As explained in Chapter III, the china clays or kaolins may usually be regarded as primary clays derived from granitic or other felspathic or felsitic rocks by chemical decomposition. Such clays are found near to their place of origin, are usually obtainable in a comparatively pure state and are generally deficient in plasticity. They may occur in beds of small or great depth, but these are not 'accumulations' in the ordinary meaning of that term. Residual clays (p. 70) also form a distinct class, as unlike the majority of argillaceous materials they are left behind when other substances are removed, usually by some process of solution. In many cases, however, the residual clays are really secondary in character, having been transported from their place of origin, together with limestone or other minerals, the mixture deposited and subjected to pressure and possibly to heat, whereby a rock-like mass is formed. This mass has then been subjected to the solvent action of water containing carbon dioxide or other substances which dissolve out the bulk of the associated minerals and leave the residual clay behind. The chief agents in the transport and accumulation of clays are the _air_, in the form of wind; _water_, in the form of rain, streams, rivers, floods, lakes and seas, or in the form of ice and snow as in glaciers and avalanches; _earth-movements_ such as the changes wrought by volcanoes, earthquakes and the less clearly marked rising and falling of various portions of the earth's crust which result in folded, twisted, sheared, cracked, inclined, laminated and other strata. These agents have first moved the clay from its original site and have later deposited it with other materials in the form of strata of widely varying area and thickness, some 'clay' beds being several hundred feet in depth and occupying many square miles in area, whilst others are in the form of thin 'veins' only a few inches thick or in 'pockets' of small area and depth. These deposits have in many places been displaced by subsequent earth-movements and have been overlain by other deposits so as to render them quite inaccessible. Others have been covered by deposits several miles thick; but the greater part of the covering has since been removed by glacial or other forces, so that clays of practically all geological ages may be found within the relatively small area of Great Britain. The Transportation of Clays.