Let us take for example the fourth analysis under the table of kaolins (§ 17), which shows 39.7% of alumina; 39.7X2.53=100.4%, or the material is practically pure kaolin. Again No. 5, under brick clays, shows 11.07% alumina; 11.07X2.53=28%, or only 28% óf the mass is true clay. If we consider the last analysis under ball clays, we have alumina 45%. 45X2.53=113.85%, or the alumina present represents an amount of kaolin 13.85% greater than the whole mass. This proves that the clay is in part composed of some compound richer in alumina than kaolin, as gibbsite, pholerite, etc. Further study of the tables in this way will be found instructive.
If the percentage of alumina in any such analysis be multiplied by 1.176, the result will be the percentage of silica, which combined with this alumina would form kaolin. If this factor be used with the tables it will be found that in quite a number of instances the silica shown in the analysis is less than that required for kaolin, which again proves the presence of some compound richer in alumina than kaolin. In the great majority of instances, however, the amount of silica found is less than that given in the analysis. This may be explained either by assum ing the presence of free silica, or that of a compound poorer in alumina than kaolin, as cimolite. Usually the first of these explanations is the true one, but the second is occasionally, we do not know how frequently, true also.
There seems little doubt that the presence of compounds richer or poorer in alumina than kaolin will, in large measure, account for differ ences in behavior of clays whose analyses show similar compositions. This cannot be demonstrated until we know more about the properties of these compounds.
Agents which Aid in the Decomposition of 19. In § 12, 13, 14, 15 we have shown how hard granitoid rocks may be decomposed, by earth-water and the acids which it contains, to form an earthy mass called clay. This process is greatly aided by disintegration produced by
alternation of heat and cold, freezing of water, mechanical action and ef fect of organisms. When rocks are heated they expand; when cooled they contract. This movement loosens the grains and allows water to enter more easily. When water in pores or cracks freezes and expands, it tends to break up the rocks or at least to enlarge the openings. Mechan ical force in bending, compressing, or stretching rocks produces strain surfaces, cleavage planes, joints and fractures, and occasionally pul verizes rocks, all of which aids the circulation of water, and so decom position. Mechanical force, by bending, breaking, compressing or stretching the rocks, also raises their temperature in the areas in which it operates, and this heat is imparted to the water and makes it more active. Heated rocks often liberate caustic acids, as those of sulphur, boron, fluorine, etc., which unite with water and are transmitted by fractures to distant rocks, where they effect marked changes. Many de posits of kaolin and poorer clays doubtless owe their origin principally to these gases.
Plant roots when small enter crevices in rocks, and as they increase in size act as wedges to widen the cracks. They also have the power to some extent to eat their way into rocks and, enlarging, force off spalls. When organic matter decays, gases are produced which are taken up by water and aid in decomposition.
Depth of Deposits of Residual Clays.—§ 20. The depth to which the decomposition of crystalline rocks into clays may be carried has never been determined. It depends on many local conditions. Numerous in stances are on record where the resulting clays showed a thickness of from 50 to 200 ft. This could only occur, however, in regions where the drainage was excellent and where the surface was protected from erosion.