tion of the importance of trace fossils in shales, the discovery of thick sequences of overpressured shales in regions such as the Gulf Coast (which have important implications for hydrocarbon migration and faulting), the ex­ tension of the principles of metamorphic facies to the realm of low tempera­ ture diagenesis by study of the organic matter in shales, and shales as ul­ timate sources for mineral deposits. Accordingly, we decided it was timely to write a book on shales. In one respect, however, ours is an unusual book. Most books in geology are produced after one or two decades of progress have been made in a field and attempt to summarize and evaluate that progress.