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, Aldershot Moors, Hartford Bridge Flat, Frimley ridges, and Windsor Forest. That rich old world was all swept away, and instead of it desolation and barrenness, piling up slowly on its ruins a desert of sand and shingle, rising inch by inch out of a lifeless sea. There is something very awful to me in the barrenness of those Bagshot sands, after the rich tropic life of the London clay. Not a fossil is to be found in them for miles. Save a few shells, I believe, near Pirbright, there is not a hint that a living being inhabited that doleful sea. But do not suppose, gentlemen and ladies, that we have yet got our gravel-pit made, or that the way-worn pebbles of which it is composed are near the end of their weary journey. Poor old stones! Driven out of their native chalk, rolled for ages on a sea-beach, they have tried to get a few centuries' sleep in the Eocene sands on the top of the chalk hills behind us, while the London clay was being deposited peacefully in the tropic sea below; and behold, they are swept out, once more, and hurled pell-mell upon the clay, two hundred feet over our heads. Over our heads, remember. We have come now to a time when Hartford Bridge Flats stretched away to the Beacon Hill, and many a mile to the south-eastward--even down into Kent, and stretched also over Winchfield and Dogmersfield hither.