A Contribution to the Geology of the Kavirondo Rift Valley

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From Mr. J. Hospers :— In Professor Shackleton's paper, as well as in Dr. Dixey's paper (1946) on “ Erosion and Tectonics in the East African rift system,” mention is made of Dr. Bullard's gravity measurements (Bullard 1946). Bullard found that the plateaus surrounding the rift valleys are very nearly isostatically compensated, but that over the rift valleys large negative isostatic anomalies occur. Not only are these rift valleys uncompensated, but there is even more light matter under them than if they were simply cut out of the plateau, leaving the compensation unchanged. Under the rift valley there is therefore an excess of light matter and the upward hydrostatic forces will, according to Bullard, exceed the downward pull of gravity; the floors of the rift valleys will therefore rise unless held under by some other force. Consequently, this settles the argument in favour of the compression theory, a wedge-shaped block (widening downwards) being forced down between reversed faults. The crucial point in this reasoning is that if a part of the crust is not in isostatic equilibrium (that is, if the isostatic anomalies are not zero or very small) it cannot be in hydrostatic equilibrium. This is true only for rectangular and not for wedge-shaped blocks. When wedge-shaped crustal blocks float on a denser substratum there is no isostatic equilibrium in the sense that the isostatic anomalies are zero, even though the block may be in hydrostatic equilibrium. In a recent paper F. A. Vening Meinesz (“ Les ‘ graben

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