To fulfil these conditions would require a work of such great magnitude that it could not be left to individual enterprise yet, notwithstanding the large number of people interested in the subject, the great and important industries dependent upon it, and the amount of information in existence, no book has yet been published in which British clays receive their proper share of attention. Usually these substances occupy a subsidiary position in works on geology or on brick or pottery manufacture, and their importance is, consequently, overlooked. This is, in part, due to the inherent difficulty of the subject and to its extent, for though clays are found in almost every part of the British Isles, their true constitution is, even now, not known with certainty, and the characteristics of many of them require much more investigation than they have hitherto received.