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 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.  We know that the neighbourhood of the Isle of Sheppey, at the mouth of the Thames, was covered with rich tropic vegetation; with screw pines and acacias, canes and gourds, tenanted by opossums, bats, and vultures:  that huge snakes twined themselves along the ground, tortoises dived in the pools, and crocodiles basked on the muds, while the neighbouring seas swarmed with sharks as huge and terrible as those of a West Indian shore.

It is all very wonderful, ladies and gentlemen:  but be it is:  and all we can say is, with the Mussulman—­“God is great.”

And then—­when, none knows but God—­there came a time in which some convulsion of nature changed the course of the sea currents, and probably destroyed a vast tract of land between England and France, and probably also, that sunken island of Atlantis of which old Plato dreamed—­the vast tract which connected for ages Ireland, Cornwall, Brittany, and Portugal.  That convulsion covered up the rich clays with those barren sands and gravels, which now rise in flat and dreary steppes, on the Beacon Hill,