This book is intended for the use of advanced students research workers and teachers in the field of petrology. Its aim is to present a unified general impression of the origin and evolution of rocks that are generally believed to have crystallized or to have been profoundly modified at high temperatures and at pressures such as prevail from the earths surface to a depth of perhaps 20 km. This impression must necessarily be marred and blurred by omissions and misinterpretations on the part. Of the authors for it is based on data (of field association mineralogy chemistry-end fabric of the rocks themselves) too voluminous to be grasped and adequately handled by one or even two students. More-over its limitations reflect the prejudices and necessarily restricted geological experience of two individuals.