Professor W. S. Pitcher asked whether account had been taken of possible variations in water content through the rocks concerned and whether the possibility of metasomatic homogenization in the centre of domes could be discounted.
In reply to Professor Pitcher the Author admitted that chemical homogenization could not be entirely discounted.
Dr E. R. Oxburgh congratulated Dr Talbot on a most interesting field study and agreed that the application of the principles of fluid dynamics to the problem of gneiss domes seemed highly appropriate. It was, however, extremely important that this be done with care and rigour. In the present case it appeared from the oral presentation that there were some serious lapses (although some of these might have been clarified in the manuscript).
The author first introduced the Rayleigh number, R = αβl4g/vk, as providing a possible stability criterion. But having done so, he did not apply it as such, but assumed the situation to be unstable and proceded to use the Rayleigh equation to derive the viscosity, by assigning values to all other variables. Having thus declined to employ the one theoretical stability test available the author nevertheless found himself able to assign a value to R in order to derive a viscosity. Even if instability had been established there was no way of ascertaining the value of R other than by use of the Rayleigh equation; it could only be stated that R exceeded some critical value, probably of the order of 103. It could, however, have