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SYNOPSISAnalyses are presented of the b 0 -spacing value of almost 200 potassic white micas from the low temperature zone of the Scottish Caledonides. These measurements, which provide a relative geobarometer, indicate a gradual transition in the facies series of metamorphism into the area of 'Buchan' metamorphism. The use of this method to accurately define the shape of isobaric surfaces is suggested. The implications of the results are briefly discussed. I N T R O D U C T I O NTwo contrasted and classical styles of regional metamorphism the 'Barrovian' and the 'Buchan', are developed within the metamorphic Caledonides of the Scottish Highlands. Barrovian metamorphism is characterised by an extensive chlorite-zone in the lowest grade areas, and by the occurrence of almandine, kyanite and sillimanite at successively higher grades; it falls within the kyanite-sillimanite facies series (Miyashiro 1961) and the grade rises from low greenschist facies in the south and south-west Highlands to upper amphibolite facies in the central Highlands. Buchan metamorphism (Read 1952) has been referred to the low pressure intermediate facies series (Miyashiro 1961), ranging also from greenschist facies to upper amphibolite facies, but distinguished by the occurrence of andalusite and cordierite in higher-grade zones, and biotite-bearing pelites in the low-temperature area.It is not possible to distinguish chronologically the Buchan event from the Barrovian one. Johnson (1962) showed that the main crystallisation of both occurred at the same point in the Caledonian deformational cycle. Moreover, the radiometric data suggest that the climax of metamorphism was synchronous across the southern Caledonides (Pankhurst and Pidgeon 1973) at least within an experimental error of 10-20 m.y.The boundary normally taken between the Buchan and Barrovian provinces is the kyanite/andalusite inversion (Chinner 1966;Porteous 1973). Other significant changes do not coincide with this boundary: for example, garnet and staurolite occur well within the andalusite zone (Porteous 1973, fig. 1). The distribution of cordierite, staurolite and garnet, with garnet and staurolite relatively more abundant in the south and west of the Buchan area (Porteous op. cit.), could suggest a transition from the intermediate Scott.