At the close of the geological period whose natural physiognomy we have thus traced, Europe was still far from displaying the configuration which it now presents. A map of the period would represent the great basin of Paris (with the exception of a zone of Chalk), the whole of Switzerland, the greater part of Spain and Italy, the whole of Belgium, Holland, Prussia, Hungary, Wallachia, and Northern Russia as one vast sheet of water. A band of Jurassic rocks still connected France and England at Cherbourg--which disappeared at a later period, and caused the separation of the British Islands from what is now France. CHAPTER XXII THE AGE OF MAMMALS The Geological Record is not perfect. There are breaks in it such as have not and may never be filled up; and it is because of these breaks that some of the divisions are made in geologic time. At present the earth's crust has only been scratched for fossils. Great parts of Asia and of Africa and of South America remain to be explored, and they may in some future generation fill the gaps of our knowledge and render superfluous some of the divisions which geologists now place in the eras of the rocks and of the fossils. But so far as we know at present there were real breaks in the history of the continents, perhaps not swift or sudden, but wholly changing the appearance and the life, vegetable and animal, of half a world, perhaps the whole world at a time. Many geologists believe that the secret of these changes lies in the core of the earth; and that, to use our old simile of the golf ball, when the tension and pressures inside the earth grow too much for its strength something gives way, and the whole world begins to change, the continents sinking under the oceans and new lands arising. We shall not again consider this idea in all its bearings, or ask whether there is any simpler explanation to be found in the never-ceasing explosive tremors of the crust; but we shall only say that the last of these great changes set in at the end of the Chalk age. After that era we arrive at the period among the rocks which, with all its subdivisions, Eocene, Miocene, Pliocene, Quaternary, is classed as Cainozoic or Modern.