sea outside. Now, the lower part of the clay, near here, contains
shallow-water shells: but if you went forty miles to the eastward,
you would find in the corresponding lower beds of the clay, deep-
water shells, and far above them, shallow-water shells such as you
find here: a fact which shows plainly that this end of the clay sea
was shallowest, and therefore first filled up.

But again--and this is a very curious fact--between the time of the
Plastic clays and sands, with their oyster-beds and black pebbles,
and that of the London clay, great changes had taken place. The
Plastic clay and sands were deposited during a period of earthquake,
of upheaval and subsidence of ancient lands; and therefore of
violent currents and flood waves, seemingly rushing down from, or
round the shores of that Wealden island to the south of us, on the
shore of which island Odiham once stood. 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