It is highly probable that the action of water does not cease with the formation of clay, but that it slowly effects an increase in the plasticity of the clay. There thus appear to be at least three kinds of primary clay, viz.: _Kaolinic_ or _china clays_ which are chiefly derived from felspar and can be isolated in a relatively pure state. They are highly refractory, but only slightly plastic. _Epigenic_ or _colloidal clays_ derived from kaolinic clays, as a secondary product, or directly from felspar, mica, augite and other alumino-silicates by 'weathering.' They are usually less refractory and much more plastic than the china clays and contain a large percentage of impurities--sometimes in the form of free silica (sand) or of metallic oxides, carbonates, sulphides, sulphates, silicates, or other compounds. Many so-called secondary clays such as pipe clays, ball clays and fireclays may be of this type, though their origin is difficult to trace owing to their subsequent transportation and deposition. _Lateritic_ or _highly aluminous clays_, of a highly refractory character, but low plasticity. They are usually somewhat rich in iron oxide which materially affects their plasticity. Unlike the china clays, pure lateritic clays are completely decomposed by hydrochloric acid. Bauxite and some of the highly aluminous clays of the Coal Measures appear to be of this type. Unfortunately these different types of clay are extremely difficult to distinguish and in many instances they have become mixed with each other and with other materials during the actions to be described in the next chapter, that it is often almost impossible to decide whether the true clay in a given specimen possessed its characteristics _ab initio_ or whether it has gained them since the time when it ceased to be a primary clay. =Secondary clays= are those which have been produced by the action of the weather and other natural forces on primary clays, the changes effected being of a physical rather than a chemical nature (see Chapter IV). The essential constituent of secondary clays has not been positively identified. In so far as it has been isolated it differs from the true clay in the primary clays in several important respects, and until its nature has been more fully investigated great caution must be exercised in assigning a definite name to it. For many purposes the term _pelinite_ (p. 149) is convenient, being analogous to the corresponding one used for material in china clays (_clayite_, p. 107). These terms are purely provisional and must be discarded when the true mineralogical identities of the substances they represent have been established. CHAPTER IV THE MODES OF ACCUMULATION OF CLAYS From whatever sources clays may have been originally derived, the manner in which they have been accumulated in their present positions is a factor of great importance both in regard to their chemical and physical characters and their suitability for various purposes.