Norman L. Bowen (1887-1956) is widely regarded within the petrological community as the premier igneous petrologist of the twentieth century. Bowen was arguably the first igneous petrologist to channel virtually his entire career toward the exposition of a comprehensive theory of the origin of gneous rock diversity. In dozens of lucidly written and tightly reasoned articles in which lines of argument and conclusions were driven home by clear illustrations, Bowen supported his theory with a wealth of quantitative experimental data, field evidence, and petrographic evidence, and he grounded the theory in sound principles of physical chemistry. The goal of this book is to trace in detail the course of those developments within the context of the various factors influencing that course.