other. The bottommost contains the hearths, burnt wood, implements, and finally the bones of an ancient race of men called the Mousterians. After they had left, the cave was filled in with dust and mud, and a new race of men came in and cooked their food on the new floor, and in turn left their implements, and in the end their bones, in this layer: these were called the Magdalenian men. Lastly, a third race, the Solutrians, came and occupied the cave, leaving their bones in the uppermost layer. Now, the Mousterians have been identified with the Australian aborigines, the Magdalenian men with the North American Indians and the Patagonians, and the Solutrians, with the Bushmen of South Africa. It is evident that a very great lapse of time has occurred between that in which these peoples occupied the south of France, and the present time; but the soil forming at the present day is being filled up in the respective countries with the bones of the buried races of Australians, North American Indians, and Bushmen; and these beds, therefore, though containing the same organic remains as the layers in the caves of the south of France, are very much more recent. The coal plants of Europe, again, migrated from the Southern Hemisphere, so that the beds containing the coal plants in South Africa are much earlier than the rocks of Europe containing the same forms. Sediments containing the same fossils, but not contemporary, are called homotaxial. Homotaxis, then, means the same fossils but the beds containing them not contemporaneous.
It has often happened in the geological history of the globe that one area has been dry land at one period and sea at another. While the area is dry land no
