Dr B. J. Walton said that Dr Oxburgh appeared to imply that convection currents were likely to be responsible for plate movements. This did not seem to be necessary if, for example, a lithospheric dome developed by radioactive heating at the base of a large initially stationary area of continental crust, with the result that the lithosphere slid off the dome under its own weight, and started to descend into the lighter asthenosphere at subduction zones. In this case the flow through the asthenosphere would be from the trench to the oceanic ridge, opposite to that generally envisaged for convection currents.

Dr R. B. McConnell referred to the outflow of heat at the mid-ocean ridges referred to by Dr Oxburgh and asked whether he thought that the present plan of the world-wide rift system was due to an original pattern of convection cells in the Earth’s mantle, as had been formerly suggested, or whether it was not more likely that the heat produced in the mantle by radioactive decomposition found its outlet by making use of segments of a pattern of deep dislocations imposed on the Earth in the early Precambrian? It had been pointed out by several authors that some of the ancestral pre-drift fractures, which were to develop into spreading oceans, can be seen to continue into the continents as deep faults which had been shown in some instances to be resurgent Precambrian lineaments, some segments of which might have served as outlets for convection heat and developed

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