None the less, clays are notably impure, and in many there is a large admixture of quartz sand. The kaolin, derived originally from the decay of other silicates, is rarely freed from a variety of minerals and rock-fragments that were associated with it in its place of origin. Grains of quartz and unaltered felspar a tenth of a millimetre in diameter distinctly "lighten" a clay soil, on account of their relative coarseness. A sandy clay is styled a _Loam_, and a fine-grained loam furnishes the ideal soil for the general purposes of a farmer. It does not retain water too long upon its surface, nor does it dry too quickly after rain. Much of what we call boulder-clay proves to be in reality a loam. T. Mellard Reade and P. Holland[41] have shown that even in clays of marine origin there may be a considerable proportion of very fine quartz sand. Calcium carbonate, usually occurring as fine rock-dust derived from limestone, or as minute shell-fragments, may be mingled with clay to form a _Marl_. The term is not a quantitative one, and may be applied to any clay that shows a brisk effervescence with cold acids. Though unpleasantly sticky when wet, marls flocculate themselves naturally by supplying calcium carbonate in solution to waters that pass through their crevices (see p. 80). The stratification of clays may be invisible throughout considerable masses, unless sandy beds are intercalated among them. Yet, when a lump of clay is dried and then placed in water, as previously described, it will often break up along parallel planes, which show that there is a regular arrangement of its particles. The fact that so many of these particles are platy becomes emphasised under the pressure of subsequent sediments, whereby the platy surfaces of the particles are brought into planes parallel with one another. The clay then becomes a _Shale_, with regular planes of fissility, which are parallel to those of bedding. A certain amount of deformation of the rock accompanies this change, flow being set up parallel with the bedding, and included fossils becoming sometimes flattened. This deformation is especially noticeable in the case of plant-remains. Shales may in time attain the density and fissile structure of true slate. The colours of clays and shales are of considerable interest. Blackness is often due to organic matter, and especially to fragments of plants, which retain their woody structure and their carbonaceous character when protected by clay from oxidation.