The net result of the processes above described Is a rock powder containing a great variety of sizes of soil grains intermingled with clay. The larger soil grains are called sand; the smaller, silt, and those that are so small that they do not settle from quiet water after 24 hours are known as clay. Clay differs materially from sand and silt, not only in size of particles, but also in properties and formation. It is said that clay particles reach a degree of fineness equal to 1/2500 of an inch. Clay itself, when wet and kneaded, becomes plastic and adhesive and is thus easily distinguished from sand. Because of these properties, clay is of great value in holding together the larger soil grains in relatively large aggregates which give soils the desired degree of filth. Moreover, clay is very retentive of water, gases, and soluble plant-foods, which are important factors in successful agriculture. Soils, in fact, are classified according to the amount of clay that they contain. Hilgard suggests the following classification:-- Very sandy soils 0.5 to 3 per cent clay Ordinary sandy soils 3.0 to 10 per cent clay Sandy loams 10.0 to 15 per cent clay Clay loams 15.0 to 25 per cent clay Clay soils 25.0 to 35 per cent clay Heavy clay soils 35.0 per cent and over Clay may be formed from any rock containing some form of _combined silica _(quartz). Thus, granites and crystalline rocks generally, volcanic rocks, and shales will produce clay if subjected to the proper climatic conditions. In the formation of clay, the extremely fine soil particles are attacked by the soil water and subjected to deep-going chemical changes. In fact, clay represents the most finely pulverized and most highly decomposed and hence in a measure the most valuable portion of the soil. In the formation of clay, water is the most active agent, and under humid conditions its formation is most rapid. It follows that dry-farm soils formed under a more or less rainless climate contain less clay than do humid soils. This difference is characteristic, and accounts for the statement frequently made that heavy clay soils are not the best for dry-farm purposes. The fact is, that heavy clay soils are very rare in arid regions; if found at all, they have probably been formed under abnormal conditions, as in high mountain valleys, or under prehistoric humid climates.