Although the great diversity of alkaline rocks, with their tion of alkaline rocks and carbonatites with rifting has long been acknowledged. However, many large, diverse relatively exotic rnineralogies. has always attracted the interest of petrologists, as have the more recently defined and important provinces are not widely known, includ­ carbonatites, it could be argued that little progress has ing the 20 or so major provinces of the U. S. S. R. and some of the major concentrations of South America. been made over the past 50 years towards formulating a comprehensive petrogenesis of these rocks. It could Although alkaline rocks are known to be characteristic also be maintained that as the alkaline varieties have the of stable, intra-plate environments, it is becoming clear most extreme compositions of all igneous rocks, so an that they also play an important role in the igneous activity concentrated at plate margins. In this respect understanding of their genesis is essential if we are to understand fully the workings of the solid earth, while also, therefore, it is concluded that thecollation of all the available regional data can make a significant contribu­ a knowledge of the most extreme products of differentia­ tion must inevitably cast light on rocks of less extreme tion to understanding these rocks. compositions. The importance of academic research on Sorensen (1974) in the Preface to 'The Alkaline Rocks' these rocks is thus clear.