A classification based exclusively on the composition of clays is equally unsatisfactory for other reasons, the chief of which is the placing together of clays of widely differing physical character, and the separation of clays capable of being used for a particular purpose. To some extent the latter objection may be disregarded, though it is of great importance in considering the commercial value of a clay. Classification based on the uses of clays of different kinds has been suggested by several eminent ceramists, but is obviously unsatisfactory, particularly as it is by no means uncommon to use mixtures of clays and other minerals for some purposes. Thus stoneware clays must be vitrifiable under conditions which may be defined with sufficient accuracy, but many manufacturers of stoneware do not use clays which are naturally vitrifiable; they employ a mixture of refractory clay and other minerals to obtain the material they require. A classification based on the origin of clays regarded from the petrological point of view offers some advantages, but is too cumbersome for ordinary purposes and suffers from the disadvantage that the origin of some important clays is by no means clearly known. The author prefers a modification of Grimsley's and Grout's classification (31) as follows: I. Primary clays. (_a_) Clays produced by 'weathering' silicates--as some kaolins. (_b_) Clays produced by lateritic action--very rich in alumina, some of which is apparently in a free state. (_c_) Clays produced by telluric water containing active gases (hypogenically formed clays)--as Cornish china clay. II. Secondary clays. (_d_) Refractory[16] secondary clays--as fireclays and some pipe clays. (_e_) Pale-burning non-refractory clays--as pottery clays, ball clays and some shales. (_f_) Vitrifiable clays--as stoneware clays, paving brick clays. (_g_) Red-burning and non-refractory clays--as brick and terra-cotta clays and shales. (_h_) Calcareous clays or marls, including all clays containing more than 5 per cent. of calcium carbonate. III. Residual clays. (_i_) Clays which have been formed by one of the foregoing actions and have been deposited along with calcareous or other matter but, on the latter being removed by subsequent solution, the clay has remained behind--as the white clays of the Derbyshire hills. [Footnote 16: A refractory clay is one which does not soften sufficiently to commence losing its shape at any temperature below that needed to bend Seger Cone 26 (approximately 1600°C.) (see p. 116).]