A study of the clay minerals is in progress and while not complete is yielding information about some of the fundamental minerals of sedimentary materials and the conditions that favor their formation. There are several kaolin minerals, but kaolinite is the only one that is likely to be found in sedimentary beds. Clays of the heidellite, montmorillonite, or other types are, however, more widespread in soils and shales than kaolinite. Calculations based on the assumption that kaolin is the typical clay-forming mineral are commonly in error. The relationship between geologic processes and the formation of different types of clays is outlined; and also the way in which geologic conditions may be reflected by base exchange.