Geology is the study of the structure and history of the earth. It is the fundamental doctrine of modem geology that the study of the present is the key to the past, and to understand thoroughly what has taken place in past ages we must first gain an acquaintance with the earth in its present condition. The processes which are now taking place, both on land and in the sea, are the same in kind, if not in degree, as those which have always taken place since the earth first attained to something resembling its present state. Instead of treating clastic rocks first and igneous and other non-clastic rocks later, it has seemed more desirable to begin with those rocks from which clastics are largely derived, before dealing with the clastics themselves. Students, who are preparing to make geology their life work, will be very beneficial from this book. But the great majority of students of geology come to this subject only with the desire to gain some knowledge of the world they live in, of the material of which it is composed, of the forces which have fashioned it, and of the laws which have governed its development.