You must know that certain sorts of shells live in deep water, and certain in shallow. You may prove this to yourselves, on a small scale, whenever you go to the seaside. You will find that the shell which crawl on the rocks about high-water mark are different from those which you find at low-tide mark; and those again different from the shells which are brought up by the oyster-dredgers from the 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 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.