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.

They are flints; but very small, dark, often almost black, and quite round and polished.  Compare them with the average flints of the pit, and you see that while the average flints are fresh from the chalk, these have plainly been rolled and rounded for years.  They are (except in their dark colour) exactly such shingle as forms the south-coast beach about Hastings and Brighton.  They are the shingle beaches of the Eocene sea, part of which are preserved under the London clay.  To the north a vast bed of them remains in its original place, on Blackheath near London; while part, in the district to the south, which the London clay has not covered, have been washed away, and carried into our gravel-pit, to mingle with other flints fresh from the chalk.

I said just now that I had proof that a great tract of the chalk-hills which are now bare, was once covered with sand and gravel.  Here, in the presence of these dark pebbles, is a proof.  But I have another, and a yet more curious one.

For our gravel-pit, if it be, will possibly yield us another, and a more curious object.  You most of you have seen, I dare say, large stones, several feet long, taken out of these pits.  In the gravels and sands at Pirbright they are so plentiful that they are quarried for building-stone.  And good building-stone they make; being exceedingly hard, so that no weather will wear them away.  They are what is called saccharine (that is, sugary) sandstone.  If you chip off a bit, you find it exactly like fine whity-brown sugar, only intensely hard.  Now these stones have become very famous; for two reasons.  First, the old Druids used them to build their temples.  Second, it is a most puzzling question where they came from.