=Residual clays= may be formed by the simple removal of other materials, the clay remaining behind, as in the decomposition of some argillaceous limestones, in which the calcareous matter has been removed by solution whilst the clay is unaffected. Such a clay is not a primary one as it has probably been derived from some distant source and, having been deposited along with the limestone ooze, has formed an intimate mixture from which the limestone has, at a later geological epoch, been removed in the manner indicated. Residual clays are seldom pure, being often rich in iron compounds, though the white clays of Staffordshire and Derbyshire are highly refractory. It is seldom necessary to distinguish residual clays from other secondary or transported ones (Chapters II and IV). =Primary clays=, on the contrary, have been derived from rocks which have undergone chemical decomposition, one of the products being clay. The most important primary clays are the kaolins, which are derived from the decomposition of felspar, but other primary clays derived from other minerals are known, though less frequently mentioned. The _kaolins_ are primary clays[10] formed by the decomposition of felspar and occur in many parts of the world. In Great Britain the most important are the china clays found in Devon and Cornwall, which occur in association with the granite from which they have been formed. The kaolins in Germany are, apparently, of similar origin, though some are derived from porphyry and not from granite; they are the chief material used in the manufacture of Dresden, Meissen, Berlin and other porcelains. The French kaolins from St Yrieux and Limousin are said by Granger (17) to be derived from gneiss amphibole. The American kaolins have, according to Ries (6), been chiefly formed from the weathering of pegmatite veins, but the origin of some important deposits in Texas and Indiana has not yet been fully explained. [Footnote 10: Some kaolins in central Europe appear to have been transported and of secondary origin.] [Illustration: Fig. 12. China clay pit belonging to the North Cornwall China Clay Co. (_By courtesy of W. H. Patchell Esq._)] The corresponding material used by the Chinese for the manufacture of porcelain bears a name which is really that of the place from whence it was originally obtained; the term _Kao-ling_ indicates merely a high ridge. According to Richthofen (18) the rock from which Chinese porcelain is made is not a true kaolin, but is allied to the _jades_. The term 'kaolin' is therefore a misnomer when applied to white-burning, primary clays generally, but its use has become so firmly established as to render it permanent.