Dr W. J. Phillips writes: In his recent paper Dr Walker (1975a) adopted Bradley's (1965) concept of isopiestic surfaces and derived theoretical isopiestic surfaces of lithostatic pressure for a model in which "a body of low density (2.3 g cm−3) acid magma is emplaced in rocks of appreciably higher density" (2.6 to 2.85 g cm−3), but is underlain by basic intrusive rocks of density, 3.0 g cm−3 (Walker 1975a Fig. 3a). Walker's hypothesis for the formation of cone sheets is based on the assumption that isopiestic surfaces of lithostatic pressure derived from this model may be superimposed on another set of theoretical isopiestic surfaces (Fig. 3b) which is derived for "the hydrostatic pressure of a basaltic magma of density 2.65 g cm−3” if it occupied the space of the acid diapir and basic intrusive rocks of the first model. Clearly the superposition of the two sets of isopiestic surfaces, Fig. 3c, is incorrect because the space can be occupied by only one of the two alternative components of the model and therefore the supposed isopiestic surfaces of the other component would be absent. This restriction invalidates the concept of excess hydrostatic pressure and consequently also the supposed new concept of the evolution of the British Tertiary intrusive centres. However it is of interest to consider a number of other points which give cause for concern.

In general, basaltic magma does not penetrate the pore spaces of the envelope rocks and therefore the isopiestic surfaces derived for the hydrostatic pressure. in

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