I write to support and amplify the conclusions (p. 54) of this author concerning the fabric of African red clays and Keuper Marl. It is clear that:

(1) The properties of undisturbed engineering soils may differ from those of a simple mixture having the same composition as the soil.

(2) The differences may arise because the mineral grains in the soil are not in random positions, but have a structure within the soil resulting from the sedimentation, chemical and stress history.

(3) The structure may be physically stable, or preserved in the soil by cementing agents derived from the chemical history. Crystalline cements may be detected optically or by x-rays, but fine amorphous cements may be detected by a chemical/mineralogical mismatch as shown by Sherwood for Keuper Marl.

(4) Sherwood has also shown that where cementing agents are present, the normal soil classification tests may give misleading results, and research based on soil chemistry may be needed.

May I add that research based on soil physics leads to similar structural conclusions for African red clays. Thus, their reflexion spectrum has been used (Coleman 1965) to show that the colouring agent (disordered alpha ferric oxide in one instance) is present mainly as a particle coating, and moisture-sorption studies suggest that the coating is porous, so that the soils may have a moisture content in the field as high as 30 per cent.

J. D. Coleman

Soil Physics Laboratory, Department of Civil Engineering, The City University, London e c

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