In this book the task of summarizing modern petrology from the genetic standpoint has been attempted. The scale of the work is small as compared with the magnitude of its subject, but it is nevertheless believed that the field has been reasonably covered. In conformity with the genetic viewpoint petrology, as contrasted with petrography, has been emphasized throughout. It is hoped that the book will appeal to students of geology who have acquired an elementary knowledge of the science, and who are going on to more advanced work; to advanced students and post-graduate researchers; and to workers in other branches of geology who desire a conspectus of the present state of the science of petrology. Further more it is believed that the book will prove useful to workers engaged in technical and applied aspects of geology, in all of which some knowledge of rocks in general in fundamental.