Earth history is a subject of fascinating interest and also of much practical value. The various features of the earth, the continents and oceans, the mountains and plains, and the multitudinous assemblage of organisms in the waters, on land, and in the air have not always been as they are today. The orderly succession of rock strata and their innumerable contained relics of strange animals and plants were not made to mystify man, nor were the ores of metals, deposits of petroleum, and other useful earth materials hidden away merely to test man's ingenuity in finding them. Rather, all these things are the product of events and conditions in the history of our planet. To know something of the probable conditions of earth origin, the almost inconceivable antiquity of the earth, the evolution of the continents, the elevation and obliteration of great mountain chains, and the remarkable record of life on the earth in past ages is to grow in understanding and appreciation of the modern world. And to acquire such appreciation is a worthy end of study. The student of historical geology, moreover, finds in this subject special opportunity for training in clear thinking, in the scientific con sideration of numerous complex problems, and in reasoning from evidences or effects to the causes that produced them. An account of earth history that narrated accurately the changing conditions and events of past geologic time, but largely omits the basic observations on which the narrative depends, may hold elements of interest. The instructional value of such an account, however, is surely very far short of one in which many observational data are given and in which emphasis is laid on the deductive interpretation of these data. From the standpoint of scientific training, the means of arriving at conclusions concerning earth history are much more important than the conclusions themselves. With this in mind, the writer has undertaken in the following pages first to describe selected items of observation in connection with the geologic record and then to consider the interpretation of these items in terms of history. Uncertainties and unsolved problems are so indicated. Maps representing the distribution of sea and land at various times in the geologic past paleogeographic maps are largely omitted because such maps are for the most part highly subjective, the data used in constructing them are generally not evident, and the sometimes very small areas of reason able control are not differentiated from the uncontrolled areas. Maps showing actual distribution of the systems and of the rocks of respective eras are useful, however. These are used here and are accompanied by numerous graphic representations of typical geologic sections that show the nature and thickness of rock formations. Historical geology is a subject of some difficulty. This is due partly to its encyclopaedic scope in space and time and the breadth of its contacts with the related fields of astronomy, physical geology, physiography, biology, and others, and partly to its profusion of unfamiliar names that designate divisions of geologic time, rock formations, and fossils. These difficulties, more apparent than formidable, cannot wholly be avoided, and it is easy to understand that, if misplaced emphasis is laid on the learning and cataloguing of a jumble of names, historical geology becomes indeed dry and uninteresting.