We know this from the
great irregularity of the beds: while the absence of that
irregularity proves to us that the London clay was deposited in a
quiet sea.
But more. A great change in the climate of this country had taken
place meanwhile; slowly perhaps: but still it had taken place.
In the lowest clay above the chalk are found at Reading many leaves,
and buds, and seeds of trees, showing that there was dry land near;
and these trees, as far as the best botanists can guess, were trees
like those we have in England now. Not of the same species, of
course: but still trees belonging to a temperate climate, which had
its regular warm summer and cold winter.
But before the London clay had been all deposited, this temperate
climate had changed to a tropical one; and the plants and animals of
the upper part of the London clay had begun to resemble rather those
of the mouths of the African slave-rivers.
Extraordinary as this is, it is certainly true.
We know that the country near the mouth of the Thames, and probably
the land round us here, was low rich soil, some half under water,
some overflowed by rivers; some by fresh or brackish pools. We know
all this; for we find the shells which belong to a shallow sea,
mixed with fresh-water ones. We know, too, that the climate of this
rich lowland was a tropical one.
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