one world to another so sheer, as that between the chalk and the
London clay above it.

But how do I know that there was a shore-line here? And how do I
know that the chalk was covered with sand-beds?

I know that there was a shore-line here, from this fact. If you
will look at the surface of the chalk, where the sands and clays lie
on it, you will find that it is not smooth; that the beds do not
rest conformably on each other, as if they had been laid down
quietly by successive tides, while the chalk below was still soft
mud. So far from it, the chalk must have become hard rock, and have
been exposed to the action of the sea waves, for centuries, perhaps,
before the sands began to cover it. For you find the surface of the
chalk furrowed, worn into deep pits, which are often filled with
sand, and gravel, and rounded lumps of chalk. You may see this for
yourselves, in the topmost layer of any chalk-pit round here. You
may see, even, in some places, the holes which boring shells, such
as work now close to the tide-level, have made in it; all the signs,
in fact, of the chalk having been a rocky sea-beach for ages.

The first bed which you will generally find upon the water-worn
surface of the chalk is a layer of green-sand and green-coated
flints. Among these are met with in many places beds of a great
oyster, now unknown in life. I cannot say whether there are any
here; but at Reading, to the east of Farnham, at Croydon, and under
London, they are abundant. There must have been miles and miles of
oyster-bed at the bottom of that Eocene sea; among the oyster-beds,
beds of a peculiar pebble, which we shall see in our gravel-pit.