Every one knows the great chalk beds which appear in the south of England, and which have given its name to the latest age of the Mesozoic. This great deposit of light-coloured and usually soft calcareous matter attains in some places to the enormous thickness of 1,000 feet. Nor is it limited in extent. According to Lyell, its European distribution is from Ireland to the Crimea, a distance of 1,140 geographical miles; and from the south of France to Sweden, a distance of 840 geographical miles. Similar rocks, though not in all cases of the precise nature of chalk, occur extensively in Asia and in Africa, and also in North and South America. But what is chalk? It was, though one of the most familiar, one of the most inscrutable of rocks, until the microscope revealed its structure. The softer varieties, gently grated or kneaded down in water, or the harder varieties cut in thin slices, show a congeries of microscopic chambered shells belonging to the humble and simple group of Protozoa. These shells and their fragments constitute the material of the ordinary chalk. With these are numerous spicules of sponges and silicious cell-walls of the minute one-celled plants called Diatoms. Further, the flinty matter of these organisms has by the law of molecular attraction been collected into concretions, which are the flints of the chalk. Such a rock is necessarily oceanic; but more then this, it is abyssal. Laborious dredging has shown that similar matter is now being formed only in the deep bed of the ocean, whither no sand or mud is drifted from the land, and where the countless hosts of microscopic shell-bearing protozoa continually drop their little skeletons on the bottom, slowly accumulating a chalky mud or slime. that such a rock should occur over vast areas of the continental plateaus, that both in Europe and America it should be found to cover the tops of hills several thousand feet high, and that its thickness should amount to several hundreds of feet, are facts which evidence a revolution more stupendous perhaps then that at the close of the Palæozoic. For the first time since the Laurentian, the great continental plateaus changed places with the abysses of the ocean, and the successors of the Laurentian Eozoon again reigned on surfaces which through the whole lapse of Palæozoic and Mesozoic time had been separated more or less from that deep ocean out of which they rose at first. This great Cretaceous subsidence was different from the disturbances of the Permian age. There was at first no crumpling of the crust, but merely a slow and long-continued sinking of the land areas, followed, however, by crumpling of the most stupendous character, which led at the close of the Cretaceous and in the earlier Tertiary to the formation of what are now the greatest mountain chains in the world. As examples may be mentioned the Himalaya, the Andes, and the Alps, on all which the deep-sea beds of the Cretaceous are seen at great elevations.