Abstract

Dr S. H. Treagus writes: Cosgrove (1976) has attempted to explain the variety in form of crenulation cleavage in terms of two main processes: instabilities of different types initiating in a stressed multilayer or fabric, and the development of these structures into zones or planes by mineral migration. These form the two main sections to his paper. It is the first section, that concerned with the buckling behaviour of anisotropic materials, which I wish to criticise.

The theory presented by Dr Cosgrove, and the basis of the first section of his paper, is Biot’s (1964) analysis of instabilities in the confined multilayer. The multilayer can be considered statistically as an anisotropic homogeneous material (Biot 1965 p. 186, Cobbold et al. 1971). Biot showed that two kinds of instability were solutions to the general displacement equation, and were governed by particular conditions relating to initial stress and elastic moduli (Cosgrove 1976 equations 7-10). The first type of instability is internal sinusoidal buckling, and the second Biot illustrates as a plane of slip or kinking. Geologists (Cobbold et al. 1971, Johnson 1970, Cosgrove 1976) have used Biot’s theory to explain the initiation of two distinct structures, buckles and kink bands, in compressed multilayered rocks. It will be shown that in his recent paper Dr Cosgrove has used the two instability solutions incorrectly to explain structures in a variety of types of anisotropic material.Cosgrove (1976) considers three orthotropic models of anisotropy, in the form of fine layering, at 0°, 45° and 90°

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