f a receding ocean, are believed by others to be the
inequalities on the bottom of a shallow sea. Not only, however, does the
general character of these successive terraces suggest the idea that
they must have been shores, but the ripple-marks upon them are as
distinct as upon any modern beach. The regular rise and fall of the
water is registered there in waving, undulating lines as clearly as on
the sand-beaches of Newport or Nahant; and we can see on any one of
those ancient shores the track left by the waves as they rippled back at
ebb of the tide thousands of centuries ago. One can often see where some
obstacle interrupted the course of the water, causing it to break around
it; and such an indentation even retains the soft, muddy, plastic look
that we observe on the present beaches, where the resistance made by any
pebble or shell to the retreating wave has given it greater force at
that point, so that the sand around the spot is soaked and loosened.
There is still another sign, equally familiar to those who have watched
the action of water on a beach. Where a shore is very shelving and flat,
so that the waves do not recede in ripples from it, but in one unbroken
sheet, the sand and small pebbles are dragged and form lines which
diverge whenever the water meets an obstacle, thus forming sharp angles
on the sand. Such marks are as distinct on the oldest Silurian rocks as
if they had been made yesterday. Nor are these the only indications of
the same fact. There are certain animals living always upon sandy or
muddy shores, which require for their well-being that the beach should
be left dry a part of the day. These animals, moving about in the sand
or mud from which the water has retreated, leave their tracks there; and
if, at such a time, the wind is blowing dust over the beach, and the sun
is hot enough to bake it upon the impressions so formed, they are left
in a kind of mould. Such trails and furrows, made by small Shells or
Crustacea, are also found in plenty on the oldest deposits.

Admitting it, then, to be a beach, let us begin with the lowest type of
the Animal Kingdom, and see what Radiates are to be found there. There
are plenty of Corals, but they are not the same kinds of Corals as those
that build up our reefs and islands now. The modern Coral animals are
chiefly Polyps, but the prevailing Corals of the Silurian age were
Acalephian Hydroids, animals which indeed resemble Polyps in certain
external feature