Abstract

Dr Garth Raybould writes: While Dr Phillips has given a lucid account of solution-transfer and its role in fibrous vein formation, which accords with the work of Durney (1972), he implies that fibre growth always takes place from the centre of the vein outwards. Raybould (1974) has described fibrous quartz veins from the Craigypistyll anticline of mid-Wales in which there is ample evidence that crystal growth took place from the walls inwards, and Durney & Ramsay (1973) have given detailed descriptions and explanations of both types of growth in veins and pressure shadows. Clearly, the growth mechanism which Dr Phillips suggests is applicable to only one type of fibrous vein. Dr Phillips attributes the formation of low-angle fractures to hydraulic lifting of impermeable beds during consolidation, and this implies that fibrous veins are concordant. Such is not always the case. Some of the Craigypistyll veins are concordant but where bedding is steep the veins become discordant in order to maintain a low dip angle. In this case the vein orientation is related to the former stress orientation within the fold. Dr Phillips suggests that fibrous veins usually show an up-dip displacement of the upper part of the vein, related to subsidence and crustal shortening during sedimentation. Conversely, the Craigypistyll veins almost always show a down dip displacement. The writer suggests that displacement may be either up-dip or down-dip, according to the position of the veins within the particular tectonic feature containing them.

Dr W.J. Phillips replies: Dr Raybould is quite

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