In the preceding chapters an attempt has been made to present outline sketches of the geography, fauna, and flora of North America as they exist now. Yesterday, we may say for the sake of emphasis, there were differences from what exists to-day in each of these great groups of facts. That is, changes are everywhere in progress. With the recognition of this idea comes logically the conclusion that similar changes must have taken place in the past, and that the geography of the earth's surface, and its flora and fauna, at no very distant time must have been markedly different from what they are to-day. To test this hypothesis the geologist studies the records preserved in the rocks in much the same manner that the historian searches the papyri or the monuments of Egypt to discover what changes in the affairs of men have occurred since the days of the Pharaohs. The changes referred to are not essentially different from those now in progress, but in reality the two are parts of a single series. For a very long time there have been continents and oceans, lakes and rivers, and the land has been diversified by mountains and hills, plains and valleys, in the same general way as at the present time. When once the idea is grasped that we are living in a geological age, and that there is no break between the present and the past, it is evident that the history of the past can be interpreted by means of the results produced by known causes. Familiar formulas which express this idea are: "The present is the key to the past"; "Geography is the geology of to-day," etc. The forces or agencies which are now modifying the earth's surface, such as the rending of rocks by changes of temperature and the action of frost, erosion and deposition by streams, the dash of ocean waves against the land, volcanic eruption, the chemical action of organic acids, movements producing upheaval and subsidence, etc., have been in action for geological eras, but their intensity has varied from time to time and from place to place. THE GROWTH OF THE CONTINENT