Earliest Known Bird Form (_Insert_) 320 18 Skeleton of Large Flying Reptile (_Upper picture_) 321 Skeleton of a Swimming Reptile of the Mesozoic Era (_Lower picture_) 321 19 Appalachians Along New River, Peneplain Upraised Again 352 20 Glacial Bowlder Left by Ice Sheet (_Upper picture_) 353 Esker, Deposited by a Stream in the Great Glacier (_Lower picture_) 353 CHAPTER I INTRODUCTION Earth features are not fixed. The person of ordinary intelligence, surrounded as he is by a great variety of physical features, is, unless he has devoted some study to the subject, very likely to regard those features as practically unchangeable, and to think that they are now essentially as they were in the beginning of the earth's history. Some of the most fundamental ideas taught in this book are that the physical features of the earth, as we behold them to-day, represent but a single phase of a very long-continued history; that significant changes are now going on all around us; and that we are able to interpret present-day earth features only by an understanding of earth changes in the past. _Geology_, meaning literally "earth science," deals with the history of the earth and its inhabitants as revealed in the rocks. The science is very broad in its scope. It treats of the processes by which the earth has been, and is now being, changed; the structure of the earth; the stages through which it has passed; and the evolution of the organisms which have lived upon it. _Geography_ deals with the distribution of the earth's physical features, in their relation to one another, to the life of sea and land, and human life and culture. It is the present and outward expression of geological effects. As a result of the work of many able students of geology during the past century and a quarter, it is now well established that our planet has a definitely recorded history of many millions of years, and that during the lapse of those eons, revolutionary changes in earth features have occurred, and also that there has been a vast succession of living things which, from very early times, have gradually passed from simple into more and more complex forms. The physical changes and the organisms of past ages have left abundant evidence of their character, and the study of the rock formations has shown that within them we have a fairly complete record of the earth's history. Although very much yet remains to be learned about this old earth, it is a remarkable fact that man, through the exercise of his highest faculty, has come to know so much concerning it.