Ever since the publication of Seger's memorable papers (7), and to a small extent before that time, it has been generally understood that china clay or kaolin represented the true essential constituent of clays, but several investigators have been so imbued with the idea that all true clay substance must have a crystalline form that they have frequently used the term 'kaolinite' to include the 'amorphous' substance in plastic clays. This is unfortunate as it is by no means proved that the latter is identical with kaolinite, and a distinctive term would be of value in preventing confusion. Other investigators have used the word 'kaolin' with equal freeness, so that whilst it originally referred to material from a particular hill or ridge in China[13] it has now entered into general use for all clays whose composition approximates to that of china clay (p. 16) in which the plasticity is not well developed. Thus, in spite of the difference in origin between many German and French kaolins and the china clays of Cornwall, it is the custom in Europe generally to term all these materials 'kaolin.' Yet they are very different in many respects from the material originally imported from China. [Footnote 13: _Kao-ling_ is Chinese for a high ridge or hill.] As the essential clay substance has not yet been isolated in a pure form from the most widely spread plastic clays, but is largely hypothetical as far as they are concerned, the author prefers the term _pelinite_[14] when referring to that portion of any plastic clays or mixtures of clays with other minerals which may be regarded as being the constituent to which the argillaceous portion of the material owes its chief properties. In china clay and kaolin the 'true clay' is identical with clayite--or even with kaolinite (p. 108)--and there is great probability that this identity also holds in the case of the more plastic clays of other geological formations, but until it is established it appears wisest to distinguish the hypothetical or ideal clay common to all clays (if there is such a substance) by different terms according to the extent to which its composition and characters of the materials most closely resembling it are experimentally known. [Footnote 14: From the [Greek: pêlinos] = made of clay.] The substances most resembling this 'ideal clay' which have, up to the present been isolated, are: (_a_) _Kaolinite._ Found in a crystalline form in china clays and kaolins (p. 107). (_b_) _Clayite._ A material of the same chemical composition as kaolinite, but whose crystalline nature (if it be crystalline) has not been identified--chiefly obtained from china clays and kaolins. (_c_) _Pelinite._ A material similar to clayite, but differing from it in being highly plastic and, to some extent, of a colloidal nature--obtained from plastic clays. (_d_) _Laterite._ A material resembling clayite in physical appearance, but containing free alumina and free silica (p. 80).