Dr P. E. Kent welcomed this presentation of detailed maps of the Suez rift valley system, which are directly relevant to interpretation of the mechanism of rift development. The irregular arrangement of fractures, with major faults fingering out into a series of small breaks, is characteristic of the eastern side of Africa. In both Tanzania and in Madagascar the largest faults, bounding the Karroo-Mesozoic troughs, die out in precisely the same way, and show similar evidence of development in association with the subsidence of the adjoining sedimentary basins. The direct development of the surface faulting is clearly by vertical movement, with lateral shift excluded as a serious factor, and is possibly related to crustal stretching.

The Author was glad to have confirmation from Dr Kent that the patterns which had been established by mapping in the Clysmic rift had their counterparts in East Africa. It was an indication that the same kind of processes which had produced the Clysmic rift had also been at work elsewhere. There was still much speculation about the nature of those processes—for example, why the tilted blocks were so often bordered each by a great crescentic fault, why antithetic faulting seemed to be associated with downwarping, or why in some parts of the Rift system there was an abundance of igneous activity, while in other parts like the Clysmic rift, it had been on a muted scale. Perhaps some of those problems could be solved by close collaboration between geologist and geophysicist over a

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