The papers contained in Vol. 135, Part 1, represent a considerable advance in knowledge of the diagenesis of sandstones. The importance and nature of mineral reactions (e.g. replacement of feldspars, modification of clay minerals) and the role of pressure solution during diagenesis, are repeatedly discussed. However, on reading the terminal written discussions to these papers, one finds some misunderstanding about the exact meaning and nature of pressure solution. Our aim is to clarify this misunderstanding by examining the mechanism of pressure solution during the diagenesis and deformation of terrigenous sediments.

In theoretical studies (Rutter 1976, de Boer 1977), pressure solution is defined as the transfer of material by diffusion through an intergranular pore fluid phase in response to stress gradients around grains. A stress gradient gives rise to a gradient in chemical potential, and diffusive transfer of material from the point of high stress to that of low stress occurs to dissipate this gradient. The distance of diffusion will be determined by (a) the scale on which the stress gradient is developed, and (b) the kinetics of intergranular diffusion (Rutter 1976). Stress gradients may arise in sediments through (i) the generation of heterogeneous stresses at point contacts of detrital grains, leading to a variation in normal stress around a grain surface, and (ii) the formation of gradients in mean stress through a rock in relation to the regular development of sedimentary or tectonic structures (cusps, nodules, spaced cleavages, folds, etc.).

A relation between the extent of pressure solution (as recorded

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