And these pebbles are nothing else than rolled
chalk flints.
That settles the matter. The pebbles could not come from Wales;
there are no flints there. They could not have been made before the
chalk; for out of the chalk they came; and the only explanation
which is left to us, I believe, is, that over the tops of the chalk
downs; over our heads where we stand now, there once stretched
layers of sand and gravel, "Tertiary strata" as I have been calling
them to you; and among them layers of this same hard sandstone.
When the floods came they must have swept away all these soft sands
and gravels (possibly to make the Bagshot sands, of which I shall
speak presently), and left the chalk downs bare; but while they had
strength to move the finer particles, they had not generally
strength to move these sandstone blocks, but let them drop through,
and remain upon the freshly-bared floor of chalk, as the only relics
of a tertiary land long since swept away; while some were carried
off, possibly by icebergs, as far as Pirbright, and dropped, as the
icebergs melted, both there, at Dogmersfield, and also, though few
and small, in Eversley and the neighbourhood.
But how came these tertiary sandstones to be so very hard, while the
strata around them are so soft?
Ladies and gentlemen, I know no more than you. Experience seems to
say that stone will not harden into that sugary crystalline state,
save under the influence of great heat: but I do not know how the
heat should have got to that layer in particular.
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