The natural "flocculation" of clays, the process by which compound grains are formed in place of individual soil-particles, is assisted by the action of water bearing certain salts in solution. Calcium carbonate is an excellent flocculator, and this fact has long led farmers to place burnt lime or powdered limestone on their lands. Sodium carbonate, on the other hand, is brought up in some dry regions by capillary action, and exercises a reverse effect, keeping the minute particles apart from one another, and thus promoting thorough clayiness in the clay. Experiment has shown that fineness of grain is responsible for most of the characters of a clay, and from this point of view the small size of kaolin flakes as compared with grains of other minerals will account for the "clayiness" of this particular mineral when it constitutes a rock. Clays, however, when shaken up in a column of distilled water, cause what seems to be a perpetual cloudiness, since it remains after the great bulk of the clay has settled down. Flocculation by salts alone removes it. Some authors have urged that a colloid substance, amounting perhaps to only one or two per cent. of the whole clay, imparts this distinctive character. Such colloids are believed to arise during the decomposition of aluminous silicates under tropical and probably alkaline influences; but they are not known to be associated with the processes by which kaolin is formed from felspars. [Illustration: Fig. 8. Shrinkage-cracks in Clay, with footprints of birds in the foreground. Tundra of Mimer Bay, Spitsbergen.] A. D. Hall[40] points out that the cloudiness is probably due to the extreme minuteness of certain of the particles. True clayiness thus depends on the proportion of grains smaller than ·002 mm. in diameter. Yet Hall and Russell look to other causes to explain the continued suspension of such particles in the water, and they suggest the presence of potassium and sodium silicates of the zeolite group, which liberate by hydrolysis a little alkali in contact with a large bulk of water. Free alkalies prevent flocculation, and so encourage suspension of the particles. To the ordinary observer, a rock possesses the properties of clay, and is a clay, if it contains more than forty per cent. of particles less than ·01 mm. in diameter. But such rocks are found, on chemical analysis, to contain a large amount of kaolin, and the old view, that clays are massive kaolins, is thus substantially correct.