Some further sub-division is necessary for special purposes, particularly in sections _e_, _f_ and _h_, but to include further details would only obscure the general scheme. Some clays will, apparently, be capable of classification in more than one section, thus a vitrifiable clay may owe its characteristic to a high proportion of calcium carbonate and so be capable of inclusion as a calcareous clay. Broadly speaking, however, if the clay is tested as to its inclusion in each section of the scheme in turn it will be found that its highest value will be in the section which is nearest to the first in which the clay can legitimately be placed. From a consideration of a classification such as the foregoing, together with a detailed study of the physical and chemical properties of the material as a whole, and also of the various portions into which it may be divided--particularly that which has been isolated by mechanical methods of purification and separation--it is not difficult to gain a fairly accurate idea of the nature of any clay. Although the present state of knowledge does not permit them to be classified with such detail as is the case with plants, animals, or simple chemical compounds, the study of clays and the allied materials has a fascination peculiarly its own, not the least interesting features of which are those properties of the clay after it has been made into articles of use or ornament. These are, however, beyond the scope of what is commonly understood by the term 'the natural history of clay.' BIBLIOGRAPHY A complete bibliography of clay would occupy several volumes. The following list only includes the more accessible of the works quoted in the text. 1. "Second Report of the Committee on Technical Investigation--Rôle of Iron in Burning Clays." Orton and Griffith. Indianapolis. 1905. 2. "British Clays, Shales and Sands." Alfred B. Searle. Charles Griffin and Co. Ltd. London. 1911. 3. "Transactions of the English Ceramic Society." v. p. 72. Hughes and Harber. Longton, Staffs. 1905. 4. "Royal Agricultural Society's Journal." XI. 5. "Die Tone." P. Rohland. Hartleben's Verlag. Vienna. 1909. 6. "Clays: their Occurrence, Properties and Uses." H. Ries. Chapman and Hall. London. 1908. 7. "Gesammelte Schriften." H. Seger. Tonindustrie Zeitung Verlag. Berlin. 1908. 8. "Tonindustrie Zeitung." 1902. p. 1064. 9. "Tonindustrie Zeitung." 1904. p. 773. 10. "Treatise on Ceramic Industries." E. Bourry (Revised translation by A. B. Searle). Scott, Greenwood and Son. London. 1911. 11. "The Colloid Matter of Clay." H. E. Ashley. U.S.A. Geological Survey Bulletin 388. Washington. 1909. 12. "Sprechsaal." 1905. p. 123. 13. "Action of Heat on Refractory Materials." J. W. Mellor and F. J. Austen. Trans. Eng. Cer. Soc. VI. Hughes and Harber. Longton, Staffs. 1906.