Variations in the water content and fabric of clay, and the porosity of sand during compaction under pressures of 0-100 kg./cm.² reflect the influence of sediment texture and composition, and are not related uniquely to increases in overburden load.

The porosity and water content of sand, silt, and clay under these pressures are inversely related to particle size. This relation commonly is strong enough to obscure the expected decrease in porosity with increasing depth of burial.

Variations in the water content of saturated clay reflect the physico-chemical influences on the sorption of water on the surfaces of clay minerals. Water content and surface area per unit mass in the common clay minerals increase in the order kaolinite-illite-montmorillonite. In montmorillonite, at pressures less than 50 kg./cm.², the water content changes with the exchangeable cation--Na-montmorillonite holds more water than montmorillonite whose exchange positions are filled with Ca, Mg, K, or Al. Increasing concentrations of interstitial electrolyte tend to increase the water content of most clay at a given pressure less than 50 kg./cm.², presumably by increasing the tendency of the clay particles to form open-work flocculated aggregates that resist compactio . The main exception to this is in very fine-grained clay saturated with Na electrolytes in concentrations less than 0.3 molar.

Most of the development of preferred orientation in clay compacted under pressures of 0-100 kg./cm.² takes place very early--at pressures near 1 kg./cm.² The most critical factor for this development may be the amount of water held by the clay. If enough water is present, the particles may slip past one another into preferred positions; if not, preferred orientation may be either poorly developed

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or absent. Large amounts of carbonaceous matter seem to enhance the development of preferred orientation, perhaps by its ability to retain large amounts of water. Specific effects of other factors on clay fabric are less well known, but one might expect the development of preferred orientation to be enhanced by decreasing concentrations of interstitial electrolytes, decreasing valence of exchangeable cations, and decreasing acidity.

The main influences on the porosity of sand are the textural characteristics of the constituent particles: size, sorting, roundness, shape, and flexibility. Well-sorted sand has greater porosity than poorly sorted sand. Angular sand has greater initial porosity and is more compressible than rounded sand of the same size. Admixtures of platy mica particles increase the porosity, compressibility, and elasticity of sand.

The influence of most of these factors--except for particle size--is inferred from laboratory experiments on simple systems. Little is known of their relative importance in complex natural sediments.

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