The subject of petrology has undergone a revolution within the last decade as a result of the recently developed awareness by geoloaiats of the role of plate tectonics in petrogenesis. Field examinations have included studies-by means of deep sea cores dredge samples and submersibl~f igneous rocles formed at mid-ocean rifts. The distribution and origin of igneous and metamorphic rocks at convergent plate junctions have been more closely defined in terms of plate tectonics. The occurrence of sedimentary materials can now be related to plate tectonic regimes. Laboratory studies are now able to duplicate most of the conditions upon and within the earth. Mineral syntheses have defined the general pressure temperature and compositional limits within which most of the common rock-forming minerals can exist. Isotopid studies can now furnish a date of formation of igneous and metamorphic rocks as well as the temperatures at which many organisms lived in sedimentary environments. In addition improved classification of igneous and sedimentary rocks now yields more meaningful subdivisions and is more closely related to the method of origin than were earlier systems. The aim has been to Present a summary of the more significant portions of both the older and the more recent literature at a level appropriate to the college sophomore or junior who is exposed to a first course in petrology. Literature sources are given at the end of each chapter for those who wish to pursue some of the subjects presented. It is assumed that the student has an understanding of elementary chemistry crystallography and mineralogy as well as some facility with the petrographic microscope (although this is not critical to an understanding of the general concepts).
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