that ancient sea were filling up and becoming dry land. For coal, or
fossilised vegetable matter, becomes more and more common as we
ascend in the series of beds; till at last, in the upper coal-
measures, the enormous wealth of vegetation which grew, much of it,
where it is now found, prove the existence of some such sheets of
fertile and forest-clad lowland as I described in my last paper.
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